Nuts are typically classified as tree nuts (often encased in a hard shell) or peanuts (actually a legume). For most of the year, Americans eat nuts and seeds somewhat sparingly (less than one-half ounce per day) because of their high fat content. Nuts account for only 2 to 3% of the total fat intake in the American diet.117 People in the Mediterranean countries, who are noted for their healthy diet, consume twice as many nuts per person as Americans do.
A number of epidemiological studies have consistently reported that frequent consumption of nuts is associated with a reduced risk of coronary heart disease (CHD). For example, in the Adventist Health Study, people who ate nuts (mostly almonds and walnuts with lesser amounts of peanuts) one to four times per week had a 25% reduced risk of CHD, while those who ate nuts five or more times a week experienced a 50% reduction in risk of CHD, compared with people who ate nuts less than once a week.118 In a 9-year follow-up study of African-American Californians, the observed mortality rate for those who frequently consumed nuts was 44% lower than in those who consumed nuts very infrequently.119 In the Women's Health Study in Iowa, those women with the highest nut intake had a 60% lower risk of CHD than the women who never ate nuts.120 In the Nurses' Health Study, women who ate nuts five or more times per week had a 39% lower risk of fatal CHD and a 32% lower risk of nonfatal myocardial infarction than those eating nuts less than once a month.121
While 73 to 95% of the calories in nuts come from fat, they are low in saturated fat. On average, nuts contain about 60% of their calories from monounsaturated fat, which facilitates a reduction in blood total- and LDL cholesterol levels, without lowering the HDL cholesterol levels. A number of clinical trials have demonstrated the effectiveness of diets that contain either almonds, pecans, peanuts, hazelnuts, macadamia nuts, pistachios, or walnuts to significantly lower blood cholesterol levels.122-129 Human feeding trials that incorporated these nuts into the diet produced significant reductions in LDL cholesterol levels ranging from an 8% decrease with macadamia nuts to a 16% decrease with walnuts. Except for the studies with almonds and pistachios, all of the clinical trials involved normocho-lesterolemic subjects. Only in the case of the pistachio study did HDL cholesterol levels increase (up 12%).129 In an analysis of the nut-feeding studies, Kris-Etherton reports that the lipid-lowering effect of the test diets was 25% greater than that suggested by predictive equations that involve fat composition.130 It was suggested that phytochemicals and other non-fatty-acid constituents in the nuts provided the additional cardio-protective effects.
Nuts are known to contain a number of vitamins (folic acid, vitamin E), minerals (potassium, copper, magnesium), dietary fiber, and other substances important for cardiovascular health.118 Almonds and hazelnuts also have high levels of a-tocopherol, whereas pecans and walnuts are rich in y-tocopherol. The tocopherols are known to prevent the oxidation of LDL cholesterol. Nuts and seeds are also a rich source of tocotrienols, which are inhibitors of HMG-CoA reductase in the biosynthesis of cholesterol. Hence, tocotrienols are effective hypocholesterolemic agents, as well as potent cancer-preventive substances.68-70 Peanuts contain substantial levels of trans-resveratrol, the protective compound in red wine and grape juice shown to inhibit the formation of blood clots. As far as phytoestrogens are concerned, nuts and sunflower seeds have a substantial lignan content, while peanuts contain small amounts of isoflavones.65,85
Nuts and seeds are also a rich source of flavonoids,117 and contain high levels of phytosterols.131 Nuts typically contain 30-60 mg of phy-tosterols per ounce, with peanuts and pistachios having the highest levels. P-sitosterol is the major phytosterol, composing about three quarters of the total phytosterol content. Plant sterols can interfere with cholesterol absorption and lower blood cholesterol levels.132,133 The esters of phytosterols from soybean oil (esters of P-sitosterol, campesterol, and stig-masterol) were shown to be as effective as their synthetic saturated derivatives (sitostanol esters) in lowering blood total cholesterol (7-8%) and LDL cholesterol levels (12-13%), without affecting HDL cholesterol levels, in healthy, nonobese subjects.134 Two new cholesterol-lowering margarines have become popular spreads. One is fortified with the esters of sitostanols (from wood pulp) and the other with soy phytosterols. Various studies show that these margarines can lower elevated blood cholesterol levels by as much as 10%.134 However, new research shows that persons using these sterol-fortified margarines experienced a 25-30% drop in their blood levels of beta-carotene, a change that might result in reduced protection against cancer and heart disease.
Nuts, especially peanuts, are a good source of saponins which are reported to have both anticarcinogenic activity and hypocholesterolemic activity.135,136 Most nuts and oilseeds are a good source of phenolic acids (such as caffeic and ellagic acids). In particular, pecans and walnuts have high levels of the cancer-preventive ellagic acid,71 the majority of which is lost when the nuts are blanched.137 Typically, nuts also contain 300-500 mg of phytates per ounce.138 Phytates are known to reduce the risk of various cancers and reduce cell proliferation.139 Nuts are also rich in tannins, which are water-soluble polyphenolic compounds.49 Tannins have antioxidant, mutagenic and antimicrobial activities. Some are reported to possess anticarcinogenic activity.140 Tannins have also been reported to decrease blood lipid and blood pressure levels and modulate immunore-sponses, depending on the amount and type of tannins consumed.140 With such an array of cancer-preventive substances in nuts, it comes as no surprise that nuts have been reported to also be protective against prostate cancer.141
The use of flaxseed can lower both total- and LDL cholesterol levels, due, in part, to its very low saturated fat content and its rich content of polyphenolics.142 Oilseeds, such as sesame seed and especially flax seed, are very rich sources of lignans.65 Plant lignans are converted to mammalian lignans (enterolactone and enterodiol) by bacterial fermentation in the colon.143 The lignan metabolites bear a structural similarity to estrogens and can bind to estrogen receptors and inhibit the growth of estrogen-stimulated breast cancer.144
The increased life expectancy and low rates of chronic diseases such as heart disease, high blood pressure, diabetes, and cancer among the southern Europeans, may be, in part, due to their physically active, simple lifestyle and unique Mediterranean diet, which is especially rich in a variety of phytochemicals such as those found in olive oil, garlic and other herbs, beans, fresh fruit and vegetables.145,146
In the Lyon Diet Heart Study, it was observed that the Mediterranean diet reduced the risk of recurrence of heart disease after the first myocardial infarction, and that the protective effect persisted for at least 4 years. The advantage of a traditional Mediterranean diet over a prudent Western diet could not be explained by the typical risk factors, such as blood pressure levels; total, LDL or HDL blood cholesterol or triglyceride levels; smoking or alcohol use. It was suggested that the critical components in a Mediterranean diet rich in vegetables, legumes, and virgin olive oil may be the abundant flavonoids and other antioxidants that have antithrombotic activity and that quench free radicals.147
Many of the non-glyceride components of virgin olive oil are known to protect against heart disease and cancer. Because olive oil is obtained by a mild extraction method, it retains substantial amounts of bioactive phytochemicals such as phytosterols, polyphenolics, and many unique flavor compounds.148 The polyphenolics, which include anthocyanins, flavonoids (luteolin, apigenin), tyrosol, 3-hydroxytyrosol, oleuropein and phenolic acids (caffeic, vanillic, p-coumaric, and ferulic acids), amount to 50-800 mg/kg depending on the cultivar, soil, ripeness of the olives and the way the oil is produced and stored.149 Olive oil may contain as much as 2500 ppm of P-sitosterol, the major sterol in olive oil, which is believed to block cholesterol absorption. Another sterol, cycloartenol, aids in the excretion of cholesterol through increased bile acid secretion.150
A variety of water-soluble polyphenolics are found in olive pulp and are responsible for the bitter taste of olives. Measurable amounts of these compounds end up in the oil extract, with extra-virgin olive oil containing the highest levels. Tyrosol and 3-hydroxytyrosol, the major polyphenolics in olives, are found at levels of 120 to 150 ppm.148 The antioxidant activities of oleuropein and 3-hydroxytyrosol are similar to those of vitamins C and
E. The polyphenolics in olive oil inhibit LDL oxidation by scavenging free radicals and chelating free metal ions.149 The polyphenolics also enhance the synthesis of prostacyclin, PGI2, and thus inhibit platelet aggregation. These phenolic compounds are antioxidants that reduce eicosanoid production by leukocytes, so as to modulate inflammation and protect the cell against cancer-forming substances. Olive oil also contains up to 150 ppm of the antioxidant vitamin E, which can also inhibit platelet aggregation. Polyphenolics have also been shown to lower blood glucose levels.151
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