Intermittent fasting: Does a new study show downsides — or not? – . Health Blog

Intermittent fasting (IF) is a timing based approach to eating. The idea is that long periods of fasting make insulin levels so low that our bodies use fat for fuel. Growing evidence in animals and humans shows that this approach leads to significant weight loss. When combined with a nutritious, plant-based diet and regular physical activity, IF can be part of a healthy weight loss or maintenance plan, as I described in a previous blog post.

A randomized controlled trial published in JAMA now claims that IF has no significant weight loss benefit and a significant negative effect on muscle mass. News outlets picked up on the story and made headlines like A Potential Downside of Intermittent Fasting and an Unintended Side Effect of Intermittent Fasting.

But what did this study actually look at and find?

In the study, 141 patients were randomly assigned to either a limited-time eating plan (TRE) that consisted of 16 hours of fasting and only ate during an eight-hour window of the day or a consistent eating time (CMT) for 12 weeks. Nutrition plan with three structured meals per day plus snacks.

The group received no nutritional or behavioral counseling, nor was physical activity recommended. There was no real control group (that is, a group that received no instructions at the time of the meal).

Interestingly, both groups lost weight. Given the headlines, I had to read and reread the results several times as they show that the IF group lost a statistically significant amount of weight from start to finish – which the CMT group didn't. The researchers reported, “There was significant weight loss in the TRE group (-0.94 kg; 95% CI, -1.68 kg to -0.20 kg; P = 0.01) and in the CMT group a non-significant weight loss (-) 0.68 kg; 95% CI, -1.41 kg to 0.05 kg; P = 0.07). ”

Translated into plain language, the IF group lost more weight than by chance: between half a pound and 4 pounds, or an average of 2 pounds. The structured meals group also lost some weight, although the amounts lost could have been random: between 0.1 and 3 pounds, or an average of 1.5 pounds. The result was that there was no significant difference in weight change between the two groups. And the researchers saw a loss of muscle mass in the IF group that did not occur in the CMT group.

Dive deeper into your studies

By the way, all of these people may have eaten fried or fast food, as well as sugary sodas and sweets – we don't know. The study makes no mention of the quality of diet or physical activity. This is not how IF should be done! And yet the IF people still lost between half a pound and four pounds.

It is important that the group of structured meals has also lost weight. While this was not significant enough to prove that it was due to this intervention, for some participants it was sufficient to barely distinguish structured meal weight loss from IF weight loss. But think about it: structured meals are an intervention. After all, some people eat more than three times a day and consume several small meals throughout the day. Telling people to limit their meals to three meals plus snacks can help some eat less.

The authors might very well have concluded that IF was indeed successful. You could also ask for a follow-up study with a real control group without intervention, as well as behavioral advice, guidance on healthy eating, and recommended activity levels for IF and CMT groups.

Does additional support make a difference?

Previous studies on IF that provided behavioral advice and guidance on diet and activity have definitely shown positive results. For example, in a previous blog post, I described a 2020 study by the American Journal of Clinical Nutrition in which 250 overweight or obese adults followed one of three diets for 12 months:

  • IF according to the 5: 2 protocol, which means food intake is drastically reduced two out of five days a week (up to 500 calories for women and 700 calories for men).
  • Mediterranean, which highlighted fruits and vegetables, legumes, nuts, seeds, whole grains, and olive oil with moderate fish, chicken, eggs, and dairy, and allowed one glass of wine per day for women and two per day for men
  • Paleo, which highlighted fruits and vegetables, animal proteins, coconut products, butter and olive oil, and some nuts, seeds and legumes.
  • And that's key: All participants were given information on behavioral strategies for weight loss, stress management, sleep, and exercise.

Everyone has lost weight. The IF group lost more than any other with an average of 8.8 pounds, the Mediterranean by 6.2 pounds and the last by 4 pounds. Compliance was better with the Mediterranean Diet (57%) and IF (54%) than the Paleo Diet (35%), and better compliance resulted in one to three pounds more weight loss. The Mediterranean and IF groups also had significant drops in blood pressure, another good result.

What about the muscle mass loss that occurred in the IF group in the JAMA study? While this needs further investigation, it is important to note that other research on IF that included physical activity guidelines did not show any loss of muscle mass.

The final result

What is there to take away? Good quality nutrition and plenty of physical activity – including strength training – are vital to our health, and nothing can replace these recommendations. IF is just a tool, an approach that can be very effective in weight loss for some people. This one negative study adds but does not reverse the literature on IF. We just need higher quality studies to better understand how IF can be most effectively incorporated into a healthy lifestyle.

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