Intermittent fasting: what the evidence says, who it may suit and who it may not

What is intermittent fasting and what methods are there?

Intermittent fasting is a way of organising meals that alternates hours or days when you eat with others when you do not eat, or eat very little. It mainly changes when you eat. What you eat still matters just as much.

The hours when you do eat are called the eating window. The four methods studied most are:

• Time-restricted eating. You eat only within a fixed window of hours each day, without counting calories.
• 16/8. The best-known version of time-restricted eating, with sixteen hours without eating and an eight-hour window. You spend part of those hours asleep.
• 5:2. You eat normally for five days and eat very little on two non-consecutive days.
• Alternate-day fasting. You eat normally one day and, the next, around a quarter of what you need [2].

Complete fasts of 24 hours, once or twice a week, belong to the same family as 5:2. They are the hardest to sustain and the ones we least recommend trying on your own.

Does it lead to more weight loss than eating less throughout the day?

It leads to weight loss, and more or less the same amount as eating somewhat less spread throughout the day. A review of 99 randomised trials compared fasting with eating without any guidelines and with continuous calorie restriction, which means eating somewhat less every day [1].

Compared with eating without any guidelines, all methods lead to weight loss. Compared with continuous restriction, the result is similar [1]. Only alternate-day fasting showed a slight advantage in the short term, which is small for how hard it is to sustain [1].

One-year trials point in the same direction. Eating within an eight-hour window with fewer calories led to the same weight loss as eating the same calories without a schedule [3]. In another trial, participants in both groups lost a similar amount of weight, although more people dropped out of the alternate-day fasting group [2].

Fasting works when it helps you eat somewhat less without making food a struggle.

The most recent Cochrane review, covering 22 trials in adults with overweight or obesity, concludes that intermittent fasting may make little or no difference compared with usual dietary advice [4]. The authors point out that the included studies last twelve months at most and that the certainty of the evidence is low [4].

Other claimed benefits, such as improving blood sugar, have less evidence behind them than it might seem. A review that brought together more than a hundred effects studied in relation to fasting found only seven supported by moderate- or high-quality evidence [5].

If you are looking for the complete approach to losing fat without regaining it, you can find it in our guide to losing weight and keeping it off.

How do you do 16/8 intermittent fasting in practice?

The key is to choose an eating window that fits your day rather than someone else’s clock. These are the three most common:

• From 10:00 to 18:00. You finish dinner early. It tends to go best with sleep, but clashes with family dinners.
• From 12:00 to 20:00. You skip breakfast and have lunch and dinner as usual. It is the most popular and the most convenient for people who work late.
• From 8:00 to 16:00. You have breakfast and a good lunch, and dinner falls outside the window. It suits people who get up early and wake up hungry.

Within the window, what counts is the plate. Put together two or three complete meals with vegetables, a portion of protein and a base of wholegrain cereal, pulses or potato, using the food groups in the lists above. An eight-hour window does not make up for a day of pastries, soft drinks and ready meals.

To get started, these are the steps we follow in consultations:

1. Start with a twelve-hour window for one or two weeks.
2. If you feel well, gradually shorten it, half an hour at a time.
3. Choose a window that fits your work and family meals.
4. After four weeks, review how you are sleeping, how you are training and how hungry you feel.

If everything is going well at that review, you can stay where you are. You do not need to reach 16/8 for it to work. And if you would rather eat less without watching the clock, you have other options in how to reduce calories without feeling hungry.

What can you have during fasting hours?

During fasting hours, you can have anything that provides no energy: water, sparkling water, mild herbal teas (chamomile, lemon balm or lemon verbena), and plain tea or coffee without sugar.

Milk, plant-based drinks, juices, broths and sugary soft drinks count as food. Sugar-free soft drinks and sweeteners are not recommended as a weight-loss strategy [6]. This advice does not apply to people with diabetes.

If you take medication that must be taken with food, your medication schedule takes priority over your eating window. Ask your doctor or pharmacist before changing it.

What about hunger, muscle and performance?

Hunger is often intense during the first few days and eases afterwards for many people. This is what people who try it tell us most often, along with some irritability or difficulty concentrating during fasting hours.

If your hunger does not ease after two or three weeks, or you spend the whole day thinking about food, this approach is not for you.

You need to be careful about muscle mass. In a trial with an eight-hour window and no other changes, fasting did not lead to more weight loss than eating throughout the day [7]. In addition, much of the small amount of weight lost was lean mass, which includes muscle [7]. That is why, with this approach, protein at every meal and strength training are necessary.

Can you do intermittent fasting if you train?

You can, with adjustments. In people who were already doing strength training, a review of trials with eight- to ten-hour windows found a modest reduction in body fat and no change in fat-free mass [8]. In other words, if you do strength training, your muscle does not seem to suffer from eating within a window.

To make it work, place demanding sessions within or close to the window, and eat afterwards. If you train fasted, keep easy sessions for the first hours and make sure you reach your first meal with protein ready.

If you compete, train twice a day or have little energy to spare, fasting adds the risk of not getting enough energy. In that case, it is best to tailor it with someone who knows your schedule, and to read about relative energy deficiency in sport. If you want to lose fat without losing muscle, we cover it in the guide to losing body fat without losing muscle.

Who should not do intermittent fasting?

Intermittent fasting is not suitable in these situations:

• Pregnancy and breastfeeding, when needs increase and it is best not to go many hours without eating.
• Minors, who are still growing and need to eat regularly.
• A history of or risk of an eating disorder, such as anorexia, bulimia or binge eating.
• Diabetes treated with insulin or medicines that can lower blood sugar too much, due to the risk of hypoglycaemia.
• Frail older people or those with a poor appetite, who are at risk of losing muscle or eating too little.
• Low body weight, or having lost weight without trying.

There is evidence concerning diabetes. In people with type 2 diabetes taking glucose-lowering medicines, fasting days doubled episodes of hypoglycaemia, or low blood sugar, even though medication was reduced [9]. If you have diabetes, discuss any change in meal timing with your doctor first, as they are the one who adjusts your medication. For the rest of the approach, see the guide to eating with type 2 diabetes.

With eating disorders, going hours without eating and compensating afterwards can trigger or worsen the disorder. If this sounds familiar, the first step is to talk to a psychology professional. You can start with the guide to eating disorders. In Barcelona, the ACAB association offers a support service.

In pregnancy and breastfeeding, this approach is not suggested: energy and nutrient needs increase and it is best not to cut back eating hours. If you are at that stage, see how to look after your eating during pregnancy.

In people over 60, the data are scarce and combine studies of uneven quality. One review found that very short windows, of ten hours or less, were associated with worse outcomes, especially cognitive ones [10]. From age 60, talk to your doctor before starting. With a poor appetite or a risk of losing muscle, eating often and with protein adds more than shortening hours.

If you are unsure whether any of these situations applies to you, ask for an assessment with a dietitian-nutritionist or talk to your doctor before starting.

What mistakes do we see in consultations?

• Compensating during the eating window. You arrive very hungry, eat larger amounts, and the balance evens out.
• Skipping protein. If you eat very little protein, you may lose muscle mass.
• Starting straight away with 16/8. Going from six small meals to an eight-hour window overnight sends hunger soaring. That is why we start with twelve hours.
• Eating dinner late and heavily. A window that ends at ten at night with a large meal can worsen sleep. There is more in eating and sleep.
• Using it to make up for overeating. “I had a big dinner yesterday, so I’m not eating until the afternoon.” That punishment logic is the one worth rethinking.
• Treating it like a test. If you do not follow the schedule one day, that is fine; the next day you go back to your usual schedule.

The people who come to our consultations often tell us that fasting worked for them for a few months and then they stopped. That is not a failure. The approach that works is the one you can maintain, with or without an eating window.

And if someone promises you quick results just by changing your meal schedule, review how to recognise fad diets.

What else do people ask about 16/8 fasting?

How many hours do you need to fast? As many as you can sustain. 16/8 is a reference point, not a goal, and a ten- or twelve-hour window is a good place to start.

Is a morning or an afternoon window better? We have no comparisons that allow us to say one time slot beats another. Choose the one that fits your sleep and your social meals.

Do you lose muscle with 16/8? It can happen if you eat little protein and do not do strength training. With protein at every meal and strength training two days a week, the risk goes down. In people who were already doing strength training, fat-free mass did not change with eight- to ten-hour windows [8].

Is it safe in the long term? Studies mostly last between six and twelve months, so we do not know what happens over more years [4]. If you keep it up, check in every few months on how you feel.

Does coffee with milk break the fast? Yes, it provides energy and counts as food. If black coffee is hard for you, it is better to move the window than to sneak in the milk.