Genetics studies show that some people respond differently to exercise than others. More than 800 genes affect exercise performance. Studies by noted geneticist Claude Bouchard showed that between eight and 13 percent of people experience negative physical changes from exercise, such as increases in blood insulin, triglycerides and systolic blood pressure and decreases in HDL cholesterol (the good cholesterol). Eric Leifer from the National Heart, Lung, and Blood Institute in Bethesda, Maryland noted that Bouchard et al. did not make comparisons in people who did not exercise, so their studies did not have adequate control groups. In an examination of nearly 1,200 people and a control group, they found no evidence of subgroups of people who responded negatively to exercise. Exercise is healthy for most people, but all people do not respond the same.