The latest diet craze to hit the globe is the Fast Diet, also known as the 5:2 Diet, or the Intermittent Fasting (IF) Diet. The basic principle is that you eat normally for five days and semi-fast for two days of the week. Does it work, or is it dangerous?
One of the main reasons The Fast Diet: The Secret of Intermittent Fasting – Lose Weight, Stay Healthy, Live Longer, written by Dr Michael Mosley and Mimi Spencer, is so popular, is that it appears to be relatively easy.

Instead of counting calories, following intricate rigmaroles, cutting out whole food groups (required by the Atkins Diet, for example) or learning how to apply a low-glycaemic index (GI) diet, this diet simply requires that you restrict your energy intake on two days of the week (preferably on Mondays and Thursdays) to 500 calories a day for women and 600 calories per day for men. This two-day semi-fast is supposed to produce a loss of about 0.46kg a week for females and slightly more for males.

The authors

In vetting this diet, the first question you need to ask yourself is whether the authors really do have the authority to write about weight loss?

Well, Dr Michael Mosley studied medicine and qualified as a doctor in the UK. However, immediately after qualifying, he joined the BBC, where he spent 25 years producing science and medical programmes for the UK broadcaster. Perhaps it’s worth asking whether someone who hasn’t practised in his field or specialised in nutrition is qualified to influence the dieting habits of others? You decide…

Co-author Mimi Spencer, on the other hand, appears to be taking the 5:2 diet a bit too far. She admits that she regards herself as an "evangelist" of the Fast Diet and confides that she’s reduced her BMI from 21.4 (already regarded as below ideal) to 19.4. Her BMI could soon become dangerously low.


According to the authors, slimmers following the Fast Diet shouldn’t overeat on the non-fasting days. On these "feeding days", women should consume about 2, 000 calories and men about 2,600 calories. As mentioned above, your calorie intake should drop significantly on fasting days.

However, there’s a body of research that indicates that humans tend to overeat after fasts or periods of severe energy restriction. Even skipping a single meal often results in overeating at the following meal, or during the rest of the day. This is why nutritionists emphasise that it’s not a good idea to skip meals – what you lose on the swings will certainly be made up on the roundabouts. So, for many slimmers, it might not be easy to stick to the daily calorie limit on feeding days.

What’s more, reducing your energy intake drastically for two days a week may cause the body to switch off its weight-loss systems to conserve energy instead of losing fat. This could very well backfire and lead to weight gain, or failure to lose any weight at all.

Also bear in mind that the human body is programmed to maintain a state of homeostasis or balance. If you deprive yourself of food, it doesn’t automatically follow that the body will only burn the energy stored in your fat depots. The body can just as easily reduce its basic metabolic rate to conserve energy in the face of perceived starvation to maintain this balance – a strategy that will certainly not result in weight loss.

Potential dangers

At first glance, the Fast Diet appears innocuous. However, there are certain considerations that must be mentioned for the safety of those folks who may be tempted to try it.

People with reactive hypoglycaemia, diabetes (particularly type 1 diabetes), or those using diabetic medications such as insulin and glucophage, pregnant women, very active individuals and anyone with an eating disorder, shouldn’t use fasting as a means of losing weight.

Many diabetics are overweight and will be tempted to try the diet. This can have dangerous consequences because diabetics constantly need to adjust their medication (both oral and injected) according to their food intake. If you’re diabetic and using oral anti-diabetic agents or injecting yourself with insulin, please discuss the concept of the Fast Diet with your doctor before you try out fasting. You might just end up in a hypoglycaemic coma induced by lack of food plus the effect of your glucose-lowering medication.

People who fast also often unwittingly ingest too little liquid as food contains relatively large amounts of liquid. This could lead to dehydration, which can also be harmful.

While fasting is an accepted practice in many of the world’s religions (Catholics fast for Lent, Jewish people for Yom Kippur, and Muslims during Ramadan), the new Fast Diet craze isn’t the same thing.