The Beneficial Effects of Tree Nuts on the Aging Brain
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Nutrition and Aging 1 (2012) 55–67 55 DOI 10.3233/NUA-2012-0007 IOS Press The beneficial effects of tree nuts on the aging brain Amanda N. Carey, Shibu M. Poulose and Barbara Shukitt-Hale∗ United States Department of Agriculture, Agricultural Research Services, Human Nutrition Research Center on Aging, Tufts University, Boston, MA, USA Abstract. Dietary patterns may play an important role in protecting the brain from the cellular and cognitive dysfunction associated with the aging process and neurodegenerative diseases. Tree nuts are showing promise as possible dietary interventions for age-related brain dysfunction. Tree nuts are an important source of essential nutrients, like vitamin E, folate, and fiber. Tree nuts also contain a variety of components, such as phytochemicals like flavonoids, proanthocyanidins, and phenolic acids, as well as monounsaturated and omega-3 and omega-6 polyunsaturated fatty acids that have the potential to combat age-related brain dysfunction. Evidence is accumulating that suggests that tree nuts and their bioactive constituents have the potential to reduce oxidative stress and inflammation, as indicated by decreased lipid peroxidation in vivo and reduced production of the free radical nitric oxide and the pro-inflammatory cytokine tumor necrosis factor-alpha in vitro. Also, tree nut consumption might have the ability to mitigate some of the cognitive decline associated with aging. Here we review the current knowledge of how the consumption of nuts may improve brain health, specifically focusing on walnuts, almonds, pistachios, and pecans. Keywords: Brain, aging, walnut, nut, polyunsaturated fatty acids, antioxidant effects 1. Introduction diseases, and immensely compromising the quality of life [3–6]. Apart from being debilitating in nature on In the brain, aging has been characterized by the elderly, these diseases are an enormous economic losses in neuronal function accompanied by behavioral burden on society with a direct annual economic cost declines such as decrements in motor and cognitive for caregiving in Alzheimer’s disease alone estimated performance, in both animals and humans. Oxidative at $183 billion in 2011 [7]. However, it is worth not- stress and inflammation are thought to be impor- ing that the clinical manifestation of these devastating tant factors in brain aging and age-related neuronal diseases takes approximately 8–10 years, and in cer- dysfunction [1, 2]. The central nervous system, partic- tain cases up to 25 years, indicating a covert initiation ularly during aging, is highly susceptible to oxidative and progression originating in the brain. In the search stress and inflammation, with both increasing in tan- for a cure for these age-related diseases, prevention dem with age, compiled with a loss of efficiency by has received much less attention; however, a grow- which the body’s natural defense mechanisms mitigate ing number of studies suggest the potential benefit these insults. In essence, aging is a condition where of boosting the brain’s endogenous defenses by sup- stressors are not countered by protective functions, plementing with fruits and vegetables rich in natural leading to a cascade of altered signaling and ultimately phytochemicals. to the injury or death of neurons, resulting in permanent Evidence is accumulating to suggest that dietary pat- modification of biological and behavioral processes, terns may play an important role in protecting the aging increasing the risk of various neurodegenerative brain from cellular and cognitive dysfunction. Dietary supplementation with a number of foods has been shown to induce a multiplicity of protective effects ∗Corresponding author: Barbara Shukitt-Hale, Neuroscience and Aging Lab, USDA HNRCA at Tufts University, 711 Washington against some aspects of age-related behavioral and Street, Boston, MA 02111, USA. Tel.: +1 617 556 3118; Fax: +1 cellular dysfunction [8]. Higher consumptions of cer- 617 556 3222; E-mail: [email protected]. tain foods or specific dietary patterns, such as the ISSN 1879-7717/12/$27.50 © IOS Press and the authors. All rights reserved 56 A.N. Carey et al. / Effects of tree nuts on brain Mediterranean diet, are associated with reduced risk Table 1 of cognitive impairment, which is thought to be due to Production and utilization of major tree nuts in the US marketplace the increase in nutrient intake [9–11]. For example, in a in 2010–2011 season [108] prospective cohort study, Gu and colleagues [12] found Tree nuts Total production Domestic Exports that, in people 65 years of age or older, a dietary pattern (lbs) utilization (lbs) (lbs) of eating salad dressing, nuts, fish, tomatoes, poultry, Almonds 1,612,084 499,353 1,188,239 cruciferous vegetables, dark and green leafy vegeta- Walnuts 436,785 216,000 301,665 bles, and fruits, while low in high-fat dairy, red meat, Pistachios 250,125 169,404 79,578 Pecans 140,407 167,432 54,405 organ meat, and butter consumption, is significantly All tree nuts 2,478,896 1,315,840 1,711,460 associated with Alzheimer’s disease risk and is strongly protective against its development. This dietary pattern reflects a diet rich in omega-3 and omega-6 polyunsat- sumed nuts had a reduced risk of fatal coronary heart urated fatty acids (PUFAs), vitamin E, and folate and disease and non-fatal myocardial infarction. Moreover, poor in saturated fat and vitamin B12. prospective data in US male physicians suggests that One of the food categories showing promise as a men who consumed nuts two or more times per week possible dietary intervention for age-related brain dys- had reduced risks of sudden cardiac death and total function is tree nuts, which are important sources of coronary heart disease death compared to men who nutrients for humans that have been used by ancient rarely or never consumed nuts [16]. Given the potent civilizations to prevent or treat disease [13]. Currently, anti-inflammatory effects that nut consumption can the dietary recommendations established by the United have in the cardiovascular system, it stands to reason States Department of Agriculture states that tree nuts that nuts may also act as anti-inflammatory agents in are an important source of dietary protein, with half the brain. Nuts contain a variety of components, such an ounce of nuts (approximately 14 grams) consid- as phytochemicals and PUFAs that may have the poten- ered a daily serving of protein, and are also important tial to combat age-related brain dysfunction. Here we sources of dietary fiber and unsaturated fats [14]. In review the current knowledge of how the consumption contrast to the botanical classification, the common of nuts may improve brain health. culinary and English usage of the term “nut” may refer to many hardened edible kernels (i.e. seed) referred to as “nuts”. In the United States, tree nuts such as 2. Polyunsaturated fatty acids from tree nuts: almonds (Prunus dulcis), cashews (Anacardium occi- Potential health benefits dentale), macadamias (Macadamia integrifolia and tetraphylla), pecans (Carya illinoinensis), pistachios Although every nut has different combinations and (Pistacia vera), and walnuts (Juglans regia) are com- levels of various nutritional components, nuts are monly consumed. Among these, almonds, walnuts, known to be good sources of fat, protein [19], and pistachios, and pecans are the major tree nuts that phytosterols [20]. However, most of the tree nuts vary are domestically grown and constitute more than 75% widely in their fatty acid composition (Table 2) and of the total consumed among all tree nuts utilized in therefore might contribute differentially to health ben- the United States (Table 1). Most current scientific efits. It should be noted that, as nuts are high in fat and research on the health benefits of nuts is from the therefore energy-dense, consumers sometimes regard field of cardiovascular research [15, 16], and a limited them as fattening foods. However, data suggest that amount of research is focused on the health benefits of adding nuts to a diet generally does not produce weight nuts for the brain. The Adventist Health Study [17], gain and may even help with weight loss [21]. Further- a prospective cohort study comparing Seventh-Day more, nuts are most known and touted for their richness Adventists who often maintain a vegetarian or vegan in unsaturated fat, which is generally considered to be diet versus non-Adventists, found that those who con- a healthier type of fat compared to saturated fat [22]. sumed nuts more than four times a week experienced The PUFAs that nuts contain have garnered the most fewer fatal coronary heart disease events compared to attention. Alpha-linolenic acid (C18 : 3n-3, ALA) is an those who consumed nuts less than one time per week. omega-3 fatty acid found in nuts and is the precursor Along these lines, the cross-sectional Nurses’ Health for the long-chain omega-3 fatty acid eicosapentaenoic Study [18] found that women who frequently con- acid (C20 : 5n-3, EPA), which is then converted to A.N. Carey et al. / Effects of tree nuts on brain 57 docosahexaenoic acid (C22 : 6n-3, DHA). The conver- fatty acids and it has been suggested that this may be sion of ALA to EPA and then to DHA involves further due to an unwillingness or inability to consume marine desaturation of the fatty acid chain by increasing the sources of PUFAs. Nuts may present a pleasant and number of double bonds and elongation through the feasible way to increase the intake of dietary PUFAs, addition of carbon atoms. Omega-3 fatty acids have and they can be added to various foods or eaten as been shown to be anti-inflammatory and to decrease a snack. Furthermore, people may be concerned about reactive oxygen species production [23]. Linoleic acid the mercury content of fish, for example King Mackerel (C18 : 2n-6, LA) is an omega-6 fatty acid found in nuts contains 110 mcg/4 ounces [14].