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Condensing a calorie-restricted diet into an 8-hour window during the day didn't do much to bolster weight loss, a randomized trial found.

Participants with obesity who followed a time-restricted diet in addition to calorie restriction didn't lose a significantly greater amount of weight compared with individuals who adhered to a calorie-restricted diet alone (-1.8 kg difference, 95% CI -4.0 to 0.4 kg, P=0.11), according to Huijie Zhang, MD, PhD, of Southern Medical University in China, and colleagues, writing in the .

During the 12-month study, both groups lost a significant amount of weight from baseline:

• Time/calorie-restricted diet: -8.0 kg (95% CI -9.6 to -6.4 kg)
• Calorie-restricted diet alone: -6.3 kg (95% CI -7.8 to -4.7 kg)

"These results indicate that caloric intake restriction explained most of the beneficial effects seen with the time-restricted-eating regimen," the group explained. "Even so, our findings suggest that the time-restricted-eating regimen worked as an alternative option for weight management."

Following the same pattern, there weren't any significant between-group differences in regards to other metabolic outcomes at 6 and 12 months -- including body mass index (BMI), waist circumference, and lean and fat mass, among others -- as both diets led to significant improvements from baseline.

But these improvements tended to numerically favor the addition of time-restricted eating. At 12 months, for example, these clinical measures were:

• BMI: -2.9 for time/calorie-restricted vs -2.3 for calorie-restricted
• Waist circumference: -8.8 cm vs -7.0 cm
• Body fat mass: -5.9 kg vs -4.5 kg
• Body lean mass: -1.7 kg vs -1.4 kg
• Body fat percentage: -4.3% vs -3.0%
• Area of abdominal visceral fat: -26.0 cm² vs -21.1 cm²
• Area of abdominal subcutaneous fat: -53.2 cm² vs -37.0 cm²

Likewise, neither diet outperformed the other when it came to cardiovascular risk factors -- which included blood pressure, pulse, triglycerides, cholesterol, glucose, insulin resistance, and others -- with both groups showing significant improvements from baseline. Again however, most measures except for pulse, LDL cholesterol, and 2-hour postprandial glucose levels favored the time-restricted eating group.

Adherence to both diets was generally high, with 97.1% of all participants completing the diets for 6 months and 84.9% making it to the end of the year-long trial. At baseline, participants were eating within an average window of 10 hours and 23 minutes.

Calling this baseline window of eating "relatively short," authors Blandine Laferrère, MD, PhD, of the Columbia University Irving Medical Center in New York City, and Satchidananda Panda, PhD, of the Salk Institute for Biological Studies in California, said the fact that the time-restricted intervention only cut this window by roughly 2 hours might be the reason why not many differences were seen.

"Persons whose habitual time period for eating is long are likely to benefit the most from time-restricted eating," they pointed out, also adding that "the reduced caloric intake may have blunted any effect of time-restricted eating independent of calorie restriction."

On top of that, the editorialists said that this group of participants was generally healthy at baseline, which may have "left little room to test whether time-restricted eating plus calorie restriction has a significantly greater effect on cardiometabolic risks than calorie restriction alone, as has been observed in studies that involved patients with metabolic syndrome."

As for future research on the topic, Laferrère and Panda suggested studies that aim to delve deeper into the specifics of time-restricted eating, including finding the best window time-frame for time-restricted eating -- including if there's a difference of an early-in-the-day versus late-in-the-day window -- plus who would most benefit from this strategy.

"From a public health point of view, time-restricted eating may turn out to be an approach to accomplish calorie restriction and improve metabolic health without the resource-intensive approach of intentional calorie restriction," they concluded.

The trial recruited 139 Chinese adults (ages 18 to 75) in free living conditions and randomized them to either the time/calorie-restricted diet or the calorie-restricted diet alone. The extent of caloric limits were sex-specific: a diet consisting of 1,500-1,800 kcal per day for men and 1,200-1,500 kcal per day for women. For the 69 participants randomized to the time-restricted group, they could consume the same amount of calories but only between 8:00 a.m. and 4:00 p.m. Outside of this window, only noncaloric beverages were permitted.

All had obesity, defined as a BMI between 28 and 45. Some exclusion criteria included chronic viral hepatitis, malignant tumors, diabetes, serious liver dysfunction or chronic kidney disease, current smoking, and either serious cardiovascular or cerebrovascular disease.

Both diets appeared to be safe, with no deaths or serious adverse events reported in either group. There was a similar rate of mild adverse events among both diet types, which included fatigue, dizziness, headache, decreased appetite, upper abdominal pain, dyspepsia, and constipation.

Men and women in both intervention groups also adhered to the same caloric restrictions: consuming a combination of 40% to 55% of calories from carbohydrates, 15% to 20% from protein, and 20% to 30% from fat. This diet accounted for about 75% of the participants' baseline daily caloric intake.

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