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Home » Intermittent fasting diet probably does not work

Intermittent fasting diet probably does not work

Last updated on February 7th, 2023 at 01:48 pm

I think over my lifetime, I have seen at least 100 different diet fads, with the intermittent fasting diet being one of the more recent. I even wrote about this diet fad previously, concluding that it might work, though the evidence was weak.

The scientific evidence keeps going back to the simple weight loss equation — calories burned must be greater than calories consumed. There is no magical way to change that, and despite the claims of diet faddists everywhere, there is no scientific way of significantly altering that equation. I have yet to see a powerful medical study that shows a method to burn more calories without actually burning them.

We are going to take a look at what is the intermittent fasting diet, and what the new data says.

yellow and red round fruit on brown and white weighing scale
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What is the intermittent fasting diet?

Intermittent fasting, also known as intermittent energy restriction, is any meal timing schedule that cycle between voluntary fasting (or reduced calorie intake) and non-fasting over a given period. Methods of intermittent fasting include alternate-day fasting, periodic fasting, and daily time-restricted feeding.

One of the more common types of intermittent fasting diet is 16 hours of fasting followed by eight hours of food. No, that doesn’t mean you can consume extra food during the eight hours of food consumption, it just means that you finish your regular meals during a restricted period.

A systematic review of energy restriction diets showed that intermittent fasting diets had similar weight loss to continuous diets but with a slight improvement in metabolic syndrome and insulin levels.

A 2019 review concluded that an intermittent fasting diet may help with obesity, insulin resistance, dyslipidemia, hypertension, and inflammation.

The biological mechanism underlying this type of diet is complex, but preliminary research seems to support the following four stages:

  • The fed stage, or absorptive stage, during satiety, when the primary fuel source is glucose and body fat storage is active, lasts about four hours.
  • The postabsorptive stage lasts up to 18 hours when glucagon is secreted and the body uses liver glucose stores as a fuel source.
  • The fasting stage, which occurs after 12-36 hours of continued fasting, is when the body transitions progressively to other fuel reserves, such as fat, lactic acid, and alanine.
  • The metabolic switch stage is when the body switches from preferential lipid synthesis and fat storage to the mobilization of fat to be metabolized into fatty acid-derived ketones to provide energy.

A 2019 review of diets, including alternate day fasting, time-restricted feeding, exercise, and overeating, found that body weight homeostasis, which is controlled by the brain, could not precisely correct “energetic errors” – the loss or gain of calories – in the short term. Thus, the intermittent fasting diet could temporarily disrupt homeostasis so individuals would mobilize more fat for energy.

Now, this all sounds very scientific, but I need to warn you that it is very preliminary research. And this does not mean there is magical burning of calories — the fundamental principle that calories in must be less than calories burned is still observed. The difference is how the body responds to temporary fasting, which appears to force the body to use fat for energy during fasting periods, which may “burn” more calories.

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New research

In a January 2023 article published in the Journal of the American Heart Association, Wendy L. Bennett, MD, MPH, an associate professor of medicine at Johns Hopkins University School of Medicine in Baltimore, and colleagues examined the effects of an intermittent fasting diet on weight loss.

This study evaluated the association between time from the first meal to the last meal with weight change. The study examined 547 adults (18 years old or older) from three health systems in Maryland and Pennsylvania with electronic health records who were enrolled in the study. Participants had at least one weight and height measurement registered in the two years before the study’s enrollment period (February through July 2019).

Here are some of the data that they found:

  • Meal timing was not associated with weight change during the six-year follow-up period. This includes the interval from first to last meal, from waking up to eating a first meal, from eating the previous meal to going to sleep, and total sleep duration.
  • The total daily number of large meals (estimated at more than 1,000 calories) and medium meals (estimated at 500-1,000 calories) were each associated with increased weight over the six-year follow-up. In comparison, fewer small meals (estimated at less than 500 calories) were associated with decreasing weight. This goes back to the point that calories consumed have got to be less than calories burned.
  • The average time from first to last meal was 11.5 hours; the average time from waking up to first meal measured 1.6 hours; the average time from last meal to sleep was 4 hours; and the average sleep duration was calculated at 7.5 hours.
  • The study did not detect an association between meal timing and weight change in a population with a wide range of body weights.

Let me get right to the conclusion — the researchers found no association between restricting eating times and weight loss.

The researchers stated that even though previous studies, like the one I reviewed previously, have suggested intermittent fasting may improve the body’s rhythms and regulate metabolism. However, this study by Bennett et al. which examined a large group with a wide range of body weights did not detect any link between intermittent fasting and weight loss.

As with most diet and nutrition studies, it is really difficult to compare the diet group to a control in a large study. You have to rely on the accuracy of the participant’s memory, which can be variable.


You might think that science is going back and forth, and you are not sure what to believe. But there are reasons to put more credence in this study since it is the largest one to examine intermittent fasting. And if you know anything from my writing, I am never comfortable with small clinical studies — random bias can give you a wide variety of misleading data.

Right now, I think that most diets eventually work because they restrict calories — maybe if you embrace the intermittent fasting diet, it works because you choose not to have a “midnight snack” and thereby reduce the number of calories you consume.

I know someone will comment (and please comment) that they’ve lost kilograms of weight because of this diet. But is it some magical component of the diet? Or is it because it forces a reduction in calories?

I know this may sound like a boring method for weight loss, but it’s kind of simple — eat a lot of fruits and vegetables and get a lot of exercise. You will reduce the number of calories consumed and increase the number of calories that are burned. And I know I’ll see a comment (please comment) that I’m failing to understand that some mysterious and magical enzymatic process burns more calories because you didn’t eat before going to sleep. Just bring a peer-reviewed paper to support it.

At this time, there really is no good evidence that intermittent fasting works. But, there may be evidence that for some people, it does work, like a lot of diets. However, it’s important to understand that it works not because of magic, but because the immutable equation of calories burned must be much greater than calories consumed. It’s quite simple.


Michael Simpson

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