| Summary: | Dietary n-3 fatty acids (FAs) found in fish oils markedly lower plasma triglyceride (TG) and very low density lipoprotein (VLDL) levels in both normal and hypertriglyceridemic subjects. The present study examined the mechanism of this effect. Ten subjects with widely different plasma triglyceride levels (82 to 1002 mg/dl) were fed metabolically controlled diets containing 20% fat. The control diet contained a blend of cocoa butter and peanut oil (P/S = 0.8). The test diet contained fish oil (P/S = 1.1) and provided 10-17 g of n-3 FAs per day (depending on calorie intake). After 3 to 5 weeks of each diet, the kinetics of VLDL-TG were determined over a 48-h period after the injection of [3H]glycerol. The fish oil diet reduced the VLDL-TG synthetic rate from 23 +/- 14.3 (mean +/- SD) to 12.6 +/- 7.5 mg/h per kg ideal weight (P less than 0.005) and increased the fractional catabolic rate (FCR) for VLDL-TG from 0.23 +/- 0.12 to 0.38 +/- 0.16 h -1 (P less than 0.005). At the same time, there was a 66% reduction of plasma triglyceride levels, resulting largely from a 78% decrease in VLDL-TG levels (398 +/- 317 to 87 +/- 77 mg/dl; P less than 0.005). There was a strong correlation (r = 0.83; P less than 0.01) between the change in synthetic rates and pool sizes, but there was no correlation (r = 0.24; NS) between changes in FCRs and pool sizes. The VLDL cholesterol: triglyceride ratio increased during the n-3 diet suggesting that smaller VLDL particles were present. These particles would be expected to leave the VLDL fraction more rapidly than larger particles producing a higher FCR. We conclude that the hypotriglyceridemic effect of fish oil appears to be caused primarily by an inhibition of very low density lipoprotein-triglyceride synthesis, but an additional, independent effect upon VLDL catabolism cannot be ruled out.
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