Vitamin D: it’s time to D-fend yourself!

Andrew Hamilton explains why paying attention to your vitamin D status is essential if you want to stay well this winter

The winter equinox is almost upon us, which means – in the northern hemisphere at least – that the inevitable scourge of upper respiratory tract infections (URTIs – coughs, colds, sore throats, flu etc) is about to begin in earnest.The good news is research shows that with the right nutritional approach, immunity can be bolstered, which can reduce the risk of winter URTIs. And one of the most powerful immune-stimulating nutrients is vitamin D.

However, there’s bad news too because numerous studies have shown that vitamin D insufficiency or outright deficiency is rife, even in the affluent West. Over 77% of Americans are considered to have a sub-optimum vitamin D status(1), while, a German study concluded that vitamin D levels were insufficient or deficient in almost ¾ of the general German population(2)! 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(3).

Improving your vitamin D status

Up until around eight years ago, little research had been carried out into how vitamin D status affects endurance athletes. But a 2013 study concluded that an optimum vitamin D intake could be extremely important for endurance athletes wishing to keep winter coughs, colds and flu at bay.

British scientists studied 267 endurance athletes – including a number of Olympic triathletes – to see what influence the athletes’ vitamin D status had on their immune function and the incidence, severity and duration of any upper respiratory tract illness (URTI – coughs, colds, sore throats, sneezing, earaches, nasal congestion fever, chills etc) episodes(4).

At the start the study 38% of the subjects had inadequate or deficient levels of blood vitamin D. By the end of the four months that rose to 55% (vitamin D levels in the body tend to drop through the winter because the sun is too weak for it to be synthesised in the skin). More importantly, a significantly higher proportion of subjects in the vitamin D ‘deficient’ status group came down with an URTI during the study period than in the ‘optimal’ status group, and the total number of ‘URTI symptom days’ and the symptom-severity score in the vitamin D deficient group was significantly higher than in the other groups.

In plain English, optimum vitamin D status in endurance athletes can boost immunity resulting in less URTI episodes and less severe episodes when they do occur. Conversely, a poor D status is likely to mean more coughs, colds and flu. More recent research on a wide range of athletes has confirmed beyond doubt that this is indeed the case(5).

The solution

Given these findings, boosting your vitamin D intake through the winter months (when there’s not enough sunlight to produce it in the skin) could be a very worthwhile strategy to keep the winter bugs at bay. Here are some tips:

  • Get your vitamin D status checked regularly – if your blood level is below 50nmol/L, consider supplementing with 1000-2000IU of vitamin D3 per day;
  • During the summer months, try to regularly expose your skin to some strong sunshine (Nature’s best way of boosting vitamin D stores in the body) for 5-15 minutes before covering up or applying sunscreen/sunblock;
  • Consider a winter break nearer the equator to boost flagging vitamin D levels;
  • Try to consume plenty of vitamin D-rich foods (eg oily fish, milk, eggs) at all times of the year.

Of course, there are a number of other proven nutritional strategies to maintain immunity and stay well through the winter, and this is a topic we have examined in depth in a number of other Sports Performance Bulletin articles. In particular, this article provides a comprehensive summary of research-proven nutritional strategies that can help to bolster immunity, reduce the incidence of URTIs and also reduce the severity and duration of symptoms when an URTI does occur. So take care of yourself and keep well this winter because it’s a guaranteed way of ensuring a flying start to next season!


  1. Arch. Intern. Med. 2009, 169, 626–632
  2. Arch Phys Med. 1945;26:641–644
  3. Nutrients 2013, 5, 1856-1868
  4. Exerc Immunol Rev. 2013;19:86-101

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