We all know exercise promotes health, reducing most people's risk of developing diabetes and becoming obese. Now light has been shed on how it does this at a cellular level. It seems exercise may be able to drastically alter how genes operate, studies show. Genes are not static. They turn on or off depending on the biochemical signals they receive from elsewhere in the body. One powerful means of affecting gene activity involves a process called methylation, in which methyl groups, a cluster of carbon and hydrogen atoms, attach to the outside of a gene and make it easier or harder for that gene to receive and respond to messages from the body. What is particularly fascinating about the methylation process is that it seems to be driven largely by lifestyle. Diet, for instance, notably affects the methylation of genes. But the role of physical activity in this has been poorly understood. Researchers ... began by recruiting dozens of sedentary but generally healthy adult men. Using new molecular techniques, researchers mapped the methylation patterns on the DNA within [their] cells. Then, under the guidance of a trainer, the volunteers began attending hour-long spin or aerobics classes about twice a week for six months. By the end of that time, the men had ... altered the methylation pattern of many of the genes in their fat cells. More than 17,900 individual sites on 7663 separate genes in the fat cells now displayed changed methylation patterns. Other studies have found that exercise has an equally profound effect on DNA methylation within human muscle cells.