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Intermittent fasting is said to deliver many health advantages, including lowering body weight, improving blood sugar, lowering blood pressure and cholesterol and reducing inflammation.

Decades worth of rodent studies support these claims. Mice who fast between meals are slimmer, healthier and live longer, regardless of the content of their diet.

Studies in people, though, are fewer in number, and most have been conducted for a short duration in small numbers of participants. Still, findings suggest that intermittent fasting has benefits.

Not all studies concur. Recent research has also hinted that the diet regimen may have unwanted consequences.

What is intermittent fasting?

Intermittent fasting is a pattern of eating that switches back and forth between eating and fasting. There are several variations.

Alternate-day fasting involves fasting one day (or cutting calories by 75 per cent) and eating whatever you like the next day. The 5:2 approach allows you to eat normally five days a week and eat 500 or 600 calories the other two days.

Time-restricted eating, a popular form of intermittent fasting, involves restricting eating to an eight-hour window and fasting for 16 hours (16:8 fasting). In some versions, the eating window is six or 10 hours.

Does it work?

Randomized controlled trials conducted with overweight and obese adults have shown alternate-day and 5:2 fasting to be effective for weight loss and improving cardiometabolic risk factors.

The effectiveness of intermittent fasting, however, isn’t superior to daily calorie restriction. A review of 11 trials, published in 2018, concluded that weight loss and cardiometabolic improvements are comparable to those by achieved by eating 25 per cent fewer calories each day.

Small studies (10 to 34 participants) have associated time-restricted eating with weight reduction and fat loss in overweight adults. Some, but not all, have demonstrated the regimen’s ability to lower blood pressure, glucose, insulin, cholesterol and/or triglycerides.

Extending the fasting period allows insulin in the bloodstream to fall, prompting fat cells to release their stored energy. A decrease in calorie intake and/or appetite may also lead to weight and fat loss.

It’s also thought that intermittent fasting can increase calorie-burning in the body; however, this hasn’t been found in human studies.

The latest study

A 12-week randomized controlled trial, published this fall in JAMA Internal Medicine, concluded that time-restricted eating was ineffective for losing weight and improving cardiometabolic risk factors.

For the study, 116 overweight participants ate their meals between 12 p.m. and 8 p.m. while the control group ate three meals at conventional times.

There was no change in cardiometabolic risk factors for both groups. Weight stayed the same for people in the control group, while time-restricted eaters lost two pounds.

Sixty-five per cent of the weight lost, though, was lean mass, more than double what’s normally expected during weight loss. Lean mass in the arms and legs, which is correlated with physical strength, was particularly lower.

Delaying meals to 12 p.m. may have caused people to eat less protein, which stimulates muscle protein synthesis.

Does the timing of intermittent fasting matter?

Evidence suggests that muscle growth and repair is regulated by the body’s circadian rhythm, its 24-hour clock, which governs things such as blood sugar, cholesterol, hormones and digestion. Eating protein in the morning may be important for maintaining muscle mass and strength.

Whether you get better results by restricting meals to earlier in the day, say 7 a.m. to 3 p.m. or 9 a.m. to 5 p.m. (early-time-restricted eating), isn’t known. But it makes sense to do so.

Our bodies are better able to metabolize food early in the day. Increasingly, research is finding that disrupting our circadian rhythm, by eating late at night for instance, can increase the risk of abdominal weight gain and cardiometabolic disorders.

Bottom line

Short-term studies suggest potential health benefits of intermittent fasting. But the long-term benefits are unknown.

If you try time-restricted eating, or you’re already doing it, include three nutritious meals in your eating window and make sure each one includes a good source of protein. Consider eating your first meal earlier in the day.

Growing kids, pregnant and nursing women, people with diabetes who take insulin or oral medications and individuals with low blood sugar should not fast.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

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