It has been established that large quantities of phytosterols reduce cholesterol absorption and LDL cholesterol concentrations. Now a recent study by a team of researchers from Washington University, St Louis, Mo, US, has found that even low concentrations of phytosterols found in corn oil reduce intestinal cholesterol absorption.
The study published in the The American Journal of Clinical Nutrition (subscription/pay per view) involved 25 healthy subjects (20 women and 5 men) with a mean serum cholesterol concentration of 5.1 mmol/L, in a randomised double blind crossover test. Free and esterified phytosterols were removed from the corn oil by hexane extraction. The subjects received an otherwise sterol free test breakfast on 2 occasions, 2 weeks apart that contained 35 mg hexadeuterated cholesterol and 30 - 35g of corn oil. The plasma enrichment of the radioactively labelled tracer was measured by negative ion mass spectrometry.
Cholesterol absorption was 38% greater after consumption of the sterol free corn oil than after consumption of a commercial corn oil with an identical fatty acid content. When corn oil phytosterols were added back to sterol free corn oil at a concentration of 150mg per test meal, cholesterol absorption was reduced by 12.1%, and by 27.9% when 300mg phytosterols were added. These results illustrate that phytosterols present at less than 1% of commercial corn oil reduce cholesterol absorption and may be part of the cholesterol lowering activity previously attributed solely to unsaturated fatty acids. The phytosterol content of oils can be affected by the refining process, with up to 20-fold differences in the concentrations of phytosterols removed during physical refining. If phytosterols are important to the diet, then refining processes may be optimised to remove a minimum concentration while maintaining quality.