Second Nature logo



Is intermittent fasting good for you?

Robbie Puddick
Written by

Robbie Puddick

Medically reviewed by

Fiona Moncrieff

9 min read
Last updated June 2024

Jump to: What is fasting? | Is intermittent fasting good for you? | Fasting for weight loss | Fasting may decrease lean mass | Fasting may not be sustainable for many people | Take home message

Whether fasting is good for you will depend on what type of fasting you do, how you do it, and what your baseline health is at the time.

Still, there’s good evidence that fasting can benefit weight loss, improve markers of heart health, reduce the risk of chronic diseases such as type-2 diabetes, and improve gut health.

There also seem to be benefits in the more subtle forms of fasting, such as time-restricted eating (TRE), where people consume all of their meals within 11 hours.

This sounds more realistic for most people and seems to align with our body’s circadian rhythm (our internal clock, which is regulated by our exposure to night and day).

This isn’t to say it doesn’t come without its downsides. There are concerns about the long-term retention of lean mass such as muscle and bone and whether it can trigger or exacerbate eating disorders.

Considering eating disorders are often made worse, or even caused, by strict dieting and rules around food, it’s unlikely having restrictions on when you can eat would be beneficial in this context.

The practice of intermittent fasting has been around for thousands of years. It’s famously practised in religions such as Islam during Ramadan, but health has always been a motivator.

Hippocrates, the ancient Greek practitioner, reportedly recommended it as a treatment for certain medical conditions.

In the past decade, fasting has made its way back into Western cultures as a tool for weight loss and health, particularly for those living with chronic health conditions.

It’s particularly popular in the online keto and low-carb communities, where people often pair these diets with a fasting approach.

Scientific research is also starting to show the positive health benefits of fasting. It’s been shown to be an effective tool for weight management, improving insulin resistance, and gut health. It may not be just another fad diet.

Check my eligibility

What is fasting?

Simply put, fasting is going for extended periods without eating any food (calories) and only consuming water (sodium and potassium supplements are often recommended in extended fasts).

Fasting essentially moves your body from a fed state of growth and energy storage to a state of burning through stored energy and cell recycling.

There are a few different types of fasting:

  • Time-restricted feeding or eating (TRF or TRE): Eating all your calories within a set window, for example, between 10:00 and 18:00.
  • Alternate day fasting (ADF): As the name suggests, you alternate the days of the week you eat. For example, eating on Monday, fasting on Tuesday, and eating on Wednesday.
  • Extended fasting: Fasting between one to seven days or longer. These are typically done on a rolling schedule, so 36 hours once a month or five days every three months.

If you’d like to join 150,000 people who’ve used Second Nature to make losing weight feel easier – click here to take our health quiz.

Otherwise, keep reading as we unpack three lines of evidence analysing the pros and cons of intermittent fasting.

1) Fasting for weight loss

One of the first documented cases of extended fasts was in 1973, when a man called Angus Barbieri was admitted into the University Department of Medicine in Dundee, Scotland. He weighed 207kg (456lbs / 32.6st) and was about to complete a 382-day fast.

From a weight loss perspective, the experiment was a success. Angus’s weight dropped by a total of 125kgs, from 207kg to 82kg (180lbs / 12.8st), he suffered no ill effects. After five years of follow-up, his weight had increased slightly to 88.9kg (196lbs / 14 stone).

To quote the case report:

“A 27-year-old male patient fasted under supervision for 382 days and has subsequently maintained his normal weight. Prolonged fasting in this patient had no ill-effects.”

The human body is highly resilient, and Angus’s experience of fasting without adverse side effects paved the way for new research on fasting in an era where obesity and chronic disease rates continue to rise.

Randomised controlled trials in larger populations have shown promising results on the potential benefits of intermittent fasting, particularly in the short term.

A study with 101 participants investigated the impact of two fasting periods on weight loss compared to a control group who went through usual care.

The first group was to complete time-restricted eating (TRE) regimen and consume all their calories within eight hours (known as 16/8), while the second group completed alternate daily fasting (ADF), where they fasted every other day.

The intervention lasted for three weeks, and after three months of follow-up, the TRE group lost an average of 4.44kg, while the ADF group averaged a weight loss of 14.8kg. Both groups also showed improved blood glucose, triglycerides, and waist circumference.

From a physiological perspective, weight loss is a simple equation of the number of calories you eat compared to the amount you burn.

The ability of fasting to induce weight loss seems to come from the fact that people significantly reduce their calorie intake when they can only consume meals within a specific eating window.

Some studies have also suggested that fasting may help prevent overeating by reducing hunger which might be associated with lower levels of the hunger hormone ghrelin and insulin levels.

This was illustrated by another randomised controlled trial that compared the effect of TRE and calorie restriction on weight loss after 12 months in 118 participants.

Both groups experienced weight loss (8kg and 6.3kg), but there was no significant difference between them.

Both groups also improved markers of heart disease, such as blood pressure, blood glucose (blood sugar levels), and waist circumference. But again, no significant differences between the groups were observed.

So while fasting diets can support weight loss, there isn’t evidence to suggest it’s more effective than eating patterns that help reduce calorie intake.

There would also be concerns about the sustainability of approaches like early TRE. How many people would want to stop eating at 4 pm and miss having dinner with their loved ones in the evening?

Key points:

  • Angus Barbieri was one of the first documented extended fasts, and its success paved the way for future research in the area.
  • Large randomised controlled trials have shown that fasting is effective for weight loss.
  • Fasting also improves markers of chronic diseases such as blood sugar control and triglycerides.
  • Research has also shown that fasting helps people reduce their calorie intake and isn’t more effective than diets that help people maintain a calorie deficit to lose weight.
  • Questions remain over how sustainable an approach fasting is, particularly when less restrictive approaches are just as effective.

2) Fasting may decrease lean mass

The effects of intermittent fasting may not always be positive. Our ability to maintain lean mass is one of the areas where the context of the individual’s situation needs to be considered.

Our lean mass, which comprises our organs, muscles, bones, and skin – is essential for our physical health, and typically reductions in lean mass are not a good thing.

A concern about fasting is whether the body ‘burns’ through any lean mass to maintain an adequate energy supply to crucial organs such as the brain, liver, and heart over a longer period of time.

A study investigated how much the different macronutrients (carbohydrates, fats, and proteins) contribute to our resting metabolism during 16 days of fasting. If more protein contributes, it’s a sign the body is breaking down lean tissue to provide this.

It showed that in lean individuals with a BMI of 20, up to 30% of the energy required for use at rest can come from protein. In contrast, only 10% of protein contributes to energy requirements in people with a BMI of 35.

It’s possible that obese individuals who have plenty of fat stores to provide energy to their vital organs during a fast don’t need to break down essential protein from their tissues.

In lean participants, this isn’t the case; the body seems to go into a type of ‘survival mode’, prioritising vital organs over bones and muscle.

Recent trials investigating the effects of alternate daily fasting (ADF) on obese and lean subjects support this.

One study investigated the impact of ADF in obese individuals. It showed that after 70 days of ADF, the participants lost 5.6kg without experiencing any loss of lean mass.

While another study on lean participants undergoing ADF for three weeks saw reductions in lean mass, particularly in bone mineral density; this reduction in lean mass was also observed in lean individuals who didn’t lose weight.

This suggests that whether or not fasting would be beneficial for you can depend on your goals:

  • If you’re overweight and obese and weight loss is your goal: fasting won’t negatively impact your lean mass.
  • If you’re lean and looking to improve your health: extended periods of fasting beyond 24 hours would likely lead to a decrease in lean mass.

Key points:

  • Research has shown that how your body responds to fasting depends on your current level of body fat.
  • Overweight individuals will maintain lean mass as their bodies have adequate fat stores to provide energy, while lean individuals need to burn more protein, and lean mass will decline.
  • Whether fasting would be beneficial depends on your current weight status and goals.

3) Fasting may not be sustainable for many people

As clinical trials have shown, fasting is an effective method to enable a calorie deficit and achieve weight loss in a research setting where people are extrinsically (externally) motivated to stick to the rules of the study.

But a recent study of free-living individuals who had a choice of which diet to follow suggests it may not be sustainable for many people in the long run.

The study recruited over 250 overweight participants, and they offered the choice of three diets to aid their weight loss:

  1. Intermittent fasting
  2. Mediterranean diet
  3. Paleo diet

After 12 months, 54% of individuals reported they were still following their fasting regime, despite being the most popular choice, with 57% of the 220 participants choosing fasting as their diet.

The fasting group achieved modest weight loss, averaging 2.8kg after 12 months. Over half as effective as other fasting trials where the individuals were in a more controlled setting where the extrinsic motivation to stick to the diet may be higher.

The psychology of weight loss and weight maintenance is often overlooked. The dogma of ‘eat less, move more’ often reigns and people are told to have more willpower and stick to their calorie deficits, and they’ll lose weight.

Research has shown that restrictive dieting practices are associated with weight gain, not weight loss.

Further research on individuals who’ve achieved weight loss maintenance suggests that they can achieve their long-term weight loss goals only after shifting to a different mindset and moving away from strict rules and restrictions.

To quote from a recent paper [bold ours]:

“In contrast to the narrative of being trapped or helpless reported by some regainers, maintainers often talked about a growing sense of liberation and personal control, along with growing confidence and self-esteem and the emergence of a ‘new me’.

Epiphaniou and Ogden (2010b) reported that ‘Following weight loss maintenance, participants described a shift in identity from that of a previously restricted individual who was restrained across a number of different domains, towards one who was liberated.’”

While fasting can support weight loss from a physiological perspective by inducing a calorie deficit, it’s unlikely to be sustainable for many people in the long run. Particularly people with a history of yo-yo dieting simply looking for the next ‘quick fix’.

Key points:

  • Fasting effectively induces a calorie deficit and supports weight loss in research settings.
  • However, research has shown that people may struggle to maintain the regime in the long run.
  • Research suggests that people who regularly attempt restrained dieting practices are more likely to be overweight and obese.
  • People who shift their mindset away from restrictive practices and are more open about their food choices tend to maintain weight loss in the long run.

Take home message

There’s been a lot of hype around fasting online, in magazines, and even in the world of research.

The evidence behind the approach to support weight loss, maintain a healthy weight, improve risk factors associated with chronic disease, and improve gut health is accumulating, and it’s very promising. For the right person, it can be health-promoting.

However, this doesn’t mean that fasting is appropriate for everyone or that it can replace a healthy diet.

If you’re going to experiment with fasting, you should combine it with healthy foods such as whole grains, lean protein, healthy fats, and plenty of fruit and vegetables.

Additionally, it’s likely that for specific individuals, such as those who are lean and/or have a history of yo-yo dieting, fasting is just another set of rules and restrictions they must follow.

They then enter a cycle of guilt and shame over their decisions when they cannot stick to them.

An approach that promotes liberation from food rules and restrictions may be a better alternative in the long run.

If you’re looking for a healthy eating plan to complement your intermittent fasting plan, click here for an NHS-trusted meal plan.

Meal Plan

Download our free, indulgent 7-day meal plan

It includes expert advice from our team of registered dietitians to make losing weight feel easier. Subscribe to our newsletter to get access today.

I've read and agreed to the Terms of Service & Privacy Policy.

You might also like

Make losing weight feel Second Nature

The first step on your Second Nature journey is to take our health quiz.

Hand holding phone

Write a response

As seen on

The GuardianThe TimesChannel 4The Sunday Telegraph
Evening Standard