It is a common misunderstanding that plants contain cholesterol, the waxy, fat-like substance found in animal products. Plants do not produce cholesterol in any significant amount, unlike animals that synthesize it for various bodily functions. Instead, plants create similar compounds known as phytosterols, or plant sterols. While these plant-derived compounds share a basic structural resemblance to cholesterol, their physiological roles and effects within the human body are distinct.
Phytosterols in Plants
Phytosterols serve as the plant kingdom’s counterpart to cholesterol, playing a comparable structural role in plant cell membranes. These fat-soluble compounds are characterized by a tetracyclic structure, which is a four-ring core, much like cholesterol’s. However, a key distinction lies in their side chain, typically at the C-17 or C-24 position, where phytosterols possess additional methyl or ethyl groups that cholesterol lacks. This subtle chemical difference significantly impacts their absorption and function in the human body.
Several hundred types of phytosterols exist in plants, with beta-sitosterol, campesterol, and stigmasterol being among the most prevalent forms. These sterols are not merely structural components; they also contribute to maintaining membrane fluidity and permeability, influencing processes like embryonic growth, and aiding in the plant’s adaptation to varying temperatures and defense against pathogens.
How Phytosterols Affect Human Cholesterol
When consumed by humans, phytosterols interact with the body’s cholesterol absorption mechanisms in a beneficial way. The primary mechanism involves phytosterols competing with dietary and endogenous cholesterol for absorption in the intestines. Both cholesterol and phytosterols require incorporation into mixed micelles, small lipid structures, to be transported across the intestinal lining into the bloodstream. Phytosterols are more readily incorporated into these micelles, effectively displacing cholesterol.
This competition reduces the amount of cholesterol available for absorption into the body. Once absorbed into intestinal cells, phytosterols are largely unrecognized by the human body’s transport systems, leading to their active excretion back into the intestinal lumen. Consequently, only a small fraction of ingested phytosterols, between 0.5% and 4%, enters the bloodstream, compared to cholesterol’s absorption rate of about 50%.
The net effect of this reduced cholesterol absorption is a decrease in blood low-density lipoprotein (LDL) cholesterol levels, often referred to as “bad” cholesterol. Studies indicate that a daily intake of approximately 2 grams of phytosterols can lead to an 8-10% reduction in LDL cholesterol. While the body may attempt to compensate for decreased dietary cholesterol absorption by increasing its own cholesterol synthesis in the liver, this compensatory increase is generally not sufficient to offset the overall reduction in circulating cholesterol levels.
Common Plant Sources of Phytosterols
Phytosterols are naturally present in a wide array of plant-based foods, though their concentrations vary significantly. Unrefined vegetable oils are among the richest natural sources, providing substantial amounts. For instance, rice bran oil can contain around 1890 milligrams per 100 grams, and corn oil approximately 990 milligrams per 100 grams. Olive oil also contributes phytosterols.
Nuts and seeds are also notable sources of phytosterols. Pistachios offer roughly 270 milligrams per 100 grams, and almonds around 160 milligrams per 100 grams. Flaxseed is another good option, containing approximately 210 milligrams per 100 grams.
Fruits and vegetables, while generally containing lower amounts than oils or nuts, still provide a modest contribution. Examples include broccoli, corn, and blueberries. The typical dietary intake of phytosterols for many individuals ranges from 150 to 450 milligrams per day, with vegetarians often consuming higher amounts. To achieve beneficial cholesterol-lowering effects, some foods like certain margarines, yogurts, and milks are fortified with added phytosterols, providing a more concentrated source.