Selenium: the overlooked nutrient for athletes?

When it comes to sports nutrition, much time and research is directed toward optimizing the intakes of key nutrients needed for performance. As regular readers will know from the previous articles (referenced here with links), these nutrients include carbohydrate, which serves as an ideal energy source during longer or high-intensity endurance events, protein for muscle growth and post-exercise recovery, and of course fluid (water), where even a small shortfall can negatively impact performance (see this article for an in-depth discussion). Fat intake also matters to athletes, not least because too much dietary fat can displace the protein and carbohydrate needed for performance/recovery, and also because athletes need to ensure an adequate intake of the essential fats (most notably omega-3), which are important for athlete health immunity and even performance (see this article).

Macro vs. micronutrients

The dietary nutrients listed above are commonly referred to as ‘macronutrients’ because they are needed in relatively large amounts every day. A shortfall or imbalance of any of these nutrients can lead to a rapid negative performance and or/recovery impact (as any athlete who’s ‘hit the wall’, struggled to recover or become dehydrated during exercise will know!). Because these macronutrients are needed in the range of a few grams to hundreds of grams per day every day, an athlete’s intake and nutrient status is shaped by the mainstay foods he or she consumes on a day-to-day basis. Perhaps more importantly and because of this, athletes are generally acutely aware (or should be!) as to the overall balance and adequacy of their macronutrient intake.

However, in addition to the macronutrients, there’s a whole class of other nutrients - so-called ‘micronutrients’ that are essential for health and performance. Although only needed in the range of a few millionths of a gram (mcg) to fractions of one gram, their role in the body’s biochemistry and ability to perform is no less important. Familiar examples of these nutrients include (links show referenced articles):

·        Iron – a mineral needed to synthesize hemoblobin in red blood cells (essential for the transport of oxygen around the body).

·        Zinc – a mineral needed to activate key enzymes involved in protein turnover and synthesis in the body. [NB: enzymes can be thought of as ‘biochemical sparkplugs’, which enable essential biochemical reactions to occur in cells that would otherwise take place too slowly or not at all).

·        Magnesium – a mineral needed for the activation of key enzymes required for the release of energy via the ATP energy system.

·        B vitamins - needed for key enzymes required for a vast number of biochemical reactions in the body, particularly in the brain, DNA synthesis and repair, energy release, protein turnover and synthesis of signalling molecules.

·        Vitamin D – needed for immunity, muscle function and bone health maintenance.

Unlike the macronutrients where athletes tend to notice quite rapid changes (deterioration) if these nutrients are lacking or become imbalanced, a shortfall of one or more micronutrients in the diet is a far more insidious process, with changes in performance taking many weeks or month to manifest. This makes identifying a micronutrient deficiency more difficult, especially as the deficiency symptoms may initially be quite diffuse and non-specific – for example, feeling ‘washed out’, an increased tendency to suffer from illness and infection, poorer recovery, unexplained aches and pains, poorer hair and skin condition etc.

To complicate matters further, micronutrient deficiencies can easily develop under the radar; unlike fat, protein and carbohydrate contents, which are typically listed on labels, the micronutrient content of foods is not. Unless you are nutritionally knowledgeable, it might be difficult to ascertain whether you are consuming enough of these micronutrients in your diet. To make matters even more complicated, the levels of micronutrients in the same foods may vary dramatically, depending on where the food was cultivated and how it was processed before reaching your dining table. And to top it off, even if you suspect a micronutrient deficiency, getting hold of a lab test to confirm it may be expensive and logistically complicated!

Selenium – the forgotten micronutrient

When it comes to sports nutrition, the trace mineral selenium is often overlooked in terms of its importance for athlete health and function. Selenium is a trace mineral that is absolutely essential for human health, functioning as a critical component of ‘selenoproteins’. These proteins are key enzymes involved in various physiological processes in the body, including the antioxidant defense system, thyroid hormone metabolism, immune function, and reproductive health.

Research has shown that selenium, when incorporated into the selenoprotein known as glutathione peroxidase, protects cells from oxidative damage by neutralizing free radicals⁽¹⁾, which in turn helps reduce the risk of chronic diseases, such as cardiovascular disease and cancers. It also shows that selenium plays an important role in enhancing immune response by supporting T-cell function and antibody production⁽²⁾ and plays a key role into converting a molecule known as ‘thyroxine (T4 – what a physician will prescribe where thyroid activity is low)’ to triiodothyronine (T3), which is the active thyroid hormone⁽³⁾.

The recommended dietary allowance (RDA) for selenium in adults is around 55mcg/day (55 millionths of a gram⁽⁴⁾, which is easily achievable IF the diet is wholesome and nourishing. Good food sources of selenium include Brazil nuts (exceptionally rich providing the 55mcg RDA in just 2-3 nuts!), and seafoods like tuna and shrimp (providing 100-200 mcg per 100g of food); meats such as chicken and beef are also decent sources, providing 20-50mcg per 100g⁽⁵⁾. The content of selenium from carbohydrate sources is reasonable for whole grains such as whole grain wheat bread and whole grain rice and oats, typically providing around 15mcg per 100g⁽⁶⁾; however, the selenium content of refined and processed foods (eg white flour products, sugary foods etc) tends to be very poor – for example a serving of cornflakes provides only 1mcg.

Selenium and athletes

As mentioned above, selenium exerts its physiological roles in humans through selenoproteins. Of the 25 selenoproteins identified thus far in mammals, two (GPX4 and TXNRD2) are classified as mitochondrial selenoproteins (found in mitochondria – the ‘energy factories’ within cells), while seven others (DIO2, SELENOF, SELENOK, SELENOS, SELENOM, SELENON, and SELENOT) are localized in the endoplasmic reticulum (the main body) of cells⁽⁷⁾. These nine selenoproteins therefore have the ability to regulate mitochondrial and cell function.

Importantly for athletes, these selenoproteins are particularly prevalent in skeletal muscle, which means muscle is vulnerable to damage when there is a selenium deficiency. Research shows that a selenium deficiency can eventually lead to impaired muscle contraction and muscle weakness/wasting, which can progress to vascular injury within muscles as well as impaired physiological function^(8,9). Given these facts, sports nutrition researchers have become increasingly interested in the role of selenium nutrition for athletes, and now a new research paper has been published on this topic.

New research

In this study, researchers from the University of Beijing carried out a review study on selenium nutrition and athletic performance, systematically reviewing all the previous data on this topic in order to come up with solid recommendations. Published in the journal ‘Nutrients’, this new study aimed to identify all the facets of selenium nutrition that could be relevant for athletes and make some best practice recommendations⁽¹⁰⁾. Although these findings are rather technical in nature, the key points are summarized below in plain English.

What they found

Here are the key conclusions from the paper:

Antioxidant protection

Selenium mediates its physiological roles in the body predominantly through selenoproteins - not a single one, but a synergistic function of multiple selenoproteins. These selenoproteins include: seleno-W, seleno-P, seleno-M, seleno-T, seleno-K, seleno-H and glutathione peroxidise. Common to all of these seleno-proteins is the ability to deactivate harmful oxygen free radicals, which left unchecked, can harm the mitochondria and endoplasmic reticulum (ER) in muscle cells. As figure 1 shows, several key selenoproteins collaborate to eliminate excess oxygen radicals, alleviate mitochondrial and ER stress, and restore cellular balance when it is disrupted by oxidative stress induced by intense exercise. Optimizing selenium intake therefore can help maintain skeletal muscle homeostasis and balance, thereby enhancing exercise performance and fitness.

Figure 1: Synergistic effects of selenoprotein networks in skeletal muscle