Rather than helping us to reach our target weight more quickly, severely restricting calories actually prevents our bodies from burning unwanted fat stores effectively - and unfortunately, this means that weight loss slows down.
Why does a very low calorie intake slow down weight loss?
Quite simply, your body goes into 'starvation mode'. This mechanism, which is thought to have evolved as a defense against starvation, means the body becomes super efficient at making the most of the calories it does get from food and drink. The main way it does this is to protect its fat stores and instead use lean tissue or muscle to provide it with some of the calories it needs to keep functioning. This directly leads to a loss of muscle, which in turn lowers metabolic rate so that the body needs fewer calories to keep ticking over and weight loss slows down. Of course, this is the perfect solution if you're in a famine situation. But if you're trying to lose weight, it's going to do little to help you shift those unwanted pounds.
So how many calories should I have to prevent starvation mode?
Unfortunately, there's no single answer to this question. As everyone's metabolism varies in the first place, so too will the point when the body starts to use muscle to provide it with calories in a 'famine-type' situation. You do not want to lose more than 2lbs a week, so your body won't go into starvation mode.
As a general rule though, most nutrition experts recommend never going below 1,000-1,200 calories a day if you're dieting on your own. It's also worth bearing in mind that the body doesn't suddenly 'enter' and 'leave' starvation mode, like crossing the border from Devon into Cornwall. It's a gradual process - so you don't need to panic if you do go below your calorie intake very occasionally.
What's the link between muscle and metabolism?
The metabolic rate - the rate at which the body burns calories - is partly determined by the amount of muscle we have. In general, the more muscle we have, the higher our metabolic rate; the less muscle we have, the lower our metabolic rate. This explains why men, who have a high proportion of muscle, have a faster metabolism than women, and why a 20-year-old has a higher metabolism than a 70-year-old - again, they have more muscle.
Ultimately, muscle burns a lot more calories than fat so when we lose muscle, our metabolic rate drops and we burn fewer calories. In fact, research shows that the body loses a proportionately high amount of muscle with a very low calorie intake and this may considerably suppress metabolism by up to 45 percent.
This explains why it's crucial to do as much as you can to protect your metabolic rate, especially when you're dieting. And this means dieting sensibly with a suitable, rather than a very low calorie intake so that you lose fat rather than muscle.
Is there anything else I can do to stop losing muscle when I'm dieting?
As well as making sure you have sufficient calories to burn fat rather than muscle, it's also possible to build muscle, which in turn boosts metabolism. The way to do this is, of course, to increase the amount of exercise you do. While aerobic activities such as jogging, swimming, fast walking and aerobic classes help to tone muscle and burn fat, strength or resistance training in particular will increase the amount of muscle you have in your body. And this is good news because for every extra 1lb of muscle you have, your body uses around an extra 50 calories a day! This means an extra 10lb of muscle will burn roughly an extra 500 calories a day without you doing anything - and that's a sufficient amount to lose 1lb in a week.
But doesn't your metabolism drop when you lose weight anyway?
Yes, your metabolic rate naturally slows down a little when you lose weight, but this isn't automatically because you've lost muscle. It's because when your body has less weight to carry around, it needs fewer calories. This means if you weighed 150 pounds to start with and now weigh 130 pounds, you need fewer calories to maintain your new weight than you did when you were heavier. Simply put, there's less of you to carry up and down the stairs, into the bath, around the supermarket and to the bus stop - and because your body doesn't have to work as hard as it did in the past, it can survive on fewer calories.
Will yo-yo dieting have damaged my metabolism permanently?
Fortunately not! The idea that yo-yo dieting permanently lowers your metabolism has been relegated to the archives. However, if you've frequently crash dieted and severely restricted your calorie intake without exercising, it's likely you'll have a lot less muscle now compared with the very first time you dieted. As a consequence, it's likely your metabolism will also be lower so that you need fewer calories to maintain your current weight. This is because when you follow a very low calorie diet, you lose muscle as well as fat. When the weight goes back on, you usually only regain fat. This means, your metabolic rate is likely to have dropped a little every time you've dieted, making it slightly harder each time for you to lose weight. The good news is you can increase the amount of muscle you have by weight training. This in turn will rev up your metabolism so that you can lose weight one final time on a slightly higher calorie intake than you've perhaps been used to.