Monday, September 13, 2021

Intermittent Fasting: New Evidence From A Top Journal

Intermittent Fasting has become a very popular dietary strategy for those trying to lose weight. What does the research say? Is it safe? Does it work? What are the side effects? A very well designed randomized trial in JAMA Internal Medicine was recently published. The results may be quite surprising to you. (Reference 1).

The Study

In this interesting study, 116 subjects were randomized into 1 of two groups:

-Half of the subjects were put into a group instructed to eat 3 meals per day.

-The other half followed an intermittent fasting protocol, where they could only eat between noon and 8:00 PM each day.

Subjects were followed for 12 weeks.

Here are some of the results of this important investigation:

-By the end of the 12 week follow-up, there were no significant differences in weight loss between the fasting group and the conventional dieting group.

-By the end of the 12 week follow-up, there were no significant differences in any metabolic measures between the groups. These variables included fasting insulin, glucose, hemoglobin A1C, triglycerides, total cholesterol, HDL cholesterol and LDL cholesterol.

-Measures of daily physical activity were significantly lower in the fasting group when compared to the conventional 3 meal a day group.

-Sleep efficiency was significantly lower in the fasting group when compared to the conventional 3 meal a day group.

-Adherence was higher in the conventional diet group. In other words, it was harder to stick to the intermittent fasting plan.

-In the fasting group, 65% of weight loss was muscle. This is a really big number. Normally, when we lose weight, 20-30% of the weight loss is muscle. This finding led the researchers to warn that intermittent fasting could lead to increased risk of sarcopenia (muscles loss). This is not good for your health or the long term ability to keep the weight off.

Conclusions And Recommendations

This is an interesting and important study that does not support intermittent fasting as a successful strategy for weight loss or health improvement.

To summarize:

-Fasting did not improve rate of weight loss.

-Fasting did not improve health.

-It was harder to stay on this diet than a conventional 3 meal a day plan.

-There were several negative side effects seen only in the intermittent fasting group. Most notably, a decrease in sleep quality, physical activity and an increased loss of muscle mass.  

I know that there are a lot of popular and convincing books about fasting out there, but at the end of the day, the research should guide us when it comes to evaluating any weight loss strategy. Although this is a well designed study in a top journal, it is only one study and is not definitive by any means.

Having said that, this is not the first study to show negative outcomes with intermittent fasting. Several other research investigations that I have previously covered in my blog showed numerous side effects to fasting. These include: an increased LDL cholesterol, glucose dysregulation, increased inflammation, increased hunger, a decreased physical activity level and a decreased energy expenditure. If you want to reference these studies, please refer to this older blog post (click here).

If you are trying to lose weight and improve your health, intermittent fasting does not look like the way to go.


1) Lowe DA, et al, Effects of time restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity. JAMA Internal Medicine; 2020; 180:1491-99.

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