DO CRASH DIETS WORK?

“If you want to try and sustain your weight loss, the worst thing you can do is try and starve yourself for three weeks”

It depends what you mean by ‘crash diet’. There is evidence that supervised food replacement diets work very well for many people. But what about the more DIY crash diets that claim to make your weight plummet? Diets like the cabbage soup diet, the grapefruit diet, and juicing and cleansing diets?

The evidence behind these is currently slim. However, there is less scientific opposition to losing weight quickly than there used to be. Australian research has indicated not only that more people achieve their weight loss goals if they lose weight fast, but also that losing weight quickly doesn’t mean you’ll regain it quickly as well. Rapid weight loss can motivate people to stick with some programmes, the researchers suggest.

But maintaining a healthy nutritional balance while on these diets can be a problem: advice from the NHS is still that “crash diets make you feel very unwell and unable to function properly… crash diets can lead to long-term poor health”.

And both our biology and lifestyles may condemn many extreme crash diets to failure. Dr Giles Yeo, principal research associate at Cambridge University’s Institute of Metabolic Science, specialises in the molecular mechanisms underlying the control of food intake and body weight.

“If you want to try and sustain your weight loss, the worst thing you can do is try and starve yourself for three weeks,” he says. “Rather than taking a huge pendulum swing that will inevitably swing back in the other direction, I think people have to find some balance to lose weight long-term.”

In particular, we have to address the fact that crash diets generally make us feel hungry. Yeo’s research examines how the brain responds to hormones and nutrients that are released from the gut into the blood. These reflect the body’s nutritional status, and the brain turns them into what we experience as ‘ fullness’ or ‘hunger’. You can find more advice for feeling fuller for longer at the end of this feature.

“One of the universal truths of weight loss is that if you want to eat less then you have to have a strategy to make you feel more full, otherwise you are simply fighting hunger for the rest of your life,” Yeo says. “What we now know is that the longer something takes to be digested, the fuller it makes you feel – because as food goes down the gut, different hormones keep being released, most of which give us a feeling of fullness. That’s why high-protein diets can work, because protein is more complex than fat or carbohydrate and goes further down the gut before it’s broken into its constituents.”

Look out for Dr Giles Yeo in the new series of Trust Me, I’m A Doctor, starting on BBC Two this January.

Verdict: Crash diets are not nutritionally balanced and will make you feel awful.