Abstract
Alterations in nonshivering thermogenesis are presently discussed as being both potentially causative of and able to counteract obesity. However, the necessity for mammals to defend their body temperature means that the ambient temperature profoundly affects the outcome and interpretation of metabolic experiments. An adequate understanding and assessment of nonshivering thermogenesis is therefore paramount for metabolic studies. Classical nonshivering thermogenesis is facultative, i.e. it is only activated when an animal acutely requires extra heat (switched on in minutes), and adaptive, i.e. it takes weeks for an increase in capacity to develop. Nonshivering thermogenesis is fully due to brown adipose tissue activity; adaptation corresponds to the recruitment of this tissue. Diet-induced thermogenesis is probably also facultative and adaptive and due to brown adipose tissue activity. Although all mammals respond to injected/infused norepinephrine (noradrenaline) with an increase in metabolism, in non-adapted mammals this increase mainly represents the response of organs not involved in nonshivering thermogenesis; only the increase after adaptation represents nonshivering thermogenesis. Thermogenesis (metabolism) should be expressed per animal, and not per body mass [not even to any power (0.75 or 0.66)]. A 'cold tolerance test' does not examine nonshivering thermogenesis capacity; rather it tests shivering capacity and endurance. For mice, normal animal house temperatures are markedly below thermoneutrality, and the mice therefore have a metabolic rate and food consumption about 1.5 times higher than their intrinsic requirements. Housing and examining mice at normal house temperatures carries a high risk of identifying false positives for intrinsic metabolic changes; in particular, mutations/treatments that affect the animal's insulation (fur, skin) may lead to such problems. Correspondingly, true alterations in intrinsic metabolic rate remain undetected when metabolism is examined at temperatures below thermoneutrality. Thus, experiments with animals kept and examined at thermoneutrality are likely to yield an improved possibility of identifying agents and genes important for human energy balance.
Highlights
Mainly as an effect of the global obesity epidemic, the scientific interest in thermogenesis has increased dramatically in recent years
The questions asked are principally: are forms of obesity due to decreased metabolism? – and can forms of obesity be treated by an increased thermogenesis?
It is important to realize that thermogenesis is functionally linked to the maintainance of body temperature and is closely linked to ambient temperature
Summary
Mainly as an effect of the global obesity epidemic, the scientific interest in thermogenesis has increased dramatically in recent years. We will restrict the discussion to mammals – mainly mice This is because, most classical studies of thermogenesis have been performed on rats, the present ability to generate transgenic mice has directed almost all current efforts to mouse systems. There is a range of ambient temperatures within which the general metabolism of the organism, in the absence of any physical activity, generates sufficient heat as a byproduct of the continually ongoing metabolism so that its predetermined body temperature can be maintained This temperature range is known as the thermoneutral zone, and at this temperature the organism demonstrates its basal metabolic rate (Fig. 1) [for a deeper overview of thermoregulation than that presented below and for further references, see e.g. Schmidt-Nielsen (Schmidt-Nielsen, 1990)]. Based on data on wild-type mice published previously (Golozoubova et al, 2004)
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