A nutritional supplement, prepared food product, or direct food additive for ingestion by mammals, and methods for preparing such products, are provided. Products of the invention comprise an oxidation-resistant fat-based composition substantially free of exogenous solubilizing and dispersing agents for phytosterols. The fat-based composition includes greater than 25% and less than 75% by weight of one or more triglyceride-based edible oil or fat, and greater than 25% and less than 75% by weight of one or more non-esterified phytosterols. The fat-based composition, when exposed to air, contains a reduced amount of oxidative by-products compared to a similar fat-based composition lacking non-esterified phytosterols. Also provided are methods for reducing plasma cholesterol in mammals, and methods for protecting plasma lipoproteins and cholesterol from oxidation by ingestion of products of the invention.