CANCER DIETARY SELENIUM LEVELS
In North America, dietary selenium levels are relatively high, hence extra may not be as useful as it would be in Europe where dietary selenium levels are lower (the difference relates to selenium levels in the soil in which vegetables are grown). In addition, selenium can be supplied as a pure chemical form or as what is known as a ‘complex’ linked to organic compounds more akin to the form obtained from food. Thus all we really know for sure is that the precise form of tablet used in the SELECT trial does not prevent prostate cancer in North American men. Other trials are still ongoing with both agents – for example, our own group is studying both selenium and vitamin E in men and women with early bladder cancer (also linked to lack of both in the diet) to see if supplements can prevent recurrence of cancer.
My own opinion is that in most cases in the developed world, the levels of most vitamins and minerals will be adequately provided by most diets, particularly given the growing tendency to over-consume calories. Any effect from supplements in this setting is likely to be small, as most diets will already contain an excess over what is really needed. This is why definitive trial proof has been so difficult to obtain. As with many things in life, what starts out looking quite simple gets more complex the closer you look at it. This uncertainty, of course, fuels the market in supplements – what could be safer than taking extra ‘natural’ vitamins and minerals? If the men in white coats (though, of course, mostly we don’t wear them any more) are not sure, why not take them just in case?
What about herbal remedies? These are, of course, attractive in the sense of being somehow more ‘natural’ than harsh, chemically produced pharmaceutical products. The logic is, however, intrinsically flawed – there is nothing inherently ‘nice’ about the natural world– watches any wildlife television show for confirmation of this. The word really has no meaning in this setting – context is everything. For example, botulism is a highly unpleasant, sometimes lethal, gut infection, but botulinum toxin is used to make people look more ‘beautiful’ and is certainly relatively safe as a medicinal product. The medicinal product is therefore much safer than its ‘natural’ source. If a herbal remedy works, it is of course because it is a drug (or more precisely, a mixture of many drugs, with varying activities and side effects). There is also nothing magic about it being ancient (as if the length of use somehow confers an aura to it). Good examples of long-used natural remedies include witch hazel (which contains abundant salicylic acid, better known as aspirin), the opium poppy (the source of morphine and diamorphine), and foxgloves. Foxgloves are a good example of an ancient source of drugs. A brew known as ‘Shropshire tea’ made from foxglove leaves was used for centuries to treat the ailment known as ‘dropsy’ – accumulation of fluid in the lower limbs, accompanied by shortness of breath, now known to be heart failure. Then 20th-century science isolated the active ingredients – a family of chemicals named after the plant – digitalis alkaloids, of which the most commonly used is called digoxin. These drugs still form a major component of the treatment of heart failure. As far as I am aware, though, no one still uses Shropshire tea in place of digoxin.
So what about herbal cancer drugs? Well, firstly, many cancer chemotherapy drugs are indeed herbal extracts – vincristine, used to treat blood and lymphatic cancers is derived from the periwinkle plant. The taxanes, used for many cancers including breast, prostate, and lung, are derived from the yew tree bark and leaves, and so on. Hence the study of the properties of herbs has been a major and fruitful source of some of our most potent drugs. Again, the natural source of these drugs would not make a good herbal medicine – for example, eating yew leaves is both difficult (they are very tough) and potentially fatal – the window between useful treatment effect and lethality is small.