The differential effect of two diets, taken in synchrony with the menstrual 
cycles for 2 wk each, on serum and bile lipids was investigated in young healthy 
women. The "normal" diet was high in cholesterol and total fat, and low in 
polyunsaturated fat and fiber; the "prudent" diet contained a high proportion of 
polyunsaturated fat and fiber, but was low in cholesterol and total fat; there 
was little difference in energy content. Both in whole serum and in low-density 
lipoprotein the concentrations of cholesterol and apolipoprotein B were almost 
30% lower with the "prudent" than with the "normal" diet; HDL-cholesterol was 
16.3% lower. Triglycerides were increased, only in the very-low-density 
lipoproteins while cholesterol and apolipoprotein B did not change much in this 
fraction. The risk to acquire cholesterol gallstones was not less with the use 
of the "prudent" diet as originally expected. While using the "prudent" diet 
five of the women had slightly higher lithogenic indices, in two there were much 
higher values (greater than 25%), and only in three the lithogenic index was 
unchanged or slightly lower than with the "normal" diet.