Millions of people in the United States alone have submitted their DNA for analysis and received information that not only predicts their risk for disease but, it turns out, in some cases might also have influenced that risk, according to a recent study by researchers at Stanford University.

The team, led by Alia Crum, assistant professor of psychology, found that when people were told of a genetic propensity for either obesity or lower exercise capacity, it altered the way their bodies responded either to a meal or to exercise. The work was published Dec. 10 in Nature Human Behavior .

“Receiving genetic information doesn’t just make you more informed,” Crum said. “What this study shows is that it can also have a physiological impact on your body in a way that actually changes your overall risk profile.”

Crum and the study’s lead author, graduate student Bradley Turnwald, said that the results don’t suggest that DNA testing is bad or good, just that when delivering information, genetic counselors or personalized genetic testing companies need to be aware that the mere knowledge of the test result could influence a person’s risk.

A brief deceit

To carry out the research, the group first took DNA samples from people who were told they were participating in a study about the relationship between DNA and diet. Later, the participants returned and 116 of them carried out an exercise test, while 107 of them ate a meal. After the meal, the researchers measured levels of molecules in the blood that indicate hunger or fullness.

Unbeknownst to the participants, Crum and Turnwald had tested the participants for one of two genes – one that has been associated with obesity and one associated with exercise capacity. During that first round of tests, the researchers could see small differences in either exercise capacity or satisfaction after the meal, depending on which version of the gene the people carried. People with the protective version of the exercise gene did have slightly better exercise capacity, for example.

A week later, when participants returned for their second test, they were given a genetic result that might or might not have been true. Some of those with genes that protect them from obesity or gave them higher exercise capacity were told they had a higher risk version of the gene, and vice versa.

People were also given reading material that helped explain the effects of having a particular form of the two genes. In the obesity group, participants read research summaries and lay research articles suggesting that one version of the gene made them produce less of a hormone that relays an “I’m full” signal to the brain. In the exercise group, participants learned that people with a particular gene variant wouldn’t perform as well during exercise.