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Does Intermittent Fasting Work for Weight Loss? What the Trials Show

Trials show intermittent fasting works about as well as standard calorie cutting, not better. Dr. Sayantan Banerjee explains 16:8, 5:2, who it suits and how to do it well.

OBESITY REMISSION

Dr. Sayantan Banerjee

10/31/20267 min read

Intermittent fasting is one of the most popular ways to try to lose weight in the US and the UK. It has been promoted by celebrities, doctors, apps and best-selling books, often with claims that it burns fat faster, resets your metabolism or works better than any other diet.

So does it work? The short answer is yes, for many people. But the trials suggest it works mainly because it helps people eat less, not because fasting has special fat-burning powers. In most head-to-head trials, it produces similar weight loss to standard calorie restriction. Its real advantage is that some people find it much easier to stick to.

This article walks through the main types of fasting, what the best trials found, who it suits, who should avoid it, and how to make it work if you choose it.

The main types of intermittent fasting

Time-restricted eating (TRE). You eat all your meals within a set window each day and fast for the rest. Common versions are 16:8 (16 hours fasting, 8 hours eating), 14:10, and 12:12. Early time-restricted eating means the window is earlier in the day, for example 7 am to 3 pm.

The 5:2 diet. You eat normally on five days a week and have about 500 to 800 calories on two non-consecutive days. It was popularised in the UK in the early 2010s and studied by researchers in Manchester, among others.

Alternate-day fasting. You alternate normal eating days with fasting or very low calorie days, typically around 500 calories.

Periodic fasting or fasting-mimicking diets. Several consecutive days of very low calorie intake, repeated every few weeks or months.

Why fasting can lead to weight loss

Weight loss happens when you take in less energy than you use over time. Intermittent fasting helps many people do that without counting every calorie. Shorter eating windows mean fewer snacks, especially in the evening. Low-calorie days lower the weekly total.

There are also possible metabolic effects. During longer fasts, insulin levels stay lower for longer, and the body shifts towards burning fat and producing ketones. Some research suggests that eating earlier in the day fits better with the body clock, when insulin sensitivity tends to be higher.

The key question is whether these extra effects make a real difference to weight when total calories are the same. That is what the best trials tried to answer.

What the trials found

Alternate-day fasting versus calorie restriction. A one-year trial published in JAMA Internal Medicine in 2017 randomly assigned 100 adults with obesity to alternate-day fasting, daily calorie restriction, or no change. Both diet groups lost a similar amount of weight, around 6% of body weight at six months, with similar results at a year. More people dropped out of the alternate-day fasting group, suggesting many found it harder to stick to.

The 16:8 pattern on its own. In the TREAT trial, published in JAMA Internal Medicine in 2020, 116 adults with overweight or obesity were assigned either to 16:8 eating, with no other instructions, or to three structured meals a day. After 12 weeks, the 16:8 group lost only slightly more weight, and the difference wasn't statistically significant. The researchers also raised a concern that a larger share of the weight lost in the fasting group may have been lean mass.

Time-restricted eating with calorie restriction. A large trial from China, published in the New England Journal of Medicine in 2022, assigned 139 adults with obesity to calorie restriction with an 8-hour eating window, or calorie restriction alone, for 12 months. Both groups lost a similar amount of weight, about 6 to 8 kg. Adding the eating window didn't produce significant extra loss.

Early time-restricted eating. Some trials of an early eating window, such as 7 am to 3 pm, have reported slightly greater weight loss or better blood pressure and glucose than a longer window, when combined with calorie reduction. These trials are small and short, and an early window is hard for many working people and families to keep.

The 5:2 diet. Several UK and Australian trials found that 5:2 led to similar weight loss to continuous calorie restriction, with similar improvements in insulin sensitivity. In a 12-month Australian trial in people with type 2 diabetes, both approaches lowered HbA1c and weight to a similar degree.

Overall, systematic reviews come to the same conclusion. Intermittent fasting is an effective way to lose weight, roughly as effective as other calorie-reduction methods, but not clearly better.

Why that doesn't make it useless

It is easy to read those results and think fasting doesn't work. That isn't the right conclusion. It works about as well as other good approaches. The best diet is the one you can keep up, and for some people, fasting is easier.

People who like fasting often say it helps because:

there are fewer food decisions to make each day
they aren't very hungry in the morning anyway
a clear cut-off time stops evening snacking
low-calorie days feel manageable when normal eating is only a day away
it fits with religious or cultural fasting they already know

People who dislike fasting often find:

they get very hungry and irritable
they overeat in the eating window
it clashes with family meals or work
it triggers binge eating or an unhealthy focus on food

Both experiences are real. The trials suggest you should choose based on what suits you.

The muscle question

Some trials have raised concern that fasting may cause more loss of lean mass than other diets, possibly because people eat less protein within a short window. This isn't consistent across studies, but it is worth taking seriously.

To protect muscle while fasting, include a good portion of protein in every meal, aim to spread it across at least two meals, and do resistance exercise two or three times a week.

Who should be careful or avoid fasting

Talk to a doctor before trying intermittent fasting, or avoid it, if you:

have diabetes and take insulin or a sulfonylurea, because of the risk of low blood sugar
take an SGLT2 inhibitor, because of a rare risk of ketoacidosis with very low calorie days
are pregnant, breastfeeding or trying to conceive
have a current or past eating disorder
are underweight, frail or elderly
have a medical condition that needs regular meals or medicines with food
are under 18

How to do intermittent fasting well

Start gently. Begin with a 12-hour overnight fast, for example 7 pm to 7 am. After a week or two, move to 14:10. Only move to 16:8 if you feel well.

Choose a window that fits your life. If family dinner is at 7 pm, a 10 am to 7 pm window may suit you better than finishing at 3 pm.

Don't treat the window as a free-for-all. Weight loss depends on what and how much you eat in the window. Build meals around vegetables, protein, whole grains and healthy fats.

Break the fast with protein. Starting your first meal with protein and vegetables helps control hunger and glucose for the rest of the day.

Stay hydrated. Water, sparkling water, black coffee and plain tea are fine during fasting hours for most people.

Keep active. Walking and strength training support fat loss and help preserve muscle.

Track progress. Weigh yourself weekly and measure your waist monthly. If nothing changes after four to six weeks, adjust what you eat in your window or try a different approach.

Listen to your body. Persistent dizziness, headaches, poor sleep or binge eating are signs the pattern isn't right for you.

Example: a 14:10 day

8 am to 9 am: water and black coffee or tea.
9 am: scrambled eggs with tomatoes and spinach, a slice of wholegrain toast.
1 pm: a large salad with chickpeas or chicken, olive oil dressing, a piece of fruit.
4 pm: Greek yogurt or a few nuts, if hungry.
6.30 pm: salmon, tofu or lean meat with roasted vegetables and a small portion of brown rice or potatoes.
7 pm onwards: water or herbal tea only.

Fasting and GLP-1 medicines

Many people on GLP-1 medicines find their appetite is so low that they naturally eat within a short window. That is fine, but make sure you still get enough protein and nutrients. If you are coming off a GLP-1 medicine, a fasting pattern may help some people manage the return of appetite, combined with high-protein, high-fibre meals.

Fasting for women and older adults

You may have read that intermittent fasting is harmful for women's hormones. Most of the human trials described above included many women, and they didn't report serious hormonal harm over the periods studied. Some animal studies suggested effects on reproductive hormones, but these don't translate directly to people. That said, women who are trying to conceive, pregnant or breastfeeding should avoid fasting for weight loss, and any change in periods should be discussed with a doctor.

Around the menopause, some women find fasting helps with evening snacking and weight. Others find it worsens sleep or mood, or makes them very hungry. Strength training and enough protein matter especially at this stage, because muscle loss speeds up with age and falling oestrogen. If you fast, make sure your eating window includes at least two meals with a good portion of protein.

For older adults, the main concern is muscle and bone. Weight loss in later life can increase frailty if it comes with loss of muscle. Gentle approaches, such as a 12- or 14-hour overnight fast with three protein-rich meals in the day, are usually safer than long fasts or very low calorie days. Older adults taking several medicines should always check with their doctor or pharmacist before fasting, since some medicines need to be taken with food and some raise the risk of low blood sugar or dehydration.

For anyone with a history of disordered eating, fasting rules can reinforce unhelpful patterns. A structured eating plan with regular meals, developed with a dietitian, is a better choice.

How my book can help

Intermittent fasting is one of the strategies covered in The Obesity Exit Strategy, alongside food structure, exercise, sleep and stress. The book explains how fasting interacts with insulin and appetite signals, and how to use it as one tool within a wider plan, rather than as a quick fix. See the book page.

Frequently asked questions

Is intermittent fasting better than calorie counting? In most trials, weight loss is similar. Choose the one you can stick to.

How much weight can you lose with 16:8? It varies widely. In trials combining an eating window with calorie reduction, people lost around 6 to 8 kg over a year.

Does intermittent fasting burn belly fat? It can reduce belly fat as part of overall weight loss, but it doesn't target the abdomen specifically.

Can I drink coffee while fasting? Black coffee and plain tea are usually fine. Milk and sugar add calories.

Is 5:2 or 16:8 better? Both have trial evidence. Choose the pattern that fits your routine.

Does fasting slow your metabolism? Any weight loss causes some slowing. There is no strong evidence that fasting causes more slowing than other diets.

What breaks a fast? Strictly, anything with calories. Black coffee, plain tea and water are generally fine. A splash of milk is a small amount, and many people find it doesn't affect their results.

Can I exercise while fasting? Most people can do light or moderate exercise in a fasted state. If you take glucose-lowering medicines, or feel dizzy, eat first or exercise after your first meal.

How long before I see results? Many people notice changes in weight within two to four weeks. If nothing changes after six weeks, review what you are eating within your window.

Can I do intermittent fasting long term? Many people do. Choose a pattern that you can sustain comfortably, and review it if your health or circumstances change.