Intermittent fasting: what it is, types and what science says
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With the same calories, fasting burns no more fat than a normal diet. What 12-month trials show, the main types and how to do it without losing muscle.
Intermittent fasting is probably the most searched weight-loss strategy of the last decade. It promises to slim you down without counting calories, to "reset" your metabolism and even to extend your life, all with one simple rule: do not eat for a few hours a day. A promise that big deserves an uncomfortable question: what do human studies actually show, and what happens to your muscle when you go 16 hours without food?
The short answer is that intermittent fasting works, but not through the magic attributed to it, and that it carries a specific risk for people who train if it is done badly. Let us look at the types that exist, what the most solid clinical trials say and how to apply it, if you decide to try it, without losing the muscle mass that is so hard to build.
What intermittent fasting is (and what it is not)
Intermittent fasting is not a diet in the strict sense: it does not say what to eat, but when. The Spanish Society of Endocrinology and Nutrition (SEEN) makes this point in its document on the trend, stressing that fasting "cannot be considered a diet", because it restricts hours, not foods. We all fast every night for 8 to 12 hours; intermittent fasting simply extends that period on purpose.
The most common protocols
There are several patterns, and not all are equally easy to sustain. Daily ones restrict eating hours every day; weekly ones concentrate the restriction on one or two days.
| Protocol | How it works | Difficulty | Who it suits |
|---|---|---|---|
| 12/12 | 12 hours of fasting, 12 of eating | Very low | A starting point for anyone |
| 16/8 | 16 hours of fasting and an 8-hour eating window | Medium | People who are not hungry at breakfast and eat dinner early |
| 5:2 | 5 normal days and 2 days with intake cut by around 75% | Medium-high | People who prefer two hard days to five days of control |
| Alternate-day | One normal day and one fasting or very-low-intake day | High | Supervised clinical settings |
| 24 hours | One or two full fasts per week | High | Experienced people with no contraindications |
By far the most popular is 16/8, also called time-restricted eating. A typical window runs from 12:00 to 20:00, so a good part of the 16 fasting hours coincides with sleep.
Does it slim you down more than a normal diet? What the trials say
This is where marketing and data need separating. The relevant question is not whether people lose weight doing intermittent fasting, which they do, but whether they lose more than when eating the same calories spread out normally.
The 12-month New England Journal of Medicine trial
The most rigorous study to date was published by Liu and colleagues in The New England Journal of Medicine in 2022. They assigned 139 people with obesity to calorie restriction with or without an 8:00-16:00 eating window, with the same calories in both groups, for one year. The fasting group lost 8.0 kg and the conventional calorie-restriction group 6.3 kg: a difference that was not statistically significant. There were no relevant differences in body fat, visceral fat or metabolic markers either.
The TREAT trial and the warning about muscle
The same pattern appeared in the TREAT trial by Lowe and his team, published in JAMA Internal Medicine in 2020. For 12 weeks, 116 adults with overweight followed either 16/8 or a three-meal schedule. The 16/8 group lost 0.94 kg and the control group 0.68 kg, with no significant difference. But there was a finding almost nobody quotes: the fasting group lost more appendicular lean mass, meaning muscle in the arms and legs.
The conclusion that matters: when calories are matched, intermittent fasting does not burn more fat than a conventional diet. It works because it helps many people eat less without thinking too much. It is an adherence tool, not a metabolic shortcut. SEEN itself sums it up this way: time-restricted eating "may slightly favour weight loss" and appears "as effective and safe as an equivalent calorie-restriction diet".
Intermittent fasting and muscle: the risk almost nobody mentions
If you strength train, this is the part that affects you most. The problem with fasting is not fasting itself, but that squeezing meals into a few hours makes it easy to eat too little protein and to distribute it poorly.
The good news is that, done sensibly, it does not have to cost you muscle. Moro and colleagues studied 34 resistance-trained men for 8 weeks in 2016, published in the Journal of Translational Medicine. Those who followed 16/8 with the same calories and protein as the control group reduced body fat and kept their lean mass and strength. The contrast with the TREAT trial is revealing: in Moro's study the participants trained and ate enough protein.
How to protect muscle if you fast
The evidence on protein synthesis allows clear guidelines, which apply whether you fast or not:
- Enough total protein. The meta-analysis by Morton and colleagues (2018) places the ceiling of benefit at around 1.6 g per kilo of bodyweight per day. If you are losing fat, aim for the upper end of the range.
- Spread over at least two or three servings. Schoenfeld and Aragon (2018) recommend about 0.4 g/kg per meal. Three protein-rich meals fit easily into an 8-hour window.
- Strength training. It is the signal that tells your body to keep muscle during a deficit.
- A window that includes the post-workout period. If you train in the afternoon, make your eating window start before and end after the session.
| Time | Moment | What to do |
|---|---|---|
| 08:00 | Fasting | Water, unsweetened coffee or tea |
| 12:00 | First meal | Complete meal with 35-45 g of protein |
| 16:30 | Pre-workout snack | Yoghurt or dairy with fruit: 25-30 g of protein |
| 17:30 | Workout | Strength session |
| 19:30 | Last meal | Dinner with 40-50 g of protein, carbohydrates and vegetables |
| 20:00 | Window closes | The 16 fasting hours begin |
Does it have benefits beyond weight?
This is the most attractive part of the pitch and also the most exaggerated. The review by de Cabo and Mattson in The New England Journal of Medicine (2019) explains the mechanism behind it: after several hours without food, the body switches from using glucose to using fat and ketone bodies, a "metabolic switch" with effects on insulin sensitivity, inflammation and cellular repair.
The honest caveat is that much of that evidence comes from animal studies or short human trials. When calories are matched in people, as in the trials above, improvements in glucose, blood pressure or cholesterol look very similar to those of a conventional diet. SEEN agrees: it warns that the number of published studies is still small, and that they are too short to draw conclusions with confidence.
Who should not do intermittent fasting
Intermittent fasting is not for everyone. SEEN states that the following should consult a specialist before adopting it: people with diabetes, heart disease, kidney or liver disease, pregnant or breastfeeding women, children and adolescents, older adults, and anyone who has or has had an eating disorder.
To that list it is worth adding people taking medication that depends on meals, and athletes with several sessions a day, who need intake spread across the day to perform and recover.
How to start if you decide to try it
If you are not in any of those groups and the idea appeals to you, the sensible way to start is gradually:
- Weeks 1-2: a 12-hour overnight fast, for example from 20:00 to 08:00.
- Weeks 3-4: extend it to 14 hours by delaying breakfast.
- From week 5: move to 16/8 if you feel well and have no anxiety around food.
During the fast you can have water, unsweetened coffee or tea. In the eating window, quality still rules: if you eat in 8 hours what you used to eat in 14, you will not create a deficit. And if you notice the clock becoming an obsession, or that you overeat as soon as the window opens, that is a clear sign this strategy is not for you.
How fasting fits into a fat-loss plan
Putting it all together: intermittent fasting is a way of organising a calorie deficit, not a replacement for one. What decides whether you lose fat and keep muscle is the same as always: a moderate deficit, enough protein and strength training.
That last piece, the one that protects your muscle while you slim down, is exactly what the Progrevia routine generator designs according to your level, your days and your equipment. You choose your meal schedule; the tool provides the training plan.
The summary worth remembering
Intermittent fasting works for weight loss, but because it helps you eat less, not because it burns more fat: with the same calories, 12-week and 12-month trials find no difference compared with a conventional diet. Its real risk for people who train is losing muscle if protein falls short. If you find it comfortable, go ahead: prioritise 1.6 g/kg of protein spread across meals, train for strength and consult a professional first if you have any health condition. If you do not find it comfortable, you are not missing out on anything.
- Cancer Minchot E, Moreno Borreguero A. El ayuno intermitente como tendencia. Spanish Society of Endocrinology and Nutrition (SEEN). 2020.
- Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. The New England Journal of Medicine. 2022;386(16):1495-1504.
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Internal Medicine. 2020;180(11):1491-1499.
- Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine. 2016;14(1):290.
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. The New England Journal of Medicine. 2019;381(26):2541-2551.
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. 2018;52(6):376-384.
- Schoenfeld BJ, Aragon AA. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition. 2018;15:10.
Frequently asked questions
Does intermittent fasting help you lose weight?
Yes, but not because it burns more fat: it works because it helps many people eat less. In the trial by Liu and colleagues (NEJM, 2022), with 139 people over 12 months and the same calories in both groups, the fasting group lost 8.0 kg and the conventional diet group 6.3 kg, a non-significant difference.
What is 16/8 fasting?
It is the most popular pattern: 16 hours of fasting and an 8-hour eating window, for example from 12:00 to 20:00. A good part of the fast coincides with sleep. During the fasting hours you can have water, unsweetened coffee or tea.
Do you lose muscle with intermittent fasting?
It can happen if protein falls short: in the TREAT trial (2020) the fasting group lost more lean mass. But in trained men who followed 16/8 with enough protein (Moro et al., 2016), lean mass and strength were maintained. The key is reaching about 1.6 g/kg of protein spread across meals and strength training.
What can you drink while fasting?
Water, black coffee or unsweetened tea, and calorie-free drinks. Any food or drink with calories, such as milk, juice or sugary soft drinks, breaks the fast.
Who should not do intermittent fasting?
According to the Spanish Society of Endocrinology and Nutrition, people with diabetes, heart, kidney or liver disease, pregnant or breastfeeding women, children and adolescents, older adults and anyone who has or has had an eating disorder should consult a specialist first.