Intermittent Fasting: Vetting popular claims

In the previous article, we understood what Intermittent Fasting is, the basic science behind it and how to add it to our daily life. However, there are popular and mind blowing claims attributed to IF which must be closely examined to see whether the claims are true and who it is true for. So now, let’s examine the claims in the light of studies conducted on them.

Intermittent Fasting: What the scientific studies reveal

The hormonal benefits of Intermittent Fasting strategy is widely acclaimed. The concept of intermittent fasting is not new. Humans have practiced it as part of religion or due to other reasons for years. Yet this is a widely debated topic among the nutritionists and researchers with highly polarized studies. It is therefore important to analyze research papers objectively to understand the proven hormonal benefits of this strategy. The main points to note while reading such a paper/article are:

  • Animal studies do not directly translate to humans.[1] Recently a presentation[2] (based on experiments of IF on rats) at the european society of endocrinology concluded that IF decreases pancreatic islet mass and increases insulin resistance. However, as it is not possible to extrapolate this effect on humans. The method of study on rats is also in question[3] by other experts as not enough details are known.
  • Effect on hormones can vary for male/female subjects as well as lean/obese subjects and age group and medical conditions of the subjects.
  • The nutrition of the test subjects during the entire day must be equated to determine the effect of fasting alone. This is a common point of failure for many studies that try to make a case for/against intermittent fasting. Substantial misreporting of calorie intake by test subjects can skew the results of studies.

Insulin, Diabetes and Fat loss

For obese diabetic subjects, studies[4] show that Intermittent fasts over periods of days have been shown to improve glucose tolerance. However, for healthy, normal weight middle aged men and women, a study[5] shows that insulin sensitivity decreases and worsens morning glucose tolerance with a longer fasting window when the food intake is maintained equal. It cites that the reduced glucose tolerance is reversible and has no long term effects. Another study[6] showed that normal weight women showed reduced glucose tolerance to alternate day fasts than normal weight men. Yet another study[7] on normal weight men showed increased glucose tolerance after being subject to intermittent fasts.

The subjects that maintained same caloric intake along with intermittent fasting maintained their weight over 6 months. This is contrary to the claim that intermittent fasting causes weight loss by itself. While there is no significant weight loss benefit, the study[8] found a significant modification of body composition, including reductions in fat mass; significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations; and a significant decrease in concentrations of cortisol.

Autophagy and Longevity

Several studies show that caloric restriction improves longevity. A study[9] comparing continuous and intermittent energy restriction was conducted on young overweight women and found both approaches to be equally effective. Autophagy is engaged to varying degrees in varying tissues even over the course of a normal day with an 8 hour overnight fast. However, a study[10] on mice suggests that a 24 hour to 48 hour fast probably have the strongest autophagy effect. For humans, it is not clear that intermittent fasting provides any benefit over caloric restriction[11] for autophagy and longevity. Though popular websites promoting fasting assert definitively that intermittent fasting promotes autophagy and longevity, this article[12] highlights the need for further studies in this area.

Growth Hormone

Another aspect of fasting is that it triggers the release of HGH(human growth hormone). It is found to be released before we wake up in the morning. Food intake is found to suppress it while continued fasting is found to increase its levels multifold via spontaneous bursts of HGH[13]. As we grow older, HGH levels drop. It has anabolic properties of triggering growth and protecting against loss of muscle mass but in fasted state[14] it has the additional effect of enhancing fat breakdown(lipolysis)and glycogen breakdown to maintain blood glucose levels[15]. The increased secretion of this hormone is the main reason behind the “fountain of youth” or anti-aging[16] perspective towards Intermittent Fasting. However, external supplementation of HGH is found to be risky[17].

However, all the studies linked above are conducted on normal weight or obese males. Not enough fasting related studies feature female subjects and measure growth hormone. Most well known articles[18] supporting intermittent fasting for women are quoting a study[19] which is not based on fasting induced growth hormone at all! They were injected doses of this hormone and the subjects are men and women aged 65 to 88 years.

Resistance Training and Intermittent Fasting

It is often recommended that heavy lifting must be timed so that you can eat after your workout. Anabolic hormones are important for muscle recovery and muscle protein synthesis. Hence it makes sense that after a resistance training session, the micro tears in the muscle will repair and grow the best if Insulin increases and Glucagon decreases (by breaking a fast). Proper protein intake is also essential for preserving and increasing muscle mass.

Studies regarding this are mostly performed on male subjects. The findings[20] show that the combination of resistance training and intermittent fasting caused more improvements in body composition than just resistance training with equivalent calorie intake. A study[21] that included female participant who strength trained, showed improvement in body composition too.

Other studies[22][23] report no particular benefit but these studies either did not have any dietary guidelines or relied on self reported calorie intake (which they mention could have substantial misreporting).

Female Hormones and Intermittent Fasting

Many articles[24] use anecdotal accounts to suggest that Intermittent Fasting would cause hormone imbalance for some women. They also quote some extreme studies[25] on rats to support their theories which cannot be equated. However, there is no scientific study that conclude this. Ramadan is a religious intermittent fast undertaken by both sexes and a study[26] on it found no significant effect on female hormone levels.

An interesting aspect is that fat tissue(adipose cells) have been found to be a major source of estrogen[27]. Fat tissue plays a very valuable role in females for energy homeostasis, lipid metabolism, immune response, and reproduction. Even after menopause, fat tissue plays an important role. Therefore it makes sense that a healthy female body preserves more fat and resists fat loss. Fasting can therefore trigger a different response in women compared to men and not enough studies test this to make a definitive analysis.


  • Intermittent Fasting can be used as a tool to improve body composition and other health factors coupled with a nutritious diet and exercise
  • Weight loss is not a direct result of IF but will happen if IF is paired with a calorie deficit diet.
  • Improvements in Insulin sensitivity and glucose tolerance cannot be said to be achieved by IF generically. Such benefits seem to be limited to obese diabetic subjects and it is unclear how long these benefits persist after the fasting.
  • The effect of intermittent fasting on hormones can depend on variety of other factors such as age, sex and body composition. However, no lasting ill-effects are found and the results vary from no effect to excellent.
  • The hormonal benefits do not outweigh a bad diet or an unsustainable lifestyle. Having said that, one must try a strategy for at least a period of 1 month to understand what works or does not work for them.

[1] “Are animal models predictive for humans? – NCBI – NIH.” 15 Jan. 2009, Accessed 28 Aug. 2018.

[2] “Intermittent fasting for three months decreases … – Endocrine Abstracts.” Accessed 27 Aug. 2018.

[3] “expert reaction to unpublished poster presentation on diabetes and ….” 19 May. 2018, Accessed 27 Aug. 2018.

[4] “T HE – Metabolism – Clinical and Experimental.” Accessed 27 Aug. 2018.

[5] “Impact of Reduced Meal Frequency Without Caloric … – NCBI – NIH.” Accessed 27 Aug. 2018.

[6] “Glucose tolerance and skeletal muscle gene expression … – NCBI – NIH.” Accessed 28 Aug. 2018.

[7] “Effect of intermittent fasting and refeeding on insulin action in … – NCBI.” Accessed 29 Aug. 2018.

[8] “A controlled trial of reduced meal frequency without … – NCBI – NIH.” Accessed 27 Aug. 2018.

[9] “The effects of intermittent or continuous energy restriction … – NCBI – NIH.” 5 Oct. 2010, Accessed 28 Aug. 2018.

[10] “Short-term fasting induces profound ….” 14 Aug. 2010, Accessed 29 Aug. 2018.

[11] “Long-Term Calorie Restriction Enhances Cellular … – Science Direct.” Accessed 28 Aug. 2018.

[12] “Fasting or caloric restriction for Healthy Aging – NCBI – NIH.” 29 Apr. 2013, Accessed 29 Aug. 2018.

[13] “Fasting enhances growth hormone secretion and … – NCBI – NIH.” Accessed 29 Aug. 2018.

[14] “Fasting enhances growth hormone secretion and … – NCBI – NIH.” Accessed 29 Aug. 2018.

[15] “Evidence for increased sensitivity of fuel mobilization to growth … – NCBI.” Accessed 29 Aug. 2018.

[16] “Growth hormone and aging: A challenging controversy – NCBI – NIH.” Accessed 29 Aug. 2018.

[17] “Growth hormone, athletic performance, and aging – Harvard Health.” 19 Jun. 2018, Accessed 29 Aug. 2018.

[18] “Fasting and growth hormone – Diet Doctor.” 24 Oct. 2016, Accessed 29 Aug. 2018.

[19] “Growth hormone and sex steroid administration in … – NCBI – NIH.” 13 Nov. 2002, Accessed 29 Aug. 2018.

[20] “Effects of eight weeks of time-restricted feeding (16/8) on … – NCBI – NIH.” 13 Oct. 2016, Accessed 28 Aug. 2018.

[21] “Time-Restricted Eating In Women – A Pilot Study – Western University.” Accessed 28 Aug. 2018.

[22] “Intermittent fasting combined with resistance training … – NCBI – NIH.” Accessed 28 Aug. 2018.

[23] “Time-restricted feeding in young men performing resistance … – NCBI.” 22 Aug. 2016, Accessed 28 Aug. 2018.

[24] “Is Intermittent Fasting Safe for Women? – OAWHealth.” 15 Jan. 2018, Accessed 28 Aug. 2018.

[25] “Intermittent Fasting Dietary Restriction Regimen Negatively … – PLOS.” 29 Jan. 2013, Accessed 28 Aug. 2018.

[26] “Effects of long-term fasting on female hormone levels … – NCBI.” Accessed 28 Aug. 2018.

[27] “Estrogen production and action. – NCBI – NIH.” Accessed 29 Aug. 2018.

Intermittent Fasting: Vetting popular claims

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