Genetic heritage, more than environment, appears to strongly dictate an individual's risk of growing up fat or thin, researchers reported in two studies in today's New England Journal of Medicine.
The studies support previous research indicating that genetic makeup is highly influential in determining how the body handles unneeded calories and distributes body fat, while the effect of childhood diet and activity levels on adult obesity may not be as great as had been believed.
Researchers cautioned, however, that the studies do not mean that genetically predispositioned people will automatically become fat or that they cannot lose excess weight. The extent of environmental influence on obesity is still unknown, experts said, advising that it remains prudent for anyone age 2 and older to limit dietary fat intake.
"Destiny, in the form of our genetic makeup, has a great deal to do with the shape of our middles," said Dr. Ethan A. H. Sims of the University of Vermont in an editorial accompanying the studies. Sims has authored previous studies on genetic influence on body fat.
The research reveals that depending on genetic fate, some individuals have an easier task than others in controlling their weight. The studies suggest, among other things, that for certain individuals there might be limits to the benefits of weight-loss programs.
In the first study, researchers at Laval University in Quebec City, Canada, found large differences in weight gain and fat distribution among sets of twins, ages 19 to 27, who were fed a regular diet plus 1,000 extra calories a day for 84 days.
In the second study, comparing twins who were raised together with those reared apart, a group of researchers in the United States and Sweden found that genetic factors may account for as much as 70% of the differences between individuals in weight.
The Laval University study takes a major step in resolving a long-simmering disagreement among researchers over differences in how people handle excess food intake, Sims said.
Twelve pairs of identical, male twins who were not overweight and were in good health were housed in a dormitory under constant supervision. The twins were fed identical diets plus 1,000 extra calories for 84 out of 100 days.
The study showed that, despite similar diets and sedentary routines, weight gains varied markedly from 9 1/2 pounds to 29 pounds.
"Some (research) suggests that some people can eat like a horse and not actually gain weight. A lot of people say that's ridiculous," Sims said. "But in our long-term study 20 years ago we found unbelievable variation in the ability to gain weight. (The Laval group) has found the same thing. The range was tremendous."
The Laval study showed that weight gain within a pair of identical brothers was similar, pointing to a strong genetic component. Even more striking, changes in fat distribution--especially deep abdominal fat--were very similar within twin brothers but varied among the entire group.
A genetic linkage to the sites of fat deposit is important because the National Institutes of Health have identified total body fat, along with distribution of fat at the trunk or abdomen, as important determinants of health.
"For a given amount of fat gain there are people predisposed to store more fat in the abdominal region," said Angelo Tremblay of the Laval University research group. Twins "will have the same distribution of fat. This is where we found the strongest genetic effect."
Studies have shown that people who store fat deep in the abdomen under abdominal muscle, as opposed to in the body fat that is visible, are at great risk for a multitude of health problems, such as diabetes and hypertension, Tremblay said.
"When you have this big belly, like we generally see in obese males, you are more at risk to exhibit the metabolic complications of obesity," he said.
Questions remain about the extent of the contribution of genetics to obesity, Tremblay said. But the second study, by researchers from the University of Pennsylvania, the Karolinksa Institute in Stockholm, Sweden, and Pennsylvania State University, indicated that childhood home environments had no significant influence on whether an individual grew up slim or obese.
The study followed hundreds of pairs of identical and fraternal twins, some reared together and some apart. Researchers found that identical twins reared apart were as similar in weight as those reared together. Fraternal twins, who do not share identical genes, were less likely to be of similar weight.
"The early family environment in which they were raised didn't contribute to the similarity of the twins in adult life," said Dr. Albert J. Stunkard, of the University of Pennsylvania, who directed the study.
"We've known for a long time that if you have two fat parents their child will be more likely to be fat. But we haven't had any explanation for it. You could say it could be home environment, that the child was taught poor eating habits. But now you can say it might be due to genetic factors."
But while the identical twins in the study were of similar weight they were not perfectly matched, Stunkard said.
"The fact that they weren't identical in weight means that environmental differences must have played a role," he said. "Although the genetic influences were strong, they were not fixed at birth. The genes are not omnipotent."
Sims added: "You're not looking at something that is inherited the way the color of your eyes is. It can vary a lot on the basis of the environment. It's not that you can't do anything about it."
The studies have important implications for individuals with a family history of obesity, Sims said.
"People differ, and they ought to ask themselves how they differ," he said. "I think if they have a good look at their mother and father they'll get a good idea. If your old man had an early heart attack and you are large around the waist and you're 18 years old, you'd better bear (the health risks) in mind."
The findings could also influence how experts approach patients in weight-loss programs, Tremblay said. For example, some studies have shown that people with genetic susceptibility to obesity might achieve less weight loss during treatment.
"It is not without reason to suggest that we reassess our objectives of obesity treatment," he said. "Maybe we have to be satisfied with a certain level of success in weight loss."
Weight-loss programs should address a possible genetic influence, said Dr. Jack Rubin, associate director of the Health Development Institute at Century City Hospital, which offers the medically supervised diet, Optifast.
"What we teach these (genetically predispositioned) people is that it's going to be a constant struggle," Rubin said. "That's why we tell our patients obesity is a disease. We can treat it, but we can't cure it."
Conversely, slender people from families with few weight problems may not need to worry about extra calories, Stunkard said.
"One of the possibly bad effects of the attention on obesity is that some people who don't even have the genetic potential become so afraid of being obese that they restrict their diet in ways that might be harmful--anorexia nervosa and bulimia are at the far ends of that spectrum," Stunkard said. "This might take off some of the pressure."