What are the real benefits of intermittent fasting?


It’s not just the fat-burning benefits that make intermittent fasting so popular. Hailed for boosting our “metabolic resilience”, studies show that fasting can lead to a whole range of health improvements.

Dr Michael Mosley, broadcaster and co-founder of the TheFast800 online programme, says: “A review of all the best available research, published in the New England Journal of Medicine in 2019, concluded that ‘intermittent fasting has broad-spectrum benefits for many health conditions, such as obesity, Type 2 diabetes, cardiovascular disease, cancers, and neurologic disorders [such as dementia]’.”

However, while fasting can be good for you, it needs to be taken on sensibly and does not suit everybody. Here we explain the benefits of fasting and the best way to do it.

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Does intermittent fasting contribute to a healthier heart? 

Recent reports of a study claiming intermittent fasting could almost double your risk of having a fatal heart attack caused alarm. Dr Mosley says: “Experts are extremely sceptical about the study that generated these headlines. The fact is that many, more rigorous randomised controlled studies, have shown the benefits of time restricted eating for heart and metabolic health.”

There are, it appears, multiple ways intermittent fasting can actually benefit the heart. Better cholesterol and blood sugar levels, both of which are outcomes of intermittent fasting, can lower the risk of weight gain and diabetes – two risk factors for heart disease.

Meanwhile, a study published in the American Journal of Medicine found that intermittent fasting appears to positively impact multiple cardiovascular risk factors including obesity, hypertension, dyslipidemia (an imbalance of lipids that can result in cardiovascular disease), and diabetes.

It accelerates cellular repair

“One of the benefits of doing intermittent fasting is that you switch from burning sugar to burning fat as your main fuel, which helps in multiple ways,” says Dr Mosley. “Giving your body a break from digesting and processing food helps trigger something called autophagy, a form of cellular ‘spring cleaning’, where old cells are broken down and recycled.”

It benefits your brain

Prof James Goodwin is the director of science and research impact at the Brain Health Network in London, and the author of Supercharge Your Brain. He says: “A systematic review of all intermittent fasting studies to date, and published in the New England Journal of Medicine in 2019, shows benefits of intermittent fasting to general health in animal and human studies, plus a high probability of benefit to the brain.”

With a background in evolutionary biology, Prof Goodwin finds the likely metabolic chemistry of hunter-gatherers very convincing.  “Our brain chemistry evolved during forced periods of non-eating, an incipient part of the lives of hunter-gatherers. Their chemistry adapted such that the brain was able to use ketones - energy rich molecules which the body produces by breaking down fat stores – as an alternative to glucose once bodily supplies ran out.”

When we do not eat for more than 12 hours, the body’s natural sources of glucose become exhausted. “The body then turns to breaking down fat stores to release ketones, the alternative energy source,” he says. “This process was entirely natural when we were hunter-gatherers when food was in short supply, eg in the winter.

“In today’s western way of life, few people ever exhaust their glucose, with the result that our bodies are constantly saturated by sugars and then insulin which controls the sugar level. Both persistently high sugar and insulin levels are harmful to long-term health.”

What’s more, the metabolic, cellular, and circadian effects when intermittent fasting may have great potential to treat or prevent brain-related diseases. Studies have found it can reduce the risk of neurological disorders, including Alzheimer’s disease, Parkinson’s disease and stroke, as well as improving mental clarity.

“One way that [intermittent fasting] helps the brain is that it increases the level of a protective substance in the brain called, ‘BDNF’ (brain derived neurotropic factor). BDNF promotes the growth of new cells in the brain.  As we age, levels of BDNF decline dramatically and contribute to the loss of brain cells,” adds Prof Goodwin.

It helps with weight loss

After a period of time without food we start to utilise or “burn” stores of body fat to maintain adequate energy balance. “Periods of between 10-16hrs are long enough to signal the body to start this process,” says Dr Fionnuala Barton, a GP and menopause specialist.

Weight gain is often a result of fat being stored around our internal organs as “visceral fat” causing further negative impact on our metabolic health. “And if we start to use some of these fat stores, we see improvements in metabolic health, including improved sensitivity to insulin – one of the main hormones responsible for maintaining normal blood sugar levels in the blood. When insulin sensitivity improves alongside lower dietary glucose burden, it becomes easier to maintain normal weight or possibly even lose weight,” she adds.

Intermittent fasting alone will not help you lose weight 

However, stresses Dr Barton, while weight loss may be achievable with the use of intermittent fasting, it is not a universal experience.

“Studies exploring this in both animal and human studies show mixed results. Weight loss is more likely when there is a caloric deficit alongside time-restricted eating. Our bodies have a number of mechanisms in place to achieve energy homeostasis, or balance, and the weight loss we see with [intermittent fasting] is more likely related to the relative caloric restriction we see when limiting our eating window.” So those late-night snacks might have to hold.

It reduces stress and inflammation

Fasting can stabilise the hormone cortisol produced by the adrenal glands, reducing stress levels. And a study from Cambridge scientists claims that fasting for 24 hours may protect us against dementia and Parkinson’s by reducing inflammation. Inflammation is our body’s natural response to injury or infection, but if it becomes chronic, it could contribute to further tissue injury and disease, according to the National Institute of Environmental Health Sciences, including the growth of tumours and cancer. And, the Cambridge researchers claim, fasting raises levels of a chemical in the blood known as arachidonic acid, which inhibits inflammation.

It may lower your risk of diabetes

Natalie Burrows, a registered nutritional therapist, says: “Fasting can be an effective way to reduce overall calorie intake which can assist with weight management or loss and, as Type 2 diabetes risk increases with higher body weight, fasting can be an effective tool in diabetes prevention.”

After fasting for approximately 12-14 hours, she says, the glucose supply in the body is reduced and the body will enter mild ketosis which assists in fat burning. “After this, when food, and therefore glucose are available, the body can be more sensitive to insulin, responding and using glucose more efficiently, resulting in lower blood glucose levels”.

It prevents dips in energy 

Surely going without food for long periods of time makes you sluggish? Indeed, research suggests that some people practising intermittent fasting can experience fatigue and low energy levels. However, a 2020 study found that intermittent fasting may actually reduce fatigue as your body adjusts to regular fasting periods.

“You might expect to feel tired and lethargic if you haven’t eaten for 12 hours, or you only ate 800 calories the day before, but intermittent fasting trips your body over into fat-burning mode and fat is a highly efficient fuel for your body and brain. This flip makes your metabolism work more efficiently, keeping energy levels stable,” says Dr Mosley.

Intermittent fasting tips

Dr Mosley is a fan of the 12-hour fast. “If you want to try time-restricted eating, fast overnight for 12 hours and eat within a 12-hour window. The simplest way to do this is to stop eating two to three hours before bed and then not eat again until 12 hours later.”

Dr Barton agrees. “The health benefits of fasting for 12 hours are very similar to those seen with longer fasts and it’s more achievable, less stressful and easier to fit around social engagements, with less risk of disordered eating,” she says.

But you still need to eat well, warns Barton. “You cannot expect to eat a nutritiously poor diet whilst intermittent fasting and still see the benefits,” she says; sadly, a large slab of chocolate cake followed by cheese may not have the desired effect.

Who should avoid intermittent fasting?

  • Anyone under the age of 18

  • Pregnant women or those breastfeeding

  • People with Type 1 diabetes

  • Anyone with a history of eating disorders

  • Women struggling with menopausal symptoms

Those under the age of 18 are not advised to do intermittent fasting, nor women who are pregnant or breastfeeding. While people with Type 2 diabetes are safe to do it, those with Type 1 diabetes who take insulin should not, as an intermittent fasting eating pattern may result in unsafe levels of hypoglycemia during the fasting period.

Anyone with a history of eating disorders should avoid it. A​ study published in Eating Behaviors looked at its impact among adolescents and young adults in Canada. Researchers found an association between intermittent fasting and the behaviours of eating disorders and other dangerous behaviours among some members of this age group.

It can have the same implications for people who have a long-term tendency towards disordered eating, says Dr Barton, who adds that for women in midlife, “intermittent fasting has the potential to be incredibly powerful, but I have also seen it mask eating disorders that some women have dealt with their entire adult lives without help.”

Burrows advises against strict fasts if you are struggling with menopause symptoms. “I don’t see menopausal-stressed women do very well with fasting, overall. There will be exceptions to this, but the additional stress of reduced food intake is often counterproductive.”

How to fast safely

For maximum benefits, start with an overnight fast of 12 hours so you’re on a 12:12 eating and fasting window and focus on fasting between meals during the day, says Burrows. “Then move to a 14:10 fast, with 14 hours of overnight fasting and still no snacking. Next is 16:8”.

“The success of fasting comes from being metabolically flexible,” she says. “To be able to switch to stored fat burning instead of relying on incoming carbohydrates for easy fuel. Carbs are the body’s preferred sources of fuel because it’s a simpler conversion, but using fat for fuel is why some people find fasting easier than others - the demand for food isn’t there because the body can tap into what it already has in ‘storage’. Fasting can help train the body to become metabolically flexible, and this is why I recommend easing into it in stages.”

But make sure you’re healthy before you start. “I encourage people to get nourished before they begin fasting. Many people are nutritionally depleted and then they deplete further through fasting due to reduced food intake. There may be an initial ‘high’ but it often has a negative impact long-term,” says Burrows.

How long will it take to see the benefits of intermittent fasting?

Some people begin to see some results within the first few weeks, including weight loss, increased energy and improved mental clarity. “Everyone has a different starting point and different medical history to work with. What works for one, and the speed at which it works, may not work for another,” says Burrows.

One study has shown beneficial results for people with Type 2 diabetes within as little as two weeks. But it is worth persevering as another study showed that after six months of intermittent fasting participants lost an average of 3.6 percent of their body weight.

“But if you’re fasting for Type 2 diabetes benefits, I would consider this as a lifestyle change and long-term commitment,” says Burrows. “Many enjoy it as a new way of life because of how they feel.”