A second study in the same journal also used data from this population to develop a genetic risk score for obesity. It can help predict, as early as childhood, who is at high risk for a lifetime of obesity and who is not.

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Together, the studies confirm a truth that researchers wish more people understood. There are biological reasons that some struggle mightily with their weight and others do not, and the biological impacts often are seen on appetite, not metabolism. People who gain too much weight or fight to stay thin feel hungrier than naturally thin people.

The study of the appetite-dulling mutation was led by Dr. Sadaf Farooqi, professor of metabolism and medicine at the University of Cambridge, and Nick Wareham, an epidemiologist at the university.

The study drew on Dr. Farooqi’s research into a gene, MC4R. She has probed it for 20 years, but for the opposite reason: to understand why some people are overweight, not why some are thin.

People with MC4R mutations tend to be obese. Researchers have recorded as many as 300 mutations in this gene, and they are the most common single-gene cause of obesity. Mutations in the gene account for 6 percent of children with severe obesity.