Intermittent Fasting Still Worked a Year Later, but Only When the Window Was Fixed
Clinical Nutrition found that adults who had done 12 weeks of time-restricted eating in a
fixed 8-hour window kept off significantly more weight a year later than adults who
received Mediterranean diet education alone. Both the early window (roughly 9 a.m. to 5 p.m.)
and the late window (roughly 1 p.m. to 9 p.m.) held up. The group that chose its own 8-hour
window each day did not reach significance. Two caveats matter: everyone in the trial also received
diet education, and the loss of fat-free mass persisted at 12 months alongside the weight loss.
Most weight-loss findings fall apart at the follow-up. People lose weight during a trial, the trial
ends, and a year later the difference between groups has quietly closed. That pattern is so common
that a 12-month result is worth more attention than the headline result that preceded it.
Which is what makes this one interesting. Researchers led by Alba Camacho-Cardenosa at the
University of Granada went back to participants a full year after a 12-week time-restricted eating
intervention had finished, and found the separation was still there.
The paper is Effects of an early, late, and self-selected time-restricted eating intervention
on weight loss maintenance in adults with overweight or obesity: A 12-month follow-up of a randomized
controlled trial, published in Clinical Nutrition in 2026 (volume 63, article 106706,
PMID 42302513), with co-authors from the University of Granada, the Granada Institute for Biomedical
Research, the Public University of Navarra, and CIBER.
What the trial actually did
Ninety-nine adults with overweight or obesity, roughly half of them women, were randomized into
four groups for a 12-week intervention:
- Usual care (n = 25) — kept their habitual eating window of 12 hours or more
- Early time-restricted eating (n = 25) — an 8-hour window starting early in
the day, around 9 a.m. to 5 p.m. - Late time-restricted eating (n = 27) — an 8-hour window starting after
1 p.m., around 1 p.m. to 9 p.m. - Self-selected time-restricted eating (n = 22) — an 8-hour window the
participant chose
One detail deserves more weight than it usually gets in coverage of this trial: every
group, including usual care, received Mediterranean diet education. So the comparison is not
fasting versus doing nothing. It is fasting added to dietary advice versus dietary advice on
its own. That makes the result more useful, not less, because dietary advice is what most people
already get from a clinician.
During the 12-week phase, the time-restricted eating groups lost roughly 3 to 4 kilograms more than
the group receiving nutritional guidance alone, according to the University of Granada’s summary of
the work.
What was still true a year later
At the 12-month follow-up, the paper reports that early time-restricted eating (p = 0.01) and late
time-restricted eating (p = 0.02) had maintained significantly greater body weight loss
compared with usual care.
On body composition, the early group came out ahead: the paper reports that fat mass loss remained
significantly better preserved in early time-restricted eating compared with usual care (p = 0.04).
The late group did not reach that threshold on fat mass, even though it did on total body weight.
The authors’ own conclusion is deliberately modest in its wording: a 12-week time-restricted eating
intervention may represent an effective long-term strategy for maintaining body weight loss up to
12 months.
The finding almost nobody is reporting: the self-selected group
Here is the result that changes how you should read this study.
The group that picked its own 8-hour window each day did not reach statistical significance
for maintained weight loss. Same eating duration. Same eight hours. Same diet education. The
only difference was that the window moved.
That is a meaningful signal, and it runs directly against how time-restricted eating is usually
sold. The popular framing is that the number of hours is what matters, and that you can float the
window to fit your schedule. In this trial, the two groups with a fixed window held their
result, and the group with a flexible one did not.
Be careful about how hard to lean on this. With 22 people in that group, a study can fail to detect
a real effect simply because it is too small to see one. Failing to reach significance is not the same
as showing there is no effect. But if you are choosing how to do this, the version with 12-month
evidence behind it is the fixed window, not the floating one.
The caveat that belongs in every honest summary
Weight loss is not automatically good news, and this paper is transparent about something most
coverage leaves out.
The reduction in fat-free mass also persisted at 12 months, in both the early group
(p = 0.051, reported as a tendency) and the late group (p = 0.04), compared with usual care. Fat-free
mass is largely muscle, bone and water.
So the weight that stayed off was not purely fat. Part of what these participants kept off, they
kept off from lean tissue. That matters more as people get older, because muscle mass affects strength,
metabolic rate, blood sugar handling and the ability to recover from illness or a fall.
It does not cancel the finding. It does mean that anyone doing this over a long period has a good
reason to pay attention to protein intake and resistance training, which is exactly the advice most
clinicians would give alongside any sustained calorie reduction.
How much confidence does 99 people buy you?
Not as much as the headline implies, and it is worth being straight about the size.
Ninety-nine participants split four ways leaves 22 to 27 people per group. Several of the key
p-values sit close to the 0.05 line, including one reported at 0.051 and described by the authors
themselves as a tendency rather than a result. A trial this size can detect a large effect reliably
and a modest one only sometimes.
What raises the value of this particular study is not the sample. It is the design and the follow-up:
randomized, with a real comparison group that also received dietary advice, and a full year of distance
from the intervention. Twelve-month maintenance data is genuinely scarce in this field, because it is
expensive to collect and often shows nothing.
What this does and does not tell you
It does suggest that a 12-week block of fixed-window time-restricted eating can
leave a measurable trace a year later, on top of standard dietary advice.
It does not tell you that fasting beats other approaches to the same calorie
deficit, because that comparison was not run. It does not tell you the effect lasts beyond 12 months.
It does not establish that early is better than late for weight, since both maintained weight loss and
they separated only on the fat mass measure. And it says nothing about people who are not adults with
overweight or obesity.
It also cannot tell you whether this is safe or sensible for you specifically. Time-restricted
eating interacts with medication timing, with diabetes management, with pregnancy, with a history of
disordered eating, and with several other situations where compressing an eating window is a decision
for a clinician rather than an article.
Frequently Asked Questions
Does intermittent fasting actually keep weight off long term?
In this trial, partly yes. Twelve months after a 12-week intervention ended, participants who had
used a fixed 8-hour eating window had maintained significantly more weight loss than participants who
received Mediterranean diet education alone (early window p = 0.01, late window p = 0.02). That is one
randomized trial of 99 people, not a settled answer, but 12-month maintenance data is uncommon and this
is a real result rather than a projection.
Is it better to fast early or late in the day?
For total body weight, this trial found no clear winner: both the early (about 9 a.m. to
5 p.m.) and late (about 1 p.m. to 9 p.m.) windows maintained significantly more weight
loss than usual care. The early group did better on one measure, with fat mass loss remaining
significantly better preserved compared with usual care (p = 0.04), which the late group did not reach.
If you have no constraint either way, the early window has slightly more support here. If your life
only allows a late window, the weight result was still there.
Can I just pick a different 8-hour window each day?
The trial included exactly that group, and it did not reach statistical significance for maintained
weight loss. With 22 people in it, that may reflect the group being too small to detect an effect
rather than the flexible approach failing. But the practical reading is straightforward: the fixed
windows are the versions with 12-month evidence behind them, so if you are choosing, choose a time and
keep it.
Do I still need to change what I eat?
This trial cannot tell you that fasting works without dietary change, because it never tested that.
Every group, including the control, received Mediterranean diet education. Time-restricted eating was
tested as something added to dietary advice, not as a replacement for it. Any claim that you can eat
whatever you like inside an 8-hour window and get these results is going beyond what was studied.
Does time-restricted eating cause muscle loss?
This trial found that the reduction in fat-free mass, which is largely muscle, bone and water,
persisted at 12 months in both the early group (p = 0.051, a tendency) and the late group (p = 0.04)
compared with usual care. So the maintained weight loss was not purely fat. That is a normal feature of
sustained calorie reduction rather than something unique to fasting, and it is the reason protein intake
and resistance training belong in the conversation alongside the eating window.
How much weight did people actually lose?
During the 12-week intervention phase, the time-restricted eating groups lost roughly 3 to 4
kilograms more than the group receiving nutritional guidance alone, per the University of Granada’s
summary. The 12-month paper reports the maintenance result as statistical significance against usual
care rather than as a single headline number, which is a more honest way to present a follow-up but
harder to quote.
Is this study large enough to trust?
It is small: 99 participants split into four groups leaves 22 to 27 people per arm, and several
p-values sit near the 0.05 threshold. What gives it value is the design rather than the size. It was
randomized, the comparison group received real dietary advice rather than nothing, and participants
were followed for a full year after the intervention ended. Long-term maintenance data is the part of
this field that is usually missing.
Should I try it?
That is a question for a clinician who knows your situation, not for a study summary. Compressing an
eating window interacts with medication schedules, diabetes management, pregnancy and a history of
disordered eating, among other things. What this research supports is narrower than a recommendation:
in adults with overweight or obesity who were also receiving dietary advice, a fixed 8-hour window for
12 weeks left a measurable difference a year later.
Sources
- Camacho-Cardenosa A, et al. Effects of an early, late, and self-selected time-restricted eating
intervention on weight loss maintenance in adults with overweight or obesity: A 12-month follow-up of a
randomized controlled trial. Clinical Nutrition. 2026;63:106706. DOI: 10.1016/j.clnu.2026.106706.
PMID: 42302513. —
PubMed record - University of Granada, research summary of the 12-month follow-up, reported July 2026 —
Institutional release
Group sizes, p-values and window definitions above are taken
from the published abstract of the 12-month follow-up. The 3 to 4 kilogram figure describes the
12-week intervention phase and comes from the institutional summary rather than the abstract, and is
attributed accordingly.
