Does intermittent fasting make sense?
Does fasting really boost weight loss?
It’s an old idea witnessing a modern revival: Fasting sure is trendy.
Intermittent Fasting, the practice of giving your digestive system a rest – for whole days or a portion of a day – is at the center of many weight loss diets. It no doubt works for many who practice it. That’s not saying much though, as practically any diet that induces caloric deficit leads to weight loss.
But is intermittent fasting more than just a way to cut caloric intake?
It’s certainly plausible. Our circadian rhythm – the inner clocks in every cell and system in our body – organizes our bodily functions, including our hormonal and metabolic pathways, in a time dependent way; even our gut microbes have circadian fluctuations.
Anecdotal intermittent fasting success stories abound as most people swear by the latest diet they practice, but in order to prove an advantage of this method it needs to be put to the test.
Which is precisely what was done in the longest trial yet.
The results of a yearlong study completed by 118 healthy overweight and obese people were published in the New England Journal of Medicine. The participants, all on a low calorie diet, were randomly assigned to spread their intake as they pleased, or to restrict eating to only 8 hours a day (8am-4pm).
At the end of one year the two groups achieved identical weight loss results: about 8kg (~18 pounds). The variability of participants’ weight loss was similar in both groups. The two groups also didn’t differ in body composition outcomes, such as waist circumference, BMI and body fat. Metabolic risk factors such as blood glucose levels, insulin resistance, blood lipids and blood pressure were also similar in both groups.
Restricting eating to 8 hours a day added nothing to weight loss or metabolic health in this long, randomized trial.
These results concur with those of a recent review including 11 randomized controlled trials. Intermittent food restriction, as in weekly fast days, was compared to continuous caloric restriction, with both methods achieving similar weight loss results and similar metabolic outcomes.
Several other reviews comparing intermittent fasting to continuous calorie restriction yielded corresponding results: an indistinguishable weight loss of 4-8 percent body weight with no advantage to periods of fasting.
Intermittent fasting’s success, alas, seems to be attributable solely to caloric restriction.
Does time restricted eating help cut calories?
The trick with maintaining a healthy weight over time is adopting habits that help you keep an equilibrium almost effortlessly – habits that control caloric intake without the need to count calories and be constantly vigilant.
As such, limiting the time window in which we eat can be helpful, not because it's a magic trick that revs up our metabolism (if only!), but rather as a tool for eating in moderation intuitively. Even the basic “nothing after dinner” rule is one that can achieve some caloric control without counting calories.
And indeed, several studies confirm what intermittent eaters testify to: limiting the hours in which you eat helps you reflexively cut calories as a blessed side effect. That time-restricted eating works as well as calorie-restricted diets is actually actionable good news.
We humans were all once intermittent eaters. We may have been looking for food all day as hunter gatherers but we didn’t find it – and days were short. Even as recently as my own childhood it was uncommon to access food between meals, and after dinner mom’s kitchen closed.
It’s perhaps wise to intermittently turn off the munching machine and give it a rest.
Dr. Ayala