Polyunsaturated fatty acids have some interesting nutritional properties. In particular, they are known to have beneficial effects on plasma levels of certain lipoproteins. Evidence suggests that polyunsaturated fatty acid consumption results in lower plasma levels of both the very low density lipoprotein (VLDL) and the low density lipoprotein (LDL) particles compared to saturated fatty acids. The exact reason for this is not clear, but generally it is accepted that multiple mechanisms may explain this phenomenon. One explanation centers on the ability of polyunsaturated fatty acids to alter the postprandial utilisation of fatty acids. In this regard, following a meal, triglycerides are removed from their chylomicron particles and taken up to adipose tissue as fatty acids. Postprandially, these fatty acids are released to circulation where some of them are repackaged into VLDLs in the liver, before re-entering circulation. As these VLDL particles lose their fatty acids, they form LDLs, the main transporters of cholesterol to peripheral tissues.
It is this process that may be altered by polyunsaturated fatty acids. It has been suggested that it is less likely for polyunsaturated fatty acids to be incorporated into VLDLs which may decrease their formation. As less polyunsaturated fatty acids are incorporated into the VLDL particles, their number in plasma drops, and this in turn creates fewer LDL particles that are normally formed as a result of VLDL catabolism. Fish oils appear to be more effective than the polyunsaturated fatty acids in vegetable oils, which may give clues as to the lipoprotein lowering effects of vegetable oils. If polyunsaturated fatty acids are not so readily incorporated into VLDL particles, then it is necessary to explain where they go. Because they do not accumulate in the body it may be assumed that they are somehow used as a source of energy. One explanation is that they are preferentially converted to ketone bodies and oxidised. This explains the weight loss and triglyceride lowering effects of fish oils, which has been reported in numerous well designed animal experiments and human studies.
Therefore polyunsaturated fatty acids may be preferentially transported to the muscles as ketone bodies where they are oxidised as fuel, rather than be incorporated into VLDL particles to be exported to the adipose tissue. Fish oils have a greater effect than the polyunsaturated fatty acids in vegetable oils and this therefore explains the superior triglyceride lowering effects of fish oils and their weight loss effects. However, there is an alternative theory that suggests that polyunsaturated fatty acids may in some way alter steroid excretion rates. In particular, diets high in polyunsaturated fatty acids can increase the excretion rates of steroids in the faeces, suggesting that bile flow may be increased. However, care needs to be exercised with this explanation, because the steroid excretory effects of such diets may relate to other factors that are present in the diet or the presence of other substances in vegetable oils. That is not to say that this effect is not real and measurable, but may not be due directly to the presence of the polyunsaturated fatty acids per se.
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