Concept

Genetics Hormones

HE A LTH EDUC A TION H E A L T H 1 0 0 V E R S I O N 3 / Factors Contributing to Overweight and Obesity

"Obesity is a complex multifactorial chronic disease developing from interactive influences of numerous factors—social, behavioral, physiological, metabolic, cellular, and molecular. Genetic influences are difficult to explain and identification of the genes is not easily achieved through studies. Furthermore, whatever the influence the genotype has on the etiology of obesity, it is generally attenuated or exacerbated by nongenetic factors. A large number of twin, adoption, and family studies have explored the level of heritability of obesity; that is, the fraction of the population variation in a trait, such as BMI, that can be explained by genetic transmission. Recent studies of individuals with a wide range of BMIs, together with information obtained on their parents, siblings, and spouses, suggest that about 25 to 40 percent of the individual differences in body mass or body fat may depend on genetic factors. However, studies with identical twins reared apart suggest that the genetic contribution to BMI may be higher, about 70 percent. From the research currently available, several genes seem to have the capacity to cause obesity or to increase the likelihood of becoming obese. Leptin and ghrelin are two hormones that have been recognized to have a major influence on energy balance. Leptin is a mediator of long-term regulation of energy balance, suppressing food intake and thereby inducing weight loss. Ghrelin, on the other hand, is a fast-acting hormone, seemingly playing a role in meal initiation. As a growing number of people suffer from obesity, understanding the mechanisms by which various hormones and neurotransmitters influence energy balance has been a subject of intensive research. In obese subjects, the circulating level of the anorexigenic hormone leptin is increased, whereas, surprisingly, the level of the orexigenic hormone ghrelin is decreased. It is now established that obese patients are leptin-resistant. However, the manner in which both the leptin and ghrelin systems contribute to the development or maintenance of obesity is as of yet not clear."

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