1. Lipid-lowering diets enriched in polyunsaturated fat decrease the serum cholesterol in hyperlipoproteinaemia, usually by reducing both the low-density-lipoprotein (LDL) and high-density-lipoprotein (HDL) cholesterol concentrations. The aim of the present study was to investigate whether the effects on LDL could be maintained but those on HDL cholesterol be diminished by reducing the ratio, polyunsaturated: saturated fat (P:S) of the diet.
2. Twenty hyperlipoproteinaemic patients (six with type IIa, eight with type IIb and six with type IV) in a metabolic ward were given two fat-modified diets during two consecutive 3–week periods in a randomized order. The diets were identical with regard to nutrient composition but differed with regard to the P:S values, which were 2.0 and 1.3 respectively.
3. The lipoprotein-lipid composition and serum apolipoprotein concentrations were similar at the end of the two dietary periods in type IIa and type IV patients but in type IIb patients a more pronounced reduction of the LDL-cholesterol concentration by 9% (P < 0·05) was achieved on the diet with the higher P:S value. The HDL-cholesterol did not differ significantly.
4. The results indicate that increasing the P:S value of lipid-lowering diets from 1.3 to 2·0 does not offer a great advantage with regard to the lipoprotein-lipid reductions achieved in moderate hyperlipoproteinaemia.