1. Serum cholesterol reduction and changes in cholesterol metabolism were studied during rapeseed oil feeding without and with increasing amounts of sitostanol trans-esterified with rapeseed oil fatty acids and dissolved in rapeseed oil mayonnaise. Fifteen mildly hypercholesterolaemic subjects replaced 50 g of their usual dietary fat by 50 g of rapeseed oil fat mayonnaise for 6 weeks followed by randomization so that eight subjects continued on rapeseed oil mayonnaise alone (control group) for 15 weeks and seven on rapeseed oil mayonnaise with a small dose of sitostanol ester (800 mg/day of sitostanol) for 9 weeks followed by 6 weeks with higher dose of sitostanol ester (2000 mg/day of sitostanol).
2. During the rapeseed oil period the reduction in serum low-density lipoprotein cholesterol was 14% from the home diet. The control-adjusted reduction by the low sitostanol ester dose was 7.4% (not significant) and by the higher dose it was 15.7%.
3. The low dose of sitostanol ester had no consistent effect on cholesterol precursors or cholestanol in serum, reduced serum levels of campesterol and sitosterol by 28.2% and 23.6%, respectively, and reduced cholesterol absorption efficiency significantly from 28.7% to 23.4%. In accordance, faecal excretion of neutral and particularly endogenous neutral sterols increased (16.7% and 19.7%, respectively), but faecal cholesterol elimination and cholesterol synthesis were only insignificantly increased.
4. During the high dose of sitostanol ester the high-density lipoprotein- to low-density lipoprotein-cholesterol ratio increased. Serum levels of cholesterol precursor sterols, indicators of cholesterol synthesis, increased up to 12%, whereas those of cholestanol were slightly decreased and those of campesterol and sitosterol values were further reduced by 30% and 25.6%, respectively.
5. Associations of serum plant sterols and cholesterol precursors with cholesterol absorption efficiency and synthesis and the sitostanol-ester-induced changes in serum campesterol and lathosterol proportions with those in serum low-density lipoprotein-cholesterol suggest that reduced cholesterol absorption efficiency was the main reason for cholesterol reduction and that there was a compensatory increase in cholesterol synthesis.
6. The findings indicate that sitostanol ester dissolved in dietary fat is apparently unabsorbable and interferes with sterol absorption so that the serum levels of cholesterol and low-density lipoprotein-cholesterol are reduced to the extent that sitostanol ester-fat mixture in reasonable daily amounts can be recommended to replace dietary fat for lowering of serum cholesterol.