Does fasting work?

Does fasting work or is it just another health fad? We asked a nutritionist whether fasting is safe and whether it really works

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It’s certainly popular, but does fasting work? From vegan and paleo to keto and pegan, diet trends are never in short supply and, with the herald of the 5:2 diet in 2012, fasting has become a hot topic in the world of health and wellbeing. The deliberate, periodic restriction of food, drink or both, fasting is usually done for the purpose of weight loss or improving one's health. 

One thing that science has shown us is that when it comes to optimal health and wellbeing, a balanced approach that incorporates both exercise and diet is favorable compared to extreme measures of either. Dietary changes coupled with the best exercise machines to lose weight and the best exercises for weight loss will set you well on your way to a healthy, balanced lifestyle.

However, if you’re on the fence about fasting, or are wondering how exactly does fasting work, our experts share their opposing opinions on this hot topic.

Does fasting work? The case for

When new diets first gain popularity, there is often insufficient evidence to determine if they are effective. Fortunately, as the public’s fascination with fasting piqued, so did the scientific interest. Over the years, research has looked into the various fasting methods, in particular intermittent fasting. This fasting method involves cycling between periods of non-eating and eating and can vary from 16-hour fasting to complete avoidance of food for one day each week, usually as a means to lose weight.

Preliminary studies show fasting could be a promising way to help people with weight loss and health, and nutritionist and author Rob Hobson agrees that there are many positives from fasting.

‘Although the research is lacking a little in terms of the health benefits of fasting, many people have found it useful as a way of dieting that doesn’t involve rules or foods that you have to omit,’ Hobson says. ‘The research we do have highlights benefits that occur in the body when it’s deprived of food. These include low levels of blood insulin (which promotes fat burning), increase in human growth hormone (which boosts fat burning and muscle gain) and the promotion of cellular repair.’

A 2021 review study led by researchers from the University of Illinois Chicago, analysed over 25 intermittent fasting research studies and found it to be a safe diet therapy, capable of improving several markers of metabolic health and producing clinically significant weight loss. Other health benefits, such as improved appetite regulation and positive changes in the gut microbiome, were also demonstrated.

There’s also good news when it comes to diabetes. ‘Some of the strongest research for fasting is around the potential to reduce insulin resistance and lower the risk of type 2 diabetes,’ says Hobson. ‘Other potential benefits include reducing oxidative stress and inflammation in the body (both of which increase ageing), lowering heart disease risk factors and protecting brain health.’

One study in the journal Nature Reviews Neuroscience suggests that intermittent fasting could keep your brain sharp, with researchers concluding that lifestyles that incorporate metabolic challenge, such as intermittent fasting, are likely to improve brain health. Another study, involving 29 healthy fasted individuals also found that intermittent fasting increased levels of the feel good and motivating hormones serotonin and dopamine.

But whilst fasting for health and weight loss does appear to show some potential, Hobson adds a word of warning: ‘It is important to still eat nourishing calories – eating anything you want on non-fasting days won’t get you anywhere if you are doing it for weight loss.’ 


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Does fasting work? The case against…

Whilst current scientific studies on fasting look encouraging, we’re still really just starting to uncover the answers to the question of does fasting work to support overall health and weight management.

Despite the fact that people have practiced fasting for centuries, usually for religious or ritualistic reasons, there are still a limited number of human research studies, which makes application of data difficult. Plus, research published in the Canadian Medical Association Journal suggests that whilst many diet and exercise trends have legitimate scientific claims, there can be a tendency for facts to get distorted when these trends achieve mainstream acceptance. According to the research, benefits can become blown out of proportion and risks downplayed as science takes a back seat to marketing.

Nutritionist Alix Woods warns against intermittent fasting, suggesting that it has the potential to interfere with our bodily functions. ‘I don’t believe in fasting,’ says Woods. ‘Without food and/or water, the body lacks all the vital micronutrients (vitamins and minerals), antioxidants and hydration for cell survival and energy.’

Alix also raises concern about potential negative side effects of fasting. ‘Symptoms of fatigue, dizziness and dehydration can occur when you fast and, for some, it may exacerbate feelings of stress and disrupt sleep,’ she says. ‘Fasting may also lead to inflammation, causing flare-ups like gout and gallstones. It is also best avoided by hypoglycaemic (low blood sugar) sufferers as the dips in blood sugar could be severe and lead to loss of consciousness.’

According to research from the journal Nutrients, depending on the length of the fasting period, people may initially experience side effects such as headaches, crankiness or constipation as their body adjusts to the altered eating pattern. Another study also found that a consistently low-calorie intake can have a negative effect on women’s menstrual cycle.

Yet many studies, including a systematic review of 40 studies published in Molecular and Cellular Endocrinology, have found intermittent fasting to be effective for weight loss. However, Alix is skeptical about this idea. ‘Yes, there are studies to say that it’s an effective way to lose weight,’ she says. ‘However, there are studies that reveal fasting results in quick fluid loss, and once you start eating normally again the weight returns rapidly.’

Alix also draws attention to the potential effects on metabolism. ‘Fasting for quick weight loss isn’t an effective or healthy way to lose weight as metabolism slows down and essential water and nutrients are lost,’ she says. ‘A slow metabolic rate hampers healthy weight loss in the future.’  

Food in bowl

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Whilst some studies do suggest that long periods of fasting can cause a drop in metabolism, research from the Journal of the Academy of Nutrition and Dietetics indicates that shorter periods of fasting may have beneficial effects on metabolism, making it a healthier way to lose weight than standard calorie restriction. One study from the American Journal of Clinical Nutrition, which looked at 11 healthy men found that a three day fast boosted metabolism by a sizable 14%.

However, Alix explains that fasting can also have implications on mental wellbeing. ‘Fasting has been found to worsen emotional and psychological issues for people with eating disorders and it may even be a trigger for binge eating and/or anorexia,’ she says.

Whilst more moderate proponents of fasting advise eating sensibly during your non-fasting days, the Canadian Medical Association Journal study raises the concern that some advocates of intermittent fasting may unintentionally encourage excessive behaviors like binge eating, particularly with fasting methods like the 5:2 diet. The implication being that if you fast two days a week, you can eat as much junk as your stomach can handle for the other five days.

This notion of intermittent fasting affecting mental wellbeing is also supported by research from the Department of Psychology at Beihang University, which concluded that fasting could lead to negative emotional states such as irritability. However, the study does note that positive psychological experiences such as a sense of reward, accomplishment, pride, and control could also be encountered.

How long should you fast for?

When it comes to fasting, there are a variety of methods to choose from. Some involve eating and fasting periods that span the course of a day, whilst others split the fasted and fed state over a week or longer.

We are all unique beings whose bodies respond differently to various external or internal changes – fasting included. Overall, you should consider why you want to practice fasting and what you hope to gain.

There is no single plan that will work for everyone and if you choose to adopt fasting, it is best to ease into it, trying out the different methods to see which one suits your lifestyle and preferences. You may also wish to consult a nutritionist or health professional, particularly if you have any medical conditions that could affect a fast.

Our guide on how to lose weight by fasting explores some of the most popular fasting methods with top tips for success.

Angela Yussuff

Angela has been a health writer for over 10 years, contributing to a range of online and print publications including Women's Health, Women's Fitness, Your Fitness, Top Santé, Healthy, and Good Housekeeping. She writes about all aspects of health and wellbeing, with a special interest in nutrition and the therapeutic application of food. A qualified nutritionist and a recipe developer, she is the founder of Dara Dara Nutrition and has developed recipes for titles including GoodtoKnow. In her spare-time she likes to throw netballs, hula hoops and yoga poses (or what poses as yoga!) and has recently taken to the bass guitar. Fortunately for her neighbors, she’s invested in some headphones.