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Fig. 6.9 Plasma non-esterified fatty acid (NEFA) concentrations after an overnight fast and following a meal. The meal was the same as described in Fig. 6.4. The plasma insulin concentration (expressed in nmol/l) is shown as a dotted line. Mean values for eight normal subjects are shown; data taken from Frayn et al. (1993).

Fig. 6.9 Plasma non-esterified fatty acid (NEFA) concentrations after an overnight fast and following a meal. The meal was the same as described in Fig. 6.4. The plasma insulin concentration (expressed in nmol/l) is shown as a dotted line. Mean values for eight normal subjects are shown; data taken from Frayn et al. (1993).

small intestine; see Section 3.3.3). The total concentration of triacylglycerol in plasma varies widely between different people (even apparently quite healthy people), depending greatly upon fitness, body build and genetic influences - but a typical figure after an overnight fast is around 1 mmol/l. It should be borne in mind that, since each triacylglycerol molecule contains three fatty acids, this is equivalent in terms of energy delivery to a concentration of 3 mmol/l of non-esterified fatty acids. For now, we shall just consider the triacylglycerol in chylomicron particles. The concentration of chylomicron-triacylglycerol also varies widely between people, but it is close to zero in the overnight-fasted state, and rises after meals to (typically) 0.4-0.6 mmol/l. (This figure will depend, of course, on the amount of fat in the meal.)

6.2.3 The postabsorptive state

After an overnight fast, the concentration of non-esterified fatty acids in plasma is around 0.5 mmol/l, and the total triacylglycerol concentration (variable between people) around 1 mmol/l. The chylomicron-triacylglycerol concentration will be close to zero - usually less than 0.05 mmol/l.

Note that the lipid fuels (non-esterified fatty acids and triacylglycerol) circulate, for the most part, in lower concentrations than glucose (whose concentration is around 5 mmol/l in this state). But it is interesting to think in terms of 'energy yield'. Some calculations are given in Box 6.1. The box shows that lipid fuels are potentially a more important source of energy than might appear from their concentrations, and that non-esterified fatty acids constitute an important energy source in the postabsorptive state.

The turnover of non-esterified fatty acids in the postabsorptive state involves their liberation from adipose tissue and their uptake by a number of tis-

Box 6.1 Glucose and lipids as energy sources

Glucose and lipid fuels in the plasma are compared in terms of their potential yield of energy in the postabsorptive state. First, we will use typical concentrations in the plasma (given in the text) and look at the potential yield of energy per litre of plasma.

Box 6.1 Glucose and lipids as energy sources

Glucose and lipid fuels in the plasma are compared in terms of their potential yield of energy in the postabsorptive state. First, we will use typical concentrations in the plasma (given in the text) and look at the potential yield of energy per litre of plasma.

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