Put simply, your resting metabolic rate is the minimum number of calories your body needs to survive if you sleep in bed all day. Your body needs energy for hundreds of bodily functions such as breathing, keeping the brain functioning and any movement your body makes. Just being alive uses up around 70% of the calories you burn every day.

According to research, the following calorific expenditure is used by the body:

Homeostasis - 70%

Digestion & Elimination - 5-15%

At this point, our body has already burned around 85% of daily calories without taking any exercise or activity into account. Running on a treadmill at a fairly steady pace for 20 minutes can burn between 150-300 calories depending on your body weight ? thus the amount of calories we burn in activity is far fewer than the body burns keeping itself functioning.

So if you consider what happens when we reduce the amount of calories you ingest but keep your activity levels the same, your body has to become more efficient at functioning with less energy and will consequently slow down your metabolic rate in order to function more efficiently.

In addition, as your body is receiving from fewer calories from your diet it will look for elsewhere for other sources of energy...your muscles. If you're wondering why it won't turn to fat just yet, this is because the body will use fat as a last resort - its long term energy store in cases of starvation ? it will only be used when all other energy reserves have been used up.

So not only will you starve your body of the adequate nutrients it needs to function, you will also be forcing it to eat into muscles for energy and you will lose lean body mass. There is also research that shows that if you do start to eat 'normally' again, most of the weight gain will be fat and that muscle mass will not be replaced by re-feeding.

So what does this mean for those of us trying to lose weight?

It means that:

1) Cutting calories alone will simply slow down your metabolic rate

2) Cutting calories will probably result in muscle loss, which will not be re-gained if you start to eat 'normally'

3) Aerobic exercise is unlikely to burn sufficient calories for effective weight loss and may also result in loss of muscle

4) An important key to losing weight is retaining your lean body mass - your muscle, which means resistance exercise should be included in your programme

So if you want a short, one sentence that says it all - here it is...a combination of diet, a small amount of aerobic training (at the right intensity) and a moderate amount of resistance training should result in the most weight loss without a loss of lean body mass.

Lea Woodward is a qualified personal trainer and director of activOne ltd in the UK. She provides personal training, diet & nutrition advice and massage therapy to clients in the Midlands and virtual training to clients worldwide. Check out the website http://www.activone.co.uk