Go to the Globe and Mail homepage

Jump to main navigationJump to main content


What you should know before going on a fast Add to ...

Fast yourself thin?

It sounds like a sure-to-fail weight-loss strategy. However, a growing number of medical professionals, nutrition experts and consumers are getting on board with the idea that food deprivation could actually work. Fasting, they say, not only helps people lose weight, but can also prevent cognitive decline and even increase longevity.

The eight-hour diet, the 5:2 diet, the intermittent diet. Numerous high-profile fasting diets have become intensely popular in recent months. Despite the demand for these restrictive diets, many medical experts are concerned about the overall effectiveness and safety of fasting.

What is a ‘fasting’ diet?

The idea that can be the key to losing weight and living a healthier life hit the mainstream last August, with the broadcast of the BBC documentary Eat, Fast and Live Longer. It followed Michael Mosley, a physician and journalist as he embarked on an intermittent fasting plan, known as the 5:2 diet. Followers of the plan eat normally for five days a week and drastically reduce calorie consumption for two days. Men consume 600 calories on “fast” days, while women consume 500 calories. (Compare that to Health Canada’s estimated daily caloric requirements for sedentary men and women between ages 31 and 50, which is 2,350 and 1,800, respectively.) After five weeks, Mosley had lost more than 10 pounds and noticed improvements in several areas, including lower cholesterol.

Several other fasting plans have been steadily gaining in popularity in recent months. Some advocate alternate-day fasting, which involves alternating between eating normally one day and fasting the next; others recommend an eight-hour diet, which involves eating only during an eight-hour window during the day and fasting for 16.

Proponents of fasting point to anecdotal evidence that people who follow these diets lose weight quickly. They also highlight numerous studies that have shown mice who live on a calorie-restricted diet live much longer than their peers. The limited research that has been conducted on fasting plans, such as alternate-day fasting, seems to show positive results. One study published in Nutrition Journal in 2010 found that 16 obese people who followed an alternate-day fasting plan for several weeks were able to lose weight.

The basic idea behind fasting, according to Mark Mattson, chief of the laboratory of neurosciences at the U.S. National Institute on Aging in Baltimore, is to put the body in a state of stress. The brain and red blood cells require glucose to function. When we don’t eat, the liver releases stores of glycogen, a type of sugar, to fuel the brain and red blood cells. Those stores are depleted in a matter of hours. After that, Mattson says, the body starts converting fat into a type of sugar the brain can use. As a result, fasting can help you burn fat and lose weight, he said. If you simply reduce the amount of calories you consume, glycogen stores are never depleted and the body isn’t forced to burn fat this way.

“What I’m saying, from the standpoint of people losing weight and losing abdominal fat, fasting is definitely better than just reducing the size of meals but still eating meals frequently,” Mattson said.

The catch

There’s one problem with the rationale behind fasting diets, according to David Lau, an obesity researcher, professor and chair of the diabetes and endocrine research group at the University of Calgary. When glycogen stores in the liver are depleted, the body doesn’t burn fat: It breaks down protein. Lau said starvation will only burn fat as fuel after about a week of starvation.

“Muscle protein breakdown occurs in the first 24 hours of starvation,” he said. “Muscle protein breakdown [is] not healthy.”

His concern is that many people are fasting without realizing they may be losing crucial muscle mass. The danger is that as protein in the body breaks down, it could lead to other unwanted side effects, such as altered immunity. In order to fight infections, the body needs to produce antibodies, a type of protein. But when the body is breaking down protein for fuel, it may not send the right signal to make antibodies, Lau said.

Britain’s National Health Service recently issued a warning to consumers about fasting diets, saying they simply haven’t been studied enough for anyone to conclude whether they are safe or effective. Although some studies have shown fasting diets appear to work, they typically involve a small number of patients or were conducted over a short time frame, meaning the results likely aren’t conclusive. People on fasting diets have also reported sleeping problems, irritability, bad breath, anxiety and fatigue during the day, according to the NHS.

The bottom line

Even in the BBC documentary, Mosley warns that more research needs to be done before fasting diets should be recommended. He urges caution to those who want to start and says it may not be feasible over the long term. Until more is known, it might be better to avoid semi-starvation and instead go for the slow and steady approach that includes cutting back on food intake and increasing physical activity.

“We need to understand weight gain and weight loss really takes time,“ said Lau. “To me, it’s really scary that some of the doctors are promoting certain diets that, to me, are really unscrupulous.”

Report Typo/Error

Follow on Twitter: @carlyweeks

Next story




Most popular videos »

More from The Globe and Mail

Most popular