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Fasting or Exercise? A 12-Week Trial Shows Each Changes Something Different
Picture two people starting the same twelve weeks. One moves breakfast later and stops eating after dinner, so all food fits into eight hours. The other keeps eating as before but turns up twice a week to train hard. At the end, the first has lost more weight. The second has barely moved on the scale. Which one did better?
A new randomised trial from Granada answers that question more carefully than most, and the answer is a little awkward for both camps. Each tool changed something real. Each tool left something else alone. And the scale, which most of us use as the judge, could see only half of what happened.
The trial: four groups, twelve weeks
The study is called TEMPUS, and it was published in Nature Communications on 14 October. Researchers at the University of Granada randomised 187 adults with obesity, aged 25 to 65 and split evenly between men and women, into four groups.
Everyone got the same baseline: a 90-minute group session once a month on Mediterranean-style eating and physical activity. The authors call this usual care. On top of that, one group ate all their food inside a self-chosen eight-hour window, seven days a week. A second group did two supervised training sessions a week. A third did both.
The training was not gentle. Each session combined a circuit of resistance exercises with elastic bands and body weight, then three or four 4-minute intervals of fast incline walking on a treadmill, hard enough to push heart rate above 85% of reserve. Trainers watched every session and heart-rate straps checked the effort. The exercise groups were also asked to add about 10% to their daily steps each week.
People stuck to it remarkably well. The eating-window groups kept to nine hours or less on about 94% of days, and the training groups attended about 92 to 93% of sessions. Whatever this trial shows, it is not a story about people who gave up.
What the scale saw
After twelve weeks, both groups with the eating window had lost about 2.2 kg more than usual care. The training-only group was 0.3 kg lighter than usual care, a difference too small to separate from zero.
Fat mass told a similar story. The eating-window group lost about 1.3 kg more fat than usual care, and the combined group about 1.8 kg more. Training alone did not produce a clear difference in fat mass.
If you stopped reading there, you would conclude that the clock beat the gym. Many headlines did. But the scale is a single number, and the body was keeping more than one account.
TEMPUS trial, 12 weeks
Same twelve weeks, three different changes
Change compared with usual care
Weight −2.2 kg
Fat mass −1.3 kg
Fat-free mass −0.9 kg
Fitness no clear change
Weight no clear change
Fat mass no clear change
Fat-free mass no clear change
Fitness +5.2 ml/kg/min
Weight −2.2 kg
Fat mass −1.8 kg
Fat-free mass no clear change
Fitness +5.7 ml/kg/min
Three ways to read the result
The fasting fan: eating inside eight hours was easy to keep and moved the scale and fat mass more than training did.
The training fan: only the groups that trained became fitter, and training alone did not take fat-free mass with it.
The useful middle: they are different tools for different jobs. The group that used both got the weight change and the fitness change, and lost no clear amount of fat-free mass.
Source: Clavero-Jimeno, …, Ruiz, Nature Communications 2026 (TEMPUS, n = 187 randomised). Values are between-group estimates against usual care at 12 weeks; fitness is estimated VO₂max from a treadmill test, body composition from bioimpedance. The eating-window bar shows typical times for illustration; the 13-hour and 8-hour lengths are the trial’s figures. Cards show categories, not a shared scale.
What only the training changed
Now look at fitness. Both groups that trained improved their estimated VO₂max by about 5 to 6 ml/kg/min compared with usual care, and lasted about two and a half minutes longer on the treadmill test. The eating-window-only group improved by 0.8, a change the trial could not tell apart from nothing.
Five ml/kg/min is not a rounding error. For people who started around 35 to 38, it is roughly a seventh more aerobic capacity in three months. And aerobic capacity is one of the quietest, most underrated numbers in health. The American Heart Association has argued that cardiorespiratory fitness belongs among the vital signs, because in large cohort studies it tracks long-term outcomes at least as closely as several of the risk factors doctors already measure. That is an association, not a promise, but it is a strong and consistent one.
Here is the uncomfortable part for anyone who judges progress by the bathroom scale. The training-only group did something genuinely valuable and the scale gave them nothing for it. Eight weeks in, a person in that group could easily have decided it was not working.
The quiet cost of the eating window
There is one more number, and it is the one the press releases treated most lightly. The eating-window-only group lost about 0.9 kg more fat-free mass than usual care, and about 1.1 kg more than the training group.
Fat-free mass is not the same as muscle. In this trial it was measured with bioimpedance, a scale-like device that passes a small current through the body, and it includes water, glycogen and organs as well as muscle. Some early weight loss on any diet is water. So the honest reading is not that eight-hour eating “burns muscle.” It is that the eating window, used alone, took some lean tissue with it, and that nothing in the eating window was asking the body to keep it.
This is not a one-off. In an earlier 12-week trial in JAMA Internal Medicine, people who ate only between noon and 8 pm lost slightly more appendicular lean mass, the lean tissue in arms and legs, than people who ate three regular meals, without a clear advantage in weight. Reviews of weight loss more broadly point the same way: when people eat less, resistance training and enough protein are the main tools for keeping lean tissue.
TEMPUS fits that pattern neatly. Add two training sessions a week to the eating window, and the fat-free-mass difference against usual care shrank to a statistically unclear 0.5 kg, while the fat loss grew.
The liver result nobody led with
The headline outcome of the trial was not weight at all. It was fat inside the liver, measured precisely with MRI. On that measure, none of the three programmes beat usual care, and none beat each other. Liver fat fell in every group, including the one that only got monthly education sessions.
That is worth a moment. It suggests that being in a trial, with regular contact, Mediterranean-style eating advice and the attention that comes with measurement, already moved the outcome the trial cared about most. The extra tools added things on top, but not on that number.
The authors are candid about the reasons. Many participants started with only modest liver fat, leaving little room to fall. The usual-care group improved more than the researchers expected. Twelve weeks is short. All fair. It still means the most rigorous measurement in the study did not separate the groups.
A year later, the groups looked alike
The researchers brought people back twelve months after the programme ended. By then the differences in weight, body composition and liver fat that had appeared at twelve weeks were no longer detectable between groups. The authors point to partial regain once the structure stopped.
This is the most common ending in nutrition research, and it is not a verdict on eating windows or on training. It is a verdict on twelve weeks. A programme that works while someone is watching, sending reminders and booking your sessions is a programme with a scaffold. Take the scaffold away and the building has to stand on its own habits.
I find this the most useful result in the paper, because it moves the question from “which method is best” to “which method will still be running next October.”
What to do with this
First, name the change you want before you pick the tool. If the goal is the scale or your waist, a consistent eating window was a workable lever here, kept on almost every day with little drama. If the goal is fitness, the eating window did nothing; only training moved it. You can read more on how eating windows compare with ordinary calorie control in our evidence review of intermittent fasting.
Second, if you narrow your eating window, do not do it alone. Two strength sessions a week was enough in this trial to make the lean-tissue loss unclear, and the protein guidance for longevity matters more, not less, when your eating hours shrink.
Third, judge each tool with its own ruler. The scale and a tape measure for the eating window. For fitness, use something repeatable: the same hill at the same pace, and your heart rate on it, which should fall over weeks as you get fitter. A wearable such as the Agen Band can log heart rate during that walk and your resting heart rate each morning as trends; any VO₂max figure on a wrist is an estimate. Our guide to VO₂max and longevity explains why that number deserves your attention.
Fourth, plan for week thirteen. Decide now which part you will keep when nobody is checking. A smaller version you actually keep is worth more than a perfect plan that ends when the supervision does. Our piece on losing weight versus keeping it off goes deeper on that second half.
The bottom line
TEMPUS is a careful trial with a modest, honest message. Eating inside eight hours moved the scale and fat mass. Training moved fitness. Eating inside eight hours alone took a little lean tissue with it, and training seemed to protect against that. None of it beat usual care on liver fat, and a year later the groups had drifted back together.
So the answer to the opening question is that both people did better, at different things, and the scale could only see one of them. Use numbers to correct the fantasy that one method does everything. Then choose the combination you can keep.
