The weighty matter of genes and environment
‘There’s plenty of evidence to back up the idea that our body weight and shape is at least partly determined by our genes,’ says Prof. Jennie Brand-Miller. ‘A child born to overweight parents is much more likely to be overweight than one whose parents are not overweight. Most of this knowledge comes from twins studies. Identical twins tend to be similar in body weight even if they are raised apart. Twins adopted out as infants show the body-fat profile of their biological parents rather than their adoptive parents.’
So how much is ‘genes’ and how much ‘environment’? And how much does that matter?
University College London (UCL) researchers writing in the American Journal of Clinical Nutrition (February 2008, Volume 87, No. 2) report that genetics and heritability may account for 77% of obesity, while environmental factors make up less that 25%. ‘Although contemporary environments have made today's children fatter than children 20 years ago, the primary explanation for variations within the population, then and now, is genetic difference between individual children,' say the authors. The researchers analysed the BMI and weight circumference in a UK sample of 5,092 twin pairs (identical and sibling) aged 8 -11, born between 1994 and 1996. They found that the correlations for monozygotic (identical) twins in their study were similar in boys and girls, and greatly exceeded those of the dizygotic (sibling) twins, suggesting a strong genetic influence.
In commenting on this study, Dr David Ludwig, Associate Prof. of Pediatrics at Harvard Medical School and Director of the Optimal Weight for Life (OWL) program says: ‘One must be very cautious interpreting studies aiming to determine what proportion of a disease is genetic vs environmental. Clearly, heritable factors can affect body weight and risk for obesity. However, our genes haven’t changed much in the last half century, as obesity rates in children have tripled in the US and elsewhere.
A disease can look predominantly genetic under one set of conditions, and primarily environmental under another set. For example, among people living at high latitudes where sun exposure is limited, most cases of skin cancer may result from genetic susceptibility. However, if those people moved to the tropics, skin cancers would increase, and most cases would be directly attributable to sun exposure (an environmental factor.)
The bottom line is that we can’t change our genes, but we can change our environment, especially the home environment where we and our kids spend much of our time. “Protecting the home environment” is in fact the primary parenting practice we recommend in our 9-week family weight loss program here at Children’s Hospital Boston.’