Weight Loss

Nutritional Pearls: Does Intermittent Fasting Improve Weight Loss?

A 36-year-old man who is trying to lose weight for his wedding presents to your clinic for advice. He asks you if there is evidence that intermittent fasting could help him lose weight.

How would you advise your patient?

(Answer and discussion on next page)

Dr. Gourmet is the definitive health and nutrition web resource for both physicians and patients with evidence-based resources including special diets for warfarin users, patients with GERD/acid reflux, celiac disease, type 2 diabetes, low sodium diets (1500 mg/d), and lactose intolerance.

Timothy S. Harlan, MD, is a board-certified internist and professional chef who translates the Mediterranean diet for the American kitchen with familiar, healthy recipes. He is an assistant dean for clinical services, executive director of The Goldring Center for Culinary Medicine, associate professor of medicine at Tulane University in New Orleans, and faculty chair of the all-new Certified Culinary Medicine Specialist program.

Answer: Intermittent fasting continues to be a popular diet strategy, but the science is still lacking.

The Research

In 2018, a German study with 136 participants that lasted a total of 50 weeks (12 weeks of active intervention) compared 3 arms of a weight loss study.1 One group followed a 5 to 2 intermittent fasting diet with 5 days of usual caloric consumption and 2 days of 25% of caloric consumption. Another group reduced its daily calories by 20% each day. On average, the group consumed about the same number of calories as those in the intermittent fasting group. A third group continued their usual diets (control). Those who cut calories––by intermittent fasting or just cutting daily calories––lost weight. The difference between the 2 calorie restriction groups was only "borderline significant," statistically speaking.

In 2020, I reported on a 12-week study in more than 100 overweight or obese men and women who were randomly assigned to a time-restricted eating group––eating only between noon and 8:00 pm––or a control group.2 Neither group was instructed to limit their caloric consumption. Those who restricted when they ate lost a little more weight than those who did not, but not only was that difference not statistically significant, those who restricted when they ate lost more muscle mass than those who ate regular meals and snacks, when desired.

Today's research does not help the case for intermittent fasting.

In a study performed in Brazil,58 women with an average age of about 31 years and an average body mass index of 33 kg/m2 (clinically obese) took part in a short-term diet designed to reduce caloric intake for just 21 days. Each participant had their resting metabolic rate and their usual levels of activity measured to identify their baseline caloric consumption.

Their diet plans were designed by dietitians to make use of ingredients the participants were already accustomed to eating. Each participant's caloric intake was cut by an appropriate amount to induce slow but steady weight loss––as customized for each participant's weight and level of activity­­––and participants met with dietitians weekly to help plan meals and identify the appropriate number of calories they should be consuming each week.

The participants were randomly assigned into 1 of 2 groups: one group consumed their daily meals throughout the day as usual, while the other, experimental group consumed their daily meals within a 12-hour window, drinking nothing but noncaloric beverages during the other 12 hours of the day.

The participants took part in the low-calorie diets for 21 days but returned to have their weight and waist circumference measured regularly for 81 days.

The Outcome

There was no statistically significant difference in the amount of body weight lost between the intermittent fasting group, who limited when they ate, and the control group, who cut calories but ate at their usual times. There were 2 differences the researchers could find between the 2 groups after the 81 days of the trial. The first was that the intermittent fasting group reduced their body fat by 0.68%––about 0.62 kg in a 90-kg person––while the control group lost 0.07% of their body weight in fat. The other difference was the intermittent fasting group lost more waist circumference than the control group––2.75% vs 2.20% (the difference for someone with a 86.36-cm waist at the start of the study would be 2.37 cm lost vs 1.90 cm lost).

No other differences were seen between the 2 groups with respect to levels of hunger, how difficult their respective diets were to follow, gut or thyroid hormone levels, and insulin sensitivity.

What This Means For You

This is a small study to begin with, and the differences between the 2 groups, both of whom we must recall cut calories to the same degree, are not large enough to be convincing one way or the other. Moreover, the participants who were instructed to fast for 12 hours per day could, if they chose, avoid eating for 12 hours after their evening meal, essentially skipping breakfast or eating breakfast a little later than usual.

In a direct comparison between reducing calories in the usual way and reducing the same portion of calories while also restricting when you eat, I am seeing no convincing difference.


  1. Harlan TS. The 5:2 diet. Dr Gourmet. Published online December 5, 2018. https://www.drgourmet.com/bites/2018/120518.shtml
  2. Harlan TS. Restricting when you eat won’t help you lose. Published online October 7, 2020. https://www.drgourmet.com/bites/2020/100720.shtml
  3. Pureza IROM, Melo ISV, Macena ML, et al. Acute effects of time-restricted feeding in low-income women with obesity placed on hypoenergetic diets: randomized trial. Nutrition. 2020;77:110796. https://doi.org/10.1016/j.nut.2020.110796