16 BBC SCIENCE FOCUS MAGAZINE COLLECTION
DO CRASH DIETS WORK?
It depends what you mean by ‘cra sh diet ’. There is evidence that
super vised food replacement diet s work ver y 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. But there is less
scientif ic opposition to losing weight quickly than there used to
be. Aus tralian research ha s 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
too. Rapid weight loss can motivate people to s tick with some
programmes, the researchers suggest.
But maintaining a healthy nutritional balance while on these
diet s can be a problem: advice from the NHS is s till that “cra sh diet s
make you feel ver y unwell and unable to func tion properly... cra sh
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.“If you want to sustain your weight loss, the worst thing you can
do is try and starve yourself for three weeks,” he says. “I think
people have to f ind some balance to lose weight long-term.”
In particular, we have to address how crash diets generally make
us feel hungr y. Dr 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 a s ‘ fullness’ or ‘hunger ’.
“One of the univer sal truths of weight loss is that if you want to
eat less then you have to have a s trategy to make you feel more full,
otherwise you are simply fighting hunger for the rest of your life,”
Dr 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 relea sed, mos t of
which give us a feeling of fullness. That’s why high-protein diets can
work , because protein is more complex than f at or carbs, and goes
fur ther down the gut before it ’s broken into it s cons tituent s.”Verdict: Crash diets are not nutritionally balanced and will
make you feel awful.“If you want to
sustain your
weight loss,
the worst thing
you can do is
try and starve
yourself for
three weeks”