Short answer

Yes, it can, if you treat the eating window as the whole plan. In a 12-month follow-up of a randomised trial in 99 adults with overweight or obesity, people who did 12 weeks of 16:8 time-restricted eating kept off more weight than a usual-eating control group a year later. The catch was body composition. Only the early eating window group kept a clearly significant fat mass reduction at 12 months, while all time-restricted groups showed a persistent loss of fat-free mass. Fat-free mass is not pure muscle, it includes water, organs and other lean tissue, but for anyone who cares about strength, glucose control and healthy ageing, losing it is the wrong direction.

The good news is that this is not an argument against time-restricted eating. It is an argument for pairing it with the two tools the trial authors themselves point to: resistance training and adequate protein. When time-restricted eating is combined with resistance training in other trials, muscle mass and strength are generally preserved while fat mass falls. So the honest answer to does intermittent fasting cause muscle loss is: fasting alone, during weight loss, can cost you lean tissue; fasting plus lifting plus enough protein is a different intervention.

This article walks through what the Granada trial actually did, what held up at 12 months, what did not, and a practical muscle-preserving protocol you can track like a self-experimenter.

What did the Granada trial actually test?

The study was led by researchers at the University of Granada, the Granada Institute for Biomedical Research, the Public University of Navarra and CIBER, and published in Clinical Nutrition in 2026. It was a 12-month follow-up of an earlier randomised controlled trial whose main short-term results appeared in Nature Medicine.

Ninety-nine adults took part. Half were women. The average age was 49 and the average BMI was 32, so this was a group with overweight or obesity, not lean athletes. Everyone received education on the Mediterranean diet. For the first 12 weeks, participants were randomised to one of four approaches:

• Usual eating: keep an eating window of 12 hours or more
• Early time-restricted eating: an 8-hour window starting before 10:00, for example 9:00 to 17:00
• Late time-restricted eating: an 8-hour window starting after 13:00, for example 13:30 to 21:30
• Self-selected time-restricted eating: choose your own 8-hour window

After the 12 weeks, the structured intervention ended. Researchers then measured weight, waist and hip circumference and body composition again 12 months later. That design matters. Most fasting studies stop at the end of the diet. This one asked whether a short, 3-month change leaves anything behind a year later, when people are mostly back to living their normal lives.

Adherence during the 12 weeks was high, reported at 85 to 88 percent. During the follow-up year, only about a quarter to a third of participants said they kept up some form of time restriction on their own. So whatever persisted at 12 months was not dependent on perfect long-term compliance. That is both encouraging and a reason for caution when interpreting the body composition signal.

What held up after a year?

Weight loss maintenance was the headline. After the 12-week intervention, the usual-eating group had lost about 1.4 kg. The early group had lost about 4.2 kg, the late group about 3.1 kg and the self-selected group about 3.8 kg.

Twelve months later, the usual-eating group had regained slightly, up about 0.4 kg on average in the preliminary conference report. Both the early and late groups were still significantly lighter than the control group, roughly 2.0 to 2.2 kg lower in that report, with the peer-reviewed follow-up describing about 2.4 to 2.6 kg more weight loss maintained than usual care and roughly half of the initial loss retained, from about 5 percent at 12 weeks to about 2.5 percent at one year. The self-selected group sat in between and did not reach statistical significance against control in every analysis.

Waist and hip results were mixed by group, with the late group showing the clearest sustained reductions in the early report. The important point for our question is not the waist tape. It is what that remaining weight loss was made of.

The muscle catch, explained plainly

Body composition is where the trial becomes more complicated, and more useful.

Only the early time-restricted group maintained a statistically significant reduction in fat mass at 12 months. All time-restricted groups, however, showed persistent loss of fat-free mass. Commentators summarising the peer-reviewed paper singled this out as a concern and noted the authors recommendation to combine time-restricted eating with resistance training and adequate protein intake to help preserve muscle.

Three things need saying about fat-free mass so this finding is not overstated.

First, fat-free mass is not the same as skeletal muscle. It includes body water, bone mineral, organs and connective tissue. Early in any weight loss, some of the fat-free change is water and glycogen. A fall in fat-free mass does not map one-to-one onto biceps lost.

Second, some lean loss is expected in almost any weight loss intervention. When people lose weight through calorie restriction alone, a meaningful share, often quoted at around a quarter to a third in the general weight loss literature, can come from lean tissue if protein is low and there is no resistance stimulus. Fasting is not uniquely muscle-wasting in that sense. It is one way of creating an energy deficit, and deficits without a muscle signal tend to cost lean tissue.

Third, this trial was not designed as a muscle-preservation trial. Participants got Mediterranean diet education, not a prescribed high-protein target and not a structured progressive resistance programme. Measuring what happens to fat-free mass when you add a window and general healthy-eating advice, without those two protections, is exactly how you find out that the window alone is not enough.

That is why the result should change your protocol, not necessarily your decision about whether to use a window at all.

Does intermittent fasting cause muscle loss in people who lift?

This is where the wider literature helps, because the Granada participants were not put through a lifting programme.

A 2024 systematic review and meta-analysis in Nutrients looked specifically at time-restricted feeding combined with resistance training. It included seven randomised or crossover trials with 164 adults, lasting at least four weeks. Compared with resistance training and habitual eating, time-restricted feeding plus resistance training reduced body mass and fat mass further, and improved fasting insulin and cholesterol measures. Crucially, it found no significant adverse change in muscle mass or strength.

In plain terms: when the resistance signal is present, the muscle outcome looks very different. The body is being told, several times a week, that this tissue is needed. Without that signal, during a deficit, it has less reason to keep energetically expensive muscle.

Protein is the second half of the signal. Across the broader resistance training literature, a widely cited meta-analysis by Morton and colleagues found that protein supplementation during resistance training improved lean mass gains up to a total intake of around 1.6 grams per kilogram of body weight per day, with a wide confidence interval. More recent reviews of weight loss in people with overweight or obesity generally land in a similar practical range, often 1.2 to 1.6 grams per kilogram per day, distributed across meals, individualised for age, kidney health, body composition and training load. This is not medical advice for any individual, but it explains why a single small protein shake on a fasting day, as tested in some alternate-day fasting work, is not a muscle-preservation strategy. Total daily and weekly protein, plus training, is what counts.

So the Granada fat-free mass finding and the resistance training meta-analysis are not contradicting each other. They are describing two different interventions that happen to share a clock.

Why would a window cost lean tissue if you do nothing else?

Self-experimenters like mechanisms, so here is the simple version without overstating it.

When you compress eating into eight hours, most people spontaneously eat less. That is part of why it works for weight. If protein ends up squeezed into one or two small meals, total protein often falls without anyone noticing. Muscle protein synthesis responds to adequate essential amino acids, particularly leucine, spread across the day. One large protein bolus does not fully compensate for two missed opportunities to stimulate it.

At the same time, an energy deficit increases the background rate at which the body breaks down tissue for fuel. Resistance training pushes in the opposite direction by sensitising muscle to amino acids and creating a demand for repair. Without training, there is less stimulus to rebuild what a deficit breaks down.

Add sleep, steps and alcohol into the picture and it gets more personal. Poor sleep, low daily movement and regular alcohol can all worsen body composition outcomes during weight loss. None of this is unique to fasting. Fasting just makes it easier to accidentally under-eat protein while telling yourself the window is doing the work.

A muscle-preserving 16:8 protocol

If you like the simplicity of 16:8 and want the Granada weight maintenance benefit without paying for it in lean tissue, build the protocol around muscle first and the window second.

1. Anchor protein before you anchor the window.
Set a daily protein target with a clinician or dietitian if you have medical conditions, kidney disease, diabetes medication, a history of eating disorders, or you are pregnant, breastfeeding or under 18, in which case fasting may not be appropriate at all. For many healthy, resistance-training adults in a deficit, the literature discussed above clusters around 1.2 to 1.6 grams per kilogram per day. Divide it across two to three meals inside your window, aiming for a substantial protein serving at the meal that breaks your fast and at your post-training meal. Track it for two weeks. Most people who think they eat enough protein do not.

2. Lift two to four times a week, progressively.
Two full-body sessions is a minimum effective starting point for many people; three or four sessions allows more volume per muscle group. Prioritise compound movements you can load safely: squats or leg press, hip hinges, presses, rows and carries. Keep loads challenging even as body weight falls. If every lift is dropping rapidly while weight is only falling modestly, that is a signal to review energy, protein, sleep and programme design rather than to celebrate the scale.

3. Choose the window you will actually keep.
The Granada data are reassuring here. Early and late windows both maintained more weight loss than usual eating at 12 months. Pick the one that fits work, family meals and training. If you train in the morning and feel dreadful fasted, a later window with a pre-training meal may produce better training quality than a theoretically optimal early window you abandon. Adherence beat timing in this trial, because nobody was still being supervised a year later.

4. Eat like the Mediterranean education in the trial, not like a free-for-all.
All groups in Granada received Mediterranean diet education. The window was added to better food quality, not used as a licence to compress ultra-processed food into eight hours. Vegetables, legumes, fruit, whole grains, fish, olive oil and fermented dairy if you tolerate it make it far easier to hit protein, fibre and micronutrients inside a shorter window.

5. Track body composition, not just weight.
Weigh yourself under consistent conditions if you find it useful, but do not judge a fasting protocol on weight alone. Add waist circumference, progress photos under the same lighting if you are comfortable with them, gym performance, and, where available, a consistent body composition method such as the same DEXA scanner or the same bioimpedance device under the same conditions. All consumer methods have error. Consistency matters more than false precision. The question you are answering is not did I lose weight, it is did I lose mostly fat while keeping performance.

6. Protect sleep and steps.
Weight loss that costs sleep usually costs muscle and hunger control as well. Keep a regular sleep window, get outdoor light early in the day where possible, and keep daily steps up on non-training days. These are boring, and they are often the difference between fat loss and tired loss.

7. Review at 12 weeks, like the trial did.
Run the protocol for 12 weeks, then review honestly. Are you lighter, is your waist smaller, are your main lifts holding or improving, is your protein average actually hitting target, and do you like this enough to still be doing some version of it in a year. The Granada lesson is that a 12-week block can leave a 12-month trace, but only if the habits built in those 12 weeks are livable.

When to be more cautious

Time-restricted eating is not for everyone, and this article is not a recommendation to start it. Be particularly cautious and seek medical advice first if you take glucose-lowering medication, insulin or blood pressure medication that may need adjustment when food timing changes, if you have a history of eating disorders, if you are underweight, pregnant or breastfeeding, or if you are an older adult already at risk of sarcopenia, the age-related loss of muscle mass and strength. For older adults, preserving muscle is not a cosmetic goal. It is central to independence, balance and fracture risk, so any intervention that risks lean loss needs extra care and supervision.

Also be cautious about extending fasts well beyond 16:8 without a reason. The trial discussed here used an 8-hour eating window. It does not provide evidence that narrower windows or multi-day fasts preserve muscle equally well, and the longer the fast, the harder it becomes to fit adequate protein and training fuel into the eating period.

Finally, treat rapid strength loss, persistent fatigue, hair loss, menstrual changes, mood changes or recurrent illness during any weight loss attempt as a stop-and-review signal, not as proof the diet is working.

The bottom line for self-experimenters

Time-restricted eating kept the weight off for a year in the Granada trial, but there is a muscle catch if you stop there.

Treat the window as an adherence tool. It can make an energy deficit simpler and, for many people, more sustainable than counting every calorie. It is not a muscle-preservation tool. Muscle is preserved by the combination that the trial authors and the wider literature keep returning to: enough protein, spread sensibly across your eating window, and progressive resistance training that gives your body a reason to keep the tissue you care about.

If your tracker, your waist measurement and your logbook all agree that fat is falling while strength holds, your protocol is working. If the scale is falling and everything else is falling with it, you do not have a fasting success. You have an under-fuelled weight loss that happens to have a clock attached.

Sources

• Camacho-Cardenosa A and colleagues. 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. DOI: 10.1016/j.clnu.2026.106706. Summary via University of Granada, 2 July 2026.
• European Congress on Obesity report of the same 12-month follow-up, EurekAlert, 10 May 2025: 99 adults, mean age 49, mean BMI 32, 12-week intervention, weight and waist outcomes at 12 months.
• Ho Y and colleagues. Synergistic Effects of Time-Restricted Feeding and Resistance Training on Body Composition and Metabolic Health: A Systematic Review and Meta-Analysis. Nutrients, 2024. Seven trials, 164 participants: fat mass fell with time-restricted feeding plus resistance training, with no significant adverse change in muscle mass or strength.
• Morton RW and colleagues. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength. British Journal of Sports Medicine, summarised in subsequent protein reviews: benefits plateaued around 1.6 g/kg/day in resistance-training adults.

Frequently asked questions

Does intermittent fasting cause muscle loss?

It can contribute to fat-free mass loss when it is used as a weight loss method without adequate protein and resistance training. In a 12-month follow-up of 99 adults, all time-restricted eating groups showed persistent fat-free mass loss, while only the early group kept a significant fat mass reduction. When time-restricted feeding is combined with resistance training in other trials, muscle mass and strength are generally preserved.

Is fat-free mass the same as muscle?

No. Fat-free mass includes skeletal muscle, but also water, bone, organs and other lean tissues. Losing fat-free mass during weight loss is a warning sign for muscle, especially if strength is also falling, but it is not a direct one-to-one measure of muscle alone.

Did early fasting beat late fasting in the Granada trial?

For weight maintenance at 12 months, early and late time-restricted eating both beat usual eating, with no clear timing winner overall. For body composition, only the early group maintained a statistically significant fat mass reduction. Both approaches remained useful options, and the researchers emphasised choosing a schedule people can sustain.

How much protein do I need to protect muscle while fasting?

Protein needs are individual. Reviews of resistance training and weight loss commonly discuss around 1.2 to 1.6 grams per kilogram of body weight per day for many healthy, training adults, spread across meals. People with kidney disease, diabetes or other medical conditions should set a target with their own clinical team rather than using a general range.

Can you build muscle on 16:8?

You can build or at least preserve muscle on 16:8 if total energy is appropriate, protein is adequate and resistance training is progressive. It is usually harder to build substantial new muscle in an aggressive energy deficit, fasting or not. For most people using 16:8 for fat loss, maintaining strength and lean mass while fat falls is the realistic win.

What is the best eating window to keep muscle?

The best window is the one that lets you train well, hit your protein target in two to three meals, sleep well and keep the routine for months. Early and late windows both maintained weight loss in the Granada trial. A window that makes you miss protein or skip training to stay fasted is the wrong window for muscle, regardless of timing theory.