Study: Intermittent Fasting and High-Intensity Exercise Produce Better Weight Management Results When Combined

Combining time-restricted eating and high-intensity functional training may boost metabolic health and weight loss more than each method alone, new research suggests.

A study published May 1 in the journal PLOS One found that inactive women with obesity who practiced not just one but both strategies had more significant reductions in body composition and lipid and glucose levels.

Time-restricted eating (TRE), a type of intermittent fasting, refers to narrowing food intake to specific windows of the day, while high-intensity functional training—or HIFT—involves aerobic and resistance exercises that mimic everyday movements, such as pulling or lifting, performed at a rapid pace. Both strategies have become popular in recent years.

“We can highlight in this study that TRE is a good solution to combat obesity, easy to implement since it does not require people to limit their overall food intake or count the total number of daily calories,” study author Rami Maaloul, PhD, of the University of Sfax, Tunisia, said in an interview published by PLOS One. “For optimal benefits, you should incorporate both TRE and HIFT into your strategy.”

A Closer Look at the Findings

According to the authors, previous research has suggested that practicing TRE or HIFT alone in the “short term” may help people lose weight and improve markers of cardiometabolic health, such as blood pressure and glucose levels.

Researchers wanted to investigate whether combining TRE and HIFT would yield even more significant health benefits.

The team recruited 64 women with obesity; the average age was 32. Researchers assigned the participants to practice either time-restricted eating only, high-intensity functional training only, or a combination of the two. 

The women who followed the time-restricted eating regimen ate only between 8 a.m. and 4 p.m. Those in the other groups exercised three days a week with an instructor. 

After 12 weeks, participants in all three groups showed significant weight loss, waist and hip circumference reductions, and positive changes in lipid and glucose levels. The people who exercised also had lower blood pressure.

Those practicing both TRE and HIFT, however, saw more pronounced benefits in general than those in the other groups.

“This study highlights another exciting positive effect of intermittent fasting, which is to augment the positive effects of exercise on metabolic health,” Julie Pendergast, PhD, an associate professor of biology at the University of Kentucky who researches metabolism, told Health.

Weaknesses in the Research

The authors acknowledged several limitations, including the small sample size, the specificity of the functional exercises, and the fact that eating habits were self-reported and not measured objectively. 

Additionally, people who practiced TRE also reported consuming fewer calories, so it's unclear whether the health benefits resulted from a shift in eating times or reduced caloric intake. Some previous research has suggested that any benefits seen during TRE may in fact be due to fewer calories consumed in light of the shorter eating window.

The study didn’t explore that question or why people who did both HIFT and TRE had the best outcomes. However, experts said a couple of factors may be at play.

According to Pendergast, “both interventions result in a metabolic shift to promote fat burning by the body.” 

Exercise also builds lean muscle, she added.

“I think the key message from this study is incorporating physical activity and training in the setting of active weight loss can be beneficial towards the maintenance of lean tissue, which is invariably lost to a degree when someone’s losing weight,” Sean Heffron, MD, a cardiologist at the Center for the Prevention of Cardiovascular Disease at NYU Langone Heart, told Health.

What to Consider Before Practicing TRE and HIFT

If you want to begin a new exercise or diet regimen, it’s best to consult with a healthcare provider first.

Maaloul said people interested in starting a HIFT routine may want to begin slowly and work their way up to a higher intensity. Doing too much too soon may cause injury.

While Pendergast said that “when practiced carefully, studies have shown [that TRE] is safe and effective,” the eating plan may not be ideal for some people, including those with diabetes or a history of eating disorders, or people who are pregnant or suspect they are pregnant.

Experts say it’s also important to focus on what—not just when—you’re eating. Restricting eating to certain hours wouldn’t negate the health effects of adopting a poor diet. 

And to lose weight and remain healthy long-term, Heffron said to consider creating habits you can maintain. That means that if you don’t enjoy intermittent fasting or high-intensity functional exercise—or find them too challenging to stick to—they may not be the best approaches for you.