Vitamins are a set of organic compounds that are required by the body in small amounts. Vitamins facilitate bodily functions by serving as cofactor or necessary component of a particular pathway/system (McArdle et al., 2016). With an exception of vitamin D, vitamin requirements are matched to daily losses via urinary secretion. Vitamin D is the only vitamin that can be synthesised by the body itself, through ultraviolet B (UVB) exposure on the skin. The vitamins can be categorised as fat- or water-soluble. Fat-soluble vitamins dissolve in fat, these vitamins rarely cause a deficiency as the body can store them in adipose tissue. Vitamin A, D, E and K are fat-soluble vitamins. Water-soluble vitamins are distributed in the bodily fluids. Excess amounts are excreted via the urine, therefore dietary requirements are higher for water-soluble vitamins (Jeukendrup & Gleeson, 2018; McArdle et al. 2010). The B vitamin family and vitamin C are all water-soluble vitamins. All vitamins are elaborated on below.
Vitamin A or beta-carotene is widely known to contribute to the health of our eyes. Rich sources of vitamin A are organic meats, carrots, sweet potatoes, pumpkin, and dairy products (McArdle et al., 2016). Vitamin A is a fat-soluble vitamin. Low intake of fat, due to dietary restrictions, can decrease overall vitamin A status. Vitamin A serves as an antioxidant by mopping up free reactive oxygen species which damage the cells and tissues. Vitamin A can lower the levels of free radicals and can thus protect the body from harm. Lastly, vitamin A regulates gene expression and thereby controls cell differentiation (Bender & Cunningham, 2021). The reference intake, according to the European Food & Safety Association, is 650 μg/day (Retinol equivalents) (European Food Safety Authority, 2017).
Vitamin B consists of several different compounds with different functions and can therefore best be described as the B-vitamin family. The B vitamins are characterised by acting on the energy metabolism in the body (Jeukendrup & Gleeson, 2018).
Vitamin B1 (Thiamine)
Vitamin B1, present in whole grains, liver, nuts, and peas, is an essential cofactor in the carbohydrate energy metabolism. Vitamin B1 is needed for one of the steps in the conversion of sugars into energy inside the cells (Bender & Cunningham, 2014). The requirements are set at 0.1 mg per MJ energy intake (European Food Safety Authority, 2017).
Vitamin B2 (Riboflavin)
Vitamin B2, like the other B vitamins, is key in the energy metabolism. The vitamin serves as an important cofactor in oxidative phosphorylation chain, as well as fatty acid and amino acid oxidation (Bender & Cunningham, 2014). Adults require 1.6 mg of vitamin B2 per day (European Food Safety Authority, 2017). As the vitamin is mostly present in dairy, like milk and cheese, deficiencies are mostly common in lactose-intolerant or vegans.
Vitamin B3 (Niacin)
Vitamin B3 can be synthesised by the body from the amino acid tryptophan. The vitamin is needed in the body to facilitate energy metabolism as well as DNA maintenance (Bender & Cunningham, 2014). It is mostly present in grains, vegetables, fruits, and dairy. It is required to take in 1.6 mg per MJ energy (Niacin equivalents) (European Food Safety Authority, 2017).
Vitamin B6 is a key cofactor for glycogen phosphorylase which facilitates glycogen breakdown, releasing glucose. This is very important during exercise, as the muscles break down glycogen to free glucose needed for energy production (Bender & Cunningham, 2014). It is required to consume 1.6 mg of vitamin B6 per day (European Food Safety Authority, 2017). Good sources are different meats and potatoes.
Vitamin B11 (folic acid)
Vitamin B11 is more commonly known as folic acid or folate. Its main functions are biosynthetic and catabolic. It facilitates the formation of new red and white blood cells and is necessary during foetal development. The daily requirement is 330 μg/day (dietary folate equivalents) (European Food Safety Authority, 2017).
Vitamin B12 is exclusively found in animal products, therefore deficiencies are most common in vegetarians or vegans. It is needed for DNA synthesis and fatty acid metabolism. Furthermore, it facilitates the formation of new red blood cells and promotes neurological function. Dietary requirements are 1.4-2.0 μg per day (Bender & Cunningham, 2014).
Vitamin C serves as an antioxidant in the body. Like vitamin A, it mops up free radicals and protects cells against structural and genetical damage (Padayatty et al., 2003). Furthermore, vitamin C supports the immune function by protecting the epithelial barrier function and enhances immune cell function (A. C. Carr & Maggini, 2017). Good sources of vitamin C are fruits and vegetables. However, there are significant losses of vitamin C during cooking and exposure to air after cutting. Dietary requirements of vitamin C are 95 mg per day (European Food Safety Authority, 2017). Research has shown that additional antioxidant supplementation (vitamin C and E) can supress muscle adaptation after strength and endurance exercise (G. Paulsen et al., 2014; Gøran Paulsen et al., 2014; Peternelj & Coombes, 2011).
Vitamin D is endogenously formed by ultraviolet B sunlight exposure on the skin. Most of the vitamin D is obtained in this way. Dietary sources of vitamin D are liver, fish, eggs, dairy, and oils (Jeukendrup & Gleeson, 2018). In the body, vitamin D plays an important role in the intestinal absorption of calcium and phosphorus and thereby promotes healthy bone formation and maintenance. It has been stated by the American College of Sport Medicine, that vitamin D affects injury prevention and rehabilitation, muscle size and signalling, stress fractures and inflammation (Rodriguez et al., 2009).
Groups at risk of vitamin D deficiency are athletes with dark skin pigmentation, early- or late-day training sessions, indoor training, excessive clothing use, or geographic locations with high latitudes and dark winter months (Larson-Meyer & Willis, 2010). Healthy vitamin D levels for athletes are set at 80 to 125 nmol/L (Cannell et al., 2009; Rodriguez et al., 2009). Under normal conditions, athletes will have sufficient exposure to sunlight to synthesise adequate amounts of vitamin D. In case of history of stress fractures, bone or joint injury, overtraining, muscle weakness or lifestyle limitations it is advised to athletes to assess vitamin D status and potentially start supplementation (Rodriquez, Marco & Langley, 2009). As vitamin D is endogenously synthesised, there is no dietary reference intake. The upper limit is set at 100 μg/day (European Food Safety Authority, 2017).
Vitamin E is one of the fat-soluble vitamins. It is absorbed, transported, and stored in fat. Vitamin E also serves as an antioxidant and can be found in all cell membranes. It prevents cellular membrane peroxidation by clearing free radicals from the cell (Herrera & Barbas, 2001). It is assumed that vitamin E can reduce the risk of cardiovascular diseases, which suggests its function differs from other antioxidants, like vitamin A and C. For instance, vitamin E can reduce atherogenesis, the formation of fatty deposits in arteries. Furthermore, vitamin E can prevent the reduction in spermatogenesis in males, resulting in a higher sperm count (Brigelius‐Flohé & Traber, 1999). Rich sources of vitamin E are vegetable oils, nuts, seeds, spinach, kiwi, and whole grains (McArdle et al., 2016). A sufficient intake of vitamin E is 11 mg/day (European Food Safety Authority, 2017).
There are two forms of vitamin K. The first form is vitamin K1 or phylloquinone, which is the dietary form found in plant-based foods. The second form is vitamin K2 or menaquinones, which is synthesised by the microbiota present in the gut. The K vitamins are cofactors in the metabolic activation of intracellular proteins (Greer, 2010). In other words, vitamin K facilitates the activation of key proteins. For example, a well-known target of vitamin K are the coagulation factors which control bleeding. Vitamin K is also an important regulator of bone health, activator of necessary proteins and regulator of transcription of bone-specific genes (Stafford, 2005). Vitamin K1 is found in green leafy vegetables like kale, spinach, and broccoli. Vitamin K2, synthesised by the gut microbiota, is found in liver, dairy, and fermented products (Stafford, 2005). The adequate level of intake is 70 μg/day, this requirement is based on data of vitamin K1 (European Food Safety Authority, 2017).
• Daily requirements for vitamin supplementation do not differ between the general population and athletes.
• An excessive intake of vitamins can even be toxic.
• It is advised for athletes to limit the supplementation of vitamins A, C and E.
• During the winter months, for some athletes, vitamin D can be prescribed.
• Dietary intake of vitamins from fruits and vegetables is sufficient, so there is no need to supplement vitamins during exercise for team, strength and (ultra-)endurance athletes.