It is often claimed that brain diseases such as dementia are a result of an ageing population, but this theory is flawed for two main reasons. Firstly, Western individuals are not living longer compared to those from the early 20th century. Average age has increased because childhood death has been reduced, and as a consequence more people live to old age. However, adjusting for the falling childhood death rate, the average age in most Western countries has not changed for 100 years or so. Secondly, there are many examples of populations around the World that have longer lifespans that those populations from the West, and yet these longer living populations suffer from no age related dementia. The Hunza tribe from the Himalayas is renowned for longevity and for their health well into old age. So too are populations that inhabit the island of Sicily in the Mediterranean. Studies of these areas show little evidence of age related mental decline or ill health in the oldest inhabitants of these populations.
Rather than age, it is consumption of the typical Western diet that sews the seeds of cognitive deterioration in Western populations. The typical Western diet is pro-inflammatory and this inflammation leads to chronic inflammation in the tissues of those that consume it. This inflammation is now thought to play a role in cancer, cardiovascular disease and dementia, three of the leading Western diseases in developed nations. The Hunza tribes, the populations of the Mediterranean and other populations that consume traditional diets are protected from Western disease because they consume anti-inflammatory foods. Many of these foods are high in plant chemicals which include antioxidants, and this reduces the oxidative stress and inflammation in the consumers. The traditional inuit diet and the diet of the Japanese is also rich in fish oils which have pronounced anti-inflammatory effects, and this may explain the health of these two populations, particularly with regard mental disorders.
The doctrine of signatures is an ancient Greek concept that herbs resemble the parts of the body they are able to treat. based on the doctrine of signature it could be expected that walnuts (Juglans regia) are beneficial to the brain, as the similarity between the hemispheres of the human brain and walnuts is evident. Evidence in the nutritional literature supports a role for walnuts in the protection of the brain, and this may relate largely to two components within the nuts. Firstly walnuts are a rich source of phytochemical antioxidants and secondly they contain the long chain essential fatty acid alpha-linolenic acid (ALA, C18:3 (n-3)). Both of these components may have specific anti-inflammatory effects in humans. In addition, walnuts are a good source of fibre, and this may have beneficial glycaemic effects. Walnuts, along with a healthy diet may therefore also regulate blood sugar levels and this can have cardioprotective effects. Cardiovascular disease may increase the risk of dementia.
In humans the ALA from walnuts is converted through the omega-3 fatty acid pathway to the longer and more unsaturated docosahexaenoic acid (DHA, C22:6 (n-3)) in the liver. The DHA may play a particular role in brain health, because DHA accumulates in cell membranes where it may have particular neurological effects. For example, correct brain levels of DHA may be required to maintain correct serotonin metabolism, and may be involved in neurotransmitter metabolism in general. DHA is known to accumulate in the brain in relatively high concentrations compared to other tissues. The ALA in walnuts is also converted to eicosapentaenoic acid (EPA, C20:5 (n-3)) and this then accumulates in the cell membranes. Here the EPA acts as a reservoir for the production of series 3 eicosanoids which can inhibit the production of pro-inflammatory series 2 eicosanoids. In this way ALA may supply the substrate that allows correct neurotransmitter metabolism and also anti-inflammatory effects to the brain.
Walnuts are also a very good source of certain polyphenols. Polyphenols have structures that confer antioxidant properties on them, and many such polyphenols in plants are biologically active in humans. Evidence supports a role for plant polyphenols in human health. Polyphenols found in walnuts include vitamin E, hydroxycinnamic acids (chlorogenic acid and caffeic acid), P-coumaric acid, ferulic acid, sinapic acid, hydroxybenzoic acids (syringic acid and ellagic acid), gallic acid, gansrin, juglone, syringaldehyde, ellagitannins and other polymeric tannins as well as flavonoids. In addition, walnuts contain carotenoids, another major group of plant antioxidants that are bioavailable in humans. Polyphenols show particular protective effects against brain damage, particularly with respect to correct calcium homeostasis in the striatum and hippocampus, areas of the brain required for primary and secondary memory functions. Melatonin is also present in walnuts, and low levels of melatonin are associated with dementia.
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