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Vitamin D and Immunity for Athletes
Written by: Stephanie Boville MSc, RD, Registered Dietitian and Sports Nutritionist
As the snow and cold weather rolls in, it is often accompanied by the common cold or worse, the flu. Endurance athletes are more at risk of illness, specifically upper respiratory tract infections (URTI) because they often engage in intense training sessions outdoors for over an hour. Athletes who are over trained are even more at risk of getting sick. Studies show that runners training for 96km/week doubled their odds of illness compared to those training only 32km/week and about 40% of marathoners experience an URTI in the 2 months of winter training leading up to a marathon race.
Along with the vigorous training, other factors such as stress, lack of sleep and inadequate nutrition can lead to immune suppression, increasing risk of illness. If we look at the IOC paper discussing the consequences athletes face when they are in a relative energy deficient state (inadequate overall energy intake), we see that one of the many systems affected is immunity. Being that my specialty is nutrition, in this blog I would like to explain how vitamin D may help boost your immune function this winter season.
Immune Function in Athletes
There are a few reasons why URTI’s are common. Although this is not a complete list of all the immunological changes that occur with heavy exercise, it will provide background as to why illness is specific to the upper respiratory system.
- Cortisol, a stress hormone, is increased temporarily with exercise which suppresses the immune system.
- Specific biomarkers of inflammation increase after exercise and have been linked to immune suppression.
- Natural Killer cell (cells that kill viruses) activity is reduced for at least 6 hours by 40-60% following exercise lasting over an hour.
- Nasal neutrophil phagocytosis (cells responsible for searching for and destroying bacteria) is decreased with intense exercise
- Nasal and salivary Immunoglobuin A (IgA) (antibodies secreted in the saliva which are responsible for protecting the immunity of the respiratory tract) concentration has been shown to be decreased by 70% for 18h after a 31km race.
- Nasal mucociliary transit time is prolonged post marathon race for several days, meaning the movement of foreign particles and bacteria out of the upper respiratory tract and away from the lungs is less efficient.
All of these aspects combined demonstrate that there is a window of increased risk of contracting an URTI after exercise because the immune system is compromised. The
last three points demonstrate why the upper respiratory tract in particular is so vulnerable, especially because the nose and throat are the first line of defense against pathogens entering the body.
Vitamin D and Immunity
There are two studies I want to discuss:
Study 1 was an observational study investigating the effects of vitamin D status on the incidence and immune function during winter training in 239 endurance athletes. They found that being vitamin D deficient resulted in significantly more symptom days and increased symptom severity of URTI when compared to optimal, adequate and inadequate vitamin D levels. They also investigated saliva samples which showed that salivary IgA concentration was significantly reduced in athletes with deficient levels of vitamin D, and that only athletes with optimal levels of vitamin D had significantly higher salivary IgA secretion rates. From the information in the paragraphs above, we see that there is one area in common, the salivary IgA. Both exercise and reduced levels of vitamin D results in decreased salivary IgA concentration and secretion rates which suggests that having an optimal level of vitamin D has an important role to play in maintaining immunity in athletes. Lastly this study found that the antimicrobial peptides in the blood were positively correlated with vitamin D status suggesting that optimal vitamin D may result in better immune function and protection.
One limitation of this study is that it was an observational study and therefore no clear links can be determined as to whether an intervention with vitamin D supplement in vitamin D deficient athletes will reduce frequency, length and severity of illness. Rather this study concludes that there is an association between reduced vitamin D status and increased illness length and severity along with reductions in immunological markers.
Study 2 investigated the effects of 14 weeks of 5000IU vitamin D3 supplements in 39 athletes on antimicrobial peptides and proteins. They found that the 5000IU dose for 14 weeks was enough to elevate blood levels of vitamin D to optimal levels and that 14 weeks of winter training without supplementation decreased vitamin D levels. 5000 IU of vitamin D per day resulted in significantly higher percent change in antimicrobial peptide concentration compared to placebo and increased the salivary IgA and antimicrobial peptide secretion rates. Therefore, their conclusion was that optimal vitamin D may help up-regulate the systems needed to protect against URTI.
Some limitations of this study include participants started at adequate levels of vitamin D and therefore the changes might underestimate the importance of vitamin D’s effect
on immunity and bigger changes might be seen when vitamin D levels start at an inadequate/deficient level. Another limitation to this study is they did not record incidents of URTI and therefore no conclusion could be made as to whether the immunological changes resulted in decreased illness.
Research has clearly demonstrated that vitamin D is of critical importance in the body’s immune system however there is more to learn about how vitamin D levels impact the frequency of illness and severity, and at what level the immune system is compromised.
From the studies above we learn that:
- Optimal levels of vitamin D increase antibacterial peptides in the blood
- Optimal levels increase salivary IgA secretion rates and concentration
- Observational studies show that illness severity and length is decreased compared to lower levels of vitamin D.
Because salivary IgA is suppressed after intense exercise and taking vitamin D increases the IgA secretion rate and concentration, this may be one mechanism that optimal vitamin D is used in to protect against upper respiratory tract infections.
Should we be supplementing?
Usually my belief is “food first, supplement second”. With that being said, there are few foods that contain high levels of vitamin D. We can make vitamin D from the sun, however we are too far north for adequate sun exposure to make vitamin D this time of year (fall through spring). Even in the summer we may make inadequate amounts of vitamin D from the sun if we wear sunscreen, have dark skin or train inside or early/later in the evening. It is likely a good idea to supplement to make sure you are getting adequate amounts of vitamin D. Not only does it play a role in immunity, but it also helps maintain bone and muscle health, and therefore it is an important nutrient.
How much vitamin D?
Health Canada has set the Adequate Intake (AI) level of vitamin D at 600 International Units (IU)/day and the upper limit at 4000 IU/day for most ages with some exceptions. First, AI means that there is not sufficient evidence to set a Recommended Dietary Allowance (set to meet the needs of 97-98% of healthy individuals) but the AI is assumed to be adequate. There is debate about the AI for vitamin D as many think this AI is set too low, and people are also supplementing over the upper limit (like in study 3 from above). In the following years this AI could see an increase.
People often think milk is the best source of vitamin D, however 1 cup of milk only provides around 100IU or 1/6 of your daily needs of vitamin D. One of the best sources is actually salmon, which, depending on the kind can provide anywhere between 200-600IU in one 2.5oz serving (the size of a deck of cards).
Depending on diet and your current vitamin D status, supplementing with 1000-2000IU should be enough to give your vitamin D levels a boost and is recommended, especially in the winter.
There are many other tactics athletes can try to maintain and help with increased immune function. If you would like to learn more, book an appointment so we can optimize your nutrition intake to support your health, well being and athletic performance!
Stephanie Boville MSc, RD
Stephanie is our Registered Dietitian and sport nutritionist. She graduated with Honours from the University of Guelph with a Bachelors of Applied Science specializing in Applied Human Nutrition. She then pursued her passion for sport performance nutrition by completing her Masters of Science degree specializing in Exercise, Nutrition and Metabolism at the University of Guelph. Here she was involved in studies investigating the nutritional adequacy of young hockey players and hydration habits of amateur, varsity and elite athletes to name a few. She then completed her internship at London Health Sciences Centre and is currently working there on the Medicine unit. She also has experience working with mental health and eating disorders. She also working towards being a Certified Specialist in Sport Dietetics.
Stephanie spent most of her childhood in the rink as a competitive figure skater, and later was involved in volleyball, track and cross country. During her university years she was drawn to lifting and has continued with this ever since. She is currently enjoying learning the art of Olympic weightlifting. Stephanie believes that every food fits in moderation and truly believes that nutrition has a huge impact on our sport performance and health.
Nieman, D.C. Exercise effects on systemic immunity. Immunology and Cell Biology. 2000. 78: 496-501.
Gleeson, M. Immunological aspects of sport nutrition. Immunology and Cell Biology. 2016. 94: 117-123.
He, C.S., Handzlik, M., Fraser, W.D., Muhamad, A., Preston, H., Richardson, A., Gleeson, M. Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes. Exerc Immunol Rev. 2013. 19: 86–101.
He, C.S., Fraser, W.D., Tang, J., Brown, K., Renwich, S., Rudland-Thomas, J., Teah, J., Tanqueray E., & Gleeson. M. The effects of 14 weeks of vitamin D3 supplementation on antimicrobial peptides and proteins in athletes. Journal of Sports Sciences. 2016. 34: 67-74.
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