Quirks & Quarks

Intermittent fasting: Why not eating (for a bit) could work for weight loss and health

Intermittent fasting may be more than just the latest weight loss trend. According to a recent review published in the New England Journal of Medicine, it could have health benefits that go beyond those usually associated with losing weight.

Fasting seems to works at least as well as other diets, and may have additional health benefits

Carol Anne Bell-Smith, who lost 66 pounds with fasting, has also experienced many health benefits. (Left, after and right, before) (Carol Anne Bell-Smith)

Intermittent fasting may be more than just the latest weight loss trend. According to a recent review published in the New England Journal of Medicine, it could have health benefits that go beyond those usually associated with losing weight.

"We think in the long run that an intermittent fasting eating pattern will likely decrease one's risk for stroke and possibly Alzheimer's and Parkinson's diseases," said Mark Mattson, one of the co-authors of the review. Mattson is an adjunct professor of neuroscience at Johns Hopkins University.

Intermittent fasting is less about what or how much you eat, and more about a change in eating patterns. Two popular regimes that were the subject of the research included in the journal review are known as "5:2" and "16:8." 

While 5:2 involves eating normally five days a week, then only eating 500 calories on the other two days, 16:8 involves only eating in an eight-hour window, then fasting for 16 hours. 

Both groups of women, in our studies, lost the same amount of weight over a six-month period. But the women on 5:2 intermittent fasting lost more belly fat and ... had a stronger anti-diabetic effect.- Mark Mattson, adjunct professor at Johns Hopkins University

Mattson said the journal asked him to review the latest science because so many patients were asking their doctors about fasting and it was important to inform doctors about the subject. 

He said this was also a good time to summarize the evidence because research on humans about intermittent fasting has been accumulating recently. As a result, "we can be pretty confident and saying that what we're finding in rats and mice applies pretty well to humans."

The review found that, beyond weight loss, other benefits to intermittent fasting could include increased physical and mental energy, improved learning and memory, reduced anxiety levels and improved mood. There was also evidence that fasting can suppress inflammation, improve blood sugar regulation and could even help extend life expectancy.

Twitter CEO Jack Dorsey practices intermittent fasting to gain a mental edge on his competitors. (David Becker/Getty Images)

Intermittent fasting for weight loss

A number of studies have shown that intermittent fasting is at least as effective as standard diets in losing weight in those who are overweight or obese. But Mattson said simply reducing overall caloric intake throughout the day, as is done in many diets, doesn't give the same health benefits as fasting.

He said when he compared the results from an intermittent fasting regime to a regular diet where the same number of calories were cut, he saw differences.

"Both groups of women, in our studies, lost the same amount of weight over a six-month period. But the women on 5:2 intermittent fasting lost more belly fat and they had significantly greater improvement in insulin sensitivity — that is, they had a stronger anti-diabetic effect," he said.

When people get used to intermittent fasting, they do tend to eat less, but Mattson said it doesn't appear to be the reason for the health benefits.

"We studied a strain on mice that when we put them on alternate-day fasting, on the day they do have food, they eat pretty much twice as much food as they would normally eat," he said.

Over the long term, those mice weren't eating more calories, yet they lost more fat and showed metabolic and neurological improvements. 

CBC Radio science journalist, Sonya Buyting, investigated the science of intermittent fasting and shared her experience of losing 28 pounds in five months with Quirks & Quarks host Bob McDonald. (Sonya Buyting)

Benefits of fasting due to insulin 

Toronto-based kidney specialist Dr. Jason Fung, author of The Obesity Code, said the logic behind intermittent fasting was clear. 

"Most people who are trying to lose weight have hundreds of thousands of calories of body fat sitting on their body. So why won't your body use it? If you don't eat, your body will use it," said Fung. 

We started intermittent fasting and within like a month, all three were off of their insulin. And sort of like, within six months, all of them were classified as non-diabetic.- Dr. Jason Fung, author of

Fung says the key to fasting is in the insulin response. Insulin levels are increased by the body when we eat. He describes insulin as a train conductor that can switch tracks to redirect where the body gets its energy. 

When insulin levels are up, the track switches to burning fuel from the food we just ate. When insulin levels are down, it switches tracks and starts burning fuel from our liver and fat stores.

After this switch to burning fat happens, our cells can go into "repair and maintenance" mode, a process called autophagy, that starts recycling components of cells that aren't functioning well. 

Therapeutic fasting for chronic illnesses

Evidence is accumulating that fasting, sometimes for longer periods of time, can have significant benefits for people with a range of serious and chronic illnesses.

In 2018, Fung published a paper in the British Medical Journal Case Reports about three individuals he treated with intermittent fasting for Type 2 diabetes, which they had for 20-25 years. 

"We started intermittent fasting and, within a month, all three were off of their insulin," he said. "Within six months, all of them were classified as non-diabetic." These patients fasted for 24-hour periods several times a week.

Dr. Andreas Michalsen, the director and head of Charity Medical University's Department for Integrative and Internal Medicine in Berlin, studies extended fasting — from five to 30 days — and treats up to 1,500 people a year for chronic illnesses.

He said the longer a person fasts, the more potent the improvements in mechanisms such as blood glucose regulation and autophagy seem to be.

"The signals are higher when you perform prolonged fasting," said Michalsen. 

Fung cautions that longer fasts come with increased risk and require medical supervision.

Scientists who study intermittent fasting believe it could help treat the obesity epidemic we face today in North America. (Jeff Haynes/AFP via Getty Images)

Critics of intermittent fasting worry it can promote disordered eating, so researchers caution that anyone prone to eating disorders should avoid fasting. 

Mattson added that those with a body mass index below 18, at the low end of the normal range, should also refrain from intermittent fasting, as well as children under 17 and women who are breastfeeding or who might be pregnant.

We asked the Canadian Society of Endocrinology and Metabolism about treating Type 2 diabetes with intermittent fasting. This was their statement in response:

"Type 2 diabetes is a chronic metabolic condition which has many contributing factors, including family history, age, nutrition, exercise, certain medications and other factors. No single factor is entirely responsible for the development of diabetes. Consequently, the approach to management involves multifactorial intervention. Nutrition is a major factor in the management of diabetes. However, nutrition management should be individualized and no single diet is appropriate for every individual. A number of diets, including Mediterranean style diet, Nordic style diet, DASH diet or vegetarian style diet have all been shown to help manage diabetes and reduce cardiovascular risk factors. Intermittent fasting can improve glycemic control, but also can place individuals at risk for low blood sugars (hypoglycemia), particularly if they are taking insulin. The Canadian Society of Endocrinology and Metabolism (CSEM) recommends that people with diabetes, especially if using insulin, check with their health care provider before considering any major change in their diet.