OVERVIEW
· Selenium, a trace mineral, was identified as being important to human health more than half a century ago but remains a little known nutrient.
· Indispensable for fertility, thyroid function, antioxidant defences, immune function and cancer prevention, selenium has a big role to play in maintaining good health.
· Selenium intake is tragically low in most European countries, leaving millions in a state of deficiency and susceptible to diseases such as cancer, and little is being done to improve the situation.
· When selenium does grab the headlines, recommended doses are normally too high and could give rise to unforeseen adverse effects.
· Discover how to get just the right amount of selenium to reap maximum health benefits, while minimizing any risks.
It’s unlikely that you know a lot about selenium. And if you do it is likely that your knowledge comes courtesy of the self-styled health gurus who erroneously trumpet the use of high-dose selenium supplements. Either way, in this chapter you’ll find out just what you need to know about selenium, and probably a tad more for good measure. This means you can do one simple thing every day that could make a big difference to your health.
Selenium, which borrows its name from ‘Selene’, the Greek goddess of the moon, is a trace mineral, which basically means we only need tiny amounts of it. What could be more straightforward? As ever, there’s a catch. If you live in the UK (or much of Europe for that matter), the chances are you’re missing out on selenium, and that spells bad news for your health, including your resistance to cancer. The flipside is that if you live in the USA, the chances are you’re doing just fine when it comes to getting selenium from your food, unless you’ve been misled into taking an unnecessary high-dose selenium supplement, in which case you could be creating some problems of your own.
Where did all the selenium go?
As little as 30 years ago, folk in Europe were getting considerably more selenium than they do today1. At that time wheat was imported from North America, where it was grown in selenium-rich soil. Yet things changed, and Europeans switched to home-grown crops instead. However, the selenium-deficient European soils meant that the staple source of selenium was lost from the ‘daily bread’ and European selenium intake has not recovered since.
Average intakes of selenium in the UK scrape in at a rather pathetic 43mcg per day for women and 55mcg per day for men2. This is in complete contrast to the USA, where intakes are more than double this, with women averaging 92.6mcg per day and men 133.5mcg per day2. So, if you’re living in the UK, it is likely that you’re not getting enough selenium and as a result are putting your health at risk.
Little selenium and the big ‘C’
Selenium has quite an impressive résumé. It’s a feisty little nutrient capable of delivering antioxidant and anti-inflammatory effects, as well as activating thyroid hormones in the body. If you don’t get enough selenium you run the risk of a compromised immune system, cognitive decline and greater overall mortality. That’s before we mention its important role in male fertility and female reproduction, auto-immune conditions and combating viruses3. Yet its greatest claim to fame is undoubtedly its role in cancer prevention.
Every year over 3.2 million Europeans are diagnosed with cancer and 1.7 million will die from cancer4.
Before we get too deep into this, we’re keen to set the record straight. If you do happen to read or hear something about selenium, chances are it’ll be referred to as an ‘antioxidant’. The way selenium works is a whole lot smarter than that. It does increase antioxidant activity in the body, but not through a direct antioxidant action (say, like vitamin C). What’s clever about selenium is that it increases the body’s inbuilt antioxidant defence mechanisms (a family of enzymes called glutathione peroxidases). So, by getting enough selenium in your diet, your body’s ‘nature-knows-best’ antioxidant system can work to its full potential.
There are lots of very plausible ways in which selenium could help protect against cancer. First, selenium helps to reduce oxidative stress, damage to our DNA and inflammation, which are all implicated in the development of cancer5. Second, selenium can up-regulate the immune system, thereby helping the body to destroy cancer cells6,7. And last, but by no means least, it appears that some metabolites of selenium, such as methyl selenol, may exert their own range of anticancer actions8.
Initial published research created a great deal of excitement about selenium as an anti-cancer agent. Out of scores of prospective studies most showed that having a higher selenium level lowers the risk of common cancers such as lung, bladder, colorectal, liver, oesophageal, gastric-cardia, thyroid and, the most well publicized of all, prostate cancer3. The real headline-maker though was the much-acclaimed National Prevention of Cancer (NPC) Trial, conducted in the US. Here the effect of 200mcg per day of selenium was examined in 1,312 individuals. The results were startling. Those receiving the selenium supplements had a 25% lower rate of cancer incidence over the seven-year period examined9. There was strong suggestive evidence of a reduction in colorectal cancer incidence, but the true benefit was seen in prostate cancer prevention, with supplementation slashing its incidence in half10.
While impressive, this trial was too small to conclude with 100% certainty that selenium is definitely giving these benefits. For that, larger, bolder and better-designed clinical trials are needed. And this is where we find a fly in the ointment. Exactly such a trial was carried out for selenium’s promising effects on prostate cancer. Contrary to what was expected, the trial yielded disappointing results. SELECT was a large-scale randomized, double-blind, placebo-controlled trial of 35,533 men from across the USA, Canada, and Puerto Rico. Participants were randomly assigned to four groups, who received either selenium (200mcg/day), vitamin E (400IU/day), selenium plus vitamin E, or a placebo. In September 2008, after a period of five and a half years, the SELECT trial participants were advised to cease supplementing, as no positive effect at all on prostate cancer incidence had been observed11. Superficially at least, this seemed to blow the selenium–cancer hypothesis out of the water. But closer scrutiny tells a different story.
Not too much, not too little
What’s going on here might sound a bit technical but it’s actually pretty straightforward, so hang in there. What these studies tell us is that selenium demonstrates a U-shaped dose response relationship with cancer. This means that, as selenium levels increase, the risk of cancer decreases, but once you get to a certain point the risk plateaus, and if you continue to increase selenium levels yet further, the risk starts to increase again. For want of a better saying, when it comes to selenium, the old adage that ‘everything is good in moderation’ applies. What that means for you is that you should aim to get your selenium levels into the zone of maximum benefit – neither too low nor too high.
Selenium and cancer – a U-shaped curve
The way that selenium works is to do with the fact it’s indispensable to a whole series of vital proteins called selenoproteins, which carry out a heap of good work in the body critical for everything from thyroid hormones and male fertility to anti-inflammatory and anti-cancer effects. There are 25 different selenoproteins in humans, of which selenoprotein P is considered particularly important when it comes to cancer prevention8,12. Plasma selenium levels of around 120–125mcg/L (levels commonly seen in the US) are deemed to optimize the expression of selenoprotein P12,13 (the UK population languishes significantly below this threshold, at typically 80–90mcg/L14). Go above these levels and no further expression of selenoproteins takes place. It’s almost as if you’ve reached saturation point.
In the words of Professor M. Rayman, Professor of Nutritional Medicine at the University of Surrey: ‘Any condition associated with increased oxidative stress or inflammation might be expected to be influenced by selenium status. There is some evidence that this is the case in preeclampsia, pancreatitis, asthma, and systemic inflammatory response syndrome.’
What the evidence tells us is that the benefit from selenium supplementation is only realized in individuals that start off with low levels. Here we’re talking about the kind of levels that are lower than we might typically see in the USA, but very common in Europe. The NPC trial, despite being carried out in the USA, was conducted in the southeastern part of the country, an area that has comparatively low selenium levels. Starting from this low baseline level, the trial found a significant benefit from selenium supplementation on cancer incidence. Yet this was not representative of the US population as a whole. SELECT’s much greater population of 35,533 was a better representation. In SELECT, 78% of individuals had ideal selenium levels before commencing supplementation, which explains why no benefit was observed when they were given a selenium supplement. In fact, research is now finding that giving more selenium to people who already have sufficient levels is harmful for their health. Instead of preventing cancer it appears the risk may increase9, and higher selenium intakes are also linked with an increased occurrence of diabetes11,15.
How much to take
As you can see, giving extra selenium to folk in the USA who already get enough is not the smartest idea, and supplementation is unwarranted. But for people living in Northern and Western Europe, who simply don’t get enough selenium in the first place, it is recommended. Even so, doses as high as 200mcg, as used in the clinical trials, are completely unnecessary. We just need enough to match our US counterparts to optimize our selenoproteins. For women, a selenium supplement of 50–60mcg daily will raise levels into this ideal range. For men, especially those wishing to prevent prostate cancer, it may be prudent to go slightly higher, up to as much as 100mcg daily. When choosing a selenium supplement, we recommend that you take it in the form of selenium yeast, which best resembles food-form selenium and was the form used in the NPC trial.
Brazil nuts are touted as an exceptional source of selenium. We don’t buy into that idea and suggest you don’t either. Not only is the selenium content of Brazil nuts highly variable (more than a 1000-fold difference), they can also contain undesirably high levels of barium and radium, a radioactive material3.
Where the message has gone wrong
While the early research made a case for the merits of high-dose supplementation, as we now know, more is definitely not better. To us, it’s amazing that so called ‘nutritionists’ in the public eye, who were outspoken advocates of high-dose selenium supplementation, continue to remain part of the ‘more must be better’ brigade. High doses are indiscriminately recommended for European and US populations alike, despite ill-effects now being associated with such amounts.
Endorsement by American nutritionists of 200mcg supplements, the dose found to be at best useless and likely harmful, remains prevalent, and it’s not difficult to find recommendations and supplements available for sale at outright dangerous levels of 400mcg. In the UK we’ve dropped the ball, too, by not understanding the ‘U-shaped’ concept. One renowned UK ‘nutritionist’ – who we won’t name and shame as he is in no way alone – recommends ‘a supplement of 200–300mcg of selenium for those who want optimal (cancer) protection’.
It’s easy to envisage selenium as harmful in large amounts when we remember that it was first identified as a toxic element!
At the other extreme, and in many ways worse, is the lack of intervention from national health authorities across Europe. Some countries, such as Finland, have recognized their responsibility to the health of their citizens and ordered selenium to be added to the fertilizers used to grow crops, thereby bolstering intakes. The majority of deficient European countries, however, are yet to embrace such action (although ironically, in the UK we’ve been supplementing the feed of our livestock with selenium to reduce their disease incidence since 1978!). With more and more evidence demonstrating the benefits of getting adequate selenium, we have to seriously question why those with the power to act to boost our selenium intake are doing precious little about it.
THE PARTING SHOT
Above all else, we urge you to remember one thing: the ‘U-shaped’ curve. The USA and Europe are completely different with respect to their selenium levels. In the USA, a substantial proportion of the population already gets their fill of selenium, and vastly expensive trials designed to give even more selenium have consequently been a flop. Here the message is clear: if you’re already getting enough selenium taking even more will have no benefits and could cause harm. If we’ve learned anything at all from the whole antioxidant debacle it’s that over-zealous and excessive supplementation is a bad idea.
In contrast, it’s the UK and Europe that urgently need the extra selenium, and right now they’re not getting it. Here, to reap the benefits of this powerful trace mineral, a modest supplement is required. Even then, remember that the dose is key and it should be a selenium supplement designed to give the body just the amount of selenium it needs for optimal disease protection – no more, no less.
SUMMARY AND RECOMMENDATIONS
· An adequate intake of selenium, sufficient to optimize the functioning of selenoproteins, appears necessary for optimal health and to reduce cancer incidence.
· While most people living in the USA are already getting enough selenium in their diet, the UK languishes significantly behind.
· Good dietary sources of selenium include fish, shellfish, meat, kidneys and liver, but even eating a healthy and well-balanced diet is unlikely to provide optimal levels of selenium in countries such as the UK.
· For the average adult in the UK, a daily selenium supplement of 50–60mcg should optimize the functioning of selenoproteins and maximize anti-cancer benefits. For the prevention of prostate cancer, a higher dose of selenium, up to 100mcg daily, can be considered.
· Most people living in the USA are already getting enough selenium in their diet and don’t need a selenium supplement.
· More is not better and excess selenium is outright toxic. Selenium is a double-edged sword, and having even marginally too much may have unforeseen adverse effects, including an increased risk of type 2 diabetes and cancer.