Grapefruits (Citrus paradisi) have been shown to be beneficial at lowering plasma cholesterol levels. This is thought to relate to their fibre content which is mainly in the form of the soluble fibre pectin. The pectin concentration of whole grapefruit is around 8 %, but the edible portion of the fruit contains only around 4 % pectin. However, a 15 gram portion of grapefruit pectin per day has been shown to be able to lower total plasma cholesterol levels by around 10 %, which is a significant enough fall to affect cardiovascular risk favourably. Pectin is chemically a polyanionic heterogeneous mixture of carbohydrates (a number of different carbohydrates with an overall net negative charge). Pectin is located primarily in the cell wall of plants where it acts to add structural rigidity to the cells. In humans cholesterol levels respond favourably to pectin supplements, although the exact reason for this is not clear. However, the effects of pectin on lipoproteins may relate to their structural interactions.
Pectin appears to be particularly beneficial at reducing levels of low density lipoprotein (LDL) cholesterol. It is known that endogenously produced glycosaminoglycans from connective tissue are also able to interact with low density lipoprotein particles. Glycosaminoglycans are also polyanionic carbohydrates, and they are known to be electrostatically attracted to low density protein particle. This is evidenced in atherosclerosis, were connective tissue and lipoprotein particles form components of atherosclerotic plaques. Experimentally the interaction between glycosaminoglycans and lipoproteins has also been observed experimentally1. The electrostatic attraction between pectin and the low density lipoprotein has also been observed experimentally under laboratory conditions1. No such interactions were observed between pectin and high density lipoprotein or very low density lipoprotein. Therefore there exists a mechanism by which pectin and other anionic polysaccharides could cause a lowering of plasma cholesterol.
Dr Robert Barrington’s Comments: These studies suggest a mechanism by which pectin could lower tissue levels of cholesterol. However, it is also known that pectin and other dietary fibres bind to bile salts in the gut and in this way increase their elimination. As bile salts are derived from cholesterol, dietary fibre may be able to lower plasma cholesterol levels through its ability to bind bile salts. This notwithstanding, the electrostatic interaction between pectin and low density lipoprotein is interesting and should be considered when assessing the effects of pectin on plasma cholesterol levels. Interestingly it has been theorised that the charge attraction between low density lipoprotein and collagen may change as the lipoprotein particle is modified through oxidation, and this may explain the presence of mainly oxidised low density lipoprotein in the artery walls of atherosclerotic plaques. Of course, there may also be other components in grapefruits that contribute to their cholesterol lowering effects.
RdB
