A plant sterol supplementation study will be conducted in individuals who are heterozygous for phytosterolemia compared to non-carriers. The study will use a double-blind, randomised, crossover, placebo-controlled design in which participants will consume 2 treatments including 1.6 g of plant sterol pills and a placebo pill, each for 4 weeks. A 4 week washout period will separate the two treatment periods. Plant sterol concentrations will be measured by gas liquid chromatography and absorption will be measured by the ratio of campesterol to lathosterol in plasma. Cholesterol absorption and synthesis will be measured by dual stable isotope technique, involving simultaneous administration of 13C-cholesterol and deuterium oxide, respectively.