Fasting: As Rishi Sunak reveals his weekly fast, here are the health benefits

Intermittent fasting has been used in religious and cultural rituals for centuries and is a focus of heated debate among dieticians and nutritionists. More recently its potential health benefits have been brought to the spotlight by the revelation that Rishi Sunak fasts for 36 hours each week.

Restricting calories, if overweight, is clearly beneficial if this leads to weight loss. Being overweight or obese is linked to multiple health risks ranging from diabetes, heart disease and arthritis to cancer. Whether we do this across the week or in intermittent bursts has been the topic of scientific dilemma and robust clinical trials would be welcomed to provide more clarity.

Back in 1982, a landmark paper reported that if you fed pet rats every other day rather than having food available all the time, they lived significantly longer. On the other hand, another laboratory experiment gave one group of mice as much food as they wanted all the time, another group had their food withdrawn every few days and a third group had their calories generally restricted. Both the mice with intermittent feeding or general calorie restriction lived twice as long as those who were free to eat as much as they wanted (like us humans). This study implied that the calorie restriction was the key rather than the fasting itself.

That said, there are some additional favourable biological changes which occur during the fasting period, such as reduced oxidative stress, less inflammation, lower blood sugar and improved insulin sensitivity. Some studies have reported that these changes have resulted in a reduced risk of several disease, albeit most needing confirmation in larger studies.

Fasting, weight loss and diabetes

One study found that women placed on an intermittent fasting regime achieved similar levels of weight loss as women placed on a calorie-controlled diet. They also experienced improvements in a number of biological indicators, suggesting a reduction in the risk of developing type 2 diabetes and other chronic diseases.

In this study, individuals fasted for 16 hours a day and then ate whatever they wanted in the eight hours between 10am and 6pm. This may be more palatable to many people trying to lose weight provided they don't cheat during their fasting period.

Fasting, general fitness and safety

There have been anecdotal reports of negative effects during fasting days, including difficulties sleeping, bad breath, irritability and anxiety, and dehydration. However, there have been very few reports of medical harm with fasting unless extreme and associated with anorexia or the abuse of weight-loss drugs. In one prospective trial, athletes randomised to intermittent fasting reduced fat mass and increased muscle bulk, compared the group maintaining their normal dietary regimen.

This suggests that claims of fasting being bad for general fitness levels are unfounded.

Fasting, mental alertness and cognitive decline

One laboratory study, involving mice that had been genetically engineered to develop changes in brain tissue similar to those seen in people with Alzheimer’s disease, saw a slower rate of cognitive decline in the group that were given an intermittent fasting diet than the one on a normal diet.

In humans, a study showed that fasting led to irritability but also positive psychological experiences such as a sense of reward, accomplishment, pride, and control.

Fasting and cancer

Laboratory studies have shown that fasting contributes to lowering the risk of certain obesity-related cancers, such as breast and uterus, by helping participants to lose weight. If overweight people with cancer find intermittent fasting acceptable, then it may well be helpful. The physiological changes which occur with intermittent fasting (such as reduced oxidative stress and inflammation) suggest there could be anti-cancer mechanisms.

In laboratory experiments, fasting did appear to suppress tumour growth and spread via a phenomenon termed ‘differential stress resistance (DSR). In humans, the best evidence of sensible, effective fasting comes from a study that evaluated a large cohort of overweight women who had completed their initial cancer treatments. The researchers discovered that those who adopted early dinners and late breakfasts, leaving 13 hours between the meals (without intermediate snacking), lost significantly more weight, had lower levels of glycated HbA1c (a marker of glucose control over time) and lower inflammatory markers. What’s more, after five years, they had a 36 per cent lower risk of breast cancer recurrence.

Blood pressure

In the laboratory study above, the blood pressure of the fasting mice also dropped a little (on average 7mmHg). However, it should be remembered that most supervised weight reduction programmes, including exercise, have achieved similar or even better results within a similar time frame.

Arthritis

Clinical evaluations have found that participants who fasted, reduced their salt intake and increased their consumption of vegetables had a significant reduction in arthritis-related pain and stiffness. Other studies have shown that fasting, followed by a diet rich in green and cruciferous vegetables, helped reduce joint pain and swelling, duration of morning stiffness and blood markers of chronic inflammation.

Type of fasting

Some people find intermittent fasting more practical than continuous energy restriction. The 5:2 is one of the most widely followed versions. This entails five days of normal eating and two days when calorie intake is reduced to 500 calories for women and 600 calories for men. Some enthusiasts of the 5:2 diet have suggested switching to a target of 800 calories on the fasting days which appears to be more sustainable in the longer term.

Another version of intermittent fasting is the 16:8 diet, which received media attention after the publication of its apparent benefits following a small randomised controlled trial led by Professor Krista Vardy in Chicago. Evidence around intermittent fasting from larger studies is limited, mainly because few formal human trials have been performed.

In conclusion

Intermittent fasting can certainly help with weight management, and many people, including our Prime Minister find it a useful form of self-control. It is generally safe and does reduce overall calorie intake. It suits some people but for others the inconvenience it causes, alongside the fact that simply eating a less calorific diet can be just as effective at reducing calorie intake, it is less attractive. There is some medical evidence that it has added health benefits over calorie restriction particularly for diabetes and high cholesterol.

Overnight fasting is more easily achievable which has demonstrated an anti-cancer benefit. Some of the biological changes that occur with fasting are similar to those which occur during and after exercise. Another sustainable way of achieving the health benefit of fasting is to extend the gaps between meals, exercise before food and avoid snacking.