At first glance, the cause of obesity seems simple: If a person eats more calories than he or she expends, the body stores the excess calories as fat. As fat accumulates, body weight climbs. According to this view, a person’s genes are irrelevant to his or her body weight. In reality, however, obesity reflects the combined action of genes and the environment.
Several genes are associated with obesity. One example is the gene that encodes leptin, a hormone that helps curb appetite. Individuals who inherit mutant alleles for this gene never feel full, leading to overeating and obesity.
The environment can also influence the expression of the genes that a person inherits. For example, scientists have found that mothers who ingest low amounts of carbohydrates—sugars and starches—give birth to children who are especially likely to become obese later in life. Evidence suggests that epigenetic modifications occur while a developing child is still in the womb, permanently altering gene expression patterns for life.
Fetuses presumably use the mother’s diet to “prepare for” the environment that they will be born into. These changes occur before birth, giving a newborn the best chance for survival. But if a fetus prepared for a low-calorie life is born into an environment where food is actually plentiful, then obesity is likely.