Brazil nuts, selenium and prostate cancer
Brazil nuts are a reasonably good source of selenium provided they are grown in South America and not China. They are also a good source of other trace minerals, fibre, vitamins and healthy oils such as omega-3. There are no studies directly linking the intake of selenium with a reduced risk of prostate cancer development or progression but there is strong circumstantial evidence that there regular consumption is a good idea after prostate cancer.
Selenium has been shown to slow the progression of prostate cancer cells when added to a culture medium in the laboratory. In humans two large studies in 1980 and 1990 showed that a low selenium status was associated with an increased risk of developing a number of cancers including prostate. They also showed that in patients with selenium deficiency, the prostate cancers they developed were more likely to be aggressive and fatal. The Harvard Health Professional Survey, for example, linked low selenium status (measured on toenail clippings!) with higher rates of aggressive prostate cancer. Both Finish and Taiwanese studies have linked blood lower levels of selenium with higher rates of lung and a liver cancer called hepatocellular carcinoma (HCC). In China, where the incidence of HCC is high, the inhabitants of one village were supplemented with selenium whilst another five villages were given simple salt. After six years, involving over 130,000 people, there was a 35% reduction in the HCC rate in the selenium-supplemented village, but no change in the others.
These data prompted the design and initiation of an excellent double-blind randomised trial in the USA called the Nutritional Prevention Study. It recruited 1312 individuals with a history of skin cancer and prescribed either placebo or 220 micrograms of selenium a day. The primary aim (end point) was to see if dietary selenium supplementation could reduce the risk of recurrent skin cancer. There was no difference in the number of skin cancers between the selenium or placebo group. However, when the data was analysed in more detail, a significantly lower level of lung, bowel and prostate cancer was seen in the selenium group and this lived up to robust statistical evaluation. Several large ongoing prostate prevention studies, including the SELECT study, are now underway across the world to try to confirm these findings and fine-tune the optimal selenium dose required. Many suspect, however, that the people who would benefit from selenium supplements are those with a dietary deficiency in the first place, for example, those living in areas with low levels of selenium in the soil.
The RDA for selenium is 60-75mcg/day and more than 200mcg/day is excessive. Taking too much selenium is likely to be harmful and increase the risk of cancer and other illnesses so it is better to rely on dietary intake which is very unlikely to produce an overload. As well as south American brazil nuts, sardines, prawns, shellfish, eggs, crab and crab liver are also good sources. USA grown wheat is relatively high in selenium, but the same cannot be said for European varieties because of the low levels in most European soils. Oily fish contains selenium, but this may well be bound to the mercury and made inert so also protects against mercury exposure such found in marine foods by forming inert metal-selenium complexes.
Omega-3 fatty acids found in fish, avocado and nuts have been shown to slow the growth of cancer cells in the laboratory in Petri-dishes, as well as in mice. Human studies have suggested that omega-3 fatty acids or even more the ratio of marine omega-3 to omega-6 fatty acid can help reduce cancer progression. The reason for this is not entirely understood, but one mechanism appeared to be the inhibition of the enzyme which is affected by aspirin [cyclooxygenase-2 (COX-2)] which is over-expressed in some prostate cancers.
Foods which naturally contain a high percentage of antioxidants have more recently been nick named ᠨref="http://www.cancernet.co.uk/diet-superfoods.htm">superfoods͊The section on diet and prostate gives an overview on the the foods recommended and not recommended after prostate cancer.