I am 28, a mother of two and weigh 20kg more than I should. Obesity runs in my family, especially on my mother's side. Is it possible for me to achieve normal weight?

Yes, you can achieve normal weight, but you are going to have to work much harder than some people because of your genes. If you have two obese parents, your risk of being obese as an adult is 80% compared with 40% if only one parent is obese, or 14% if neither parent is obese. The extra 20kg you are carrying came by eating more calories than you were expending over several years. Your genetic background, however, meant those extra calories were chanelled into fat storage instead of being burnt up as heat through increased metabolism - a genetic trait enjoyed by some lean individuals. So genetic factors are real, but personal lifestyle choices do have a major role to play and for most people are more powerful than heredity at explaining shape. To maintain a significant weight loss and stay lean, an individual prone to obesity will need to do 60- 90 minutes of moderate physical activity, equivalent to brisk walking, a day. Although 30 minutes of vigorous exercise several days a week is sufficient for aerobic fitness, leanness comes at a higher cost. The upside of this is you'll experience a lower risk of heart disease, cancer, depression and diabetes than others who work little for their leanness, plus you'll give your children a vital health message. The bottom line is you can achieve normal weight no matter what your genetic background; just be prepared to put in the work.

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