DOES THERMOREGULATORY FEEDING OCCUR IN NEWBORN-INFANTS - A NOVEL VIEWOF THE ROLE OF BROWN ADIPOSE-TISSUE THERMOGENESIS IN CONTROL OF FOOD-INTAKE

The physiological significance of the extensive deposits of brown adip ose tissue (BAT) in newborn human infants has been the subject of much experimentation and discussion. Because of its large thermogenic capa city, 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 ove r a rather limited range of ambient temperatures, from thermoneutralit y (32-34 degrees C) down to common ''room'' temperatures (24-28 degree s C). During such mild ''cold-exposure'', in response to a decrease in their skin temperature, their sympathetic nervous system activity inc reases, and they can more than double their resting metabolic rate, pr incipally by thermogenesis in their BAT, This review puts forward an e ntirely new role for BAT thermogenesis in the cyclic feeding pattern o f newborn infants during their first months of life. BAT thermogenesis is proposed to be an integral element in a physiological thermoregula tory feeding control mechanism in which extended periods of very gradu al cooling are interspersed with episodes of increased sympathetic ner vous system activity, increased heating via BAT thermogenesis, arousal , and feeding. The cry with which the baby attracts its mother's atten tion is an integral part of the mechanism, as is the nutritive sucklin g reflex and the behavior of the mother. Initiation of feeding is attr ibuted to a transient dip in blood glucose concentration that is due t o stimulation of glucose utilization in the BAT. Termination of feedin g is attributed to the high temperature brought about by the stimulate d BAT thermogenesis. The duration of the urge to feed extends from the time of the cry to the time of the peak rise in temperature, when fee ding stops. There is no clear circadian 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 mech anisms are consistent with the limited information on phenomena attend ing spontaneous feeding in the newborn human infant and with what is k nown about the physiological control of feeding in rats. In rats, ther moregulatory feeding is defined as a feeding episode that occurs durin g a transient but marked increase in sympathetic nervous system activi ty that has several consequences. It stimulates BAT thermogenesis and increases body temperature. It produces a transient decline in blood g lucose concentration secondary to the increased uptake of glucose by t he stimulated BAT; this signals the initiation of the feeding episode. Subsequently the high temperature induced by BAT thermogenesis signal s termination of the feeding episode. The size of the meal is determin ed by the balance between the capacity for BAT thermogenesis (heat pro duction) and ambient temperature (heat loss). BAT thermogenesis is her e viewed as an integral part of a physiological feeding control mechan ism 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 therm oregulatory feeding hypothesis supports the current practice of ''feed ing-on-demand'', i.e,, entirely in accordance with the physiological o scillations in body temperature generated by the baby, determined by i ts thermal environment, mediated by oscillations in its BAT thermogene sis, and signaled by its demand for food, Whether the hypothesis has i mplications for feeding premature infants housed in incubators, usuall y fed on schedule rather than on demand, requires investigation.