Fasting is the latest diet trend (yes, really), but is it a good idea?

It seems we’ve been riding a wave of “non-diet” diets as of late: paleo, veganism and gluten-free nutrition lifestyles have become more important than cutting calories or losing weight. But now there’s a new way of eating that’s gaining momentum among celebs and gym rats alike: fasting. Or more specifically, intermittent fasting. Although it’s not that new to some of us, since many people fast for religious reasons. And the health benefits of fasting were touted by philosopher kings Hippocrates and Plato. Who knew?

There are many experts and diet books touting fasts, but arguably the most popular method today is the 5:2 diet, or The Fast Diet, by Michael Mosley, which advocates eating normally for five days, followed by two days during which you consume no more than 500 calories (men can consume 600). Other methods include adhering to strict time periods during which you fast. So you eat during the same eight hours each day, for instance, while fasting through the other 16, for instance (referred to as time-restricted fasts).

But before you start snubbing meals, there’s some things you should know. “In the long term, the research on the 5:2 diet [and most other fasting diets] is very preliminary, and mostly done in animal studies,” says Andrea Falcone, a registered dietitian and certified fitness professional who practices in the Toronto area. “Studies that are done in humans are very short-term, and there are small sample sizes. At this point in time, we have great anecdotal evidence, but we don’t know the long-term effects in humans.” In other words, tread carefully, friends.

The reason fasting is gaining momentum, though, is because proponents claim that it can help improve insulin sensitivity, reduce disease risk (like heart disease, cancer and diabetes), boost cognitive function, as well as help you lose weight. “It’s another quick weight-loss solution—it is. I’m not going to lie about that,” says Falcone. “Anyone who goes on a calorie-restricted diet is going to lose weight for the first one, two, up to four weeks. And we can see results rather quickly, just like anything, but is that a lifestyle you can follow for the rest of your life? Most people I’ve seen who have started something like this haven’t been able to hold onto it long-term, and the weight will come back.”

Jimmy Fallon, Hugh Jackman and Benedict Cumberbatch have reportedly jumped on the fast train. The way it works in your body is that normally when we need energy to function, our body looks to glucose, from carbs, to feed our cells. But when we fast, we don’t immediately go searching our bodies for this type of “food”; instead, our bodies favour burning our fat, which gets transformed into ketones, a source of energy that is, arguably, thought to work more efficiently. Falcone isn’t convinced, though: “Ketones can be hazardous to providing energy to our brains. Carbohydrate [glucose] is the safest molecule to pass the blood-brain barrier to fuel our brain.” And getting through a day at work with brain fog just isn’t going to fly. In a New York Times article published online expert Valter Longo explained it “may slow cell growth and development, which in turn helps slow the aging process and reduces risk factors for disease.”

This sounds appealing, obviously, so would Falcone recommend fasting? “Definitely not,” she says. “It really does, at the end of the day, depend on the client and their goals. Because everyone is different and everyone will adapt to a diet or a lifestyle very differently.” Especially hangry types and those prone to low blood sugar: do yourself a favour and keep eating regular meals.

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