Exercise and Not Genetics Is Major Determinant of Weight, Study Finds

May 16, 1996

By Jeffery Kahn, JBKahn@LBL.gov


You can out-run your genes for weight but not your genes for good cholesterol. That's the conclusion reached as a result of a study of identical twins, research that is part of the ongoing National Runners' Health Study.

The Life Sciences Division's Paul Williams, who heads the study, says that conventional wisdom holds that weight and height have about the same degree of inheritance. That was not true in this case.

Said Williams, "Exercise appears to be able to nullify a genetic tendency toward being overweight. It also elevates the level of good cholesterol but not enough to eliminate its genetic determination."

The study was funded by the National Institutes of Health and involved 35 pairs of identical twins identified after they read about the study in Runner's World magazine. Each twin pair consisted of a runner and a more sedentary sibling. The runner ran an average of 39 miles per week while the sedentary twin averaged less than five miles per week.

Much to their surprise, researchers found that the twins' weights were virtually unrelated. Rather than genetics, exercise appeared to be the determining factor of each runner's relative weight.

Researchers classified each of the 25 male and 10 female twin pairs according to their body mass index (BMI), a measure of weight to height. BMI is computed by dividing weight in kilograms by the square of height in meters. For instance, a 5-foot, 5-inch tall woman (1.65 meters) who weighs 150 pounds (68 kilograms) has a BMI of 25. According to the 1995 Dietary Guidelines for Americans, individuals with a BMI over 25 are over-weight.

Based upon their BMI, 13 of the sedentary twins were overweight compared to only two of the runners. None of the runners with an overweight sedentary twin were themselves overweight.

Williams said researchers decided to go a step further, assessing the similarity of the twins' weights. Height, which is strongly inherited, was strongly correlated in the twins whereas weight was not. Height had a correlation of 0.9 whereas (because of weight differences), the twins' BMI had a correlation of only 0.2.

The twins also were measured for high-density lipoprotein (HDL) cholesterol, the so-called good cholesterol. Men and women who have high levels of HDL are generally at less risk of having a heart attack. The recommend approaches to raising HDL levels are exercise and low body weight.

Berkeley Lab researchers found that the running twin had higher levels of good cholesterol -- on average, ten percent more -- than their sedentary twin. However, despite substantial differences in exercise and weight, the twins' HDL levels were surprisingly similar (a correlation of 0.7).

Said Williams, "These results underscore the importance of the genetic determinants of the good cholesterol. That's not to discount the importance of promoting vigorous exercise or weight loss. After all, we can exercise more and lose weight, but there's not much we can do about our genetic make-up."

Williams strongly endorses vigorous exercise for weight reduction and reducing heart disease risk and says that further research involving twins is required. Twins interested in finding out if they qualify can contact the study team at 510-486-5991.

Williams' co-authors are Berkeley Lab's Davina Moussa, Elizabeth Noceti, Patricia Blanche, Richard Terry, Yan Huang, and Ronald Krauss.