There is a general misconception that high levels of serum cholesterol and triglycerides are the cause of cardiovascular disease. Evidence from nutritional research does show that high levels of low density lipoprotein (LDL), total cholesterol and triglycerides in some sub-groups of individuals, are association with cardiovascular disease. But while raised levels of lipids increase the risk of developing vascular problems, there is no proven cause and effect. More likely, raised levels of serum lipids are indicative of metabolic disorder caused by low quality diet. The number of foods that positively affect serum triglyceride and cholesterol levels is growing as more research is performed. Common properties of these food tend to be that they high in antioxidants, in their raw or unrefined state and usually replace calorie dense foods such as sugar and fat. Tree nuts are one such food that can significantly improve serum lipid levels in humans.
For example, researchers1 fed 16 healthy subjects with a mean age of 41 years a diet that consisted of 36% of energy from fat for 3 weeks. The diet was supplemented with peanuts, coconut cubes and a coconut confectionary bar, and was designed to match the normal polyunsaturated fat to monounsaturated to saturated fat ratio of the typical Australian diet. The subjects then switched to a diet that provided 46g of mainly monounsaturated fatty acids from almonds, which was followed for the next 3 weeks. Then once again the subjects were switched to an alternative diet for 3 weeks, this time being supplemented with 46g of mainly polyunsaturated fatty acids from walnuts. The three diet periods were matched for energy intakes and major dietary components, except the fat ratio. The body weight of the subjects remained constant throughout the study.
The results showed that after 3 weeks on the almond diet, there were reductions in total and LDL of 7 and 10%, respectively. After 3 weeks on the walnut diet these values had changed to of 5 and 9% of baseline. There was no significant difference between the almond and walnut diet in terms of blood lipid changes. High density lipoprotein levels did not change following either the almond or walnut diets. The study authors concluded that the changes in the fat ratios of monounsaturated and polyunsaturated fatty acids were likely the reason for the alterations in serum lipids. However, both almonds and walnuts contain high levels of polyphenolic antioxidants that may be responsible, at least in pert, for their beneficial effects. For example, walnuts contain high levels of the phenolic substance ellagic acid, and almonds contain high concentrations of quercetin, kaempferol and isorhamnetin.
That monounsaturated fatty acids have little effect of plasma lipids has been shown in some studies comparing extra virgin olive oil with refined olive oil, both good sources of monounsaturated fats. Only extra virgin olive oil produces beneficial effects on lipid levels suggesting that it is not the monounsaturated fatty acids causing the effect but some other component, possibly the polyphenolic compounds present in high amounts in extra virgin olive oil, but only in low amounts in refined olive oil. In addition, strawberries have also shown beneficial effect of lowering LDL and total cholesterol, and they like walnuts are a good source of ellagic acid. Care must therefore be taken in order to interpret the changes in blood lipids seen in studies comparing food types. For example, it has been noted that the modest 5 to 10% reduction in lipid levels cannot explain their 30% reduction in the risk of cardiovascular disease with the consumption of certain nuts.
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