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by Sue Quinn

Whether you’re trying to lengthen your night-time break from eating by having an earlier dinner or later breakfast, or going a step further and following an intermittent fasting plan, some scientists believe there are benefits beyond weight-loss to giving your digestive system a break. They argue that for many people it can improve metabolic and overall health.

What is ‘fasting’ and how does it work?

A good rule of thumb for those wanting to fast to improve their metabolic and overall health is to leave at least 12 hours between meals – ideally more – on a regular basis, according to Dr Adam Collins, Principal Teaching Fellow in Nutrition at the University of Surrey.

Time-restricted eating is often used for weight-loss, and some people increase the daily fasting period to 14 hours or 16 hours. There are other forms of ‘intermittent fasting’, including the 5:2 approach, which involves very restricted eating on two days of the week, with normal eating on the other five. The 4:3, or ‘every other day’, approach involves similar restrictions on alternate days. “Whichever one you choose, make sure you’re doing it consistently”, Dr Collins advises. (Intermittent fasting should not be followed by anyone who is underweight.)

Do we all experience the same benefits?

No regime will be right for everyone all the time, says Collins. “So much depends on what you eat, and to some extent when you eat it”, he adds. “It also depends on whether you’re going to follow eating with activity or inactivity, and whether you’ve accumulated lots of calories throughout the day.” Every person’s body reacts differently to eating and fasting.

Another contributing factor is your – natural cycles of sleep and activity. Many digestive, metabolic and cellular processes follow a daily rhythm based on cycles of light and dark, just like your sleep patterns. Nutrition scientists are now investigating whether eating out of kilter with our circadian rhythm – having a large meal at night when your body is signalling for you to sleep – is not optimal for .

What are the benefits of not being full all the time?

Regular breaks between eating give our bodies time to carry out valuable ‘housekeeping’, according to Dr Collins. After a meal, you absorb the glucose from carbohydrates in food for energy, and either use it immediately or store it for later. In a ‘fasted’ state, which typically starts 10–12 hours after your last meal, the body is depleted of this form of glucose. Then the liver begins to break down stored fat into fatty acids called to use as fuel. This process is known as and is a reason why fasting can lead to weight loss.

But emerging science suggests fasting activates certain chemicals and processes that confer health benefits beyond . It’s also thought to encourage the growth of beneficial gut bacteria that could be good for us in many different ways.

The problem is, most of us are too full for too much of the time to experience these benefits, says Collins. Eating three meals a day plus snacks, as many of us do, means we’re in a “constantly fed state”. As a result, our bodies continuously process new intakes of food instead of dealing with fat reserves.

“All the things we associate with poor metabolic health and disease risk – cardiovascular disease, insulin resistance, diabetes and low energy – are essentially a manifestation of fuel mismanagement”, he says. “Eating in a grazing pattern means you’re not able to store, liberate and utilise fuel in the right way.”

Fasting is a highly complex area, where research is ongoing and robust clinical trials are lacking, according to Collins. Most intermittent fasting research has been conducted in animals, and the benefits to humans are not yet proven.

Short periods of fasting, such as skipping breakfast (or eating an earlier dinner), may benefit your gut microbes, explains Professor Tim Spector of King’s College London in his book Spoon-Fed. Although research in the field is in its early stages, accumulating evidence suggests that during fasting, certain species of microbes replicate and feed off carbohydrates in the gut lining. This makes the gut barrier (which plays a role in the immune system) healthier and more efficient.

One explanation for this could be that gut microbes follow a like you do. “Microbe communities, like us, may also need to rest and recuperate as part of a daily circadian rhythm – which could be important for our gut health”, writes Spector.

Everyone’s body responds differently to skipping meals, so Spector suggests experimenting by giving breakfast or another meal a miss to see how you feel. Leaving a minimum of 12 hours between eating, even if it’s only occasionally, could improve your mood and energy levels in the short term (and possibly lead to weight loss in the longer term), he argues.

Benefits to fighting inflammation

Inflammation is your body’s normal response to containing and fighting infection and repairing tissue damage. But chronic inflammation can occur when this immune response lingers, causing ‘oxidative stress’. This is linked to , Type-2 diabetes, cancer, multiple sclerosis, inflammatory bowel diseases and .

Intermittent fasting has the potential to manage or prevent chronic inflammation, according to some researchers. A published in the prestigious New England Journal of Medicine found that in some groups, ketones produced during fasting improve the ability of cells to suppress inflammation and repair damage.

A separate by a team at Mount Sinai hospital in the US found intermittent fasting reduced the release of pro-inflammatory cells called monocytes, and put them into sleep mode. Given the wide range of diseases caused by chronic inflammation, researchers said there was “enormous potential” in doing more research into the anti-inflammatory effects of intermittent fasting.

Further, several studies suggest intermittent fasting might help reduce in overweight and obese adults, which can cause high insulin and blood-sugar levels and increase the risk of developing Type-2 diabetes. However, there’s conflicting evidence about whether intermittent fasting is advisable for people who already have Type-2 diabetes. Karen Davies, Senior Clinical Advisor at Diabetes UK, stresses it’s important for anyone with diabetes to seek medical advice before changing their diet. “For people with Type-2 diabetes who are taking certain medications or insulin, intermittent fasting can increase the risk of low blood-sugar levels”, she adds.

Research is ongoing into whether these benefits stem from the weight loss or the fasting.

Good metabolic health means having optimum levels of blood sugar (glucose), fat (triglycerides), ‘good’ cholesterol (HDL), blood pressure and other markers that indicate your risk of heart disease, stroke and other chronic conditions.

“Certainly, from a metabolic point of view we definitely see benefits in intermittent fasting”, says Dr Collins. “You’re better at clearing, storing and utilising carbohydrate and fat [when you’re on an intermittent fasting regime], so there are improvements in insulin sensitivity, blood lipids (fats) and other cardiovascular risk factors.”

Such improvements are evident in lean people as well as those who are overweight, he says. This is possibly because healthy-weight people can still carry harmful deposits of fat in their internal organs, including the liver.

But the effects of weight-loss are difficult to untangle from other processes at play during fasting. Dr Collins is conducting more research to find out whether metabolic health can be improved in people on intermittent fasting programmes without reducing their overall calorie intake.

Insufficient evidence yet, say some scientists

Various studies, including The New England Journal of Medicine , have suggested intermittent fasting might also be useful in treating some conditions, including Alzheimer’s disease and Parkinson’s disease, slowing the growth of some types of cancer, improving tissue damage repair and enhancing athletic performance.

Krista Varady, Professor of Nutrition at the University of Illinois, Chicago and a world expert in intermittent fasting for weight-loss, says “there’s good evidence to show the ‘every other day’ and 5:2 diets are the most effective fasting patterns for weight-loss and health benefits. With time-restricted feeding, the weight-loss seems to be a little less effective, about half as much as the other two”. But she urges caution about the benefits. “Many of the claims about the benefits of intermittent fasting are overblown”, she says. “A lot of the results of studies in animals just haven’t been replicated in humans”.

Emerging evidence suggests that regular periods of fasting may have health benefits beyond weight-loss, but the science is not yet proven. Intermittent diets are not suitable for people at risk of, or with a history of, eating disorders. Diabetics and those other with a pre-existing medical condition should seek medical advice before going on any form of fasting.

Originally published May 2021