Is intermittent fasting a healthy and effective way to cut calories?

Whether 5:2, alternate day or time restricted, fans of intermittent fasting diets hail their weight loss, health and wellbeing benefits. Are they worth the hype?

Mark Twain once said a “little starvation can really do more for the average sick man than can the best medicines and the best doctors”.

There may be something to Twain’s belief, as studies increasingly suggest skipping meals or fasting may benefit us, particularly in reducing systemic inflammation, promoting weight loss and reduce cardiovascular risk factors such as hypertension.

Celebrities like Hugh Jackman swear by fasting diets, while BBC science presenter Dr Michael Mosley created a sensation in 2012 when he devised his 5:2 diet.

But recent conflicting studies suggest this diet may be effective yet harmful, which begs the question, is intermittent fasting the best way to lose the weight?

What are fasting diets?

Fasting diets come in various forms.

The 5:2 diet  allows you to eat normally five days a week and restrict calorie intake to a quarter (500 to 600 calories) on two days.

Alternate-day is one day of fasting, one day normal eating.

There’s also time-restricted fasting, or eating only during certain hours of the day, like the so-called Warrior Diet, which involves minimal calorie intake for 20 hours a day, and eating normally only between 6pm and 10pm.

• Consider this: 10 things to know about 16:8 intermittent fasting

What are the benefits of intermittent fasting?

The 5:2 diet is suggested to reduce fasting blood glucose, while the alternate-day fast is associated with improving cardiovascular health and time-restricted eating may better metabolic dysfunction.

Among these studies,  weight loss  is seemingly the main benefit.

A 2018 study by the UK’s University of Surrey found that one group doing time-restricted fasting lost on average more than twice as much body fat as those in a control group that ate meals as normal.

The American Heart Association also stated in 2017 that there was “evidence that both alternate-day fasting and periodic fasting may be effective for weight loss, although there are no data that indicate whether the weight loss can be sustained long term”.

Dr Mosley argues that not only does fasting lead to weight loss, but also cellular repair and improved brain function.

However, time-restricted fasting may have unwanted consequences on women’s reproductive hormones, a 2022 study suggests.

Researchers discovered a decrease in DHEA, a hormone responsible for increasing testosterone and estrogen levels, in premenopausal and postmenopausal females.

Another US study found young and lean males who are physically active may find a reduction in testosterone levels with intermittent fasting.

Resulting health complications in these men may include a decline in metabolic health, muscle mass and libido.

A dietitian’s view on fasting diets

Dietitian Kate Gudorf says fasting appears to produce weight loss – as does any form of calorie restriction.

Some studies have found that fasting can reduce fasting glucose levels, HDL (bad) cholesterol, body-mass index, triglycerides (fats in blood) and some inflammatory markers, she says.

Additionally, the reduction of waist circumference and improvement in insulin resistance are also some of the many side effects of intermittent fasting, suggests a recent study.

“The question is, are these outcomes a testament to fasting diets, or because weight loss is produced, which feeds into these other benefits? At the moment we lack good data on fasting diets,” says Kate.

She recommends instead simply eating a healthier diet, with more vegetables and fruit, meaning both fewer calories and meeting nutritional needs.

Increasing fruit and vegetable while decreasing refined grains consumption has been associated with better diet quality and long-term weight loss maintenance.

“Weight loss is not always a good indication of diet quality … it’s much easier to eat fewer calories if you simply eat a healthy diet, and nutritionally you would be meeting all your needs.

Source: The 5:2 Diet Book, by Kate Harrison

Written by Mike Bruce. Updated by Melissa Hong, November 2022.