Walnuts are nutritionally interesting. Like most nuts, walnuts are a good source of fibre, are high in protein and fat, and low in carbohydrates. Walnuts also contain high concentrations of vitamin and minerals, although the mineral content is dependent on the soil on which the tree is grown. Unlike other nuts, walnuts contain high amounts of polyunsaturated fatty acids (most nuts are high in monounsaturated fatty acids), and also contain a specific antioxidants, ellagic acid. These nutritional nuances may explain the beneficial effects of walnuts against cardiovascular disease. It is perhaps a little unfair to single out walnuts as being cardioprotective, because almonds and pistachio nuts may also have particular cardioprotective properties. In addition, unsalted peanuts, actually a legume and not a true tree nut, have also been shown to possess cardioprotective effects. However, the effects of walnuts of cardiovascular disease are perhaps the most widely reported and therefore worthy of discussion.
Epidemiological studies have investigated nut consumption. A number of large scale trials have been conducted and shown positive associations with cardiovascular biomarkers. One study showed a 40 % reduction in the risk of cardiovascular disease from consuming nuts more that 5 times per week, or an average 8.3 % reduction for each serving of nuts consumed, and consuming nuts 1 to 4 times per week was associated with a 30 % reduction in cardiovascular disease. However these studies did not investigate walnuts specifically, but clinical trial involving walnuts nuts have shown beneficial cardioprotective effects. For example in one study 84 grams of walnuts per 2500 kcals daily decreased low density lipoprotein by 16 %. In another study, feeding 37 grams of walnuts and 15 grams of walnut oil per 2390 kcals daily resulted in a 12 % reduction in LDL and an 18 % reduction in triglycerides. Another study feeding 42.5 grams of walnuts per 2412 kcals daily resulted in a 9 % reduction in LDL.
Walnuts therefore appear to be able to favourable affect lipoprotein ratios in humans. The subjects in such studies have included both healthy and hyperlipidaemic subjects, and both groups appear to benefit from walnuts. Nuts are rich sources of antioxidants including carotenoids, vitamin E and other phytonutrient antioxidants. Walnuts also contain the antioxidant ellagic acid, and this may explain the benefits of walnuts on the endothelial lining of arteries. It is known that oxidative stress can disrupt the production of nitric oxide in the endothelial lining of arteries. As nitric oxide is required for the correct dilation of blood vessels in response to flow, oxidative stress can disrupt this process which may lead to damage to the arteries and high blood pressure. When one group of subjects were fed 18 % of energy from walnuts in a Mediterranean style diet, they experienced a relative 64 % improvement in endothelial function. Walnut consumption has also been shown to lower blood pressure.
That walnuts can improve endothelial function and lower blood pressure may relate to their effects on oxidation and inflammation. For example, consumption of 37 grams of walnuts and 15 grams of walnut oil per 2390 kcals daily resulted in a significant decrease in plasma C-reactive protein concentrations. C-reactive protein is considered a marker of systemic inflammation, suggesting that walnuts are beneficial at reducing the inflammatory response. Evidence also suggest that other biomarker of inflammation including E-selectin, soluble vascular cellular adhesion molecule (sVCAM), tumour necrosis factor alpha (TNF-α) and soluble intercellular adhesion molecule (sICAM) are reduced with walnut consumption. Walnuts may also increase the excretion of cholesterol possibly through reverse cholesterol transport by the high density lipoprotein (HDL) particle. Walnuts therefore show a number of cardioprotective benefits possibly relating to their unique nutritional profiles.
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