The benefits of fasting – what’s the truth?

Fasting is a go-to for many wellness enthusiasts – Liz included. With studies reporting a variety of benefits – from better brain function to enhanced longevity – it seems there are a number of wellbeing wins to gain from the practice.

But what’s the truth? Here we take a look at what the science says about the benefits of fasting.

What is fasting?

Intermittent fasting is the practice of going without food for a period of time. Fasts come in many forms, the most extreme being water-only. These involve abstaining from food entirely and can last upwards of 48 hours.

Time-restricted eating is a more popular model that includes alternate-day fasts (eating one day and not the next), OMAD (one meal a day) and the ‘warrior diet’ – eating within a four-hour window, followed by a 20-hour fast.

The most popular (and perhaps practical) approach is 16:8, which involves eating within an eight-hour window before fasting for the remaining 16 hours of the day (or night).

Other fasts make use of low-calorie days rather than the absence of food altogether. These include the 5:2 (popularised by Dr Michael Moseley), which stipulates two fasting days per week, with calories limited to 500-800 kcal on those days.

The benefits of fasting


As early as the 1930s, scientists found that rats on a calorie-restricted diet lived twice as long as other rats.

These results are yet to be replicated in humans – partially due to the challenges associated with tracking fasters over a lifetime. What does seem to be clear, however, is that fasting may reduce our risk of heart disease – one of the biggest killers in the UK. It does this by reducing ‘bad’ cholesterol levels, blood fat levels, high blood pressure and inflammation.

Fasting’s longevity-boosting credentials may also have something to do with a process called autophagy.

In the absence of calories, our body activates genes associated with preserving resources. This involves ‘recycling’, repairing and clearing out dead cells in the body – known as autophagy. This might have a positive impact on lifespan, research suggests, as dead and damaged cells are at an increased risk of developing into cancer.

Mental health

Some evidence suggests fasting may improve mental health, but the data is very limited. Research shows that levels of brain-derived neurotrophic factor (BDNF) increase when we fast. This is a protein involved in the regulation of serotonin – often dubbed our happy hormone.

Research also shows that people with depression tend to have lower levels of BDNF.

Weight loss

There’s plenty of evidence to suggest that fasting results in short-term weight loss. Studies show this is true of all different types of fasts. This includes time-regulated eating, water-only fasts, as well as a 5:2 approach.

Anecdotally, many also report finding fasting an easier weight-loss strategy than traditional calorie restriction, as there are fewer opportunities to eat. It’s harder to overeat in a four-hour window than it is over 24 hours.

Gut health

There’s evidence fasting allows ‘good’ gut bugs to clean the lining of our gut and keep it healthy. This strengthens our gut wall and reduces inflammation and/or a ‘leaky gut’. Fasting also appears to improve the makeup of our microbiome. It encourages ‘good’ bugs to flourish and less favourable ones to find another home

Metabolic health

There’s some evidence that fasting can help reduce our risk of metabolic syndrome and Type 2 diabetes. Studies suggest that benefits occur even in those who don’t lose weight while fasting. Fasting can also boost levels of specific bacteria called Prevotellaceae and Bacteroidetes, which are associated with better metabolic health

Is fasting for everyone?

It’s important to mention that experts don’t recommend fasting for some groups of people. These include those with a history of disordered eating, as the practice is linked to an increased risk of bulimia and binge-eating. It’s also advised that elderly adults and people who are pregnant or breastfeeding avoid fasting and instead prioritise eating plenty of good-quality calories.

As with any dietary intervention, if you have a pre-existing health condition it’s best to speak to a doctor before adjusting your habits.

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