Do fasting diets work?

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 we increasingly hear that skipping meals or even fasting is good for us.

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

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 fasting 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.

What are the benefits of intermittent fasting?

Weight loss seems to be the main one.

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.

fasting diets

A dietitian’s view on fasting diets

Accredited practising 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.

“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 Ms Gudorf, of the Dietitians Association of Australia.

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

“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.

“Basically Australians are not eating very well and getting about a third of their energy from ‘junk food’.”

What does a 5:2 fasting day look like?

  • Good portions of vegetables, especially leafy green veg
  • Smaller portions of lean meats, fish or eggs, baked or roasted
  • Soups, or salads dressed with vinegar and herbs
  • Avoid processed carbohydrates like white bread, pasta and rice
  • Cut out sugary foods, and most fruits – berries are best
  • Plenty of water, or drink black coffee or tea and herb teas
  • If you take milk in hot drinks, include those calories

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

Written by Mike Bruce