How many days training did you lose last winter from colds,coughs, flu or generally feeling under the weather? If it was more than one or two, read on for some recommendations on how to support your immune system through boosting your nutrient intake.
If you are training hard, you may be at increased risk of picking up minor illnesses or infections, especially upper respiratory tract infections (URTIs) which can cause you to miss training sessions. Prolonged bouts of strenuous exercise, particularly if performed without carbohydrate intake and/or adequate energy intake, may compromise your body’s immune system.In addition, heavy exercise leads to the release of high levels of stress hormones such as cortisol which reduce your body’s ability to fight opportunistic infections such as colds and flu. Experiencing emotional or psychological stress adds to this. Lack of adequate rest and recovery, including sleep and post-training nutrition, also plays a role in depressing immunity and leads to increased risk of infection. 1
When assessing your risk of catching an infection, it’s important to look at what’s going on across your whole life, not just your training. For example, if it’s a busy, stressful period at work, perhaps combined with travel, disrupted sleep and irregular meals, and you are trying to stick to a demanding training schedule on top, that’s a recipe for going down with a cold or worse, so it’s worth doing everything you can to reduce that risk.
There are certain nutrients which play key roles in immune function. Insufficient amounts of vitamins A, C, E, B6, B12 and folic acid in your diet can contribute to impaired immunity, leading to decreased resistance to infection. It is now established that vitamin D plays an important part in immunity and has been shown that vitamin D insufficiency is common in athletes,particularly if exposure to natural sunlight is limited, as it is in winter in the UK. 2 A study published in 2013 measured vitamin D levels in 225 endurance athletes at the start and finish of a 16 week winter training block. The results showed that a far higher proportion of the athletes who were deficient in vitamin D presented with the symptoms of upper respiratory tract infections, such as the common cold, than the athletes who had optimal vitamin D levels.3
Several minerals also influence immune function, notably zinc, iron and magnesium, and stores of these may be depleted by hard exercise.So, if you would like to complete your winter training infection-free, it is important to include plentiful sources of these minerals in your diet. Omega 3 fatty acids and flavonoids, a type of nutrient found in many plants, have also been shown to play a role in the immune system, eg through reducing oxidative stress and inflammation. 1 I recommend that sources of these are consumed regularly, see the list of food sources below.
There is currently much research interest in the role of gut bacteria, known as the microbiome, in immunity. The growth of beneficial bacteria, or probiotics, helps to inhibit the growth of harmful bacteria. Probiotics can also influence immune function by interacting with the immune cells found in the gut, such as Natural Killer cells. Taking a probiotic supplement has been shown in a review of 12 trials in the general population to reduce URTI incidence by 50% and shorten symptoms by 2 days. 4 A review of research looking at probiotic use in athletes found that 5 out of 8 studies reported reduced URTI frequency or duration. 5
So, to support your immune system, I suggest eating foods from across the list below on a regular basis, paying attention particularly to your post-training recovery meals, which should include fruit/vegetables as well as starchy carbohydrate and protein. Try to get as much variety as possible across each week, rather than eating the same small number of foods too regularly:
- For vitamin C: peppers, oranges, kiwifruit, berries, potatoes.
- For vitamin E: avocados, almonds, seeds,oats, brown rice
- For vitamin A (beta-carotene): orange and dark green vegetables and fruit, eg butternut squash, sweet potato, mango, apricots, broccoli, spinach, rocket
- For folic acid: leafy green vegetables, eg broccoli, spinach, cauliflower, kale
- For B vitamins: lean meats, dairy products,whole grains. Note that vitamin B12 is only found in foods of animal origin, so vegans may need to take a supplement or consume foods fortified with vitamin B12.
- For vitamin D: eggs, oily fish eg salmon,mackerel, shitake mushrooms. Note that vitamin D is only found in very small amounts in food and it may be beneficial to supplement during the winter months when the sun is not strong enough for your body to make its own vitamin D. See below for recommendations on testing and supplements.
- For zinc: eggs, turkey, almonds, cashewnuts, oysters
- For magnesium: wholegrains, nuts, seeds,leafy green vegetables
- For iron: lean red meat, lentils,chickpeas, cashew nuts, dried apricots, wholegrain bread
- For omega 3 fatty acids: salmon, mackerel,anchovies, trout, sardines, fresh tuna, ground flaxseed, chia seeds, flaxseed oil, walnuts, pumpkin seeds.
- For flavonoids: green tea, berries, apples,onions, black tea
- For probiotics: fermented foods, such as live yogurt, kefir, kombucha, sauerkraut, sourdough bread, miso soup. It’s also important to eat a wide range of plant foods to provide prebiotic fibres which stimulate growth of probiotic bacteria in the gut.
In addition to your diet, consider the following strategies:
- Take on board 30-60g per hour of carbohydrate in the form of energy drinks or gels during training sessions over one hour.This has been shown to lower cortisol and cytokine release during exercise,which are known to suppress immunity. 6 This is more important than increasing the overall amount of carbohydrate in your diet.
- If you are prone to colds and infections, or recovering from illness, avoid doing fasted workouts, which put an additional stress on the body. Eat a banana or drink 250ml of diluted fruit juice prior to pre-breakfast sessions. The potential adaptive benefits from training fasted may be outweighed by days lost to training through illness later. If you do choose to train fasted or in a low carbohydrate state to enhance training adaptations, do this no more than twice a week and avoid high intensity in these sessions. Also ensure that you include at least 60g of carbohydrates in your post-training meal and in your next meal too.
- Take a test to measure your vitamin D levels (ask your GP or visit http://www.vitamindtest.org.uk/ for a home kit). If a test shows you are vitamin D deficient, please seek the advice of a healthcare professional on supplementation. If the result is ‘inadequate’or ‘insufficient’, take a vitamin D3 supplement giving c 2000 IU daily. If your vitamin D levels are in the normal range or you decide not to test, take 400-1000 IU (10-25µg) of vitamin D3 from November to March to help reach optimal levels. During that time, vitamin D cannot be obtained from sunlight in the UK.
- Consider taking a daily probiotic supplement over the winter containing several different strains of beneficial bacteria such as Lactobacillus acidophilus,Lactobacillus casei, Lactobacillus fermentum, Lactobacillus salivarius and Bifidobacteria. Supplements typically contain 10-30 billion CFUs (colony forming units of bacteria). If you have taken a course of anti-biotics, a higher dose is suggested to help restore a positive microbial balance.
- Try a colostrum supplement. Several studies suggest that several weeks of bovine colostrum(collected from the first milk of a cow after calving) is associated with a reduction in URTI incidence in athletes. For example, when 60g of colostrum was taken for 8 weeks and compared with a whey protein placebo. 7 If you are subject to drug-testing, be aware that while colostrum is not on the list of banned substances, it is not recommended by WADA due to its potential impact on IGF-1 levels which might lead to an adverse finding.
- Take a 500mg vitamin C supplement daily in the week prior to winter races. A 2007 review found that, while ineffective in the general population, vitamin C did reduce the likelihood of developing a cold among people exposed to high physical stress, such as marathon runners. 8 However, bear in mind that taking high doses of anti-oxidants such of vitamin C are thought to adversely affect adaptations to training and have not been shown to prevent immune dysfunction during exercise, so I do not suggest taking vitamin C as a supplement every day throughout the winter. It’s worth noting that a 2013 review did not find any reduction in the incidence of colds when vitamin C was taken on a regular basis, although the duration of colds was shown to be shorter compared with not taking vitamin C regularly. 9
- If you do develop a cold, try sucking zinc acetate lozenges for a maximum of one week, at least 75mg of elemental zinc per day. A meta-analysis showed a 44%reduction in cold reduction. 10 Be aware that you may experience a bad taste in the mouth when taking zinc lozenges, but they have been shown to be effective in a way that zinc swallowed as a tablet or liquid has not.
- The herbal remedy Echinacea has been shown to lessen symptoms severity or duration so may be worth taking if you do develop a cold. It may lower risk of developing an infection in people who are susceptible to recurrent URTIs. 11
Two more tips, not about nutrition, but important. Don’t forget to wash your hands regularly to help prevent picking up an infection, particularly before eating and after going to the toilet. And never share a drinks bottle with training partners.
And lastly, here’s a recipe that includes several of the immune-supporting foods on my list above: Warm Salmon & Rice Supper.
JoScott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sport. Based in London, she also works as a Registered Nutritional Therapist, conducting one-to-one consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit www.nutritionforendurancesports.co.uk.
1 Bermon S et al. Consensus Statement on Immunonutrition and Exercise. Exerc Immunol Rev. 2017; 23:8-50. https://www.ncbi.nlm.nih.gov/pubmed/28224969
2 Larson-Meyer DE & Willis KS. Vitamin D and athletes. Curr. Sports Med. Rep. 2010; 9(4): 220-226. https://www.ncbi.nlm.nih.gov/pubmed/20622540
3 He etal. Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sports athletes. Exerc Immunol Rev. 2013; 19: 86-101 http://www.ncbi.nlm.nih.gov/pubmed/23977722
4 HaoQ, Dong BR, Wu T. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2015; Feb3 (2). CD 006895. https://www.ncbi.nlm.nih.gov/pubmed/25927096
5 Pyne et al. Probiotics supplementation for athletes – clinical and physiological effects. Euro J Sport Sci. 2015;15: 63-72 https://www.ncbi.nlm.nih.gov/pubmed/25339255
6 Halson SL et al. Effect of carbohydrate supplementation on performance and carbohydrate oxidation following intensified cycling training. J Appl Physiol.2004; 97 (4):1245-53 https://www.ncbi.nlm.nih.gov/pubmed/15155717
7 Brinkworth GD & Buckley JD. Concentrated bovine colostrum protein supplementation reduces the incidences of self-reported symptoms of upper respiratory tract infection in adult males. Eur J Nutr. 2003; 42:228-232. https://www.ncbi.nlm.nih.gov/pubmed/12923655
8Douglas et al. Vitamin C for preventing and treating the common cold (review). Cochrane Database of Systematic Reviews. 2007; July 18 (3). CD000980. https://www.ncbi.nlm.nih.gov/pubmed/17636648
9 HemilaH & Chalker E. Vitamin C for preventing and treating the common cold(updated review). Cochrane Database of Systematic Reviews. 2013; Jan 31 (1). CD000980. https://www.ncbi.nlm.nih.gov/pubmed/23440782
10 Hemila H. Zinc lozenges may shorten the duration of colds: a systematic review. Open Respir Med J. 2011; 5: 51-8. https://www.ncbi.nlm.nih.gov/pubmed/21769305
11 Schapowal A, Klein P & Johnston SL. Echinacea reduces the risk of recurrent respiratory tract infections and complications: a meta-analysis of randomised controlled trials. Adv Ther. 2015;32:187-200. https://www.ncbi.nlm.nih.gov/pubmed/25784510