Could the secret to weight loss actually be the centuries-old method of fasting used by yogis? Health editor Hilly Janes looks at the latest on-off diet craze

Any products in this article have been selected editorially however if you buy something we mention, we may earn commission

Since TV presenter Dr Michael Mosley drastically cut calories for two days each week for a TV documentary there has been a buzz again around the idea of fasting.

Mosley tried the 5:2 plan, a five days on, two days off weight loss regime in the BBC Horizon programme Eat, Fast, Live Longer. Similar to alternate day fasting (ADF) plans such as the DODO (day on day off) and UpDayDownDay (same idea), it's all about eating what you like some of the time, then cutting right back. A few years ago the 80:20 diet also let you eat what you liked a couple of days a week. Meanwhile of course yogi and monks have been saying fasting is good for mind and body for centuries.

Times Magazine Associate Style Editor Prue White (read her article in Get the Gloss here ) followed the example Mosley set on the BBC Horizon programme and tried the same 5:2 diet, eating what she fancied five days a week, then surviving on about 500 calories for the other two. Both lost weight, in Mosley’s case about a stone in five weeks. And the presenter, who studied medicine, found that blood markers indicating an increased risk of cardiovascular disease (heart attacks, strokes etc) and diabetes also dropped.

What’s the big idea? You eat less and you lose weight. Doh! But the problem with most diets, where you cut calorie intake every day, is that eventually the body learns to survive on less and conserves its fat stores instead of burning them up to create energy. Wish I’d been able to stick at one long enough to find out.

With on-off fasting regimes, this response doesn’t seem to kick in, so that the weight loss triggered by the fasting days, when calories are about 20 per cent of the recommended daily intake – about 2,000 for women and 2,500 for men – continues.

But don’t on-off fasters just compensate on the days when they can eat normally? Not according to Dr Krista Varady, a health science researcher at the University of Illinois at Chicago, who told the Horizon programme that in her studies ADF-ers ate only 110 per cent of their usual intake on “feed days” (trough not required). It would be quite challenging to eat 4,000-5,000 calories a day in any case, unless you are Bradley Wiggins.

The magic ingredient on scientists’ lips is IGF-1, a hormone which triggers cell growth. On fasting regimes IGF-1 levels drop, prompting the body to repair and renew cells rather than make new ones – a kind of biological make do and mend. Reduced IGF-1 also appears to reduce high blood sugar levels, associated with weight gain and type 2 diabetes, the cost of which at £12 billion a year may soon bankrupt the NHS. In a remarkable recent study at Newcastle University, people with type 2 diabetes on a diet of 600 calories a day actually reversed their condition.

New brain cells also seem to grow as a result of calorie restriction, believes Professor Mark Mattson, a neuroscientist at the United States National Institute on Aging. His experiments on skinny mice show that they live longer with normal memory and learning abilities than mice fed sugary, fatty food.

This all may be cutting-edge lab research at the moment, but those yogi and monks have always believed that fasting brings mental clarity. And when we live in an “obesogenic environment”, where cheap, fatty, sugary food is all around us 24 hours day, we have more in common with Mattson’s tubby rodents than their skinny relations.

Could intermittent fasting work because it is easier on the willpower? Jane Ogden, professor of health psychology at the University of Surrey, says that diets fail because we are not good at denial: “But if you can say ‘I am not going to eat that now, but I can eat it tomorrow’, it’s less of a challenge.”

Intermittent fasting: the rules


  • Expect it to be easy at first. You may feel hungry, a little light-headed and get a headache.
  • Do it if you are pregnant or breastfeeding, or there is a health-related reason that means you should consult your doctor first.
  • Make your children do it.
  • Have restriction days when you are very physically active.
  • Go mad and eat rubbish on “feed” days.


  • Plan ahead so you don’t get caught out by work/social events.
  • Work out when it best suits you to eat in the day, there are no rules.
  • Drink lots of water or herbal/black/green tea to stay hydrated and alert.
  • Eat balanced meals: a little protein, a few carbs (including fruit) and dairy, fats, lots of veg.
  • Check your personal daily calorie requirements with an app or tool such as the Calorie Calculator    and calculate 20 per cent of your needs – it varies from individual to individual.

The independent nutritionist’s view: Martin MacDonald

Martin MacDonald ( ) is one of the UK's leading nutritional scientists and lecturers whose clients who have achieved incredible results with his version of intermittent fasting. Since there is yet very little literature on the 5:2 diet, GTG's Editor Susannah Taylor asked for his opinion.

We shouldn’t be afraid of fasting if done correctly
“People are afraid of hunger,” says Martin, “But our bodies are designed to be able to go without food for lengthy periods of time.”

Don’t presume you can over indulge on your ‘eat days’
“This diet isn’t going to work if what you consume on the ‘eat days’ is ridiculously calorific,” he warns. “Two days deficit isn't enough to outdo a horrific diet. It shouldn’t be about ‘eating what you like’ as ridiculous amounts of junk could completely negate any positive effects. You need to make sure you are eating healthy, good foods on the other days too.”

Don’t undereat
Martin’s concerns with the 5:2 diet is that it doesn't necessarily encourage good habits on the five eating days. While he warns against overeating on "eat" days, it’s important not to undereat which he says could lead to disturbances in hormones and metabolism.

Eat protein to keep your metabolism revving
Martin explains that ideally your 500 calorie days should emphasise protein so that you don’t lose muscle as this could slow your metabolism.

Is the 5:2 diet sustainable in the long run?

Martin's concern is that the biggest reason why diets fail is because they don't encourage a long term way of eating. His concern with the 5:2 is that it's not sustainable long term, which he says "could lead to weight gain eventually." Could this buzz be like the Atkins where everybody lost weight, and they all put it back on again? Only time will tell.

Follow Martin on Twitter @MacNutrition or visit his website at

Clips from the BBC Horizon programme Eat, Fast, Live Longer can be viewed at