Pairing intermittent fasting with exercise leads to greater fat loss than exercise alone.
Time-restricted eating has become a popular trend for those trying to lose weight, but its effectiveness is still up for debate.
However, a new study from the University of Mississippi, published in the International Journal of Obesity (Nature Publishing Group), offers promising evidence. Researchers found that healthy adults who followed an eight-hour eating window while maintaining a regular exercise routine lost more body fat, without losing lean muscle, compared to those who exercised alone.
“We saw that this did lead to more fat loss and reduced body fat percentage over time when healthy adults were following both exercise with time-restricting eating compared to those who were only exercising for at least 4 weeks,” said Nadeeja Wijayatunga, assistant professor of nutrition and hospitality management.
“It is important to note lean mass preservation.”
Wijayatunga and Michael Hays, a tactical dietitian and recent graduate of the University of Mississippi, launched their study in response to the growing popularity of time-restricted eating. They conducted a systematic review and meta-analysis, examining data from 15 studies over the past decade that explored the effects of combining time-restricted eating with exercise.
Time-restricted eating is a form of intermittent fasting, a broad category of diets that cycle between periods of eating and fasting.
“People like time-restricted eating because they feel it’s easier to adhere to because they don’t have to think too much,” she said. “It’s all about time, not calorie-counting or watching out for certain foods.”
Why Time-Restricted Eating Appeals to Many
Intermittent fasting has quickly become one of the most popular diets in the United States, with 12% of Americans having tried it, according to the 2023 International Food and Health Survey.
The science surrounding time-restricted eating, however, is still developing, Hays said.
“For some people, this may be a good technique to help with body composition goals,” Hays said. “It’s just another tool, but more studies need to come out to really understand how this works in humans.”
While the difference between those who exercised while following a time-restricted diet and those who used exercise alone was slight, the two groups in the studies were already very healthy individuals, Hays said.
“In most cases, these were healthy adults,” Hays said. “They were already physically fit and already had exercise routines. When you already have athletic, lean people and you decrease their body fat percentage, that’s significant.”
Addressing Concerns About Lean Mass Loss
Time-restricted eating has been criticized for possibly leading to a loss of lean mass – all the muscles, organs and other tissues that make up the human body.
“We need healthy muscles,” Wijayatunga said. “Muscles are really important for the body and for your metabolism. If we lose muscle, it may impact our metabolic systems, and it just decreases mobility overall.”
In their study, Hays and Wijayatunga found that when paired with exercise, time-restricted eating did not lead to a reduction in lean mass, even for those who lost body fat.
“That’s why you want to couple diet with exercise,” Hays said. “When you’re losing weight, you never want to lose lean tissue. You want to lose fat.”
While the results are promising, Wijayatunga warned that much research is needed to confirm the findings. While this study focused on healthy, active subjects, there is a need to understand how this would impact people who are not in shape and exercising regularly.
“That’s something that needs to be studied,” she said. “What I’d recommend is implementing healthy habits in a way that you – as an individual – can maintain, with guidance from a health care professional.”
Reference: “Effects of time-restricted eating with exercise on body composition in adults: a systematic review and meta-analysis” by Harry M. Hays, Pouria Sefidmooye Azar, Minsoo Kang, Grant M. Tinsley and Nadeeja N. Wijayatunga, 10 January 2025, International Journal of Obesity.
DOI: 10.1038/s41366-024-01704-2
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