Fat

Our fat stores are usually larger by one to two orders of magnitude. This is not surprising. We looked in Chapter 1 (Section 1.2.2.2) at the considerable advantage, in weight terms, of storing excess energy in the form of hydrophobic triacylglycerol molecules, in the lipid droplets characteristic of adipocytes. A typical figure for body fat content is about 15-30% of body weight. (This figure is higher on average in women than in men.) Thus, a typical fat store is of the order of 10-20 kg. The energy content of fat is around 37 kJ/g, so we store the equivalent of around 500 MJ in the form of fat. A typical daily energy expenditure (to be discussed further in Chapter 11) is around 10 MJ, so we store sufficient energy for about 50 days of life; in fact more, since, as we shall see, one of the prominent aspects of the metabolic adaptation to starvation is that daily metabolic rate (energy expenditure) is reduced. This accords well with recorded times for survival of starvation victims of up to 60 days for initially normal people. A few obese people have starved, voluntarily, for therapeutic reasons for considerably longer periods. (They were closely monitored medically, and given necessary vitamin and mineral supplements.)

However, storage of most of our energy reserves in the form of fat poses a biochemical problem. As we have seen, some tissues and organs require glucose, and cannot oxidise fatty acids. Fatty acids cannot be converted to glucose in mammals because acetyl-CoA formed from fatty acids is oxidised completely to CO2 in the tricarboxylic acid cycle and is therefore unable to contribute to the gluconeogenic pathway. Only the glycerol component of triacylglycerol can form glucose, and this is a minor component in terms of numbers of carbon atoms. As we shall see, one 'strategy' adopted by the body during starvation is an increased conversion of fatty acids into water-soluble intermediates, the ketone bodies (see Fig. 4.5), which can be used by tissues normally requiring glucose, particularly the brain.

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