Many obese persons who have failed in their attempts to lose weight by dieting and exercising tend to blame it all on their genes. Indeed, heredity plays a big part in any tendency to overweight. However, the current epidemic cannot be blamed entirely on changes in the genetic makeup of the population. A recent study has investigated this. Pairs of women twins were studied, to see what effect differences in physical activity had on their amounts of body fat. About half the pairs were identical (monozygotic) twins. Because identical twins share 100% of their genes, any differences in body fat between one twin and her sister must be due to non-genetic, or environmental, factors. Environmental factors include, for instance, diet, exercise, smoking, hormone use, socio-economic class, and so on.

Over 940 female twins in the United Kingdom were enrolled; they included 241 sets of identical (monozygotic) twins. Body weight, height, and fat (total-body fat and central abdominal fat) were measured. Physical activity was assessed by replies to questionnaires about activity in general, and sport and exercise programs in particular. Daily energy (carbohydrate) and fat intake was also assessed by a questionnaire on food habits, and there were also questions about smoking, hormone replacement treatment and the subject's socio-economic situation.

The average age of the twins was 56, and over half of them were normal weight; 30% were overweight, and 7% were classified as obese.

Analysis of all the twins (identical and non-identical) showed that physical activity was associated with less total-body and central abdominal fat - in those who reported that they did vigorous weight-bearing exercises, total-body fat was on average 5½ kilograms (12 lb) less, and central abdominal fat almost ½ kilogram (1 lb) less than in the others. After correcting for possible effects of age, diet, smoking, hormone replacement treatment and socio-economic factors, physical activity was the most important environmental factor associated with less fat.

When just the identical twin pairs were analyzed, and corrections made for smoking and hormone treatment status, the twin that exercised more (doing moderate-intensity sports or exercises) had, on average, one kilogram less total-body fat. If one twin in a set was overweight, higher levels of physical activity were still associated with almost 4 kilograms (8½ lb) less total-body fat and ½ kilogram (1 lb) less central abdominal fat.

The study is the second done by this group of investigators. In the first, they showed that, in identical twins, there was no connection between the energy (carbohydrate) and fat intake and the amount of body fat. Taken together, the two studies show that physical activity is the most important factor in determining fat amount in healthy women. In addition, they show that having a strong family history of obesity still doesn't stop women from reducing their body fat by physical activity. Exercise - following a well-planned and supervised program - can indeed lessen body fat in women who have a hereditary tendency to overweight.

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Source

Genetic and environmental influences on total-body and central abdominal fat: the effect of physical activity in female twins.
K. Samaras, PJ. Kelly, MN. Chiano, Ann Intern Med, 1999, vol. 130, pp. 873--882