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Physical activity and adipose tissue metabolism

Is has become clear that adipose tissue is not just a passive depot for chemical energy in the form of triacylglycerol, which may be mobilised when needed as non-esterified fatty acids and glycerol. The tissue is also a very active endocrine organ secreting numerous cytokines that help regulate, e.g., appetite and insulin sensitivity and possibly also the metabolic rate and fuel selection in various organs as the skeletal muscle and liver. Besides, it may play a role in the activity in the immune system. However, the main function of adipose tissue is still to store excess triacylglycerol in situations when the organism is in positive energy balance, e.g., postprandially, and to mobilise triacylglycerol when the organism is in negative energy balance, e.g., during exercise. Lipid deposition in and lipid mobilisation from adipose tissue are brought about by a highly co-ordinated interplay between the activities of the hormonesensitive lipase (HSL) located intracellularly, and the lipoprotein lipase (LPL) located extracellularly bound to the endothelial cells in the capillaries. The activities of these enzymes are first of all regulated via sympathoadrenergic mechanisms and insulin. However, it has been recognised that other mechanisms also have significant roles. In the current issue of Scandinavian Journal of Medicine and Science in Sports, the role of growth hormone is discussed in the review by Lange (2004). Growth hormone is a lipolytic hormone, but it is not yet clear whether under normal circumstances it plays a role during exercise per se, while it is likely that it may play a role in the lipolytic events taking place in the postexercise recovery period, as there is a time lag of about 2 h from peak growth hormone concentration till peak lipolysis is reached in adipose tissue as described by Djurhuus et al. (2004). Interleukin-(IL-)6 secreted by exercising muscles has been proposed to play a role in the exercise-induced lipolysis in adipose tissue; however, this hypothesis remains speculative, and it is unlikely that it plays a major role as the circulating IL-6 concentration reached even under prolonged exercise only seems to have modest lipolytic effects in adipose tissue (Lyngs et al., 2002). Recently, on the other hand, it has been shown that atrial matrierretic peptide (ANP) plays a noticeable role in the control of lipolysis in adipose tissue (Galitzky et al., 2001). Thus, adipose tissue potentially may influence the metabolism in other organs by secretion of various cytokines, and, vice versa, substances secreted by, e.g., exercising skeletal muscles and the heart can influence the tissue itself. This interplay between the various organs co-ordinating the metabolism will probably be a core research area in the coming years, since it is highly likely that a breakdown of this coordination can explain many of the diseases connected with the sedentary western lifestyle. The regulation of the fuel selection during exercise in humans is still not fully understood, but generally lipid mobilisation from adipose tissue is normally enhanced beyond utilisation during exercise. The review by Lange (2004) deals with some aspects of this regulation, and some of the differences between various animal species in fuel selection are described. The main focus is on the role played by adipose blood flow in the supply of fuel derived from adipose tissue. Adipose tissue blood flow is intimately connected to the metabolism in the tissue both in situations in which lipid is deposited and in situations in which lipid is mobilised. Postprandially, adipose tissue blood flow increases concomitantly with an increased clearance of lipoprotein in the tissue, and this increase may be seen as a way to ensure an increased supply of substrate to LPL. During exercise and fasting, adipose tissue blood flow also increases, facilitating the fatty acid mobilisation from the tissue. Adipose tissue blood flow can increase about threefold both during lipid deposition and during lipid mobilisation. There are, however, significant regional differences between the blood flow levels in the various adipose tissue depots. The blood flow is twoto threefold higher in the visceral adipose tissue depots than the blood flow in the subcutaneous depots, and also, within the subcutaneous depot, significant differences exist. In the review by Lange (2004), it is proposed that the increase in adipose tissue blood flow during exercise may be one of the factors limiting fat utilisation when the exercise intensity increases. In this connection, it must be recognised that the arterial fatty acid concentration is still increased during exercise at intensities above 80% of the peak power output. It is therefore unlikely that it is the fatty acid supply to the exercising muscles that limits the fat oxidation in this Scand J Med Sci Sports 2004: 14: 72–73 COPYRIGHT & BLACKWELL MUNKSGAARD 2004 Printed in Denmark .All rights reserved DOI: 10.1111/j.1600-0838.2004.00388.x