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Should You Try Intermittent Fasting?

A review of Alternate Day Fasting, the 5:2 Diet, and Time Restricted Eating.

Key points

  • There are three types of Intermittent Fasting.
  • A review of Intermittent Fasting research shows mild weight loss after 12 weeks.
  • Weight loss from Intermittent Fasting comes from reducing total caloric intake.

If you’ve been trying to lose weight have you tried intermittent fasting? Several bestsellers have touted fasting strategies to “lose weight effortlessly,” “supercharge fat loss,” “reset your metabolism,” and so on. Is it all hype or could there be any special benefit from fasting? Is fasting any better than just cutting calories?

A new review offers preliminary answers. Researchers from the University of Illinois at Chicago looked at 21 outcome studies of the three types of intermittent fasting:

  • Alternate Day Fasting: a fast day when you consume fewer (0-500) calories followed by a day when you can eat whatever you want
  • 5:2 Diet: on two fast days you consume 500-1000 calories while on the other five days you’re free to eat normally
  • Time-Restricted Eating: although the specifics vary, the general idea is that you only eat during a 4- to 8-hour period. You’re limited to water or non-caloric beverages (but not artificially sweetened beverages) the rest of the day

The results were mostly favorable; fasting resulted in mild weight loss, usually between 3 to 8 percent over the course of the 8- to 12-week programs. Another positive finding is that there was little evidence that fasting caused people to feel weak or have difficulty concentrating although some headaches were reported during the first two weeks of fasting. Most of the studies lasted for less than 12 weeks; the long-term effects of fasting are unknown.

Comparing the different strategies, Time Restricted Eating was the least effective with reported weight loss of 3 to 4 percent. Also, both the Alternate Day Fasting and 5:2 Diet produced significant visceral fat loss but Time Restricted Eating didn’t.

Although it results in mild weight loss it doesn’t seem to be any “magic” to fasting. The research suggests that weight loss from intermittent fasting comes from reducing total caloric intake. On fast days, fewer calories were consumed while there didn’t seem to be increased consumption on non-fast days. Likewise, there were no differences in usual food choices on non-fast days.

The researchers don’t recommend intermittent fasting for children aged 12 or younger and for people with a history of eating disorders. They recommend caution for adolescents, people over age 70, or with Type 1 or Type 2 diabetes. If you’re not in one of these groups and you want to lose weight, should you try intermittent fasting?

My interpretation is that for some folks, the structure of an intermittent fast might be easier to implement compared with the usual diet that requires you to give up specific foods or count calories. You shouldn’t expect to “lose weight effortlessly” or “reset your metabolism,” but if you’re tired of restricting your food choices or counting calories it might be worth a try.

References

Varady, K.A. et al. (2022). Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endonchronology, 18, 309-321.

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