Page last updated at 14:39 GMT, Monday, 15 March 2010

Why diets are doomed to fail

By Eleanor Bradford
BBC Scotland health correspondent


Are diets doomed to fail?

Surveys have suggested one in five of us started a diet in January but the average dieter abandons it on 18 March.

In a week of special features I'll be exploring why it is so hard to diet and whether there is anything we can do to make it easier.

Since January, I've been following two typical slimmers, Lisa Jane Laurie and Lyndsay McIntyre.

Lisa Jane is dieting for her wedding in July, and Lyndsay is her bridesmaid.

Lisa Jane has chosen to use the Weightwatchers diet, which involves counting the number of 'points' in everything she eats and sticking to a set limit.

Lisa Jane Laurie
Weight: 17stone 11lbs
Target:  14 stone 11lbs
Diet: Weightwatchers

Lyndsay is using the Cambridge Diet, which replaces breakfast and lunch with shakes.

The BBC has not influenced their choice of diet.

Lyndsay has set herself a tough target of 8st (50.8kg) to lose but she is confident she will succeed.

"The big incentive is that Lisa Jane has asked me to be her bridesmaid.  I do not want to look at the pictures and be the fat bridesmaid" she said.

Although setting out to lose less than her bridesmaid, Lisa Jane has piled on the pressure by ordering her wedding dress two sizes too small.

"I need to lose 3.5in (8.9cm) around my waist to fit into my dress. It's a wee bit of pressure I didn't need, but it will give me a shove in the right direction."

At the moment the girls' willpower is strong but, according to scientists at the Rowett Institute at the University of Aberdeen, their diets are destined to fail.

They have studied more than 50 types of diet and Professor Julian Mercer, head of obesity research, explained the common problem.

He said: "Before you go on a diet, your weight is fairly stable and the energy you take in as food is more or less the same as the energy you're burning off.

"That's controlled by the brain and is largely a subconscious process.

"When you go on a diet you're cutting back - usually quite quickly - on the amount of calories you're taking in.

Lyndsay McIntyre
Weight: 21 stone 8lbs
Target: 13 stone 8lbs
Diet: Cambridge

"The energy you're using up exceeds what you're taking in and you'll lose weight, but your brain is really worried about this and starts to reduce your metabolic rate.

"So you're not losing any weight any longer, but you're still on a diet and you're now becoming really hungry and miserable."

The result, inevitably, for most people is that they fall off the diet.

The problem, though, is that their metabolic rate has now been reduced.

"You'll not only put the weight back on, but because it takes a while for your metabolic rate to adjust, you'll probably overshoot and end up heavier than when you started," said Prof Mercer.

If this leaves you feeling thoroughly depressed, don't panic.

By understanding what's going on in our bodies, we can follow diets which have a greater chance of success.

"It's important to recognise that your physiology is designed to prevent you from starving to death," said Prof Mercer.

"And you're working against that when you go on a diet."

Over the next four days I'll be finding out more about what is going on in our bodies and the best ways to control our weight.

On Tuesday I will be going in search of the 'holy grail' of dieting - a diet which doesn't leave you feeling hungry, but which still makes you lose weight.

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