Daylight robbery: Fortify yourself!

Recent research suggests that vitamin D is more important for performance than previously thought. Andrew Hamilton updates the latest thinking on vitamin D for sports performance, and how athletes can ensure they’re getting enough for both performance and health

Vitamin D is an essential nutrient, belonging to a group of fat-soluble steroid-like compounds of which there are two major forms: vitamin D2 and vitamin D3. Vitamin D3 is the most active form in the body and is produced in the skin after exposure to UVB (ultraviolet) rays from the sun. In the diet, vitamin D can be only be found naturally in a small range of foods. These include oily fish (such as salmon, trout and sardines), eggs, milk (which is fortified) and some meats (see table 1). By contrast, vegetables, fruits, grains, cereals, pulses, nuts and seeds contain very little vitamin D. The US National Institute for Health (NIH) recommends a dietary vitamin D intake of 600 international units (IUs) per day(1). However, dietary needs are strongly influenced by your exposure to sunlight.

Table 1: Some good sources of vitamin D

Food Serving size Vitamin D content (IU)
Milk and milk substitutes
Milk (fortified) 1 cup 120
Soy milk, fortified 250mL (1 cup) 88
Yogurt, fortified 175g (3/4 cup) 58-71
Meat and eggs
Egg, yolk, cooked 2 large 57-88
Beef liver, cooked 75g (2 ½ oz) 36
Fish and Seafood    
Salmon, red, canned 75g (2 ½ oz) 530-699
Salmon, Atlantic, fresh 75g (2 ½ oz) 342
Mackerel, canned 75g (2 ½ oz) 219
Trout, cooked 75g (2 ½ oz) 150-210
Sardines, canned 75g (2 ½ oz) 144
Fats and oils
Cod liver oil 5mL (1 tsp) 427
Margarine 5mL (1 tsp) 25

Source: USDA and “Canadian Nutrient File 2010”

As UVB rays and diet both provide vitamin D, your body’s ‘store’ of this nutrient (vitamin D status) will be a function of both your dietary intake and recent sun exposure. Although there is no universal agreement, most health authorities classify vitamin D status (tested via a blood sample) as follows:

  • *Deficient – below 30ng/L
  • *Sufficient – 30-75ng/L
  • *Optimum – 75-125ng/L

Why do athletes need vitamin D?

As well as bone health, a growing body of evidence has found that optimum intakes of vitamin D in adults provide significant protection from degenerative diseases such as heart disease, multiple sclerosis and a number of different cancers(2,3). For athletes in hard training however, there are other very good reasons why your vitamin D intake matters – not just for health, but for performance too.

*ImmunityA growing body of research suggests that sub-optimum vitamin D status harms immunity, but also that boosting vitamin D intake improves immunity. For example, when scientists studied 267 endurance athletes over the winter period, they found that a significantly higher proportion of those who were vitamin D ‘deficient’ came down with an upper respiratory tract infection (URTI – coughs, colds, sore throats etc) and suffered worse symptoms than those in the ‘optimal’ status group(4). Another study found that supplementing 5000IUs per day with vitamin D₃ for four weeks during the winter boosted immune function and reduced the incidence and severity of URTIs in taekwondo athletes(5). Other recent studies have provided strong evidence that 1) poor vitamin D status is linked to a higher incidence of URTIs and that 2) increasing vitamin intake, especially over the winter period, reduces the incidence and severity of URTIs(6).

*Injury riskVitamin D is needed for the formation of strong, healthy bones, and research clearly shows that sub-optimum intakes of vitamin D can greatly increase the risk of stress fracture in athletes(7). There’s also good evidence that a poor vitamin D status increases the risk of muscle and other soft tissue injuries in athletes – particularly in the lower limbs and core muscles(8).

*Exercise performanceSome research suggests that performance might be improved in athletes with an optimum vitamin D status(9) and that higher intakes of vitamin D can improve some power and strength measures in athletes(10). However, the evidence on performance is somewhat patchier, and some studies have found no benefits(11,12).

Vitamin D status and sunshine

Numerous studies have shown that vitamin D insufficiency or outright deficiency is rife, even in the affluent West(13). The situation is no better when it comes to sportsmen and women; studies on athletes from the UK, US, Australia, Scandinavia and the Middle East have all found high rates of vitamin D insufficiency or even out-and-out deficiency(14). Why are so many people – including athletes – sub-optimum in vitamin D? There are two main reasons:

  • There are relatively few vitamin D rich foods in the diet, which means it’s easy to go short.
  • Indoor lifestyles combined with advice to avoid the sun and cover up reduces the opportunity for vitamin D synthesis in the skin when exposed to strong sunlight.

The role of sunshine in ensuring optimum vitamin D status cannot be overstated. In the summer months, sunlight exposure is an incredibly efficient way of supplying vitamin D. With just ¼ of the body’s skin exposed (eg wearing a T-shirt and shorts), the average fair-skinned person can synthesize enough vitamin D for a day with just six minutes exposure to the midday sun! However, the UVB content in sunlight is a function of season and latitude; during the winter months in the northern states, there is very little UVB in sunlight – even on the brightest winter’s day. And the further north you live, the fewer months there are with useful UVB. In southern Florida, vitamin D synthesis is possible all year round, while in Seattle, it’s possible from March to October. But in Alaska, vitamin D synthesis is only possible from early May to late August (see figure 1)(15). This explains why a lot of athletes enter the winter period with an adequate vitamin D status, but by the mid-late winter period, many have become sub-optimum or even outright deficient(16).

Figure 1: Sunshine, season, latitude and vitamin D synthesis

Zones show when and where the UVB content of sunshine is too low (below 1000W/m2) to synthesize meaningful amounts of vitamin D in skin – and therefore when supplementation of vitamin D (1000-2000IUs per day) is recommended(17). Athletes should still ensure their diet contains plenty of vitamin D rich foods at all times though!

How can I ensure my vitamin intake is sufficient?

Here are some tips for athletes who want to ensure maximum health and performance throughout the coming winter and beyond:

  • #Try to consume plenty of vitamin D-rich foods (oily fish, eggs and vitamin D-fortified produce) at all times of the year (see table 1).
  • #Use the ‘sunshine zone’ map (figure 1) to determine when vitamin D supplementation should be considered.
  • #To confirm your vitamin D status, considered getting it checked – either through your physician or using a home blood-testing kit.
  • #If your blood level is below 75nmol/L, consider supplementing with 1000-2000IU of vitamin D3 (not D2) per day until your status is improved – regardless of season.
  • #During the summer months, try to regularly expose your skin to some strong sunshine for 5-15 minutes before covering up or applying sunscreen/sun block. Regular sun exposure can help build up your body’s vitamin D stores in readiness for the winter months ahead.
  • #Consider a winter break nearer to the equator to boost your flagging vitamin D levels!


  2. Mol Nutr Food Res. 2010 Aug;54(8):1164-71
  3. BMJ 2014;348:g1903
  4. Exerc Immunol Rev. 2013;19:86-101
  5. Int J Environ Res Public Health. 2018 Sep 14;15(9). pii: E2003
  6. J Nutr Health Aging. 2018;22(4):491-500
  7. PM R. (2010) 2:945–949
  8. Arthroscopy. 2018 Apr;34(4):1280-1285
  9. Int J Environ Res Public Health. 2018 Dec 3;15(12). pii: E2724
  10. Int J Environ Res Public Health. 2019 Sep 14;16(18). pii: E3422
  11. PLoS One. 2018 Apr 9;13(4):e0195284
  12. Nutrients. 2019 Aug 21;11(9). pii: E1960
  13. Arch. Intern. Med. 2009, 169, 626–632
  14. Nutrients 2013, 5, 1856-1868
  15. J Clin Endocrinol Metab. 1988 Aug;67(2):373-8
  16. Int J Sport Nutr Exerc Metab. 2017 Feb;27(1):6-10
  17. Nutrients. 2016 Sep; 8(9): 533.

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