Effects of short-term dietary glucose and fructose on rat serum triglyceride concentration

The effect of dietary fructose on serum triglyceride (TG) concentration was studied in rats. First, after only 3 days the mean serum TG level in rats fed fructose was higher (124±13 mg/100 ml; N = 8) than in rats fed glucose (84±6; N = 7) or starch (85±7); N = 8). Second, because rats are nocturnal feeders we measured the serum TG and insulin concentrations throughout a 24-hour period. The rats fed fructose had large diurnal fluctuations in TG; the highest concentrations occurred at night. The TG levels in rats fed fructose were greater than those in rats fed glucose at all times during the 24-hour period except at 1700 hours. Serum insulin concentrations were the same in rats fed fructose or glucose. Thus, the effect of fructose on serum TG was not mediated by a change in serum insulin. Third, we measured the effect of fructose on the ratio of TG (mg) to protein (mg) in the chylomicron and very low density lipoprotein serum fraction. This ratio was slightly lower in the rats fed fructose (17±0.5; N = 9) than in rats fed glucose (22±2.2; N = 7). The excess TG in the serum of rats fed fructose was not disproportionately composed of large lipoproteins. Fourth, we determined if the effect of dietary fructose could be maintained when the release of TG from the liver was prevented by dietary orotic acid. The serum TG concentrations were the same in rats fed orotic acid with either glucose (44±5; N = 8) or fructose (46±6; N = 8). Thus, the TG released from the liver was necessary for dietary fructose to increase serum TG.