A study of genetically diverse flies may help explain why some people can eat a steady diet of junk food and stay skinny, while others quickly gain weight. Researchers put 146 genetically distinct strains of fruit flies on each of four different diets–a nutritionally balanced diet, a low calorie diet, and high fat or high sugar diets–and then measured body weight and other metabolic traits.

According to the findings, published last month in the journal Genetics, some flies were highly sensitive to the different diets, while others maintained the same weight regardless of what they were fed. Diet alone contributed very little to variability in weight. Instead, genetics and the interaction between genetics and diet played a major role.

The findings could give you an easy excuse for failed diet attempts. According to a statement from the Genetics Society of America, “This study strongly suggests that some individuals can achieve benefits from altering their dietary habits, while the same changes for others will have virtually no effect.”