Skip to main content

Intermittent Fasting Cuts Less Fat than Traditional Daily Diet in Trial

Alternate-day intermittent fasting leads to less fat loss than traditional energy-restricting diets. | James Betts

People who followed an alternate-day intermittent fasting regimen lost less fat than those who followed a matched "traditional" diet that restricts daily energy intake, according to a new, three-week randomized trial involving 36 participants.

The new study — published in the June 16 issue of Science Translational Medicine — is one of the first to tease apart the effects of fasting and lower energy intake in lean individuals. The results seem to suggest that alternate-day intermittent fasting may offer no fasting-specific health or metabolic benefits over a standard daily diet.

"Ultimately, fasting did result in weight loss in our experiment, so it is true to say that it was an effective approach in that regard," said James Betts, professor of metabolic physiology at the University of Bath in the United Kingdom and senior author of the new study.

"However, we can confidently say it was not better than standard dieting in achieving that outcome and there were no other benefits specific to fasting within the context of our study."

But the study's authors caution that longer studies with larger groups are needed. The results also indicate that a drop in physical activity by those who did alternate-day fasting might explain some of the differences in fat loss between the study participants.

A Diet Strategy That Seems to Work

Intermittent fasting, which involves cycling through voluntary fasting and non-fasting periods, has become one of the most popular approaches to losing weight. People who follow intermittent fasting diets will pick a daily or weekly period where they can eat regular-sized meals, but will otherwise fast and avoid consuming calories.

There are many different intermittent fasting schedules, ranging from the popular 5:2 diet, where someone eats five days a week and fasts for two days, to alternate-day fasting, where someone eats one day and fasts on the next day. Other schedules instead center on fasting for part of each day, such as only eating in a daily eight-hour time window.

Many participants report that intermittent fasting schedules are relatively easy to adopt and stick by. Some researchers theorize these fasting schedules jibe with the human body's natural tendencies , as humans may have evolved to go without food for long periods in prehistoric times.

Other theories suggest that fasting can trigger beneficial changes in the body's energy processes that encourage weight loss. But few studies have examined the fasting-specific effects of intermittent fasting or compared its effects to diets that simply reduce daily net calories.

"Even a cursory analysis of online bookstores reveals tens of thousands of diet books telling us how we can make use of these techniques." Betts said. "By contrast, there's really only a handful of scientific studies telling whether there's anything particularly special about fasting."

Most previous studies have only looked at the effects of fasting on a short timescale of a few hours, such as when a participant misses a meal, according to Betts. These durations are too short to elicit fasting-specific benefits, if they exist, so his team sought to study the effects of complete fasting over at least 24 hours.

To further separate fasting and energy restriction, the researchers also came up with a unique study design. Instead of just having two groups of fasters and non-fasters, each of which restricted their calories, the scientists also included a third group of people who would follow a fasting schedule but still eat the same amount of calories as normal.

diagram of fasting interventions
The study's diet interventions and their outcomes. | James Betts

Iain Templeman, a Ph.D. student at the University of Bath and lead author of the new study, Betts, and colleagues recruited 36 lean participants — 21 women and 15 men — for their study. The researchers then split the participants into three groups of 12 and had them follow different diets for three weeks.

The first group followed an alternate-day fasting diet with energy restriction. This group ate 150% of their habitual energy intake every other day, while completely fasting in between. In contrast, the second group ate the same amount as the first group but didn't fast, eating 75% of their habitual energy intake each day.

Finally, the third group followed a similar alternate-day fasting schedule as the first group but had no restrictions in energy intake. This last group essentially ate 200% of their daily energy intake every other day.

"I'm not recommending anyone necessarily adhere to these diets in the real world, but this [setup] does generate the experimental model we need to look at the effects of fasting and energy restriction independently, and in combination," Betts said.

Fasting Regimen May Fall Short Due to Less Activity

After three weeks of the diet, the second group showed the greatest losses of both total weight and fat, with an average weight loss of 1.91 kilograms (4.2 pounds) and an average fat loss of 1.57 kilograms (3.5 pounds).

Meanwhile, the first group of alternate-day fasters lost weight as well — an average of 1.6 kilograms (3.5 pounds) — but lost less fat, with an average fat loss of .74 kilograms (1.6 pounds). The last group showed no significant drops in either weight or fat.

The scientists didn't just weigh their participants, but also used a type of x-ray to compare muscle and fat tissues. The x-rays revealed that only half of the weight loss in the alternate-day fasters was from fat loss, with the rest coming from fat-free mass such as muscle tissue.

In terms of general health, the scientists didn't find any key differences in health markers, appetite hormones, metabolic molecules, or the activity of genes in fat cells across the three groups.

Templeman and Betts' team also found that the alternate-day fasters tended to be less active than before starting the diet, particularly on their fasting days. They suggest that the drop in activity, perhaps as part of a behavioral adaption to less nutrition, may explain part of the difference in fat loss between the groups.

"I would say that anybody out there who's experimenting with intermittent fasting should be aware that what might look like successful weight loss according to your bathroom scales, you need to think … whether you may be losing muscle mass too," Betts said.

The researchers ultimately suggested that individuals considering alternate-day fasting should make sure to include opportunities for physical activity to maintain their energy expenditure.

Betts mentioned that his study has some limitations too. He noted that taking other types of measurements, such as of muscle protein synthesis, would have been helpful to understand why some participants seemed to lose muscle during the three-week period.

"I expect many diet books that advocate intermittent fasting are based on the idea that there are benefits unique to fasting, so our work at least shows that those effects are not apparent in every context," he concluded.

[Credit for associated images: Yeonsang/ Flickr; James Betts]