The role of brown adipose tissue in diet-induced thermogenesis

Abstract

It is somewhat remarkable that the importance of brown fat and diet-induced thermogenesis (DIT) in energy metabolism both gained prominence at more or less the same time, after both had remained obscure and ignored for many years. The concept of DIT (in its earlier guise of Luxuskonsumption) dates back to the turn of this century, while that of brown adipose tissue (BAT) has even earlier origins, dating back over 300 years. The importance of BAT as an effector of non-shivering thermogenesis (NST) began to emerge in the 196Os, but it was not until 1978 that its full thermogenic potential was realized (Foster & Frydman, 1978). DIT received even less attention than BAT, and it is only in the last decade that it has emerged as an important adaptive component in the regulation of energy balance. However, an early intimation of a connection between DIT and brown fat came from a study (Stirling & Stock, 1968) that suggested that rats exhibiting DIT were similar in several respects to rats exhibiting cold-induced NST. One of these similarities, the increased thermogenic response to injections of noradrenaline, suggested that DIT, like NST, involved increases in sympathetic activity. It was not until over a decade later that it was proposed (Rothwell & Stock, 1979) that the increased sympathetic activity resulted in activation of BAT thermogenesis, and that this was the source of DIT. Since 1979, there have been numerous studies relating brown fat function to various dietary influences, and the role of defective BAT function in the aetiology of obesity. Developments in the area have now reached a point where there are now several novel BAT-selective adrenergic agonists being developed by the pharmaceutical industry for the treatment of obesity (Arch, 1989; Holloway, 1989). The widespread interest and rapid developments in this area are both gratifying and reassuring to the relatively few groups that actively contributed to the early research on thermogenesis, BAT and obesity. However, the importance of DIT and the contribution of brown fat to DIT have been highly controversial topics (e.g., Hervey & Tobin, 1983; Cox & Lorden, 1986; Ma et al. 1987; Maxwell et al. 1988). This is not entirely unexpected since most new ideas tend to generate controversy, and it is important constantly to assess developments and fresh experimental findings as they arise. Thus, the object of the present presentation w i l l be to consider to what extent the original proposal (Rothwell & Stock, 1979) concerning the role of brown fat in DIT is still justified. Before considering the evidence in detail, it should be noted that there are two major difficulties that tend to frustrate a clear interpretation of results from many experimental studies. The first is that, due to both conceptual and technical difficulties, it is not an easy task to assess levels of DIT. The other problem is that the quantitative assessment of BAT thermogenesis in vivo is also very difficult and demanding, and relies heavily on measurements on the most accessible brown fat depots. Furthermore, these measure-ments are usually made under conditions that do not equate to the animal's normal physiological status, where the level of DIT may be quite different from that at the time the BAT assessment is made. Evidence for DZT DIT strictly refers to those adaptive changes in heat production that play a regulatory role in maintaining energy balance and body composition. It differs from what is