Vitamins and minerals are micronutrients found in a variety of plant and animal foods which are essential for hundreds of biological processes throughout the body. Many of these processes are vital for exercise and athletic performance, such as energy storage and utilization, protein metabolism, oxygen transport, bone metabolism and immune function. Despite the importance of these nutrients, deficiencies are relatively common.
Deficiencies vary widely between different populations, due to exposure and adherence to different diets. For example the typical western-type diet (high in animal protein, saturated fats and refined carbohydrates) is the most adopted diet in first-world adult populations and shows deficiencies in phosphate and magnesium.(1) A Mediterranean-style diet has been suggested to be superior to other diets for micronutrient intake.(2)
Supplementation is popular within athletic circles, but is it really necessary? Athletes will generally have a higher caloric intake than untrained individuals, and we would hope a better understanding of nutritional requirements, so do we still find mineral deficiencies in athlete populations?
Iron deficiency is more common in female athletes than male, although it has been reported in males involved in weight controlled sports such as wrestling.(3)
There are a number of factors to take into account here. Men are able to store more iron within the body, therefore women need a higher intake. Women also lose iron during menstruation.
Only 10% of iron that is consumed is then absorbed by the gut. Heme iron, which is found in fish and meat is more easily absorbed than non-heme iron, which is found in fruit, veg, cereals, rice etc. This explains why vegans and vegetarians are more likely to be iron deficient despite actually generally having a higher iron intake. There is some limited evidence that Vitamins A and C may increase absorption of non-heme iron from the gut.(4, 5)
In athletes with a low iron level causing anaemia (low ferritin, low haemoglobin), iron supplementation has been clearly shown to increase ferritin and Hb levels, improve VO2 max and reduce blood lactate levels after exercise.(6)
There is conflicting evidence as to whether iron deficiency that is not severe enough to cause anaemia (low ferritin, normal haemoglobin) has any measurable detrimental effect on performance, and whether supplementation is of any benefit in this group. For example in an 8 week study of 31 female athletes with low ferritin levels but normal Hb, iron supplementation was shown to increase ferritin and Hb levels, but with no improvement in VO2 max or reduction in blood lactate levels.(7)
Studies from Cornell University however have shown that untrained women with low ferritin and a normal Hb who received an iron supplement during training had significantly greater increases in VO2 max and 15-kilometer cycling endurance performance compared to the placebo group.(8)
Iron supplements will not enhance performance in athletes with normal ferritin and normal haemoglobin levels. However, endurance athletes using ‘live high, train low’ altitude training to stimulate red blood cell production require increased iron levels, and therefore may benefit from iron supplementation.(9)
Calcium is important for cellular signalling and bone health. Calcium deficiency leads to thinning of the bones (osteoporosis) as they are broken down to release calcium preferentially for use in cell signalling pathways. Sustained high intensity activity causes a reduction in sex hormones, which coupled with low body fat and BMI are risk factors for developing osteoporosis. Exercise itself also induces moderate calcium loss. This causes an increased risk of stress fractures and other skeletal injury.
Athletes particularly at risk of this are females involved in weight controlled sports, such as gymnasts, dancers and elite marathon runners. The combination of disordered eating, amenorrhoea due to low BMI/oestrogen levels and osteoporosis is known as the female athlete triad.(10, 11, 12)
There is currently no evidence that calcium supplementation has any direct effect on athletic performance, however only aerobic capacity has been investigated.
Sodium depletion is most likely to occur during exercise, due to excess sweating rather than due to low dietary intake. This occurs more commonly during endurance events in hot climates. In addition to excess sweating, sodium depletion can also occur due to excessive consumption of hypotonic fluids.
Blood sodium concentration is largely controlled by the kidneys. Therefore if the kidneys are damaged, this can lead to alterations in blood sodium levels. Rhabdomyolysis, where muscle damage causes the release of myoglobin into the bloodstream, can result in kidney damage as myoglobin is toxic to the kidneys. (13)
The best way to avoid sodium depletion during exercise is to drink an isotonic solution, which contains the correct amount of sodium. The simplest form of this is the ‘home oral re-hydration solution’- one litre of water with half a level teaspoon of salt and six level teaspoons of sugar.
Magnesium is interesting, because although magnesium deficiencies in athletes are not particularly common (again, gymnasts/dancers/wrestlers were most at risk), evidence shows that even marginal magnesium deficiency impairs exercise performance and amplifies the negative effects of strenuous exercise such as oxidative stress.(14)
Studies show that magnesium supplementation may have a positive influence on muscle performance, inflammation and immunological blood markers, but there is no evidence that it improves endurance performance related outcomes.(15)
Zinc deficiency on the other hand is relatively common. Some research has shown that up to 23% of male and 43% of female athletes have a serum zinc level below the normal range. However there is little evidence that mild zinc deficiency in adults has detrimental effects on performance, or that supplementation leads to any improvement in performance. The main correlation that may be of concern seems to be between zinc deficiency and lack of appetite.(16, 17)
Phosphate (commonly supplemented as sodium phosphate) has been shown to improve a range of parameters, including sprint time, power output, VO2 max, resting heart rate, and biomarkers of metabolic demand and cardiac function.
In female team sport athletes sodium phosphate was shown to improve both total and best sprint times for all sets and overall, with around a 6% improvement compared to placebo and beetroot juice.
Another study showed improvements in power output by around 30 watts, and finishing time during a 1km time trial in well-trained cyclists following six days sodium phosphate (4g/day) supplementation, compared to placebo.
In elite off-road mountain cyclists, the loading phase of sodium phosphate increased VO2 max (5.3%), and reduced resting heart rate (9.6%), max heart rate (2.7%), and heart rate at lactate threshold (1.7%), all of which were maintained following lower dosing, compared with placebo.
Interestingly, ‘moderately’ trained individuals supplemented similarly showed no improvement in aerobic capacity or power output. This suggests that sodium phosphate may only improve aerobic performance when individuals are well-trained or elite athletes, indicating that the dietary intake of phosphorus may be adequate outside of extreme physiological demands. (15)
Athletes are unlikely to be deficient in copper, selenium or chromium. (3)
- Female athletes are more at risk of mineral deficiencies than male athletes, particularly those involved in weight restricted sports
- The most likely minerals for athletes to be deficient in are calcium, iron and zinc
- A balanced diet is the most important source of minerals, and supplementation should only take place after assessing and optimising nutrition
- Supplementing deficiencies does not necessarily lead to improvements in performance
- Supplements that have been shown to improve performance in elite athletes (where a deficiency was present) are iron, magnesium and phosphate
- Over-supplementation can also cause side effects and negatively impact on performance