Intermittent fasting, an eating pattern that cycles between periods of fasting and eating, has become a popular way to lose weight. There’s no need to count calories, track macros or ditch entire food groups.
But does it work? According to researchers from the University of Illinois Chicago, the answer is yes. At least for the short term.
The team’s research update, published March 16 in Nature Reviews Endocrinology, evaluated the effectiveness of intermittent fasting for losing weight, lowering blood pressure and reducing blood sugar and cholesterol. Its safety was also assessed.
Here’s what to know about this eating regimen, plus dietary advice if you’re going to follow it.
Types of intermittent fasting
The review focused on three types of intermittent fasting that have been studied the most: alternate day fasting (ADF), the 5:2 diet and time-restricted eating (TRE).
ADF involves fasting one day, which can mean drinking water only or consuming roughly 500 calories, and then eating without restrictions the next day. The 5:2 diet entails fasting for two days per week (500 to 1,000 calories) and eating freely for five days. TRE requires fasting for at least 12 hours each day; a person’s eating window is confined to a specific number of hours, usually four to eight.
Intermittent fasting and weight loss
Findings to date from randomized controlled trials reveal that all three types produce weight loss in adults with obesity.
ADF and the 5:2 diet promote similar degrees of weight loss (4 to 8 per cent of starting weight) over eight to 12 weeks. Weight loss appears to peak at 12 weeks; longer studies lasting up to one year don’t show any greater weight loss.
Weight loss during TRE is less pronounced: a modest 3 to 4 per cent of starting weight over 8 to 12 weeks. No study has found TRE to result in clinically important weight loss (more than 5 per cent of starting weight), which is associated with improved blood LDL cholesterol and insulin sensitivity.
The main reason why people lose weight with intermittent fasting is because these diets lead to a 10 to 30 per cent drop in overall calorie intake.
When it comes to maintaining lost weight, four trials have shown that ADF and the 5:2 diet are effective. No study has investigated whether TRE can prevent weight regain.
Compared with traditional diets (such as daily calorie restriction), ADF and the 5:2 diet produce similar degrees of weight loss. So far, TRE has not been compared with daily calorie restriction.
Other health benefits?
There’s conflicting evidence that intermittent fasting improves blood pressure. It may do so, through weight loss or fasting itself, only in people who have high blood pressure to begin with.
While three trials found intermittent fasting to reduce blood LDL cholesterol, most showed no effect.
Fasting diets have not been shown to lower fasting blood sugar, but most studies involved people who didn’t have diabetes. Research suggests intermittent fasting has no effect on insulin sensitivity.
Is intermittent fasting safe?
Evidence from clinical trials suggests that ADF and TRE do not cause constipation, irritability, fatigue or dizziness. Headaches, however, may occur during the first two weeks of fasting.
Keep in mind that most studies of intermittent fasting don’t assess adverse effects. As a result, definitive conclusions about the safety of these diets are lacking.
Growing children, people who are pregnant or nursing, and people with disordered eating or a history of an eating disorder should not fast. Adults older than 70 should be cautioned about intermittent fasting since its effect on age-related muscle loss is unclear.
If you have a health condition and/or you need to take medications at certain times during the day, consult your doctor to determine whether intermittent fasting is safe for you.
Dietary advice for intermittent fasting
If you’re considering intermittent fasting (or you’ve already incorporated it into your lifestyle), include fruits, vegetables and whole grains in your daily diet to increase fiber. Most studies have revealed the diets of intermittent fasters to be quite low in fiber.
When starting out, meet daily fluid requirements to help prevent headaches (nine cups for women, 12 cups for men).
Include at least 50 grams of protein on fasting days (ADF, 5:2 diet) to help lessen hunger and lean muscle loss. If you’re doing TRE, include three daily meals, each containing at least 20 to 30 grams of protein.
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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