Phytosterol Absorption – Mechanism and Relevance

What is Phytosterol Absorption?

Phytosterol absorption describes the process by which plant sterols (known as phytosterols) are taken up from the small intestine into the bloodstream after dietary ingestion. Phytosterols are structurally very similar to human cholesterol and occur naturally in plant-based foods such as nuts, seeds, vegetable oils, legumes, and whole grains.

Compared to cholesterol, the intestinal absorption rate of phytosterols in humans is markedly lower: while approximately 50% of dietary cholesterol is absorbed, the absorption rate of the most common phytosterols -- including beta-sitosterol, campesterol, and stigmasterol -- is only 0.5 to 5%. Despite this limited uptake, phytosterols have significant physiological and clinical relevance.

Mechanism of Absorption

Phytosterols are absorbed via the same intestinal pathway as cholesterol. In the small intestine, phytosterols are emulsified by bile acids and incorporated into micelles, which are then taken up by enterocytes (intestinal epithelial cells). Within the enterocytes, two key transport proteins play a central role:

• NPC1L1 (Niemann-Pick C1-Like 1): This transporter mediates the uptake of sterols from the intestinal lumen into enterocytes. It accepts both cholesterol and phytosterols, but shows a structural preference for cholesterol.
• ABCG5/ABCG8 (ATP-binding cassette transporters): These efflux transporters actively pump the majority of absorbed phytosterols back into the intestinal lumen before they can enter the bloodstream. This mechanism largely accounts for the low bioavailability of phytosterols.

Phytosterols that are not effluxed by ABCG5/ABCG8 are packaged into chylomicrons and transported via the lymphatic system into the bloodstream.

Factors Influencing Phytosterol Absorption

Several factors can affect the absorption rate of phytosterols:

• Chemical structure: The side chains of individual phytosterols determine their affinity for transport proteins and thus their rate of absorption. For example, campesterol is absorbed somewhat more efficiently than beta-sitosterol.
• Food matrix: The form of the food (e.g., oils vs. solid foods) and the total fat content of the meal influence micelle formation and bioavailability.
• Genetic factors: Mutations or polymorphisms in the genes encoding ABCG5/ABCG8 can lead to significantly increased phytosterol absorption, as seen in the rare condition known as sitosterolemia (phytosterolemia).
• Form of administration: Fortified foods (e.g., phytosterol-enriched margarines) or dietary supplements can influence the amount absorbed.
• Interaction with ezetimibe: The medication ezetimibe inhibits the NPC1L1 transporter, thereby substantially reducing the absorption of both cholesterol and phytosterols.

Clinical Relevance

Cholesterol-Lowering Effect

Although phytosterols themselves are only absorbed in small amounts, they competitively inhibit cholesterol absorption in the intestine. They displace cholesterol from micelles, reducing its uptake into enterocytes. This mechanism is used therapeutically: phytosterol-enriched foods (e.g., margarine, yogurt) can lower LDL cholesterol levels by approximately 8 to 15% when 1.5 to 3 g of phytosterols are consumed daily. This effect has been scientifically recognized by the European Food Safety Authority (EFSA).

Sitosterolemia

In the rare autosomal recessive disorder sitosterolemia (also called phytosterolemia), the ABCG5/ABCG8 transporters are non-functional. As a result, phytosterols are absorbed in abnormally high amounts and accumulate in the blood and tissues. Affected individuals develop xanthomas (fatty deposits in the skin) at an early age and face a significantly elevated cardiovascular risk.

Cardiovascular Risk from Elevated Phytosterol Levels

There is ongoing scientific debate about whether elevated blood phytosterol levels -- resulting from higher-than-normal absorption rates -- may independently increase cardiovascular risk. However, the evidence remains inconclusive and is the subject of ongoing research.

Dietary Sources and Intake Levels

The average daily intake of phytosterols through a normal diet in Western countries ranges from approximately 200 to 400 mg per day. Particularly rich sources include:

• Vegetable oils (e.g., rapeseed oil, corn oil, sunflower oil)
• Nuts and seeds (e.g., walnuts, sesame, flaxseed)
• Legumes (e.g., soybeans, lentils)
• Whole grain products
• Fruits and vegetables (in smaller amounts)

Despite the low absorption rate, these amounts result in measurable phytosterol concentrations in plasma, which can serve as biomarkers for an individual's cholesterol absorption capacity.

References

1. Ostlund RE Jr. - Phytosterols in human nutrition. Annual Review of Nutrition, 2002; 22: 533-549.
2. EFSA Panel on Dietetic Products, Nutrition and Allergies - Scientific Opinion on the substantiation of health claims related to plant sterols and plant stanols. EFSA Journal, 2012; 10(5): 2692.
3. Berge KE, Tian H, Graf GA et al. - Accumulation of dietary cholesterol in sitosterolemia caused by mutations in adjacent ABC transporters. Science, 2000; 290(5497): 1771-1775.