The development of obesity in animals: the role of dietary factors.

Obesity in laboratory rodents can be difficult to define, but increases in body fat content above 'normal' can be achieved by surgical or chemical lesions in the ventromedial hypothalamus, presentation of highly palatable diets (e.g. high-fat, high-sucrose or varied diets composed of human food items), or by forced-feeding. The responses to these treatments vary remarkably, depending on factors such as age, sex, early nutritional experience, genetic background, diet composition, feeding frequency and time of day or, even, of year. Many young animals can show marked increases in voluntary energy intake, but avoid obesity by simultaneously raising metabolic rate (diet-induced thermogenesis), while others show spontaneous obesity, either as they age, or at certain times of the year. It now seems that variations in both energy intake and expenditure are involved in the regulation of energy balance, and this has made the interpretation of experimental results more complicated. However, recognition of this dual control has helped to resolve many of the thermodynamic and metabolic anomalies in the literature. It is considered equally valid to adopt the same approach when describing human energy balance regulation.