Intermittent fasting for women – Is it really that bad?

I decided to do this post in English (Danish translation may follow ;o) as the suitability of intermittent fasting for women has been quite a hot topic in the blogosphere. In particular, several high-profile bloggers have posted blogs specifying how Intermittent fasting (IF) is inappropriate for women or at least more inappropriate than for men, the most notable one being Stefani Ruper’s post at PaleoforWomen (PfW). That post in particular has been quoted in a large number of other prominent health/fitness blogs, like Marks Daily Apple,, some more horribly one-sided than others. As Stefani’s post has been the source of most of this IF for women-criticism, this post will deal with the points she put forth in her original blog post.

I have really made an effort to be thorough in this post as I hope this be the post everyone links to in response to the post at PfW ;o) This post is kind of science-heavy, but I hope you’ll endure. Make a cup of coffee and sit back, while Dr. Muscle goes elbow deep (in the science, that is ;o)


First of all, let me state that I do not have an agenda claiming that fasting diets are better or more suitable for weight loss than other regimes. Also, I do not claim that the response to fasting diets are similar between genders. Actually, I’m pretty sure that there are gender differences. However, I’m quite sure that IF is not significantly worse for health or weight loss in women than men, as is stated in a number of these blog posts.

I found it necessary to write this post, since Stefani’s post has become a mainstay of IF-criticism and I think her arguments are actually quite weak. Also, her experiences from the PfW community does not match my own. As my Danish readers probably knows, I’m a co-author of a Danish book (“1-dagskuren”) and for that book we interviewed almost 100 people practising various forms of IF. Amongst those, we encountered very few adverse reports reminiscent of Stefani’s (the most common complaints were gastric problems upon resuming eating and headaches on fasting days most likely attributable to dehydration and even those weren’t that common).


Stefani starts of telling that her post is based on her experiences in the PaleoForWomen community, where she has experienced a lot of women having problems with sleeplessness, anxiety and irregular periods, in order words, generalized stress symptoms and impaired reproductive health – bad stuff. This set her on a quest into the scientific literature, which raised a number of specific points, that can essentially be broken into three separate areas:

  • CLAIM 1: A particular scientific study (the Heilbronn study) comparing men and women showing that males have favourable metabolic reactions and female have unfavourable reactions to alternate-day fasting.
  • CLAIM 2: The metabolic fasting responses observed in scientific studies are not representative to women because they predominantly use male subjects.
  • CLAIM 3: She claims that the gender-specific neuroendocrine response (especially  increased stress and impaired reproductive health) to alternate-day fasting in rats is also applicable to humans.

I will try to deal with these specific points the best I can, but in order to do so, we need to go through the very basics of fasting.

Basics of fasting metabolism

In normal (non-fasting) humans, the body runs on approximately 60-70% fat and 30-40% carbs in the resting state. This distribution changes towards a higher reliance on glucose, as the exercise intensity increases.
In women, resting metabolism is skewed slightly towards more to the “fat side” than is the case for men. This is one of the reasons it is said that female metabolism is better suited for ultra-endurance events. During fasting, the body’s carbohydrate stores are quickly depleted (

 Short-term fasting

During short-term fasting it has also been shown that women more readily adapts to metabolizing fat as the primary substrate. Thus, in short-term fasting, women have larger decreases in insulin and larger increases in glucagon, cortisol and growth hormone (all of these hormones serve to mobilize energy stores), and lower levels of circulating amino acids, compared to men (Mittendorfer et al 2001, Merimee et al 1973, Merimee et al 1978, Soeters et al 2007). All of these traits support the notion that women are better at changing to fat utilization in the short run. While one might think that the cortisol increases would possibly result in a greater breakdown of muscle, the lower circulating levels of amino acids indicates that this is not the case, since the only source of plasma amino acids during fasting, are derived from tissue breakdown. This is further confirmed by lower excretion of total nitrogen in urine (urine nitrogen is produced during protein breakdown).
In short, these gender differences in humans generally reflect the conditions in rodents, as described in an early review by Katherine Hoyenga (Hoyenga et al 1982). While I don’t know if this difference is present in all other vertebrates, I suspect that it’s partly related to the fact that in mammals, the mother should be able to mobilize fat efficiently during breastfeeding and that this effect has some carryover during times of famine.

What about long-term fasting or caloric restriction?

There are few studies looking into human sex differences in the response to sustained fasting. The only one I have been able to dig up, is the study by Runcie and co-workers (Runcie et al 1974) with 30 days of fasting in 76 subjects of both genders. This study shows that sustained fasting is tolerable across genders and that the relative weight loss is comparable between men and women. Unfortunately, the scientists in that study did not look into hormone levels, subjective experiences or habitual activity levels. The authors did comment that the females appeared slightly more heterogeneous with respect to habitual activity levels, both before and during the fasting period, but did not claim that the average female response appeared different from the male’s. Also, there are a number of newer, shorter duration fasting studies (3-14 days). In general they report weight loss (DUH!) and an improved metabolic profile.

An interesting point should be made at this time; following sustained fasting, glucose tolerance upon resuming feeding is impaired for a short time (Horton et al, 2001). While there is no science on how long this effect persists, committed fasters with blood glucose meters have told me that this happens only during the first meal following a fast. Thus, although, the metabolism seems to benefit overall, there is a transient impairment of glucose control.

Glucose and insulin response to glucose challenge following 72 hour fasting (Horton et al, 2001)

Glucose and insulin response to glucose challenge following 72 hour fasting (Horton et al, 2001)

Furthermore, there are experiences from long-term caloric restriction that may also help elucidate this topic. In this field there is two studies of particular interest: The BIOSPHERE 2 project and the CALERIE study. BIOSPHERE 2 was a project in which 8 adults (scientists) were enclosed in a sealed habitat, essentially a huge greenhouse, and stayed there for 2 years, without receiving external supplies. During their stay in the habitat, the researchers came to realize that the amount of food the habitat could supply was not adequate for their normal energy expenditure and thus they came to rely on involuntary caloric restriction for the majority of the 2 years (Walford et al 2002).The CALERIE trial is a clinical trial being done in the USA under the National Institute of Aging, where several hundred normal weight, adult subjects are undergoing 2 years of 25% caloric restriction. The trial finished in 2012 and the first publications with results are in preparation as I write this, but there are already several publications describing the inclusion process and study design. Both of these projects have thus resulted in a number of scientific studies and amassed considerable public media awareness. As far as I know, there’s been no mention of the women suffering more than the men during their enclosure or being more prone to generalized stress. While this is not evidence by itself, it should be noted that if serious adverse reaction like prolonged loss of the menstrual cycle arose, it would have to be reported, as these were both official clinical trials registered under and the NIA.

You might think, “what is the relevance of sustained fasting and chronic caloric restriction” and the answer is that with intermittent fasting, some of the same metabolic and endocrine adaptations manifest, depending on the total fraction of time you are fasting. Therefore, if human females were generally more susceptible to the negative aspects of fasting during prolonged fasting or caloric restriction it would be more likely that this would also be the case during IF… and this ties beautifully into the next paragraph…

Then what about intermittent fasting?

Well, there are several kinds of intermittent fasting. The type most frequently used in scientific studies is alternate-day IF (ADF), while the types being used in the public are mostly either 1-2 weekly fasting days or part day fasts, like the 16:8 fast. As far as the state of the art goes, there is a bunch of studies on ADF, one or two on intermittent fasting reminiscent of 5:2 or 6:1 fasting in humans and only one reminiscent of part-day fasting (e.g. 16:8 or 19:5 fasting) at the time of writing.

Well, the results from the ADF IF studies in overweight individuals have generally shown  improvements in the metabolic phenotype in men AND women (improvements in cholesterol, LDL particle size, fasting insulin and glucose and blood pressure) (Eshginia et al, 2013; Varady et al 2009;  Varady et al, 2010; Varady et al, 2011; Johnson et al 2007). Also, the 6:1 fasting study (done in women) by Klempel (Klempel et al 2012) showed solid improvements in health biomarkers, as did the 5:2 study (done in women) by Harvie (Harvie et al, 2011).

In summary, fasting is safe and IF diets does work for weight loss and health biomarkers for men AND women. Also, they appear to have similar health effects to other energy restriction modalities resulting in similar weight losses.

Getting to Stefani’s points

Ok, having run you through the basics I’m ready to address the specific critique points that Stefani took up at the PfW blog:

  • CLAIM 1: the Heilbronn study shows that males have favourable metabolic reactions to alternate-day fasting.

The Heilbronn study is an alternate day fasting study conducted with normal-weight adults (8 men, 8 women) for 3 weeks with baseline measurements at day 21 (an eating day) and day 22 (a fasting day). this resulted in an average weight loss of 2.1 kgs. It resulted in two publications, one dealing with the effects of a glucose challenge and one dealing with fasting state numbers and energy expenditure numbers (Heilbronn et al, 2005a and 2005b). In the former of these studies, they showed some apparently rather unsettling results regarding glucose control which forms a major part of Stefani’s IF-criticism. In the study women appeared to have impaired glucose clearance following a glucose challenge (no change in men), and no change in insulin response (improvement in men). See the figure below:

Figure from Heilbronn et al 2005, showing an impaired glucose control response on a fasting day following 3 weeks of alternate-day fasting

Figure from Heilbronn et al 2005, showing the response to a glucose challenge before the study and after (on a fasting day) following 3 weeks of alternate-day fasting, showing an improvement in insulin response in men (no change in women) and and impaired glucose clearance in women (no change in men) (Heilbronn et al, 2005a and 2005b)

While this may in fact be true, I think the results in overweight and obese women from other studies, where ADF generally improves the metabolic phenotype should warrant some scrutiny. Specifically, two points comes to mind. Firstly, if you look at the curves above you’ll notice that for the insulin response  in men before the study (“pre men”) the levels peak rather high, and that the variation is also quite big. This means that there is high variation amongst the men and most likely, that just a few of them of them have a very big insulin response, indicating that they were less well off, metabolically.

Indeed, if you look at the baseline characteristics (below), you’ll see that the men were considerably less metabolically fit than the women, being slightly overweight, having higher relative body fat percentages (25% is normal for women, 22% is kind of high for men), having higher resting glucose and higher resting insulin. Having a resting insulin of 10.5 is, although not diabetic, certainly not “good” and just as certainly, a lot poorer than the women, explaining why the men seemed to improve and the women didn’t.

Baseline characteristics of the participants in the Heilbronn study

Baseline characteristics of the participants in the Heilbronn study

Indeed, another study using younger, leaner healthier men subjected to ADF for two weeks and then measuring insulin and glucose dynamics using an euglycemic clamp, showed no improvement in insulin sensitivity or glucose clearance in men following IF (Soeters et al, 2009). An euglycemic clamp is a more controlled way to exposed people to a glucose challenge – clamping is normally considered a stronger scientific model.

Secondly, considering that women are already more adept at mobilising fat than men and that we know that even short periods of fasting can induce transient impairment of glucose clearance, it is rather likely that the impaired glucose clearance observed in women is caused by the transient fasting effect, and that this effect is masked in the men because of their higher glucose response at the beginning of the study due to their poor metabolic phenotype. If you look at the fasted state insulin and glucose numbers, they’ll show that both genders does seem to improve, but with a striking difference at baseline.

Changes in fasting insulin and fasting glucose in the Heilbronn study

Changes in fasting insulin and fasting glucose in the Heilbronn study, from baseline (white) to day 21 (eating day, gray) to day 22 (fasting day, black)

In any case, I do not think that the results from the Heilbronn study can be used to say that IF harms female glucose clearance or metabolism as such, unless a similar result can be produced a day later (dodging the transient effect) and hopefully in more health-wise comparable groups.

Summarizing the Heilbronn study, I’m 99.9% sure that the reason the men seemed to improve while the women didn’t, was because they were overall less metabolically healthy and I also think that it is very likely that the impaired glucose clearance following fasting in women is the transient post-fasting impairment described earlier.

  • CLAIM 2: That the metabolic fasting responses observed in scientific studies are not representative to women because of them using predominantly male subjects

Actually this criticism is two-pronged, with some of it related to metabolic effects of fasting and some of it related to the neuroendocrine effects of fasting.

As for the metabolic effects (as I have already discussed), IF works for women, as far as both weight loss and health biomarkers go. If you don’t believe me, please write me and I’ll help you acquire the original research articles cited previously.

As the only study that compares the metabolic responses of normal weight men and women, subjected to IF, concludes that women fares badly on IF, but does so on REALLY wobbly ground, it looks slightly like cherry picking on Stefani’s behalf claiming that IF is not appropriate for women. There is SOLID evidence that it is safe and works for pretty much all relevant health biomarkers for overweight men AND women, refuting her second claim.

  • CLAIM 3: She claims that the gender-specific neuroendocrine response (especially  increased stress and impaired reproductive health) to alternate-day fasting in rats is also applicable to humans.

With basis in several rodent studies, Stefani claims that fasting is a lot worse for females than males. Particularly, according to her, IF leads to anxiety, sleeplessness and impaired fertility. I can’t help but finding it slightly ironic that she dismisses the applicability of studies using male humans, but have no problems using rodent studies. As you may already have guessed I’m gonna question if the neuroendocrine response to fasting in rodents is representative of the response in human women.

First, I’ll be going into the part about a generalised stress response to fasting. When looking for relevant human trials, sadly there are no human studies looking into the effects of fasting on biomarkers of subjective stress. However, as amenorrhagia is considered a serious adverse effect in weight loss trials, it is rather surprising it has yet to be mentioned in any of the trials that have been published thus far. A few studies does show a bigger or earlier increase in epinephrine and cortisol in response to fasting, but not all, and certainly not to a degree, that could cause a physiological response akin to that Stefani describes in rodents. And some studies show bigger responses in males than in females – what gives? However, considering the better ability of females to mobilize fat as substrate, it would make sense for them, at least normally, to have either bigger adrenaline/epinephrine responses or enhanced sensitivity.

When going into the part about reproductive health it is somewhat easier. Rodents are known to have a more extreme responses to caloric restriction than mammals higher up the food chain. This is likely because for rodents, successfully breeding as big a litter as possible has been an important evolutionary selection criterion, and hence limited access to food would down-regulate fertility, especially in females. For humans and the other primates this is not the case, as years of nurturing is more important for producing viable offspring, than temporary limited access to food. Thus, it has actually been shown that in primates, females are less sensitive to short-term energy restriction than males (Lado-Abeal et al 2005). I’ll take the liberty of quoting from the abstract:

The changes in plasma LH and cortisol levels in fasted rhesus macaques are similar to those observed in humans and suggest that gonadotropin and corticotropin secretion are more resistant to short-term energy deprivation in female than in male primates.

In humans, it has been shown that 72 hour fasting does interact with the LH secretion during the follicular phase, but not enough to perturb follicle development or cycle length in normal-weight women (Olsen et al, 1995). However, it has also been shown that the same kind of intervention does lead to some disturbance of the menstrual cycle in lean women (BF<20) (Alvero et al, 1998). However, it can hardly come as a surprise that already lean women are more susceptible to the stressors of fasting and weight loss.

Summarizing, while I cannot refute, Stefani’s claims about anxiety and sleeplessness, I CAN say that it is unlikely that the rodent studies she’s citing reflect the human fasting condition. Also, the lack of any mention of these problems in the fasting literature (which, contrary to Stefani’s claims otherwise, does contain a lot of female subjects), reinforces this notion. As for the effects on reproductive health, I can refute her claims quite well. As long as we are not dealing with already lean women fasting for weight loss, there is little risk of perturbing the menstrual cycle. But then again, lean women should generally be wary of being in energy deficits no matter what (fasting or not).


Once again, I’d like to state that I’m not crusading for fasting. Actually, I’m pretty sure that it doesn’t matter that much whether you cut 25% from each meal, your number of meals/day or the number of days you eat. I have dealt as best I could with the individual points Stefani put up in her IF-criticism and this leads me to what started her out; i.e. her experiences from the PfW community. I cannot help but wonder if the fasting women she is referring to were also doing IF, Paleo and possibly even working out at the same time and thereby establishing too large energy deficits or too large diet/training metabolic stress loads and hence having bad experiences with fasting. This would seem to match better with the available evidence (and also the experiences I have made).

Also, I get the feeling that she’s pulling some kind of feministic agenda down on top of the science, (“What? are the studies mostly on men?! That must mean they’re hiding something”), however that is just speculation on my behalf. However, everyone should be able to see that it’s just silly to refute male studies, while gladly accepting them from a  different species, especially, when the applicability of this species is questionable (but I guess she didn’t know that).

Summarizing (again ;o), for non-lean women IF is a safe and tolerable approach for weight loss. The available science does not support the notion that it damages reproductive health or causes anxiety or sleeplessness. However, as with all things, certain individuals may be more susceptible to others. Also, lean women trying to get leaner are always at risk of menstrual cycle irregularities with or without fasting.


Alvero, R. (1998). Effects of Fasting on Neuroendocrine Function and Follicle Development in Lean Women. The Journal of clinical endocrinology and metabolism, 83(1), 76–80. doi:10.1210/jc.83.1.76

Eshghinia, S., & Mohammadzadeh, F. (2013). The effects of modified alternate-day fasting diet on weight loss and CAD risk factors in overweight and obese women. Journal of diabetes and metabolic disorders, 12(1), 4. doi:10.1186/2251-6581-12-4

Harvie, M. N., Pegington, M., Mattson, M. P., Frystyk, J., Dillon, B., Evans, G., et al. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International journal of obesity (2005), 35(5), 714–727. doi:10.1038/ijo.2010.171

Heilbronn, L. K., Civitarese, A. E., Bogacka, I., Smith, S. R., Hulver, M., & Ravussin, E. (2005a). Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obesity research, 13(3), 574–581. doi:10.1038/oby.2005.61

Heilbronn, L. K., Smith, S. R., Martin, C. K., Anton, S. D., & Ravussin, E. (2005b). Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr, 81(1), 69–73.

Horton, T. J., & Hill, J. O. (2001). Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal. J Appl Physiol, 90(1), 155–163.

Hoyenga, K. B., & Hoyenga, K. T. (1982). Gender and energy balance: Sex differences in adaptations for feast and famine. Physiology & Behavior, 28(3), 545–563. doi:10.1016/0031-9384(82)90153-6

Johnson, J. B., Summer, W., Cutler, R. G., Martin, B., Hyun, D.-H., Dixit, V. D., et al. (2007). Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radical Biology and Medicine, 42(5), 665–674. doi:10.1016/j.freeradbiomed.2006.12.005

Klempel, M. C., Kroeger, C. M., & Varady, K. A. (2013). Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. Metabolism: clinical and experimental, 62(1), 137–143. doi:10.1016/j.metabol.2012.07.002

Klempel, M. C., Kroeger, C. M., Bhutani, S., Trepanowski, J. F., & Varady, K. A. (2012). Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutrition Journal, 11, 98. doi:10.1186/1475-2891-11-98

Kumar, S., & Kaur, G. (2013). Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis. PloS one, 8(1), e52416. doi:10.1371/journal.pone.0052416

Lado-Abeal, J., Robert-McComb, J. J., Qian, X.-P., Leproult, R., Van Cauter, E., & Norman, R. L. (2005). Sex differences in the neuroendocrine response to short-term fasting in rhesus macaques. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma, 17(7), 435–444. doi:10.1111/j.1365-2826.2005.01323.x

MERIMEE, T. J., & FINEBERG, S. E. (1973). Homeostasis During Fasting. II. Hormone Substrate Differences Between Men and Women. The Journal of clinical endocrinology and metabolism, 37(5), 698–702. doi:10.1210/jcem-37-5-698

MERIMEE, T. J., MISBIN, R. I., & PULKKINEN, A. J. (1978). Sex Variations in Free Fatty Acids and Ketones during Fasting: Evidence for a Role of Glucagon. The Journal of clinical endocrinology and metabolism, 46(3), 414–419. doi:10.1210/jcem-46-3-414

Mittendorfer, B., Horowitz, J. F., & Klein, S. (2001). Gender differences in lipid and glucose kinetics during short-term fasting. American journal of physiology Endocrinology and metabolism, 281(6), E1333–E1339.

Olson, B. R. (1995). Short-term fasting affects luteinizing hormone secretory dynamics but not reproductive function in normal-weight sedentary women. The Journal of clinical endocrinology and metabolism, 80(4), 1187–1193. doi:10.1210/jc.80.4.1187

Runcie, J., & Hilditch, T. E. (1974). Energy provision, tissue utilization, and weight loss in prolonged starvation. British medical journal, 2(5915), 352–356.

Soeters, M. R., Lammers, N. M., Dubbelhuis, P. F., Ackermans, M., Jonkers-Schuitema, C. F., Fliers, E., et al. (2009). Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. Am J Clin Nutr, 90(5), 1244–1251. doi:10.3945/ajcn.2008.27327

Soeters, M. R., Sauerwein, H. P., Groener, J. E., Aerts, J. M., Ackermans, M. T., Glatz, J. F. C., et al. (2007). Gender-Related Differences in the Metabolic Response to Fasting. The Journal of clinical endocrinology and metabolism, 92(9), 3646–3652. doi:10.1210/jc.2007-0552

Varady, K. A., Bhutani, S., Church, E. C., & Klempel, M. C. (2009). Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. American Journal of Clinical Nutrition, 90(5), 1138–1143. doi:10.3945/ajcn.2009.28380

Varady, K. A., Bhutani, S., Klempel, M. C., & Lamarche, B. (2010). Improvements in LDL particle size and distribution by short-term alternate day modified fasting in obese adults. The British journal of nutrition, 105(04), 580–583. doi:10.1017/S0007114510003788

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  1. Bert on 2013-09-19 at 14:24

    I agree with your analysis. The fact that women did not have a reduction in insulin response only says that the men were abnormally high to start with. Also, one should also be cautious about interpreting lab results that are sampled during weight loss, as it is a dynamic, unsustainable condition. The people in the study were losing weight, but it is the steady state "lifestyle" values before and after weight loss is complete that should be compared, as those are the physiologic conditions to which the individual's body will be exposed for decades.

    • incognitodk on 2013-09-19 at 14:39

      I agree strongly with your points. Pretty much all weight loss trials show the same biomarker improvements irrespective of modality ;o)


  2. majbrit on 2013-09-19 at 15:18

    Very important information – thank you!

    • incognitodk on 2013-09-19 at 16:22

      well thank you too, Majbrit ;o)


  3. chris on 2013-09-20 at 00:19

    great read!

  4. Jonas on 2013-09-20 at 07:19


  5. Brodie on 2013-09-21 at 19:17

    Thanks for your thorough analysis. I have been wondering for a while whether the talk about negative effects for women using intermittent fasting were true…

    • incognitodk on 2013-09-23 at 07:56

      Thank you Brodie

  6. Dave on 2013-09-22 at 01:07

    Very interesting and thought provoking Anders, thank you.

    • incognitodk on 2013-09-23 at 07:56

      You're very welcome ;o)


  7. Ann Nony on 2013-10-21 at 00:25

    Your post was exactly what I was looking for — an analysis of the interpretation of scientific studies that may be driven by an agenda. I am trying IF and as a perimenopausal woman, was kind of frightened off the whole concept after what I read in the sources you listed. After reading your article, I feel much more reassured. Thank you!

    • incognitodk on 2013-10-21 at 07:54

      Hi Ann.
      Thank you for your comment. While fasting may not be for everyone, which is true for all kinds of lifestyle interventions, there is no scientific argument making it unsuitable for women per se. gogogo ;o)

  8. Ahamed Shego on 2013-10-28 at 08:52

    Weight loss is a tough process. Following the best plans is highly important to lose weight significantly. Thing is I'm looking forward to follow some of these new ways to weight loss.

  9. dsf on 2013-11-24 at 08:36

    Great blog.

  10. Underweight on 2013-11-28 at 02:50

    I have been doing alternate day IF on and off since 2007. I'm female, 5'7", my natural weight pre-IF was 105-115 lbs, depending on my activity level (I'm a runner), with 25% body fat – I've never been very muscular, though I'm stronger than I look. During my first three months of IF, I alternated 36 hours of complete water fasting with 12 hours of unregulated eating that usually amounted to about 2000 calories, and I dropped down to 90lbs and 16% body fat. While I looked quite thin, I was in fact completely physically healthy and felt fantastic, full of energy, mentally sharp, and athletically I demonstrated improved strength and stamina. What particularly amazed me, was how easily and rapidly I built muscle, even in my relatively weak upper body. My only side effect was constipation, and I suffered no menstrual irregularities.

    I'm now 32, and since I began menstruating at age 13, I have never skipped a period or failed to ovulate (to my knowledge – I haven't always used urine test strips, but I have distinct ovulatory pain every month). My cycle is like clockwork, unless I'm double-ovulating that month, which started in my 20s and has been increasing in frequency as I age. I even ovulate on hormonal contraception (including almost a dozen combined BCPs, 2 progestin-only pills, the Patch, and the Nuva Ring), and had several pregnancies and abortions in my early 20s before I figured out a form of effective birth control (copper IUD).

    I eventually increased my caloric uptake on eating days to 3000-4000 calories and gained back some of my initial weight loss, though much of it was muscle, returning to my natural weight range, around 110lbs, after a year of IF. Since then I have been doing IF on and off, always feeling far better on than off. In 2012, having not fasted at all in the previous year, I removed my IUD and 3 months later I intentionally conceived (twins). Unfortunately one was ectopic and I miscarried them both.

    In the last 6 months I have been fasting again; I started slowly with reduced calories on my fasting days and gradually reduced them further. For the past month I have been doing under 300 calories during my fasting days. Once again, I feel dramatically better. Usually winter is hard on me psychologically, but going into it fasting, I'm finding myself more energetic and upbeat than usual. I haven't weighed myself in over a year, but I've found I do better when I don't weigh or measure myself. My clothes fit well and I feel good. I've been at 20% body fat since the end of my first year fasting – without intense exercise or calorie restriction, that seems to be my new normal while fasting. No matter what, I've kept more muscle.

    Hopefully my experience with IF as s

    • incognitodk on 2013-12-02 at 10:56

      Good to hear you've found something that works so well for you ;o)


  11. Bernadette on 2013-12-28 at 22:05

    I am really confused about IF…I am a 57 year old post menopausal woman who has been doing IF for almost a year. In the interest of the health benefits I fast all day with drinks and eat my 500 cals in the evening. I have dropped over 18 lbs. I was hoping to reduce my cholesterol which has always been on the high side. I had my bloods done two weeks ago and to my shock and disappointment it had gone up! My LDL has risen above 4.5 my HDL is good at 2.9 but my fasting glucose has risen to pre type two diabetes plus my blood pressure is up. I was so disappointed because I feel great and delighted to have shed the weight. I don't know what to do now. I wondered if all day fasting has put stress on my adrenals? Any help would be appreciated

    • incognitodk on 2013-12-31 at 12:18

      Hi Bernadette
      First of all, i think you should have your blood work redone. There may be significant day to day variations with these measurements and particularly with fasting. When you have it redone I think you should look into whether or not you have any of the common deficiencies, like Vitamin D or iron. If those are low, they could contribute. If your biochemicals are clean and the metabolic markers remain poor, I suggest you try to eat ketosis friendly on your fasting days (or abstain totally from eating) and when you resume eating following a fast, take it easy on the carbs in the first couple of meals (something like 20% carbs on the first and 40% on the second meal). Do this for 2-4 weeks and have your blood work redone again and see if it helps.
      And one final thing. Take a brisk walk of 30 minutes every day. Everyone should do this ;o)

    • katherin on 2014-06-21 at 16:21

      i would very much like to hear if you did re-do your bloodwork and whether there was any change as i have about the same characteristics as you. i did IF for several months a year ago and would like to try to start again, but I don't want to bring my cholesterol up any further than it already is.

      • incognitodk on 2014-06-23 at 10:16

        No, I didn't but now my bloodwork is exemplary, but I'm very physically active and eat decently


  12. saran on 2013-12-31 at 12:06

    Thank you so much for writing this. I've been consistently doing IF (18/6 or 14/8) on a keto diet for the past 2 months. I recently read Stefani's post and it really concerned me. Your article was very helpful! I really appreciate you walking us through the basics before getting into the specifics.

    Given that my cycles are fine and I feel no negative side effects whatsoever, I'm going to stick with the keto and IF, which has been helping me immensely 🙂

    • incognitodk on 2013-12-31 at 12:19

      sounds good Saran. That was the purpose of the post, so I'm happy it's reaching someone ;o)


  13. Sheila on 2014-06-29 at 18:40

    I’m so new to this (2 weeks) but I will share my results to date. I have done the 2 days 24 hour full fast. I have decided to use IF and ADF. I can’t work out yet but as I heal I plan to add weights and small cardio as I recover from the knee surgery. What is true for me now..
    I am a, over weight (5’6’159lbs) 50 yr woman and in the last year I am starting to go months in between my cycle. I started late with my periods as a teen too and noticed over the years that if I was stressed (regardless of body fat /weight) I would stop having a cycle a month or two at a my early years it made for a few SCARY moments before the improved pregnancy test now available, LOL. I’m not upset if my cycle is thrown of by IF,for me it’s not too far off of normal.
    I have noticed sleepless that is a bit scary. I wake up after 3 hours of sleep and I’m wide awake it’s not hunger it’s just awake, the discomfort comes from eye dryness and then I feel foggy in thought and over all energy. Right now I’m recovering from knee surgery so the lack of sleep is not negatively impacting my life , because I’m off of work. However, I would be stressed about it if I was working. I can’t speak for all women but the anxiety about that can add to the stress and the stress can impede weight loss.
    i read this blog and wanted to see if you have heard this before? i think the important fact to keep in mind is that each woman is different and the same some of this may not work but it does not mean you drop it you must balance the IF for you, your body and what feels positive.

    • incognitodk on 2014-06-30 at 10:01

      Hi Sheila
      I think that fasting is a stress to the body. If that is combined with physiological or psychological stress from other sources, this can lead into a stress syndrome of sorts, which does not facilitate beneficial changes in body composition. I also wake quite early after fasting and have slightly lower sleep requirements, but if this leads to a sense of not sleeping enough, then you should probably cut down to fasting to some sort of within-day IF until you've grown slightly more tolerant. Stay hungry as we say ;o)


  14. Danielle on 2014-07-07 at 09:02

    Thank you so much for this information! I am just starting out and hoping for the best! Just wanted to say also, that you should consider adding Pinterest to your sharing options. If I missed it, I apologize! But I would like to revisit this in the future.
    Thanks again!

    • incognitodk on 2014-07-07 at 18:31

      Hi Danielle.
      Thank you very much for your kind words. You are definitely right about sharing options and it is something i have planned to change in the very near future.

  15. Daryl on 2015-02-06 at 23:23

    Thank you for writing this and clarifying some of the finer points about this diet.

    • incognitodk on 2015-02-12 at 09:47

      You are very welcome. Thank you for your comment


  16. Babyghost on 2015-03-02 at 11:43

    Thankyou for the clarrification, i didn’t understand everything but in the end it is not perse damaging for woman to IF right? I don’t sport because i study fulltime and i don’t like to workout since i overtrained last year. I am IF for 3 days now. I do 16/8 and honestly i got hungry before bed and very verh hungry the next day before my first meal. I will see how this workout after one month 😉

    • incognitodk on 2015-03-09 at 09:53

      You are right. From a scientific point of view, it should not be dangerous to healthy adults, male or female, to not eat for for extended periods of time once in a while, as long as bodyweight doesn't drop too low. As for your own IF, try longer fasting windows or eating slightly less carbs. That may make the fasting easier.


  17. Laura G on 2015-04-05 at 10:59

    I'm glad to find an article that looks a bit more in depth at the science behind the effects on men vs women. One thing that frustrates me is that most of the stuff online about women and fasting focuses primarily on weight loss (even some that make downright unhealthy claims, like: fasting can help you drop 3 dress sizes in a week, er, no thanks, I don't even want to think about the damage that kind of extreme weight loss would do to my body) which makes it seem as if women, or the people writing for them only care about weight loss. I'm not sure about other women but personally I am far more interested in the long term health benefits and as of yet have not found any evidence that these are not possible for women (though if anyone has any evidence to contradict this please let me know). I would like to try fasting and see how it works for me and if anyone has any advice it would me very much appreciated, like I said I'm not that fussed about weight loss, I want a plan that I can use for the foreseeable future. I am also open to any evidence why I should consider not fasting (I am 26 years old, short, average weight and the only issue I have is irregular periods (so it would be hard to tell if fasting effects this or not). I am looking for information for and against as I want to make an informed decision. Thanks all and Happy Easter 🙂

    • incognitodk on 2015-05-09 at 15:44

      Thank you Laura and have a nice lent ;o)


  18. Ashley on 2015-11-23 at 17:31

    my research on IF is not hugely extensive however, I really feel after doing IF MYSELF and being perfectly fine, that the issue is CALORIC RESTRICTION while doing IF. If you starve your body… you will more than likely experience hormonal imbalance, missed period's, hair loss etc. (the symptoms associated with anorexia) along with several other side effects. IF is NOT "the" diet, it is simply a tool that works with your regimen. If you eat nutrient dense foods and you don't restrict calories I really don't see why a fasting protocol would cause any issues. Again, if you are fasting 18+ hours a day, and eating 500 calories or less… I see exactly why your body would be experiencing issues. I personally eat between 12:00PM and 8:00PM, eating 1600-1800 calories a day (sometimes MORE on training days) and the fat is melting off, along with less anxiety, better sleeping patterns, great workouts… if anyone wants to talk about this further, good luck to everyone.. 🙂 again, everyones body is different, but a lot of us have trouble eating enough calories, in fear of gaining wait, when really we need enough calories especially when doing fasts. Just my two cents.

    • incognitodk on 2015-11-25 at 17:50

      Hi Ashley
      I agreee most of the way. However, intermittent fasting on an ad libitum diet does tend to to put you at a caloric deficit. And if that caloric is sustained it can and will mess with part of the physiological function as all diets will. Also, even short term deficit tends to slightly delay ovulation, which is perfectly normal.

  19. Jane on 2016-01-22 at 14:56

    Hi, I to was sceptical about Stephanie Rupers post so came across your blog after doing some further research.
    I am a 51 year old 5' 7"woman, not yet menopausal. I don't work out regularly but do light exercise built into most days (walking, housework, yoga). I work as a web designer so at the desk for a good part of the working day ( I work from home so can break up my day more easily with movement).

    I started my version of the 5:2 diet 6 weeks ago combined with missing breakfast most days as I tend to normally not eat breakfast until 10.30am+ anyway.

    I start each day with a herbal tea (no sugar) & water, then eat fairly healthy the rest of the day, (salads, good protein for lunch (salmon, egg), veg & protein for dinner, all home cooked meals for the most part without added sauces etc – also do not eat much wheat based products at all, I do still have some under 100 cal snacks most days – usually fruit ices, nuts, or small pack of crisps, a biscuit etc.

    On the 5:2 'fast' days I never stick to 500cal but average around 750cal.

    I started out at 73 kgs six weeks ago, & am now 67.8kg, I did eat whatever I wanted over the Christmas break as well as the usual boxes of chocolates etc & gained 1kg during that time with a few low cal days between the food fest days, but stared losing weight again as soon as I resumed my fast routine.

    I find the weight loss is about 1kg per week, with weight loss stable for about 3 or 4 days before the next fast day & a drop, whereas before my weight was up & done each day. I am the lightest I have been in a decade, although never very overweight I was up to 80kg 5 years back & hovered around the low 70's for the last few years after initially losing 10kg on low carb healthy eating regime.

    I don't feel tired or have trouble sleeping. Only side effect is a bit of constipation.

    What interests me most is blood glucose effects as I was fasting 5.6 glucose last year & 8 years prior 5.8 – the doctor says it's pre diabetic but I believe it's stress induced as I get very stressed out doing any blood tests & cortisol can increase blood sugar levels – all my other markers were OK. triglycerides very good, cholesterol average & blood pressure excellent for years. Using the home glucose meter I having shaking hands if I think about it too long which is a stress response, so I know I'm prone to that. I pointed out to the doctor it had actually come down.0.2 in 8 years which she reluctantly admitted – I asked why it was not considered pre diabetic 8 years ago & was advised that the cut off was lowered since then – probably to sell more drugs IMO! I think doctors are often too keen to treat rather than cure.

    Also, all the diabetes people say losing 5-7% body weight reduces risk of Type 2 diabetes by a large percentage – so even if blood glucose went up slightly during the time you are on the 5:2 or whatever, wouldn't the resulting weight loss if you were overweight to some degree be more beneficial than any temporary glucose issues?

    I think what I'm doing works very well for me, I rarely get fatigued during the day or feel super hungry on fast days or the day after. I will keep at it until I reach my goal weight of around 65kg, then play around with what works best for the glucose levels etc after that.

    Sometimes all the 'science' online & even that done by scientists is contradictory & confusing – it's annoying, but to me at least part of the day without food or days with lower calories sounds like a logical fit to how we developed over thousands of years.

    Now I just need to train myself to exercise more & build more muscle before I get into menopause proper.

    Hope this info is useful to other women my age wondering about IF. 🙂

    • incognitodk on 2016-01-25 at 11:11

      Thank you for your comment ;o)


  20. IT Works Products on 2016-01-24 at 14:58

    why do so many questions make generalized assumptions about half of the human population? This goes for the "why do boys/men do [insert random generalisation here]" questions as well as the women ones.

    In this case, not all women are the same, some take criticism, some don't. Until you've met every single woman on the planet and devised some sort of test or experiment to measure who can and can't take criticism and how you would go about proving it, would it really be so hard to just shy away from tarring everybody with the same brush

    • incognitodk on 2016-01-25 at 11:13

      having done clinical studies myself, I can guarantee you that scientist do everything they can to reduce natural variation in their subjects. Using men is part of that as it excludes the menstrual cycle as a contributor of variation. Also men are in general easier to recruit to studies that involves a lot of needles. Don't know why.


  21. Kim on 2017-01-24 at 16:11

    Loved your thoroughness. I hate “shoulds”. This was a great article, I drown out people who thinks everyone should do the same thing. I don’t know how one can think less food in the body is harmful. If you are fasting and then eating horrible food on other days, to me that’s what’s not healthy. It makes sense to me to give your body rest.

    • Anders Nedergaard on 2017-02-12 at 21:30

      thank you very much ;o)

  22. Elisa on 2017-06-12 at 14:48

    As a female who has been doing if for 5 years , thank you for this post !

    • Anders Nedergaard on 2017-09-13 at 14:08

      you are very welcome ;o)


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