Vitamins are organic compounds that support a number of functions within the body. Vitamins are required for reproduction and growth in addition to roles in energy metabolism, muscle contraction and maintenance of general health. Vitamins are not able to be synthesised by the body and as such are required to be consumed in small daily amounts, ranging from micrograms to milligrams. Vitamins are classified as fat-soluble (A, D, E and K) or water-soluble (B group and C). Vitamins C, E and beta-carotene (a precursor to Vitamin A) also function as antioxidants.
Exercise training requires increased activity of energy production pathways and result in greater presence of free radicals, or reactive oxygen species. This has led to speculation that athletes may have higher requirements for associated vitamins (primarily B vitamins, vitamins C and E as well as beta-carotene) than the general population. Despite the logical physiological reason for increased requirements of these vitamins in people who exercise a lot, athletes have rarely been shown to have vitamin deficiencies, with the notable exception of vitamin D, which will be discussed in detail later. The lack of observed deficiencies in most vitamins could be explained by the fact that athletes consume more food to meet their additional energy requirements and this increased food intake is likely to meet any additional requirements for vitamins. Assessing vitamin deficiency is complex and some biomarkers are not always diagnostic of nutrient depletion or any associated reductions in body functions. Diurnal fluctuations and strict homeostatic control of blood concentrations of various biomarkers are two reasons that make the objective assessment of the vitamin requirements of athletes quite difficult. Therefore, many challenges occur when developing specific vitamin intake recommendations for athletes and active individuals. The current guidelines for the general population, such as the Recommended Dietary Intake listed in the Nutrient Reference Values are still quite frequently used as a starting point for athletes with relatively minor adjustments made for specific circumstances that might increase selected vitamin requirements. Vitamins that are of particular importance to athletes are vitamins C and D.
Athletes are at higher risk of upper respiratory tract infections such as the common cold, particularly during periods of high training stress. High dose vitamin C supplements may be able to improve outcomes associated with the common cold. A meta-analysis of 29 studies involving a total of 11, 306 participants showed no evidence of a reduction in incidence of the common cold in the general population. Despite this, results from a 642-member sub-group of marathon runners, skiers and soldiers on subarctic exercises were more optimistic of the opportunity to reduce incidence, symptom severity and duration of the common cold. The outcomes of this review provide cautious support for the targeted use of prophylactic vitamin C during periods of high training load.
Musculoskeletal injury (involving muscle, tendons, ligaments, cartilage and bone) is common in sport and comes at a significant cost to performance, among other factors. The consumption of vitamin C and gelatine may facilitate greater collagen synthesis and strengthening of the extracellular matrix of tissues prone to injury. Vitamin C is an essential cofactor for enzymes involved in collagen synthesis and gelatine has a similar amino acid profile to collagen because it is made by processing animal bones and cartilage. A recent study, involving 8 recreationally active young men, investigated the ability to augment collagen synthesis following oral intake of vitamin C-enriched gelatine prior to intermittent activity. Circulating glycine, proline, hydroxyproline and hydroxyglycine levels increased, peaking one hour after supplementation. Improved collagen content and ligament mechanics were also observed. Although the results of this study are very promising, the methods used involved in-vitro testing of engineered ligaments. Therefore, more research is still required to demonstrate the effectiveness of vitamin C and gelatine intake for injury prevention and faster rehabilitation. However, these supplements are quite cheap, easily accessed, safe for most athletes and are now becoming quite popular in practice.
Vitamin D is an important nutrient for bone health, plays a role in immune function and is also involved in muscle growth and repair processes. Many athletes have poor vitamin D status. While athletes have often been shown to have low dietary intake of vitamin D, it is likely that insufficient exposure to UVB light due to long hours spent training indoors and/or increased use of sun protection are also contributing factors. Measuring vitamin D status is complex and there is currently much debate about exact serum concentrations that indicate insufficiency and deficiency. Low serum concentrations of vitamin D are associated with impaired muscle function. However, improved performance is not always observed when individuals with low vitamin D status receive vitamin D supplements. It is also unclear if any adverse effects result from long-term supplementation with high doses of vitamin D. Given the importance of vitamin D for many functions in the body and the impact low vitamin D status can have on performance and well-being, more research is required to provide specific guidelines and recommendations for athletes, particularly young athletes with immature skeletal systems and athletes in locations that have limited sun exposure.
Acute exercise results in the production of free radicals, which are highly reactive molecules that can be produced during the metabolic reactions associated with energy production from carbohydrate, fat and protein. Free radicals have the potential to cause damage to cell membranes and impair immune function. These molecules have been linked to an increased risk of cancer, cardiovascular disease and diabetes. However, they also play a role in cell signalling and muscle adaptation to exercise.
Antioxidants are molecules that inhibit oxidation and reduce the potential for free radicals to cause damage in the body. Vitamin C, E and beta carotene (a precursor to vitamin A) can all function as antioxidants. However, there are many other non-vitamin antioxidant compounds that are ingested in food.
Endogenous antioxidant systems in the body become more effective in response to a period of exercise training. This adaptation is possibly one reason why exercise reduces the risk of many chronic diseases. Interestingly, the supplementation of exogenous antioxidant vitamins such as vitamin C and E has the potential to disrupt endogenous antioxidant systems and supplementation with these vitamins has sometimes been shown to be associated with an increased risk of early death from chronic disease. Supplementation with exogenous vitamin C and E has also been shown to disrupt the normal physiological adaptations to training, which could potentially have a negative impact on performance. However, the studies conducted in this area often use quite high doses of supplements and no negative effects are seen when athletes consume foods that are naturally high in antioxidants. In fact, extracts from foods that are naturally high in antioxidants such as blackcurrants and tart cherries have been shown to improve recovery after exercise and these may be beneficial during periods of high training stress.
The broad guideline to consume a wide variety of nutrient dense, minimally processed food appears to be just as relevant for athletes as it is for the general population when it comes to ensuring the adequate intake of vitamins and antioxidant compounds.
Knowledge Check – Vitamins
Minerals are inorganic substances, which alongside vitamins are considered essential to life. Minerals can be broken down into macrominerals and trace elements. Macrominerals include sodium, potassium, calcium, phosphorus and magnesium, and are required by the body in amounts >100mg/d. Trace elements are required by the body in smaller amounts, generally <20mg/d and include iron, zinc, copper, chromium and selenium. Mineral losses in sweat may increase the requirements of athletes, however, these losses can be highly variable and it is often difficult to determine exact mineral requirements of athletes. With the exception of iron (discussed below), specific mineral deficiencies are rarely observed in athletes. The increased food consumed by athletes to meet their increased energy requirements may be sufficient for many athletes to meet any increase in mineral requirements.
Exercise training can increase iron requirements above those of sedentary individuals. The exact amount of additional iron required by athletes is not known precisely and is likely to depend on the specific circumstances of individuals. Periods of rapid growth, training at high altitude, menstrual blood loss, haemolysis resulting from frequent foot strikes, or injury have the potential to negatively influence iron status in athletes. In fact, it has been observed that over one third of active women have low iron status. Iron consumption by athletes is often below recommended intakes for healthy individuals and low iron status can negatively impact on both health and exercise performance. Regular monitoring of iron status in higher risk groups such as female endurance athletes and athletes following vegetarian or vegan diets should be considered if available. However, it can be difficult to accurately monitor iron status in athletes because plasma volume expansion can dilute some biomarkers. Serum ferritin, a biomarker indicating iron stores, is also increased during stress, illness and injury which can frequently occur in athletes and this can limit the ability of ferritin to identify athletes at risk of iron deficiency. Athletes with low iron status or deficiency may benefit from high dose iron supplementation, but this is only recommended when close monitoring and the support of a health professional is available. It is prudent to recommend that athletes consume a variety of iron rich foods in order to prevent low iron status.
Calcium is an important micronutrient for athletes because exercise can increase bone mineralisation and calcium requirements may be slightly higher in exercising individuals. Dietary intake assessments of athletes often find calcium intake to be below general population recommendations. This is particularly important for athletes who haven’t yet reached peak bone mass, which usually occurs between 20 and 30 years of age. It is therefore important for many athletes to increase dietary calcium intake. Dairy foods are often the first consideration and this food group is also rich in other nutrients important for bone health such as phosphorous, vitamin D and protein. However, many other foods such as green vegetables, fish with edible bones and nuts can also be high in calcium.