Is Intermittent Fasting Safe?

This dieting trend is said to inhibit dementia and other age-related diseases. Here are the facts.

What is intermittent fasting?

Throughout the course of a 24-hour day, your body naturally cycles between feeding and fasting modes while you’re awake and when you’re asleep. People who practise what’s called intermittent fasting attempt to extend the fasting phase in order to enhance the body’s hormonal regulation processes that kick in when you’re not eating. This way, say the diet’s proponents, you can maintain a healthy weight and, therefore, avoid the risks associated with obesity, including Type 2 diabetes and heart disease.

Intermittent fasting can take several forms. Time-restricted fasting, also called the 16:8 schedule, involves limiting breakfast, lunch and dinner to an eight-hour window—say, 10 a.m. to 6 p.m. A more extreme schedule (which would require a doctor’s okay, especially if you take medication) entails eating as you typically would for five days a week and fasting for two (when you’d still consume plenty of water, and optionally up to 500 calories).

What happens in the body when someone fasts? And what are the benefits beyond maintaining a healthy weight?

The diet’s main benefit comes from how fasting affects insulin, the hormone that regulates blood sugar—and which rises when we eat. Insulin allows our cells, including fat cells, to absorb glucose from our blood. When we’re not eating, our insulin levels go down, allowing the fat cells to release their energy stores. And if those levels drop far enough for long enough, we lose weight.

Research from the last two decades has also shown that intermittent fasting reduces blood pressure, cholesterol and markers of inflammation. Scientists aren’t sure yet why this happens, but a 2015 study on lab mice suggests that when we switch from a fed to a fasting state, changes happen on a cellular level that can extend life, reducing rates of cancer and fostering immune system and organ rejuvenation. As well, researchers have found that intermittent fasting stimulated the production of a nerve protein that plays a critical role in memory, learning and the generation of new nerve cells—which could help slow age-related cognitive decline.

That all sounds promising, but are there dangers as well?

So far, studies on intermittent fasting have mostly been short-term—a few weeks to a few months—and observed adults under the age of 60, or lab animals. For older adults, some experts worry that the natural aging-related decline in muscle and bone health could be worsened by intermittent fasting—or any weight-loss program, for that matter. “Anything that extends your lifespan should also extend your years of good health,” says Stuart Phillips, a professor in the department of kinesiology at McMaster University. He suggests people who attempt fasting stay as physically active as possible, since losing muscle mass can make daily living much more taxing as we age.

And since the diet restricts food intake, intermittent fasting is not recommended for anyone with a history of disordered eating, or if you’re pregnant or breastfeeding. Fasting for an entire day may cause some people with low blood pressure to feel light-headed or unsteady on their feet, and diabetics may need more careful monitoring to make sure blood sugar levels don’t drop dangerously low.

If I want to try it, how should I start?

Your first step should be to speak with your family doctor to make sure intermittent fasting is safe for you. If you get their okay, start slow, gradually narrowing your daily eating window.

Next, find out 50 surprising ways to lose weight without exercise.

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