Does Thermoregulatory Feeding Occur in Newborn Infants? A Novel View of the Role of Brown Adipose Tissue Thermo genesis in Control of Food Intake

Abstract

The physiological significance of the extensive deposits of brown adipose tissue (BAT) in newborn human infants has been the subject of much experimentation and discussion. Because of its large thermogenic capacity, its function has usually been viewed as preparing the infant for producing heat in response to cold exposure at birth. Newborn infants are indeed capable of precise thermoregulation for a limited time over a rather limited range of ambient temperatures, from thermoneutrality (32–34°C) down to common “room” temperatures (24–28°C). During such mild “cold‐exposure”, in response to a decrease in their skin temperature, their sympathetic nervous system activity increases, and they can more than double their resting metabolic rate, principally by thermo genesis in their BAT. This review puts forward an entirely new role for BAT thermo genesis in the cyclic feeding pattern of newborn infants during their first months of life. BAT thermo genesis is proposed to be an integral element in a physiological thermoregulatory feeding control mechanism in which extended periods of very gradual cooling are interspersed with episodes of increased sympathetic nervous system activity, increased heating via BAT thermo genesis, arousal, and feeding. The cry with which the baby attracts its mother's attention is an integral part of the mechanism, as is the nutritive suckling reflex and the behavior of the mother. Initiation of feeding is attributed to a transient dip in blood glucose concentration that is due to stimulation of glucose utilization in the BAT. Termination of feeding is attributed to the high temperature brought about by the stimulated BAT thermo genesis. The duration of the urge to feed extends from the time of the cry to the time of the peak rise in temperature, when feeding stops. There is no clear Orcadian rhythm in core temperature in newborn infants, and meals occur at fairly frequent intervals both day and night in infants that are fed on demand. These physiological mechanisms are consistent with the limited information on phenomena attending spontaneous feeding in the newborn human infant and with what is known about the physiological control of feeding in rats. In rats, thermoregulatory feeding is defined as a feeding episode that occurs during a transient but marked increase in sympathetic nervous system activity that has several consequences. It stimulates BAT thermo genesis and increases body temperature. It produces a transient decline in blood glucose concentration secondary to the increased uptake of glucose by the stimulated BAT; this signals the initiation of the feeding episode. Subsequently the high temperature induced by BAT thermo genesis signals termination of the feeding episode. The size of the meal is determined by the balance between the capacity for BAT thermo genesis (heat production) and ambient temperature (heat loss). BAT thermo genesis is here viewed as an integral part of a physiological feeding control mechanism that links thermal balance with energy balance. The phenomenon is referred to as thermoregulatory feeding to distinguish it from feeding originating from other causes. As applied to human infants, the thermoregulatory feeding hypothesis supports the current practice of “feeding‐on‐demand”, i.e., entirely in accordance with the physiological oscillations in body temperature generated by the baby, determined by its thermal environment, mediated by oscillations in its BAT thermo genesis, and signaled by its demand for food. Whether the hypothesis has implications for feeding premature infants housed in incubators, usually fed on schedule rather than on demand, requires investigation. 1995 North American Association for the Study of Obesity (NAASO)