Diurnal changes in serum triglycerides as related to changes in lipolytic enzymes, (apo) lipoproteins and hormones in normal subjects on a carbohydrate-rich diet

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Abstract
Normal subjects in steady state on a carbohydrate-rich diet (3 equivalent meals a day at 9.00, 13.00 and 17.00 h), show a wave-like serum triglyceride (TG) pattern with a peak at 14.00 h. Post-heparin lipoprotein lipase (LPL) activity increased from a mean value of 49 mU/ml .+-. 13 (SD) in the fasting state to 127 mU/ml .+-. 18 in the fed state (P < 0.005). This was due to an increase in adipose tissue LPL activity which, at 16.30 and 21.30 h, was significantly higher than basal levels (128.3 .+-. 81.5 and 87.7 .+-. 23.2 vs. 43.3 .+-. 9.3 mU/g, P < 0.05 and P < 0.01, respectively). Skeletal muscle LPL activity was low (5.8 mU/g .+-. 2.3, mean .+-. SD) and showed no diurnal change. The observed changes in TG-hydrolyzing capacity in the course of the day might explain the TG-pattern. High density lipoprotein (HDL) subfractions HDL2 and HDL3, were separated by density gradient ultracentrifugation and had mean hydrated densities of 1.088 and 1.135 g/ml, respectively. While HDL2 showed no diurnal change, HDL3-cholesterol and -phospholipid significantly increased during the day (P < 0.005 and P < 0.001, respectively), reaching their highest levels in the evening. Since the rise in HDL3-lipids follows the fall in serum TG, this provides further indication that the metabolism of these fractions is mutually related.