Time-restricted eating added no weight loss to an equal calorie cut in a 100-person severe obesity trial

Time-restricted eating, fitting the day’s meals into a window of eight hours or so, is a form of intermittent fasting. A 2026 Cochrane review notes that intermittent fasting “has been publicised in blogs and news articles but studies show inconsistent effects on health.” A randomized trial in Rio de Janeiro put time-restricted eating to a direct test in people with severe obesity, and the eating window added nothing measurable to the weight they lost.

The MACRO trial, run at the Institute of Nutrition of Rio de Janeiro State University and published on October 15 in the British Journal of Nutrition, gave 100 adults with a body mass index above 40 the same reduced-calorie diet for 12 weeks. Half ate only between 10 am and 6 pm; the other half could eat at any time of day. After 12 weeks, those with the eating window had lost 4.3 kg on average and those without it 3.8 kg, a difference small enough to be chance. The eating-window group did lose slightly more fat, but their resting metabolic rate also dropped further. The authors conclude that “our findings suggest that combining ER with the TRE strategy does not enhance weight loss,” using ER for energy restriction and TRE for time-restricted eating.

Why the MACRO trial matched the calories

On average, time-restricted eating does tend to bring weight down, though individual trials vary. A 2025 meta-analysis found about 2.8 kg more weight loss in studies using non-isocaloric controls, meaning comparison groups that were not asked to eat less. The open question is why. Most people who shrink their eating window also eat less, so a trial that compares an eating window with normal eating cannot separate the timing from the cut in calories. Some researchers argue that eating in step with the body’s daily clock adds a benefit of its own, and the way to test that is to hold calories equal and change only the timing.

That is what the Rio de Janeiro team did. Both groups were given diets that provided 60% of total energy expenditure, a substantial cut of 40% below what each person was estimated to burn. The only difference was the clock. Before the trial began, the team registered its expectation: that the eating window would produce weight loss with a difference of at least 2 kg between groups over the continuous diet.

The people in the trial matter too. Earlier trials enrolled a wider range of weights; the US trial described below took adults with a body mass index of 27 to 43. MACRO adds to that evidence with a group the earlier trials barely covered: it took only people above 40, the threshold for severe obesity, which corresponds to about 116 kg (256 pounds) for someone 1.70 m (5 feet 7 inches) tall.

What the MACRO time-restricted eating trial measured

The researchers randomized 100 adults aged 20 to 60, 50 to each group, and followed them closely. The team tracked weight loss, body composition changes and metabolic responses at the start and after 1, 5, 9 and 12 weeks. Body composition was measured by DXA, a low-dose X-ray scan that separates fat, lean tissue and bone. Resting metabolic rate, the energy the body burns at rest, was measured by indirect calorimetry, which calculates energy use from the oxygen a person breathes in and the carbon dioxide they breathe out. The team also tracked appetite ratings and blood markers.

Those repeated, direct measurements are the trial’s strength. Weight alone cannot show whether the weight lost was fat or muscle, and estimated metabolic rate cannot show whether the body is slowing down to defend its weight. Both were measured here, several times over.

What happened to weight, fat and metabolism

Both diets worked. After 12 weeks, both groups experienced reductions in body weight, fat mass and visceral fat, the fat stored deep in the belly around the organs, with no significant change in fat-free mass, meaning no shift large enough to rule out chance in the lean tissue that includes muscle.

The eating window made no clear difference to the main result. The groups lost 4.3 kg and 3.8 kg, half a kilogram apart, a gap well within what chance alone could produce and a quarter of the 2 kg difference the team had set out to find. Visceral fat fell by 270 g with the eating window and 188 g without it, again a difference the trial could not separate from chance.

Fat mass told a slightly different story. The eating-window group lost 3.5 kg of fat against 2.4 kg on the continuous diet, about a kilogram more, and that difference did pass the usual statistical threshold, if only just. The authors highlight it in the paper’s title, which says the eating window “modestly improves fat mass loss.” It is one secondary measure among several, though, and with total weight loss no different, it is hard to know what a kilogram more fat over 12 weeks would mean in the long run.

Metabolism moved in the other direction. The eating-window group showed a greater decline in RMR, resting metabolic rate, than the continuous-diet group, and a trend towards greater adaptive thermogenesis that fell just short of the statistical threshold. Adaptive thermogenesis is the body’s extra slowdown during weight loss, beyond what the lost weight alone would explain. The authors read the two results together as a warning: the greater fall in metabolic rate suggests limited long-term benefit of the TRE strategy in people with severe obesity, because a body burning less energy at rest makes weight harder to keep off.

That reading has support from other work. In a 2024 study of 44 participants with severe obesity on a very low-energy diet, the bigger the drop in resting energy use beyond what weight loss predicted, the stronger the drive to eat following weight loss. Whether the extra slowdown in MACRO would show up as faster regain, the trial was too short to test.

How the result fits earlier trials of time-restricted eating

The null result is in line with the strongest earlier trials. In 2022, a Chinese team randomly assigned 139 patients with obesity to the same calorie target with or without an eating window of 8 am to 4 pm, for a full year. Weight loss was 8.0 kg against 6.3 kg, a gap the authors could not separate from chance, and the window was not more beneficial for body fat or for markers of metabolic risk either. In the 12-week TREAT trial in the United States in 2020, an eight-hour window without any calorie target produced no more weight loss than three meals a day, and its authors concluded that time-restricted eating on its own is not more effective in weight loss than eating throughout the day. That trial also found the eating-window group lost more lean mass, the opposite of the MACRO pattern.

A broader review agrees. The 2026 Cochrane review pooled 22 studies with 1,995 participants and, setting intermittent fasting against regular dietary advice, found little to no difference in weight loss over up to a year.

Not every synthesis reads the evidence the same way. The same 2025 meta-analysis, which pooled 30 studies and came from a chrononutrition group at the Federal University of Uberlândia in Brazil, reported about 1.5 kg more weight loss and 1.5 kg more fat loss with time-restricted eating even when calories were matched, and its authors argued that circadian effects may enhance the impact of calorie restriction. MACRO’s fat result leans that way; its weight result does not.

What the Rio de Janeiro weight loss trial leaves open

The trial was short. Twelve weeks shows the start of a diet, not the year or more that matters for weight, and the registered design added an 8-week phase at full energy intake after the 12 weeks of intervention, which the 12-week results do not cover. A slower metabolism matters most during that kind of period, when people stop dieting.

It was small and local. One hundred people at one clinic in Rio de Janeiro make an effect as large as the 2 kg the team expected unlikely, but cannot rule out a small one. The fat difference was modest and came from one of several outcomes, and the metabolic slowdown was measured but not followed far enough to see what it did.

What the MACRO results mean for people trying time-restricted eating

For people with severe obesity, the trial offers no evidence that adding an eating window to a calorie-reduced diet speeds weight loss, and some evidence that it may make the body conserve energy a little more. It does not show harm: both groups lost weight and fat without a significant loss of lean tissue. The Cochrane reviewers suggest that the choice of a fasting approach may come down to individual practicality and sustainability, and for people who find an eating window the easiest way to eat less, the calorie cut is still doing the work. Treatment for severe obesity, which can include medicines and surgery as well as diet, is a decision made with a doctor.

In this trial, the clock did not add to the calories: the same calorie cut produced about the same weight loss whether or not meals were squeezed into eight hours.