Limiting carbohydrate stores: alleviating the problem in the marathon runner, a hibernating lizard, and the neonate brain

The marathon runner is always potentially short of carbohydrate, and the best runners use more lipid and thus conserve carbohydrate. It turns out that training enables more effective use of intramuscular lipid, thus bypassing the problem of transporting lipid fuels across cell membranes. The bobtail lizard (Tiliqua rugosa) hibernates in winter and relies on "on board" fuel supplies during this period. One way that the bobtail conserves carbohydrate is by having a dearth of those tisses that rely on carbohydrate for fuel and consequently produce lactate. This strategy is reflected in the low resting lactate turnover of the bobtail, which is only 0.1% of that of a mammal. The neonate rat brain encounters a profound hypoglycaemia in the first 3 h of life. It counters this problem by using lactate for the first 1–2 h, and ketones for the next 2 h. These three situations demonstrate (i) that carbohydrate depletion as a problem is widespread in the animal kingdom, (ii) that the solutions to the problem involve many biological systems, and (iii) that solutions are more constrained within than between species.