Quantitative aspects of L(+)-lactate metabolism in human beings

The production rate of endogenous L(+)-lactate in the resting human being can be calculated from results of experiments during which isotopic or unlabelled lactate is infused. The merits of the different experimental techniques are discussed, although all published methods have given similar results, namely a resting production rate of about 1.3 mol . (70 kg body wt)-1 . 24 h-1. This result is similar to that found in sheep, but lower than that in dogs and rats. L(+)-lactate production in human subjects is unchanged after fasting for 36-168 h, a finding which contrasts with some animal experiments. It is not known whether this represents a true species difference or a difference in diet. Net splanchnic uptake of L(+)-lactate in the resting human being is about 0.4 mol 24 h-1 although the normal liver has the capacity to metabolize more than this. The kidney is an important site of L(+)-lactate utilization in rats, but there is no information about its role in normal human beings. There are conflicting reports concerning the uptake of L(+)-lactate by resting skeletal muscle, but there is no evidence that resting muscle can metabolize lactate.