Time-Restricted Eating in Severe Obesity: The MACRO Trial

The MACRO randomized clinical trial, published online on 15 October 2026 in the British Journal of Nutrition, tested whether an eating window adds anything to a calorie-restricted diet in adults with severe obesity. After 12 weeks, the group eating between 10am and 6pm lost about the same weight as the group with no time limit, lost somewhat more fat, and showed a larger fall in resting metabolic rate.

The authors read that mix of results cautiously. Their own conclusion is that the eating window did not enhance weight loss and that the metabolic changes suggest limited long-term benefit in this group.

What the study did

The trial comes from researchers at Rio de Janeiro State University and collaborators in Brazil. It enrolled 100 adults aged 20 to 60 with a body mass index above 40 and randomized them into two groups of 50.

Who	100 adults, 20 to 60 years old, BMI above 40
How long	12 weeks
Group 1	Time-restricted eating with energy restriction: all food between 10am and 6pm
Group 2	Continuous energy restriction: same calorie target, no time window
Calorie target	Diets provided 60% of total energy expenditure
What was measured	Body composition by DXA scan and resting metabolic rate by indirect calorimetry, at baseline and after 1, 5, 9 and 12 weeks; appetite ratings; blood markers

The design matters. Both groups were prescribed a calorie deficit. The only planned difference was the clock. That lets the trial ask a narrow question: once calories are already restricted, does squeezing meals into eight daytime hours help?

What it found

Both groups lost body weight, fat mass and visceral fat (the fat around the abdominal organs) over the 12 weeks, and neither showed a significant change in fat-free mass.

Comparing the two:

• Weight loss was similar. The eating-window group lost 4.3 kg and the continuous group 3.8 kg, a difference that was not statistically significant (P = 0.46).
• Visceral fat loss was similar. The figures were 270 g and 188 g, again not a significant difference (P = 0.24).
• Fat mass loss was greater with the eating window. That group lost 3.5 kg of fat mass against 2.4 kg (P = 0.040).
• Resting metabolic rate fell more with the eating window (P = 0.032).
• Adaptive thermogenesis showed a trend in the same direction (P = 0.061), which did not reach statistical significance.

Resting metabolic rate is the energy your body uses at rest. It normally falls when you lose weight, because there is less of you to maintain. Adaptive thermogenesis is the name for a drop in energy use that goes beyond what the change in body size would predict. Researchers watch both because a body that burns less at rest may find it harder to keep weight off.

How the authors read it

The title of the paper says time-restricted eating with energy restriction "modestly improves fat mass loss". The conclusion is more guarded. The authors write that combining energy restriction with time-restricted eating does not enhance weight loss, despite the modestly greater fat loss after 12 weeks. They add that the greater decline in resting metabolic rate and the greater increase in adaptive thermogenesis suggest limited long-term benefit of the strategy in people with severe obesity.

So there are three findings to hold together, and none of them cancels the others: the same weight loss, a bit more fat loss, and a bigger metabolic slowdown.

What it does not show

• It does not show long-term outcomes. The trial lasted 12 weeks. The caution about long-term benefit is the authors' inference from metabolic measurements, not an observation of people regaining weight.
• It does not tell you how large the metabolic difference was. The abstract reports that the decline in resting metabolic rate was greater and gives the P value, but not the size of the difference.
• It does not test an eating window on its own. Both groups ate at a prescribed deficit. A time window without a calorie target is a different intervention, and this trial cannot speak to it.
• It does not test other windows. Only 10am to 6pm was studied. Later or longer windows may behave differently.
• It applies to severe obesity. The participants all had a BMI above 40. The results may not carry over to people at lower body weights.
• The fat mass difference is modest. The authors' own word is "modestly", and a P value of 0.040 sits close to the usual threshold.

What this means if you fast

An eight-hour window like the one in this trial is what most people call 16:8 fasting. If you use it, the fair reading of MACRO is this: when total calories are already held down, the window itself did not produce extra weight loss in 12 weeks. Many people find a window useful for a simpler reason, which is that it gives the day a structure. This trial neither supports nor undermines that, because both groups were given the same calorie target.

The metabolic finding deserves attention without alarm. A fall in resting metabolic rate during weight loss is expected. A larger fall in one group, measured in one 12-week trial, is a signal for researchers to follow up, not a verdict. If you are choosing a pattern, how to choose the right fasting schedule for your lifestyle and how to structure your eating window for sustained energy cover the practical side.

One thing needs saying plainly. This was research in patients with severe obesity, on prescribed diets, with scans and metabolic testing along the way. It is not a treatment plan to copy alone. Severe obesity is a medical condition and weight loss at that level belongs with a clinical team.

Fasting is not right for everyone, and this post is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting.

Tracking an eating window with QoFast

QoFast is a fasting tracker app for iPhone and Android, for intermittent fasting and multi-day extended fasts. Its intermittent plans are 16:8, 18:6, 20:4, OMAD and custom, and the timer works offline and survives restarts, with water and mood logging and a fasting history alongside. QoFast gives general information, not medical advice, and is free to download with an optional Pro subscription on the App Store and Google Play.

FAQ

Does time-restricted eating increase weight loss when calories are already restricted? Not in this trial. Over 12 weeks, weight loss was similar with and without the 10am to 6pm window.

Did the eating window help with fat loss? Fat mass loss was greater in the eating-window group, 3.5 kg against 2.4 kg. The authors call the difference modest.

Does time-restricted eating slow your metabolism? Resting metabolic rate fell in the trial, and it fell more in the eating-window group. The abstract does not give the size of the difference, and the study was limited to adults with severe obesity on a calorie-restricted diet.

Is 10am to 6pm the best eating window? The trial did not compare windows, so it cannot say.

Sources

• Campos TAM, Brasil RLF, Oliveira MA, Feitosa ACF, Bueno NB, Mulder AP, Bezerra FF. "Time-restricted eating with energy restriction modestly improves fat mass loss in severe obesity: the MACRO randomised clinical trial." British Journal of Nutrition, published online 15 October 2026. DOI: 10.1017/S0007114526108629. No PubMed record was available on the date of this post.