The Fasting Diet: Any Merit?

The approach is simple: you don’t have to eat well all the time—just eating well sometimes will be enough to get you slimmer and healthier. At least, that’s the strategy suggested by The Fast Diet, a new book just released in North America that explains how you can eat whatever you want for five days a week, then dramatically cut down for two days (during those two days, women can have 500 calories per day and men can have 600). You repeat that cycle every week. This intermittent fasting diet is referred to as the “5:2 diet.”

According to Dr. Michael Mosley and Mimi Spencer, the authors of The Fast Diet, this is the only diet that will work.

“There is nothing else you can do to your body that is as powerful as fasting,” say Mosley and Spencer in the introduction to The Fast Diet. “Fasting is nothing new. As we’ll discover in the next chapter, your body is designed to fast. We evolved at a time when food was scarce; we are the product of millennia of feast or famine. The reason we respond so well to intermittent fasting may be because it mimics, far more accurately than three meals a day, the environment in which modern humans were shaped.”

Mosley created the diet after being told by his physician that he was overweight and had a higher risk of heart disease or diabetes. He played “guinea pig” and tried intermittent fasting. Eventually Mosley found that the 5:2 diet worked best for long-term weight loss, even if he did have to get used to hunger pangs,.

So, is there any truth to these statements?

Although The Fast Diet has gained popularity as the Bible” to fasting diets, this type of diet has actually been around for a long time. Researchers have tried to understand if this extreme diet could be effective in helping you lose weight. Although the authors of the book point out that the body is used to fasting (Jews fast a few times a year, and Muslims fast for a month during Ramadan, for example), that’s a far cry from saying that it’s healthy to fast two days a week—every week.

But so far, there are very few studies out there that have tested the theory behind the fasting diet. One study, published in the Journal of Diabetes & Metabolic Disorders, put 15 overweight or obese women on an alternate-day fasting diet (one day of fasting, then one day of a normal diet) for eight weeks. After the study was over, the participants’ body weight decreased and cholesterol was lowered, as was their blood pressure. The problem is that very few people participated in the trial, and there was no control group, either.

The National Institute on Aging claims that scientific studies have shown that intermittent fasting can benefit the brain and heart function. A review of the human and animal studies on alternate-day fasting diets, published in the American Journal of Clinical Nutrition, found:

• A lower risk of diabetes in the animal studies

• Some cardiovascular benefit in the animal studies (lower cholesterol and triacylglycerol concentration, lower heart rate, and lower blood pressure)

• A decrease in lymphoma in the animal studies

• The human studies did not show any of these same results

Not only is there a lack of research backing this phenomenon, the fasting diet can also put you at greater health risk if you fail to get enough nutrients and vitamins from the limited amount of food you’re consuming. Our verdict: this is just another diet trend.

“Can we prevent aging?” National Institute on Aging;, last accessed March 20, 2013.
Eshghinia, S., et al., “The effects of modified alternate-day fasting diet on weight loss and CAD risk factors in overweight and obese women,” Journal of Diabetes & Metabolic Disorders 2013; 12(4).
Varady, K., et al., “Alternate-day fasting and chronic disease prevention: a review of human and animal trials,” American Journal of Clinical Nutrition July 2007; 86(1): 7–13.