My friend and I have embarked on a keep-fit and weight-loss plan: we go to the gym twice a week and do 10,000 steps daily. She's lost 8lb, all of it off her tummy, which is where I want to lose it; I've lost only 2lb, and that off my bum! What am I doing wrong?

Please don't jump to the conclusion that you're doing something wrong, because this may demotivate you. Studies show that, through healthy diet and exercise, we can change body shape by about 60%; the rest is down to genes and how they affect metabolism. So, if you're naturally an apple or pear shape, your genes will make it hard to change to another shape. An appreciation of this fact is a vital step to looking at your diet and exercise as a health investment, and not as a way to create the "perfect" body.

What's more, everyone responds differently to any training programme, due to what is called susceptibility of training. Your friend may simply be more susceptible to a quicker body shape change than you are.

Research with sets of twins following the same training regime revealed a weight-loss variation of 4-17lb. This is due to genes, and to the concentration and distribution of two hormones that either encourage fat storage or encourage it to be burned off. Your friend may have a greater concentration of the fat-releasing hormone in her tummy, while you have it in your hips and bottom.

An erratic dieting history can also slow an individual's response rate to training, but you can combat this by being as consistent with your efforts as possible. Avoid very low-calorie diets and mad, excessive exercising, because this may deter your rate of response even further.

So don't fret, embrace your body shape and curves, and your consistent efforts will reward you - the tale of the tortoise and the hare relates to how our bodies work, too.

Joanna Hall is a fitness expert (www.joannahall.com). Send your exercise questions to: Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).