Considerable attention is currently being paid to the secular changes in food intake and physical
activity that underlie the increase in the prevalence of obesity that is apparent inmany societies. While
this is laudable it would be unwise to view these environmental factors in isolation from the biological
factors that normally control body weight and composition and the compelling evidence that interindividual
differences in susceptibility to obesity have strong genetic determinants. This is
particularly important, as it is only in the past decade that we have begun to obtain substantive
information regarding the molecular constituents of pathways controlling mammalian energy balance
and therefore, for the first time, are in a position to achieve a better mechanistic understanding of this
disease. Population-based association and linkage studies have highlighted a number of loci at which
genetic variation is associated with obesity and related phenotypes and the identification and
characterization of monogenic obesity syndromes has been particularly fruitful. While there is
widespread acceptance that hereditary factors might predispose to human obesity, it is frequently
assumed that such factors would influence metabolic rate or the selective partitioning of excess
calories into fat. However, it is notable that, thus far, all monogenic defects causing human obesity
actually disrupt hypothalamic pathways and have a profound effect on satiety and food intake. To
conclude, the evidence we have to date suggests that the major impact of genes on human obesity is
just as likely (or perhaps more likely) to directly impact on hunger, satiety and food intake rather than
metabolic rate or nutrient partitioning. At the risk of oversimplification, it seems that from an
aetiological/genetic standpoint, human obesity appears less a metabolic than a neuro-behavioural
disease.