The mechanistic cause - calories in, calories out
People store fat when their energy (calorie) intake, through food and drink, is higher than their energy output, or calories burned. The majority of energy used depends on the basal metabolic rate – the energy needed to keep the organs functioning. However, the metabolic rate is quite stable within individuals and can hardly be influenced. A small additional amount of energy is burned in thermogenesis – keeping the body warm, digesting food and responding to stress. The most modifiable element of energy expenditure is physical activity.
Thus, people get fatter when they start eating more or start moving less. Very small changes in energy intake and expenditure result in very large extra storages of body fat. For instance, the average increase in body weight in The Netherlands is less than 500 grams per year, caused by changes in energy intake or energy expenditure of less than 10kcal a day. As an illustration, 10 kcal is small part of a sugar cube, or a few minutes walking a day. Individuals hardly notice these changes.
Even though both are considered important, the extent to which the cause of obesity is a result of increases in energy intake through food and drink versus reduced energy expenditure remains hotly debated. Understanding the complexities of human energy balance is not an easy task. Various factors need to be considered, for example that both energy intake and expenditure may be influenced by changes in body fat stores; that people may eat more as they increase levels of physical activity and that food intake itself may affect energy expenditure. Biologically complicating the picture is the body fat tissue itself and the interaction between fat tissue and other tissues like skeletal muscle, which is responsible for most of the variability in energy expenditure. Scientists increasingly understand adipose, or fat, tissue function and have discovered it pumps out fat-promoting hormones and other proteins that affect energy intake and body fat metabolism.
