How does fasting affect the body, and what are the different types of fasting? Nutritionist Kerry Torrens explains the benefits and drawbacks of this dietary practice.
With studies suggesting a break from food may boost health, promote longevity and keep those extra pounds at bay, nutritionist Kerry Torrens takes a look at the history of fasting, the science behind some of the claims and how fasting might work with our modern lifestyles.
The origins of fasting
Despite being popularised by today’s diet world, the practice of fasting actually dates back centuries and is thought to be one of the oldest therapies in medicine. In the 5th century BC, Hippocrates, the father of Western medicine, recommended abstaining from food to aid the healing process. In Ayurvedic medicine, fasting once a week is thought to promote digestive clearance.
Fasting plays a central role in cultural and religious practices, with all major religions utilising a fast in one form or another. Christian Lent and Muslim Ramadan are two well-known examples. Whether it involves the abstinence of food and drink or a lighter, lower calorie form of eating, many argue that going without food for periods of time is something we have evolved to do.
The health benefits of fasting
With numerous health claims linked to fasting, its draw is understandable. However, it’s worth noting that for non-weight related benefits, the evidence is mixed (but promising). This is because some areas of research have only be explored with animals, so it’s unclear how the benefits translate for humans.
That said, by lightening your normal eating pattern, you give the body time to focus on other important functions, including disease prevention. Similarly, fasting appears to improve the body’s ability to manage inflammation and, as such, may help chronic conditions such as heart disease, multiple sclerosis and rheumatoid arthritis.
Several studies have also supported the use of fasting as a means of improving blood sugar control and reducing the risk of diabetes – although gender may play a part here and more studies are needed.
Improving immunity through fasting may help the body’s control of cancer. One study demonstrated that a nightly ‘fast’ of more than 13 hours could be an effective means of reducing the risk of recurrence in those diagnosed with early-stage breast cancer. This should always be undertaken with your doctor’s supervision.
Studies in animals suggest fasting may protect against, or improve outcomes in, Parkinson’s and Alzheimer’s, as well as improve brain function by supporting memory and brain processing. Similarly, animal studies suggest fasting may protect brain health and increase the generation of nerve cells.
Many dieters turn to fasting as a manageable approach to weight loss. Studies show that controlling the times we eat, or undertaking short-term fasts, can aid weight reduction, fat loss and improve blood lipids. That’s not all – other studies have shown fasting to increase the ability to switch metabolism to fat burning, preserve muscle mass and improve body composition in overweight people.
Of particular note, fasting may promote levels of the human growth hormone that plays an important role in growth and repair, metabolism, weight loss, muscle strength and exercise performance. Furthermore, fasting, and in particular adopting a diet low in protein, has, at least in animal models, been associated with extended life expectancy.
Types of fasting
A fast can take many forms, although typically you can expect the practice to be performed over a 12-72 hour period. One popular form, known as intermittent fasting, involves cycling between periods of fast and periods of eating. Ranging from a few hours to a few days, this may involve alternate day fasting, when you eat a low-calorie diet (say 500 calories in a day) and then eat normally the next, whole day fasting, where food is restricted for a 24 hour period, or time-restricted feeding, where you choose a set number of hours each day within which to eat. Opting for an ‘eating window’ is thought to be a gentler way to introduce fasting – this may be as simple as delaying breakfast.
Popularised versions of fasts include the 5:2 diet, which involves eating no more than 500 calories in a day, twice a week. This is a simple, flexible plan, although for some it may be too restrictive and hard to stick to. Since its original initiation, author Dr Michael Mosley has updated the plan with the Fast 800, which advocates 800 calories for two days and a lower carb diet on the remaining five.
Another scientifically backed plan is the Longevity diet formulated by Dr Valter Longo. The plan involves five days of fasting two to three times a year. On these days the diet comprises of 400 calories of vegetables and another 400 calories of nuts, seeds and their oils.
Linked to an array of health benefits and taking many forms, fasting is a dietary practice which appears to fit most lifestyles. However, if you decide to adopt a fast, make sure you stay well-hydrated and when you eat, the food is nutrient-dense and well-balanced. If fasting for extended periods, minimise physical activity and ensure adequate rest. It’s also wise to avoid fasting during stressful periods – it requires resolve and willpower.
Fasting is not for everyone. It’s advisable to speak to your GP or healthcare professional before starting a new dietary regime, especially if you’re under 18 years old, elderly, have a pre-existing medical condition (including diabetes and high blood pressure) or are on medication. Fasting isn’t recommended for people who are underweight, have an eating disorder or are pregnant or breast-feeding.
Please note: if you're considering any form of diet, please consult your GP first to ensure you can do so without risk to your health.
This article was published on 10th June 2020 by Kerry Torrens.
Nutritionist (MBANT) Kerry Torrens is a contributing author to a number of nutritional and cookery publications including BBC Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).
All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.
Have you tried an intermittent fasting diet, or do you have any further questions about them? We'd like to hear from you in the comments below...