The benefits and risks of intermittent fasting

Experts weigh on whether giving your gut a break can really improve your health.

Experts weigh on whether giving your gut a break can really improve your health.

(Credit: Getty Images)

A few years ago around Christmas, after slathering a steak with half a stick of truffle butter, I found myself doubled over with stabbing stomach pains. I vomited on and off for five sleepless hours - and my stomach never fully recovered.

Instead, I learned that acute food poisoning can sometimes trigger a form of irritable bowel syndrome or IBS. This meant months of bloat, nausea and frequent bathroom trips. Inconclusive tests and failed elimination diets made me desperate for a solution. Then my mom told me about a diet she heard might help: intermittent fasting.

Intermittent fasting, or IF for short, is one of the most popular diets in North America. Put simply, it's the daily cycle of eating and not eating. According to a recent survey, of the 36 per cent of Americans that followed a specific diet last year, 10 per cent chose intermittent fasting, trumping other popular options like the paleo, Whole30 and ketogenic diets.

"IF is having its moment," says Dr. Melissa Lem, a Vancouver-based family doctor and medical writer. "That said, one major positive aspect and difference from other trendy diets is that it doesn't require eating or restricting certain foods, which means that it's fairly easy for anyone to do."

Most medical professionals have reached consensus on what to eat: more vegetables, fibre and good natural fats; moderate amounts of protein; minimal processed foods; and less sugar. The importance of when to eat — and how this could impact health — is only just beginning to be understood.

Personally, intermittent fasting has helped me lose weight and get my IBS symptoms under control. Though the science is young, research shows intermittent fasting has the potential to improve health in a number of other ways. But the diet isn't for everyone.

Here's what the experts have to say about intermittent fasting.

How does it work?

Food is fuel. When we eat, insulin levels rise, turning blood sugar into energy and helping the body store any energy that we don't immediately need. We store about a day's worth of energy in the liver and muscles for quick access. Anything extra gets turned into fat.

When we don't eat for a while, our bodies break down this stored energy for power. That's the theory behind intermittent fasting. Go long enough without eating and you'll start burning fat. That's why, with intermittent fasting, the emphasis is less on what you eat and more on when you eat.

Right now, we seem to be eating all the time. According to Ipsos research, 67 per cent of eating in Canada happens between meals.

"We went from a culture — for example, in the '70s — where nobody snacked," says Dr. Jason Fung, author of The Complete Guide to Fasting. "The average number of meals a day was three. By 2005, it was near six."

Fung says that these habits developed thanks, in part, to the mistaken belief that frequent, smaller meals will increase metabolism and help with weight loss. In fact, he says, constant snacking keeps insulin levels high, increasing the risk of developing insulin resistance.

What are the health benefits?

Fung has seen first-hand how poor eating habits can negatively affect health. As a kidney specialist in Toronto, he regularly watches patients progress from obesity to Type 2 diabetes and kidney disease. Fed up with just managing symptoms, Fung prescribes intermittent fasting to help his patients improve their metabolism, lose weight and reverse their Type 2 diabetes.

"It struck me as very inefficient to be concentrating on what drugs to give when you get kidney disease, when you actually should be focusing on the other end of the spectrum," he says.

Studies have also shown intermittent fasting can help lower low-density cholesterol (LDL) and blood pressure. Animal studies indicate the potential for IF to slow age-related cognitive decline, including Alzheimer's; protect against inflammation; have a regenerative effect on the intestines; and boost the body's ability to repair itself through a process called autophagy.

Fasting's purported anti-inflammatory and gut-repairing potential is what intrigued me about it. Lem says that, given evidence for reduced inflammation in fasting mice, it's not too much of a stretch to say that my improved human IBS symptoms could be partially attributed to fasting.

But she cautions that the cycle of eating and fasting could also increase IBS symptoms for others: "Keep in mind that because people doing IF typically eat larger volumes of food during feeding times, some people with IBS may experience more abdominal pain and bloating when they're not fasting."

Of course, findings from animal studies don't necessarily translate to humans, and the few human fasting studies there are have only looked at short-term impacts. More tests would need to be done in animal models that more closely resemble humans, and in humans themselves - and that poses its own challenges.

"Diet research, especially in humans, is messy and tricky," says Dr. Aylia Mohammadi, an expert in inflammatory bowel disease and the role of diet in gut health. But, she adds, there are some new, "well-designed human trials in the pipeline" that may reveal more about how diet can trigger or mitigate inflammation.

As we learn more, we'll have a better understanding of how fasting impacts our immune system. Then, says Mohammadi, we can be specific about who intermittent fasting might benefit.

Three common types of fasting

When it comes to fasting for health and weight loss there are three common approaches.

Time-restricted eating limits eating to a 4 - 12 hour window of the day. Often, people choose to eat all of their meals in an eight-hour period and fast for 16. Aligning these eating hours with our internal circadian rhythm and eating earlier in the day (like from 7 a.m. to 3 p.m.) may increase the potential for other health benefits, regardless of whether or not you lose weight.

Intermittent or periodic fasting includes alternate day fasting and the 5:2 diet. In alternate day fasting, dieters fast for 24 hours every other day, drinking only water on fast days, but eating without restriction otherwise. With the 5:2 diet, popularized by Dr. Michael Mosley, dieters eat only 500 - 600 calories two days per week, and whatever they like the remaining five days.

Finally, there are extended fasts, including a 72-hour fast that has been reported to mitigate immune system damage.

Not so fast

If you're going to try fasting, particularly if you're taking medication or looking to use fasting as a way to treat health issues, you should consult with your doctor first.

Fung suggests giving yourself time to get used to IF and staying busy. He says drinking water, coffee, tea or broth can help, as can sticking to a nutritious diet and not using fasting as an excuse to over-indulge.

"As long as you're reasonably healthy and informed, from a medical perspective I don't see any particular problem with trying IF," says Lem, who has, unintentionally, done it herself during busy ER shifts. That said, she adds, intermittent fasting isn't for everyone.

Children, endurance athletes, people who are underweight or on the low end of the spectrum, and women who are trying to conceive, are pregnant or are breastfeeding should avoid it. So should people with a risk for malnutrition, such as those with vitamin deficiencies.

All three experts advise against those with a history of eating disorders trying IF. "Developing a healthy relationship with food should come before attempting to restrict it," says Mohammadi.

Fasting can also impact your social life (you may miss out on the occasional happy hour or brunch) and some people may experience physical side effects like fatigue, headaches and irritability. Stress impacts health, too, so if fasting stresses you out, then it could be counterproductive.

Despite its ability to help people lose weight, in practice, intermittent fasting can be just as hard to stick to as other diets. Mohammadi suggests considering your preferences, goals and lifestyle when choosing an approach to nutrition.

Here, the experts agree: the best diet is the one you can stick to.

Eva Voinigescu is a freelance journalist and producer. She writes about health and science, careers, and culture.


To encourage thoughtful and respectful conversations, first and last names will appear with each submission to CBC/Radio-Canada's online communities (except in children and youth-oriented communities). Pseudonyms will no longer be permitted.

By submitting a comment, you accept that CBC has the right to reproduce and publish that comment in whole or in part, in any manner CBC chooses. Please note that CBC does not endorse the opinions expressed in comments. Comments on this story are moderated according to our Submission Guidelines. Comments are welcome while open. We reserve the right to close comments at any time.

Become a CBC Member

Join the conversation  Create account

Already have an account?