Epidemiological studies have shown that regular consumption of nuts is associated with a significant reduction in the risk of heart disease. Clinical trials have shown that nut consumption can have favourable effects on serum lipids (fats such as cholesterol in the blood), particularly by reducing the levels of low density lipoprotein (LDL) cholesterol; the type of cholesterol in the blood which is involved in the development of atherosclerosis (the narrowing and hardening of the arteries which leads to heart disease and strokes). Therefore, reducing LDL “bad” cholesterol levels can significantly reduce the risk of developing heart disease, which is why nut consumption has such a protective effect. It has been suggested that nuts may exert their cardio-protective effects through the actions of their fatty acids, the type of dietary fibre, the very low ratio of the amino acids lysine to arginine, the effects on platelet function and prostaglandin metabolism as well as their high antioxidant capacities. [1-4] The exact mechanisms by which nut consumption is able to exert these protective effects however is not yet fully understood. As well as being a great source of non-animal protein, (therefore important for vegetarians) nuts are also very rich source of micronutrients such as vitamin E and certain minerals including selenium. A variety of nuts have been shown to have heart-heathy effects, though certain nuts such as Almonds and Walnuts are suspected to be among the best. Caution must be taken where allergies exist, (more common with peanuts than tree nuts) though for most adults, a handful of nuts per day would make an excellent heart-healthy addition to the diet. Although the type of fat in nuts is a healthy one, people on a calorie-restricted diet should take this into account.
The Adventist Health Study is a prospective cohort investigation of 31,208 white Californian Seventh-Day Adventists who were studied for more than a decade. Extensive dietary information was obtained at the beginning of the study, along with the values of traditional coronary risk factors. The results of this study found that those who consumed nuts frequently (more than four times per week) experienced substantially fewer fatal heart attacks and nonfatal heart attacks when compared with those who consumed nuts less than once per week. In fact, frequent nut consumption decreased the risk of both fatal and nonfatal heart attacks by approximately 50%. In this study, wholemeal bread was also seen to have a protective effect, whilst frequent beef consumption appeared to increase the risk of heart attacks. 
Several years later, associations between traditional risk factors, consumption of selected foods, and both coronary heart disease and all-causes of death were evaluated in the oldest portion (over 84 years old) of the Adventist Health Study group. Among this subgroup there were 1397 deaths, 364 of which were due to Coronary Heart Disease (CHD). Diabetes, regular consumption of beef and regular consumption of doughnuts appeared to increase the risk of CHD and all causes of death. The two protective factors that were observed were regular exercise and regular nut consumption. Compared with those who consumed nuts less than once per week, those who consumed nuts 5 times per week had a 20% reduced risk of all causes of death, and a 40% reduced risk of death from CHD. 
Another cohort study in California involved 3,299 African-Americans who were surveyed in 1974 and followed for more than a decade. When looking at all causes of death that occurred throughout this time, it was found that fruit and salad consumption decreased the risk whilst high blood pressure, being diabetic and heavy meat consumption appeared to increase the risk. The only dose-responsive factor found however, was nut consumption. Compared with those that rarely ate nuts, those who had moderate nut consumption had a 40% reduced risk of death whilst those that consumed nuts the most frequently had a 55% reduction in all cause mortality. 
The largest cohort study in the U.S is the Nurses Cohort which involved 86, 016 women between 34 to 59 years of age with no history of heart disease, stroke, or cancer at the beginning of the study in 1980. These women were studied for 14 years, during which time 1,255 hearts attacks occurred, 861 being non-fatal and 394 being fatal. After looking at the dietary and lifestyle surveys that they had been filling out throughout this time, the researchers found that compared with those who rarely consumed nuts, the women who ate 5 or more servings of nuts per week had 45% less heart attacks, both fatal and non-fatal. Statistical adjustments for age, smoking, intakes of dietary fats, fibre, vegetables, and fruits as well as other known risk factors and protective factors, did not alter these results. What’s more, an inverse association was seen throughout subgroups of this study, stratified by levels of smoking, use of alcohol, use of multivitamin and vitamin E supplements, body mass index, exercise, and intake of vegetables or fruits. This was one of the most important studies to provide evidence that nut consumption can significantly protect against coronary heart disease; the single most common cause of death in the western world. 
Among the same group of women, 3,206 of them were diagnosed with type II diabetes throughout the time they were followed. It was also found that nut consumption was inversely associated with the risk of type 2 diabetes; that is, the more frequently nuts were consumed, the lower the risk of diabetes. Consumption of peanut butter was also inversely associated with type 2 diabetes as well. 
Another large cohort study in the U.S is known as the US Physicians' Health Study, involving 21, 454 male participants who were followed for 17 years. An assessment of the men’s dietary and lifestyle habits based on regular questionnaires they had submitted found that compared with men who rarely or never consumed nuts, those who consumed nuts 2 or more times per week had a 57% reduced risks of sudden cardiac death and a 30% reduced risk of CHD. This association remained even after statistical adjustment for known cardiac risk factors and other dietary habits. 
In the Iowa Women’s Health Study, 34,111 postmenopausal women with no known cardiovascular disease were followed for approximately 12 years. During the 12 year follow-up, 3726 women died, 657 from CHD. Assessments were made of the frequency of consumption of nuts and other foods, as well as other CHD risk factors. Compared with those who rarely consumed nuts, those who consumed 2 or more servings per week had 20% fewer deaths from CHD. A modest inverse association was also found between frequency of nut consumption and all causes of death. 
Clinical Trials: The effects of nuts on serum lipid (blood cholesterol) levels.
Using a randomized crossover trial design, 22 men and women with normal cholesterol levels replaced half of their habitual fat (approximately 14% of approximately 29% energy) with either whole almonds or almond oil for 6 week periods. After this time, both whole almonds and almond oil consumption resulted in a 14% reduction in plasma triglyceride levels, a 4% reduction in Total Cholesterol (TC) levels, and a 6% reduction in LDL “bad” cholesterol levels, whilst HDL “good” cholesterol levels increased by 6%. Because this beneficial effect was seen in both those consuming whole almonds and those consuming almond oil, it was suggested that the protective component of almonds was found in its fatty acid content. 
In a randomized crossover study, 27 men and women with high cholesterol (hyperlipidaemia)were given either a meal containing almonds, a meal half of which was almonds, the other half whole wheat muffins, or simply whole wheat muffins alone (control) for a month. After this time, the people consuming the half almond meal had significant improvements in their cholesterol status, as shown by reductions in LDL “bad: cholesterol, LDL:HDL ratio (the ratio between bad and good cholesterol), whilst those who consumed the whole almond meal had even more substantial improvements, as shown by even greater reductions in LDL cholesterol levels, LDL:HDL, lipoprotein(a) and oxidized LDL concentrations. No significant changes occurred in those consuming only the whole wheat muffins (controls).  The authors of this study concluded:
“Almonds used as snacks in the diets of hyperlipidemic subjects significantly reduce coronary heart disease risk factors, probably in part because of the non-fat (protein and fibre) and monounsaturated fatty acid components of the nut.”
In another randomized crossover study, 25 healthy middle aged men and women were fed 3 diets, all containing the same amount of calories, for 4 weeks each after being fed the same diet for a 2-week run in period. The subjects were given either a control diet containing no almonds, a low-almond diet (in which almonds contributed to 10% of the total energy intake), or a high-almond diet, (in which almonds contributed 20% of the total energy intake). Blood tests revealed that the low almond diet showed beneficial effects by reducing the total cholesterol (TC) levels, LDL cholesterol, LDL:HDL ratio and apolipoprotein A and apolipoprotein B levels. The high almond diet also showed these beneficial effects, only even more substantially. Compared with the control diet (no almonds) , the high-almond diet reduced total cholesterol by 4.4% (and average of 0.24 mmol/L) LDL cholesterol by 7% (an average of 0.26 mmol/L) and apolipoprotein B by 6.6% (or 6.6 mg/dL). It also increased HDL “good” cholesterol by 1.7% (average of 0.02 mmol/L) as well as decreasing the ratio of LDL to HDL cholesterol by an average of 8.8%. The high almond diet contained approx 28g of almonds. All diets contained approx 2000-kcal. These results provide further evidence that moderate consumption of almonds can be beneficial by lowering the risk of cardiovascular disease. 
Forty-five men and women with high serum cholesterol levels averaging 251 mg/dL followed one of three diets; almond-based, olive oil-based, or dairy-based (the total amount of fat in each diet was the same) for 4 weeks. Blood tests revealed that the almond-based diet resulted in significant reductions in Total Cholesterol (TC), LDL-Cholesterol and the TC:HDL ratio. HDL “good” Cholesterol levels did not change. The dairy-based (control) diet resulted in significant increases in Total Cholesterol, whilst the olive-oil based diet resulted in no significant changes. The body weight of the subjects did not significantly change either. Because Olive oil is rich is mono-unsaturated fat just as nuts (including almonds) are, the authors of this study suggested that the more favourable cholesterol-altering effects of the almond-based diet may be due to a combination of numerous bioactive constituents found in almonds. 
In another study, 30 healthy medical students aged 18 to 19 years consumed hazelnuts in addition to their normal daily diets for 30 days. Other than the hazelnuts, their dietary habits during this period did not significantly change. They gave blood samples at the beginning and then at the end of this time. The results of these blood tests showed that after they had been consuming hazelnuts for a month, their average total cholesterol (TC) and LDL “bad” cholesterol levels, as well as their LDL:HDL ratio had significantly dropped, whilst their HDL “good” cholesterol levels had significantly risen. Moreover, malondialdehyde (a toxic by-product of lipid peroxidation used as a marker of free radical damage) was found to be significantly lower and antioxidant potential values were higher after the 30 days of hazelnut consumption. The authors noted that this indicated that peroxidation reactions were largely prevented by antioxidant constituents of the hazelnuts. 
A randomized crossover trial of three 30-day diets was conducted in 30 volunteers aged 18 to 53 years. Each was fed a "typical American" diet high in saturated fat (37% energy from fat); an American Heart Association Step 1 diet (30% energy from fat); and a macadamia nut-based monounsaturated fat diet (37% energy from fat) in random order. Blood tests were given throughout this time to evaluate the effects of each diet on cholesterol levels. 
The results are as follows: Total cholesterol (TC) level was 5.20 mmol/L (201 mg/dL) after the typical American diet, 4.99 mmol/L (193 mg/dL) after the Heart Association diet, and 4.95 mmol/L (191 mg/dL) after the macadamia nut based diet. LDL “bad” cholesterol levels were 3.37 mmol/L (130 mg/dL) after the typical diet, 3.21 mmol/L (124 mg/dL) after the Heart Association diet and 3.22 mmol/L (125 mg/dL) after the macadamia nut diet. HDL “good” cholesterol levels were 1.43 mmol/L (55 mg/dL) after the typical diet. HDL “good” cholesterol levels however were not higher in the macadamia nut diet. The authors of this study concluded: “The macadamia nut-based diet high in monounsaturated fat and the moderately low-fat diet both had potentially beneficial effects on cholesterol and low-density lipoprotein cholesterol levels when compared with a typical American diet.”
Another study conducted to assess the cholesterol-lowering potential of macadamia nuts involved 7 middle aged men with high cholesterol (hypercholesterolemia) who were given between 40 and 90g of macadamia nuts per day, (equivalent to 15% of their energy intake), for 4 weeks. Blood tests taken before and after the study revealed that total cholesterol (TC) levels decreased by 3%, LDL “bad” cholesterol decreased by 5.3%, and HDL “good” cholesterol levels increased by 7.9% after the 4 weeks of macadamia nut consumption. Plasma triglyceride and homocysteine concentrations however did not change throughout this time.  The authors concluded:
“This study demonstrates that macadamia nut consumption as part of a healthy diet favourably modifies the plasma lipid profile in hypercholesterolemic men despite their diet being high in fat.”
A randomized, double-blind, 5-period crossover study design involving 22 people was used to examine the effects of several different cholesterol-lowering diets on serum lipids and lipoproteins.  Three of these diets consisted largely of foods high in monounsaturated fatty acids (MUFA’s), the other low in both saturated and mono-unsaturated fatty acids. The 3 mono-unsaturated fatty acid rich diets were:
1. An olive oil based diet (34% fat; 7% SFAs, 21% MUFAs).
2. A peanut oil based diet. (34% fat; 7% SFAs, 17% MUFAs)
3. A diet based on peanuts and peanut butter. (36% fat; 8% SFAs, 18% MUFAs).
The other diet used which is known to have favourable effects on cholesterol levels which is low in both saturated and mono-unsaturated fat, was an American Heart Association/National Cholesterol Education Program Step II diet, which consisted of 25% total fat, 7% of which was made up of saturated fatty acids (SFAs) and 12% made up of monounsaturated fatty acids (MUFAs).
These four diets were compared to an average American diet which consisted of 34% fat; 16% SFA’s & 11% MUFA’s.
All three of the high-MUFA diets lowered total cholesterol by 10% and LDL “bad” cholesterol by 14%. This response was comparable with that observed for the Heart Association Step II diet.
Elevated triaglycerol levels are a risk factor for the development of heart disease. Triacylglycerol concentrations were 13% lower in subjects consuming the high-MUFA diets and were 11% higher with the Heart Association Step II diet than with the average Americans diet.
Based on these results, the authors concluded that the Olive Oil, Peanut Oil and the Peanut / Peanut Butter diets decreased cardiovascular disease risk by an estimated 25%, 16%, and 21%, respectively, whereas the Step II diet lowered CVD risk by 12%.
An Eight-week, randomized, controlled study of pecan consuming group vs non-pecan consuming group (controls), involved 19 people with normal lipid levels, 10 of which had been randomly assigned to the pecan treatment group (7 women, 3 men, average age of 45years ) and 9 to the control group (8 women, 1 man, average age of 37 years). The pecan consuming group consumed 68 g pecans per day for 8 weeks plus self-selected diets. The pecans contributed 459 kcal and 44 g fat daily. The control group avoided nuts and consumed self-selected diets. Blood tests of all participants were collected at the beginning of the trial, half way through, and then again at the end. The results showed that LDL “bad: Cholesterol levels were lowered in the pecan consuming group from 2.61 mmol/L at the beginning, to 2.46mmol/L by the end. Whilst total cholesterol (TC) was also lowered in the pecan consuming group, so too was HDL “good” cholesterol. Even though total energy, total fat, monounsaturated fat and polyunsaturated fat (as well as insoluble fibre and magnesium), were significantly higher in the pecan consuming group than in the control group, body mass indexes and body weights were unchanged in both groups. 
A randomized, controlled crossover study investigating the effect of pistachio nuts on cardiovascular disease risk factors, involved 10 patients with moderately high cholesterol levels who were given 3 weeks of dietary modification with 20% caloric intake from pistachio nuts. After three weeks, there was a decrease in total cholesterol, an increase in HDL “good’ cholesterol, and a decrease in the LDL:HDL ratio. Triglycerides and LDL levels decreased, but not significantly. Body weight and blood pressure remained constant throughout the study. 
Sixteen middled aged male volunteers with normal cholesterol levels consumed a diet providing 36% of energy as fat (92 g fat/d) for 9 weeks. For the first 3 weeks, they were given a diet with a ratio of polyunsaturated to monounsaturated to saturated fatty acids (P:M:S) to match the average Australian diet. For the next three weeks, they were given an ordinary diet which included 84 g/day of almonds (rich in mono-unsaturated fatty acids) whilst for the final 3 weeks they were given the same ordinary diet, including 68 g/day of walnuts (rich in poly-unsaturated fatty acids) instead of almonds. Compared with the standard Australian diet, the almonds reduced total and LDL “bad” cholesterol by 7% and 10%, respectively, whilst the walnuts reduced total and LDL cholesterol by 5% and 9%, respectively. 
Thirteen post-menopausal women and 5 men (average age 60) were given a habitual diet, a low fat diet, a habitual diet plus walnuts or a low fat diet plus walnuts. During the low fat plus walnut diet, total cholesterol (TC) concentrations decreased by 0.58 mmol/L when compared with the Habitual Diet, and by 0.46 mmol/L when compared with the low fat diet. LDL“bad”cholesterol concentrations also decreased by 0.46 mmol/L when compared with the low fat diet. 
During 2 periods of four weeks, 21 men with mildly high cholesterol levels were asked to consume two low fat diets (fat 30% total energy), one containing, on average, 78 g/d walnuts, the other containing no nuts at all. Oth total and LDL “bad cholesterol levels decreased in both diets, whilst HDL “good” cholesterol levels increased in both as well. When comparing the walnut and low fat diets only apo B was significantly lower on the walnut diet. Despite the walnut diet containing more total energy and total fat, the subjects did not gain any weight. 
Blood serum components, such as lipoproteins, coagulation factors, and homocysteine have been associated with cardiovascular disease. To determine the effects of walnuts, as part of a low-fat, low-cholesterol diet, on serum cardiovascular risk factors, 67 patients with borderline high total cholesterol followed a low-fat, low-cholesterol diet for six weeks before being randomly assigned to continue the diet or have 64 grams/day of walnuts in conjunction with the diet. After six weeks, the patients' diets were switched (crossover). Blood results taken throughout the study showed that serum lipids demonstrated a significant reduction in triacyglycerols and favourable trend with decreases in total cholesterol (TC), LDL “bad” cholesterol, and a slight increase in HDL “good” cholesterol. No significant effects on homocystine or the coagulation factors occurred throughout the study. 
In another trial, 18 healthy men were randomly placed on two mixed natural diets, each diet to be followed for four weeks. Both diets conformed to the National Cholesterol Education Program Step 1 diet and contained identical foods and macronutrients, except that 20 percent of the calories of one diet (the walnut diet) were derived from walnuts. After 4 weeks on the walnut diet, the average total cholesterol (TC) levels were 12.4% lower than the non-walnut diet, LDL “bad” cholesterol levels were 16.3% lower, however HDL “good” cholesterol was also 4.9% lower than the non-walnut diet. The LDL:HDL ratio was also lowered by the walnut diet. Average blood-pressure values did not change during either dietary period. 
Another randomized, crossover feeding trial involved 55 middle aged men and women with high cholesterol levels. The patients were given a cholesterol-lowering Mediterranean diet for 6 weeks, and a diet of similar energy and fat content in which walnuts replaced approximately 35% of the energy obtained from monounsaturated fat for another 6 weeks (or reverse order). Compared with the Mediterranean diet, the walnut diet reduced total cholesterol by 4.1%, LDL “bad” cholesterol by 5.9%, and lipoprotein (a) levels by 6.2%. LDL particles were enriched with polyunsaturated fatty acids from walnuts, but their resistance to oxidation was unchanged. 
Nuts and Cardiovascular Disease: In Conclusion
It has been suggested that the obvious cardio-protective effect of nut consumption is beyond what would be predicted from the fatty acid content alone. Nuts are a rich source of many other nutrients and bioactive components, similar to other whole-plant foods, such as fruits and vegetables. 
WARNING: Peanuts and Allergies .
Due to a certain type of protein it contains, one of the most common food allergens is peanuts. Common allergic reactions to peanuts include hives, angioedema, wheezing, throat tightness, vomiting, and diarrhoea; however a very small portion of people with this allergy may have reactions so severe that it could result in anaphylactic shock and even death. Studies indicate that probably just over 1% of the U.S. population suffers from peanut allergies, and only a very small percentage of those people would be capable of suffering potentially life threatening reactions. 
Nonetheless, because of this potential for harm, foods which may contain traces of peanuts must carry warnings, and people who are susceptible to serious allergic reactions should carry self-injectable epinephrine (adrenaline) for emergency situations. Given this potential for danger, if you have never tried peanuts before and do not know if you have a peanut allergy, it is best to seek medical advice before making peanuts part of your diet. The allergen in peanuts is a certain protein, and thus is not present in great quantities in peanut oil; however those that are extremely sensitive to peanuts should avoid peanut oil as well, as it may contain tiny traces of this protein. It is generally only peanuts that are responsible for such allergies and other nuts are usually not associated with allergies.
Vitamins and Minerals
Nuts are very rich sources of the antioxidant vitamin E, as well as certain minerals such as calcium, copper, magnesium, manganese and selenium. Brazil nuts are an extremely rich source of Selenium; in fact, 1 ounce (28g) or 6-8 Brazil nuts contains over 500mcg of Selenium.  This amount is actually above the Upper Intake (UI) level of 400mcg/day.  The Recommended Dietary Intake (RDI) for Selenium is approx 85 mcg/day for men and 70 mcg/day for women,  so 2 or 3 Brazil nuts per day is plenty to ensure optimum intake.
There are several forms of vitamin E, however the form named alpha-tocopherol has been identified as having the greatest significance to human nutrition. In our body’s cells, lipids (fats) undergo damage by free radicals, which are highly destructive substances with an unpaired outer electron. Vitamin E and the mineral Selenium, work in concert by protecting cell membranes from free radical damage by giving up their own electrons to the radical, thus saving the molecules in our cells from being damaged. Vitamin E has also shown to be able to protect LDL “bad” cholesterol from becoming oxidized. [33-35] This is important because the oxidization of LDL cholesterol is an important step in the formation of atherosclerosis; the narrowing and hardening of the arteries which causes heart disease and stroke. Other potentially cardio-protective effects of vitamin E have been shown by its ability to inhibit platelet aggregation (which causes blood clots) and dilating blood vessels.
Several studies have found a lower incidence of cardiovascular disease as well as Alzheimer’s disease among people who consume diets high in vitamin E, however results are mixed. [36-38]
A recent pooling of 9 prospective studies which included information on intakes of vitamin E, carotenoids, and vitamin C was carried out. During a 10 year period, 4647 heart attacks occurred among the 293, 172 subjects involved in the studies, all of whom were free of heart disease at the beginning. Those who consumed that most vitamin E had a slightly reduced risk (16%) which is not all that significant. 
Most studies have found that taking large supplemental doses of vitamin E is of no benefit. A recent meta-analysis which looked at 19 clinical trials involving vitamin E supplements for various diseases, including heart disease, kidney disease and Alzheimer's disease, found that the people who took vitamin E supplements containing 400 IU/day or more were slightly more likely to die from any cause than those who did not take vitamin E supplements at all. 
Another review of 14 controlled trials involving more than 170,000 people found that antioxidant vitamin supplements including vitamin E, offered no protection against cancer. In fact, people taking both beta-carotene and vitamin A or vitamin E supplements had in increased risk of premature death by 30% and 10%, respectively.  This may be because vitamin E supplements may contain very high doses which would be impossible to obtain naturally through food. Such high does are not natural and may have adverse effects, as demonstrated by these 33 studies involving over 360 thousand people.
Another important factor is that vitamin E supplements generally only contain alpha-tocopherol, which is only one of 8 forms of vitamin E. The other forms include beta-, gamma- and delta-, tocopherol, as well as alpha-, beta-, gamma- and delta- tocotrienols. All forms are present in food sources such as nuts, but not in supplements.
Preliminary evidence suggests that gamma-tocopherol or its metabolites may also protect against free radicals. [42,43] It has also been found that taking alpha-tocopherol supplements lowers gamma-tocopherol levels in the blood.  In a recent prospective study, higher levels of gamma-tocopherol in the blood were associated with a significantly reduced risk of developing prostate cancer, whilst no protective associations for higher alpha-tocopherol levels were found unless gamma-tocopherol levels were also high. 
These are very preliminary findings however, and it is not yet known precisely why high dose vitamin E supplements do not offer the same benefit that dietary vitamin E does. It may well be the fact that foods high in vitamin E such as nuts, oils and spinach contain various other protective components as well as all the different forms of vitamin E, whereas vitamin E supplements do not. Until further evidence is available, one would be unwise to take large supplemental doses of vitamin E, whilst ensuring adequate amounts from dietary sources such as nuts, olive and sunflower oils, spinach and avocado.
It should also be noted that doses higher than 1,000 mg/day of alpha-tocopherol of any form, may interfere with normal blood clotting which could result in haemorrhage (internal bleeding).  Because of this risk, it is frequently recommended that vitamin E supplementation be discontinued several weeks prior to surgery to decrease the risk of haemorrhage. Large dose vitamin E supplements should not be given to children. 
Selenium and Cancer
An inverse association has been observed between the average selenium intake / blood serum levels, and the incidence of certain cancers in several different countries world wide.  Evidently, populations living among low selenium containing soils may have inadequate selenium intake, thus being more predisposed to certain cancers. Studies among persons consuming adequate selenium however do not appear to show any relationship.
A case-control study in Finland collected blood samples from 95 men with lung cancer and 190 without (controls). Comparisons found that the average serum selenium levels were lower in those who had developed lung cancer than those that did not. It should however be noted that Finland is known to have very low selenium soil levels. 
Another case-control study in the U.S. found an inverse association when they compared toenail selenium content between 181 males with prostate cancer and 181 without (controls).  Similar results were found in a study comparing serum selenium levels of 249 men diagnosed with prostate cancer with 249 matched controls.  The Baltimore Longitudinal Study of Aging registry also found that lower serum selenium levels were associated with an increased risk when comparing those of 52 men diagnosed with prostate cancer and 96 age-matched controls. 
A larger case-control in Canada involving 414 cases of breast cancer, 232 cases of prostate cancer and 402 cases of colon cancer found no association between toenail selenium levels and the risk of breast or colon cancer, but an inverse association was found in regards to colon cancer. 
A double blind, placebo-controlled intervention study known as the Nutritional Prevention of Cancer (NPC) Trial, involved giving 200mcg of selenium daily or placebo to 1312 older residents with a history of recurrent non-melanoma skin cancer. After 7.4 years, the results showed that the selenium supplemented men had a 51% decreased risk of prostate cancer compared to those on the placebo, however this protective effect appeared greatest in men whose serum selenium levels were significantly lower to begin with.  Conversely, a more recent follow up from this study actually found a 25% increased risk in squamous cell carcinoma (a type of skin cancer) among those given selenium compared to the placebo group. 
Intervention trials have found that selenium supplements decreased the risk of liver cancer among a population at high risk due to a very high prevalence of Hepatitis B, in Quidong County, China.  It should however be noted that approx 58% of liver cancers in Quidong, China, contain a genetic mutation which is rarely seen in liver cancers in Western countries.  Moreover, certain areas of China are known to have very low selenium levels; thus a significant portion of these residents may have had abnormally low selenium levels to begin with.One study found that breast cancer patients in the U.S and Japan had lower serum selenium levels than women without breast cancer,  however one of the largest epidemiological studies in the U.S which followed over 60,000 female nurses for several years found no association between toenail selenium levels and the risk of any cancer. 
Evidently, higher supplemental doses of selenium may not necessarily be chemo-protective; however sub-optimal dietary selenium intake may increase the risk of developing certain cancers. Therefore it is important to include adequate selenium containing foods in ones diet. Wholegrain products, fish / seafood, meat and nuts are among the highest selenium containing foods, however by far the richest source of selenium are Brazil nuts. An average serving of Brazil nuts (6-8 nuts) provides in excess of 500mcg of Selenium,  which is more than the Upper Limit. Although studies in human adults have found that selenium toxicity does not occur below 900mcg/day, the Upper Limit has been set at 400mcg/day until further evidence is collected to ensure safe consumption of higher amounts.  The Recommended Dietary Intake (RDI) for Selenium is approx 85 mcg/day for men and 70 mcg/day for women,  so 2 or 3 Brazil nuts per day is plenty to ensure optimum intake. The levels of Selenium in common foods have been determined by the U.S. Department of Agriculture and can be accessed here: http://www.nal.usda.gov/fnic/foodcomp/Data/SR16-1/wtrank/wt_rank.html
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