6. Ambient temperature and diet-induced thermogenesis

One such factor could be the increasing ambient temperature in houses with the advent of central heating. The effect of a higher ambient temperature could depend on both metabolic and behavioural factors. A higher temperature could make a child more soporific, thereby reducing physical activity, or it could reduce the resting metabolic rate. There is increasing evidence in adults that heat transfer through the abdomen after a meal affects the thermogenic response to a meal. Thus, wrapping an adult's abdomen to insulate it and reduce heat transfer has an appreciable effect by increasing the hepatic vein temperature and reducing total body oxygen consumption. If a similar effect is observed in infants, then environmental temperature could affect diet-induced thermogenesis, particularly if the baby's clothing was excessive.

It has also been recognized that environmental temperature has a profound effect on resting metabolic rate in the neonate (HULL, 1966), and this mechanism is considered to depend on the activation of brown adipose tissue (BAT). In the young infant such thermogenesis should not be discounted and in one study was particularly evident when fat was infused (SWYER et al., 1978). This fat-induced thermogenesis is consistent with the known substrates for BAT metabolism; experimentally the fatty acid content of the diet can modulate BAT function (MERGER and TRAYHURN, 1984).

Whether this can occur in newborn babies is unclear. However, Fomon, in further studies using modified milk-formulae with either a high carbohydrate or fat content, found not only that energy intakes were similar on formulae containing either 57% or 29% of the energy as fat, but that so were the rates of weight gain and growth in height (FOMON et al., 1976). Thus, the overall energetics of body maintenance and growth were not discernibly different on diets where the type of fat (58% corn and 42% coconut oils) was the same in the two preparations. This implies that at this early age the control of energy metabolism is not affected by the level of dietary fat. This conclusion has to be interpreted with caution, however, because in adults under carefully controlled calorimetric conditions, eating extra fat is less thermogenic than extra carbohydrate, and obesity-prone individuals may be less responsive than lean adults (JAMES, McNEILL and RALPH, 1990). Altering the fatty acid composition of the diet also alters the fasting respiratory quotient in adults for reasons which are as yet unclear (JONES and SCHOELLER, 1988).

We may conclude that if ambient temperature does prove to be important, then there are further problems when specifying the energy needs of babies in different countries where the ambient temperatures are markedly different and where the effects of buildings, central heating and the cultural differences in the approach to infant clothing may prove important.