Does intermittent fasting work? For men yes – but it might be a bit harder for women…

I really enjoyed watching BBC’s Doctor in the House, which helped put functional medicine into the mainstream media. As an Applied Functional Medicine in Clinical Practice graduate I use functional medicine approaches in my clinical practice all the time. In the first episode Dr Chaterjee advised the diabetic father of the house to consider intermittent fasting for weight loss. The patient did fantastically on this: he lost weight, felt better and vastly improved his blood sugar profile.  Intermittent fasting has become pretty fashionable recently – so what evidence is there for fasting?

The first interesting thing to say is that restricting calories intermittently, say a couple of times per week, seems to be more successful than restricting calories everyday.

This study [1] compared intermittent fasting (i.e. calorie restriction below 650 kcals for 2 days per week) to daily calorie restriction (eating less than 1500kcal per day). The intermittent fasters lost 6.4kg versus the daily restrictors’ 5.6kg, and the intermittent fasters saw greater improvements to their insulin levels. A second study [2] also found that intermittent fasting was superior to daily restriction: 65% of those in the intermittent fasting group lost >5% of their body weight versus only 37% in the daily restricted group and the insulin reduction and body fat reduction was significantly greater in the intermittent fasting group. Plus, people were more likely to stick to intermittent fasting – the compliance rate was 76%, whereas only 39% stuck to the daily energy restricted diet.

We don’t yet know the ideal intermittent fasting regime or how many calories should ideally be consumed on fasting days but the take home message from these studies is that:

  1. ‍Just 1 day per week of restriction maintains weight loss!
  2. You can eat what you want on non diet days and still lose weight. On the intermittent fasting days one can eat 650kcal in any form and still lose weight, although I would strongly advocate that even these 650 kcal are made up of healthy food to keep all the rest of your metabolic pathways in balance!
  3. Intermittent diets have health benefits even if you are a healthy weight.
  4. Intermittent fasting not only helps weight loss but helps abdominal fat loss which is the most dangerous type.
  5. Intermittent fasting increases your metabolic rate.
  6. Intermittent fasting causes less muscle loss than constant restriction of calories.

The benefits of intermittent fasting appear to go beyond weight loss. Intermittent fasting has beneficial effects on our hormones: blood levels of insulin drop significantly, which helps in diabetes(1) and blood levels of growth hormone may increase as much as 5-fold. intermittent fasting may improve inflammation (which is the pathology that underlies so many diseases) and there are beneficial changes in several genes and molecules related to longevity and protection against disease.

However, I have to put a caveat in here. In my experience most men get on well with intermittent fasting but some women who have tried intermittent fasting have found it caused sleeplessness and anxiety.  A 2007 study in the American Journal of Clinical Nutrition [3] reported that three weeks of alternate day fasting found that in women but not men blood sugar control actually worsened. This doesn’t necessarily mean that as a woman you shouldn’t try intermittent fasting but perhaps not if you are having other hormone problems at the same time, as this may worsen things.

The menopause really changes things when it comes to weight! When oestrogen levels drop this turns on a fat making enzyme called ALDH-1A1 which causes us to store fat especially around the waist, and central fat is the worst kind of fat. Central fat is visceral fat which is linked with type 2 diabetes, heart disease and cancer. Diets high in unhealthy fats (cakes and cookies) further increase ALDH-1A1.  Normal levels of oestrogen suppress the effects of the enzyme. Women in the menopause have to work hard at weight loss, but I would recommend the following:

  • If you’re having hormone problems you may find you need to eat 3-5 small meals a day, which is fine if you  make them healthy – vegetables, oily fish, whole grains, spices, protein (which boosts hunger fighting hormone peptide YY) nuts (which stimulate CCK which turns down appetite and slows down gastric emptying) and plenty of water.
  • Always eat breakfast – a heavy breakfast, a moderate lunch and a light dinner have been the time-tested recipes for weight loss. In an Israeli study people were put on a low carb diet with the same overall calories, with one group given a small breakfast allowance and the second a big breakfast allowance. After 16 weeks both lost weight but by 32 weeks the small breakfasters had regained some of their weight whereas the big breakfasters continued to lose.
  • Eat good fats with vegetables as the fibre slows fat digestion and the fat helps absorb vitamins from the vegetables.
  • Eat fruit but separately from meals (30mins before or 2 hours after) so that you are not eating fat and sugar together.
  • Don’t eat too late.


[1] The effects of intermittent or continuous energy restrictionon weight loss and metabolic disease risk markers

[2] Intermittent energy restriction versus daily energy restriction

[3] Alternate day fasting and chronic disease prevention