The Evidence Behind Intermittent Fasting

For the past few years, Medcan clients have been asking questions about intermittent fasting (IF). People want to know if it’s effective for weight loss, or has health benefits, such as lowering blood pressure or blood sugar, for example. Recently, I had the opportunity to interview one of the world’s leading researchers of intermittent fasting, Dr. Krista Varady, a native Canadian and professor of nutrition at the University of Illinois, Chicago.

Dr. Varady is the author of more than 100 studies, as well as a book titled, The Every Other Day Diet. Most recently she was the lead author on a comprehensive scientific review of the research on intermittent fasting. Published in Nature Reviews Endocrinology, the evidence suggests that this eating pattern can be an effective way to lose excess weight and, possibly, improve health.

What is intermittent fasting?

Intermittent fasting (IF) describes an eating pattern that cycles between periods of fasting and eating. Different types of IF exist. The ones that have been studied the most include alternate-day fasting, the 5:2 diet and time-restricted eating.

Alternate-day fasting switches between a fast day and a so-called feast day. Fasting days can involve drinking only water or consuming about 500 calories. The 5:2 diet is a variation of alternate-day fasting; people consume 500 to 1,000 calories per day for the two fast days and then eat normally for five days.

Time-restricted eating has people fast for at least 12 hours a day; most followers compress their daily meals into an eight-hour window.

Why intermittent fasting is a popular weight loss strategy. And does it work?

“It’s the sheer simplicity of the diet that people really love,” says Dr. Varady. “They don’t need to track energy, calorie intake, or carbs, or anything like that.” She explains, “This is a totally different approach, where instead of focusing on macronutrients, like carbs and fat, we’re really just focusing on meal timing.”

Dr. Varady’s review concluded that alternate-day fasting and the 5:2 diet both promote a modest weight loss of 4 to 8 per cent (of starting weight) over eight to 12 weeks. That’s similar to what’s seen with traditional weight loss diets (e.g., daily calorie restriction).

Weight loss is less pronounced with time-restricted eating. Research shows that these intermittent-fasters lose 3 to 4 per cent of their starting weight over 8 to 12 weeks.

The observation that weight loss from intermittent fasting is comparable to simply reducing daily calorie intake was recently highlighted in a 12-month randomized controlled trial published in the New England Journal of Medicine. Among people with obesity who ate a lower-calorie diet, whether between the hours of 8 a.m. and 4:00 p.m. or their usual eating hours, there was no difference in amount of weight lost or metabolic health measures such as blood sugar, cholesterol, triglycerides or sensitivity to insulin.

When it comes to weight loss from IF, there’s no magic. Weight loss results because people who fast consume, overall, 10 to 30% fewer calories. “The main that I see consistently in our studies is people just eating less,” Dr. Varady says. “They’re eating about 300-500 calories less per day . So they’re losing a pound a week or so.”

Does fasting make you cranky and irritable?

Going without food for a whole day, even 16 hours, can make some people feel moody and less mentally sharp. According to Dr. Varady, however, “We actually find that people have a boost of energy on fast days.” Then she qualified her answer: “The first 10 days are a little rough, I’m not going to lie. In all of our studies… there is that adjustment period. Our bodies do take a few days to adapt. But once people are past that, they actually feel great.”

Dr. Varady speculates that this energy boost “might be linked to some type of evolutionary thing, where if we don’t have food, our bodies will give us energy to go out into our environment and find food.”

Side effects of intermittent fasting

“The only thing we really see in the first 10 days is an increase in headaches,” says Dr. Varady. “That tends to happen because people aren’t consuming enough water.” She explained that much of the food we eat, such as fruits, vegetables and dairy, contains a high proportion of water. By eating less food, we’re depriving the body not only of calories, but also the water that food contains—which can result in dehydration and headache.

To avoid experiencing headaches, Dr. Varady suggests consuming at least 1.5 litres of water per day.

Intermittent fasting isn’t for everyone

There are people who shouldn’t pursue intermittent fasting, according to Dr. Varady. Children under 12, women who are pregnant or lactating, people with a history of eating disorders and people who are underweight shouldn’t do it. She also doesn’t recommend it for people over 70. “When somebody hits 70,” she says, “their lean mass starts to go down quite a bit. So I probably wouldn’t recommend it for them just because we don’t want to exacerbate sarcopenia, or (age-related) muscle mass loss.”

Practical tips if you’re considering intermittent fasting

Dr. Varady always reminds her study participants to “keep in mind that it does get easier after about 10 days.”

“The quality of your diet also is really important,” she says. “That can help prevent chronic diseases long-term. So we always try to have people eat more fruits and vegetables, whole grains to boost fibre and micronutrient intake.”

For those pursuing alternate-day fasting, Dr. Varady suggests consuming 50 grams of lean protein on fasting days. “That helps people maintain muscle mass,” she says. “And then also it can help lower hunger, and cravings.” Regarding beverages, “During the fasting period, we often recommend black coffee black tea.” Cream in coffee is fine, as long as it’s not too much, according to Dr. Varady.

Bottom line: Intermittent fasting may help you lose weight but not more so than daily calorie restriction. At least that’s true for alternate-day fasting and the 5:2 diet. Time-restricted eating, however, appears to be a less effective method for weight loss.

If you’re considering adding intermittent fasting to your lifestyle, I recommend consulting with a trained dietitian who can provide strategies to ensure that you’re meeting your daily nutrient needs.