Effects of increasing dietary polyunsaturated fatty acids within the guidelines of the AHA step 1 diet on plasma lipid and lipoprotein levels in normal males

Abstract

We attempted to ascertain the effects of polyunsaturated fatty acids by conducting two studies in normal young men, in which monounsaturated fats were replaced by polyunsaturated fats within the guidelines of the American Heart Association step 1 diet. Study A employed a randomized parallel design in which subjects first consumed an average American diet (AAD) containing 37% of calories as fat (saturated fat, 16% calories; monounsaturated fat, 14% calories; and polyunsaturated fat, 7% calories). After 3 weeks, one third of the subjects continued with the AAD, one third switched to a step 1 diet in which total fat calories were reduced to 30% by replacing saturated fat with carbohydrate, and one third switched to a polyunsaturated fat-enriched (Poly) diet with the same 30% fat calories and a reduction of monounsaturated fat from 14% to 8% and an increase of polyunsaturated fat from 7% to 13% of calories. The randomized period lasted 6 weeks. Total and low-density lipoprotein (LDL) cholesterol levels on the step 1 and Poly diets were reduced compared with levels on the AAD (P < .001). Total and LDL cholesterol did not differ between the step 1 and Poly diets, although comparison between the two diets is limited by the small study groups. Serum apolipoprotein (apo) B levels fell on the Poly diet compared with the AAD. Total high-density lipoprotein (HDL), HDL2, and HDL3 cholesterol levels were not significantly affected by the diets. Postprandial lipid and lipoprotein concentrations did not significantly differ either. In study B, a randomized crossover design was used in which all subjects ate the step 1 and Poly diets for 5 weeks each with a 4-day break between diets. In the eight subjects studied, the values for fasting plasma total, LDL, and HDL cholesterol; triglycerides; apoB; and apoA-I were essentially identical at the end of each diet period. Postprandial triglyceride areas obtained after ingestion of a large, standard fat load were also the same. Finally, LDL apoB and HDL apoA-I turnovers were unaffected by replacement of monounsaturates with polyunsaturates. In summary our results indicate that modest exchanges of monounsaturated for polyunsaturated fats do not significantly affect LDL or HDL levels or metabolism, which supports the view that reducing saturated fats is the key to lowering total and LDL cholesterol.

Fig 1

Diagrams showing the two study designs. A, Study A used a randomized parallel design in which all subjects consumed an average American diet (AAD) for 3 weeks before being randomly assigned for an additional 6 weeks to a continued AAD; to the American Heart Association step 1 diet (STEP 1), in which 7% of calories from saturated fat was replaced by carbohydrate, with monounsaturated and polyunsaturated fats unchanged from AAD; or to a diet (POLY) in which 7% of calories from saturated fat was replaced by carbohydrate and 6% of calories from monounsaturated fat was replaced by polyunsaturated fat. B, Study B used a randomized crossover design in which all subjects were assigned randomly to either the step 1 or the Poly diet for 5 weeks and then switched to the other diet for an additional 5 weeks. There was a 4-day break between the two 5-week periods.

Fig 2

Total plasma cholesterol and triglyceride concentrations during study A at weeks 1,2,5, 7, 8, and 9 are platted as percent change from the values at the end of week 3 of the baseline period, during which all subjects ate the average American diet (AAD). The samples at week 5 were obtained 2 weeks after the start of the randomized diet periods. Subjects consuming the experimental diets (Step 1 or Poly; see “Methods” for definitions) had lower total plasma cholesterol levels by that time.

Fig 3

Bar graph showing the plasma total (TC), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) cholesterol concentrations presented as the mean changes between the values obtained in study A at week 3 (end of the baseline period) and week 9 (end of the randomized period) for the average American diet (AAD), American Heart Association step 1 (Stepl), and Poly groups (see “Methods” for definitions of diet groups). Because of greater week-to-week variability, the change in plasma triglyceride (TG) levels was calculated from the difference between the value at week 3 and the mean of values obtained at weeks 7, 8, and 9. Total and LDL cholesterol levels were significantly lower in the step 1 and Poly groups compared with AAD (P < .001 for each comparison). There was no significant difference between the reductions in the step 1 and the Poly groups

Fig 4

Bar graph comparing the areas under the plasma concentrations of triglycerides (TG) and total (TC), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) cholesterol after study A subjects had eaten a lunch typical of each of the three diets. The areas were calculated from the changes at 2, 4, and 6 hours after lunch compared with prelunch levels. Postlunch TG levels were significantly greater in the Poly group (P < .05) than in either the average American diet (AAD) or the American Heart Association step 1 (Stepl) groups (see “Methods” for definitions of diet groups). There was no difference in postlunch TGs between the AAD and step 1 diets, and postlunch total, LDL, and HDL cholesterol levels did not differ in the three diet groups.