Elite athletes need to take special care of their immune health as they are at high risk of immunodepression and related illnesses due to the stressors of hard training sessions, competitions, sleep difficulties, long travel, etc.
The J-curve theoretical model by Dr Nieman below demonstrates the relationship between exercise and the risk of upper respiratory tract infections (URTIs). Moderate amounts and intensity of exercise may reduce the risk, but doing too little or too much, as many athletes do, actually increases the risk.
Nutrition has recently been attracting interest as a way to combat exercise-induced immunodepression. A Consensus Statement was recently published, highlighting this interest and increasing body of knowledge.
Despite all the nutrients, foods and supplements that are said to maintain or reinstate immune function, only a small number are supported by research. Some key points of the Consensus Statement follow below:
Carbohydrates are known to boost performance and there are evidence-based guidelines for restoring glycogen stores before exercise, and maintaining blood glucose levels during exercise, to ensure proper carbohydrate availability for the working muscles. However, carbohydrate availability is also known to play a role in preventing exercise-induced immunodepression. Carbohydrates are fuel for the immune cells and ingesting carbohydrate during prolonged, fatiguing exercise appears to be more effective at minimising immune dysfunctions than increasing total daily carbohydrate intake. This could be explained by the maintenance of blood glucose levels which, in turn, blunts the release of cortisol. However, carbohydrate ingestion does not seem to prevent or reduce the incidence of URTIs.
Although not mentioned in the Consensus Statement, it’s important to take the quality of carbohydrates into consideration, e.g. a Snickers bar cannot be equated to fruit, rolled oats or sweet potato.
Some evidence from in vitro, animal and epidemiological studies suggests that many saturated fatty acids and omega-6 polyunsaturated fatty acids contribute towards inflammatory processes. Arachidonic acid from meat, eggs and plants (synthesised from linoleic acid) has been shown to be particularly pro-inflammatory and cause immune dysfunctions. However, little is known within the context of exercise and more human interventions are needed.
I disagree that omega-6s are necessarily pro-inflammatory: eating sunflower or pumpkin seeds, for example, are not going to switch on inflammatory processes. Due to the opposing effects of omega 3s and 6s, a healthy diet should contain a balanced omega 6-to-omega 3 ratio, i.e. 1:1 to 4:1 as opposed to the typical Western diet ratio of 15:1 to 17:1. Over-consuming one type of omega fatty acid will lead to a decreased conversion of the other to its longer-chain form. The consumption of margarine and refined vegetable oils, such as sunflower and canola, has risen dramatically over the years. Although widely considered ‘heart healthy’, they have in fact contributed to the increased ratios seen today and throw off the inflammatory balance completely. It’s really more about the quality and the balance between omega 3s and omega 6s, rather than omega 6s themselves as pro-inflammatory agents. You can read more about this here.
Conversely, there is strong evidence to support the long-chain omega 3s, DHA and EPA (from oily fish, krill oil and some algae), as anti-inflammatory agents, which translates into less exercise-induced muscle damage and muscle soreness in untrained individuals. The data is not that clear-cut for athletes, but from a health-based perspective, we know that omega 3s provide an array of benefits including fighting against inflammation and autoimmune diseases, improving quality of sleep and promoting brain health, which are all important for an athlete’s health too.
Plasma glutamine levels can decrease by approximately 20 to 25% after prolonged, fatiguing exercise. Glutamine supplementation has been shown to decrease exercise-induced gut permeability and some reports suggest it can decrease the incidence of exercise-induced URTIs. Preliminary evidence indicates that glutamine could help produce heat shock proteins that might provide protection against exercise-induced immunodepression. Although evidence is lacking with regard to enhancing immune function, it has recently been proposed that the body stores sufficient glutamine to tackle immune function decrements after prolonged exercise. A decrease in plasma glutamine levels may also act as a marker for immunodepression.
Branched-chain amino acids (BCAAs)
Plasma BCAA levels decrease after prolonged, fatiguing exercise as BCAAs are absorbed by the working muscles and oxidised to provide energy and nitrogen for glutamine synthesis. Although BCAA supplementation is thought to increase glutamine production, glutamine release from the working muscles remains unchanged unless a significant amount of BCAAs is ingested. However, a few studies show that long-term BCAA supplementation in athletes prevents exercise-induced decrements in plasma glutamine levels or an increased neutrophil number following an endurance event. More research is needed to make recommendations for athletes.
Zinc, magnesium and iron
These are considered to be the vital minerals for proper immune function. Requirements of these minerals are higher in athletes as exercise has a significant effect on their metabolism and also increases their loss through sweat and urine. An athlete’s diet must contain adequate amounts of these minerals.
Zinc deficiency (< 0.70 mmol/l) is highly prevalent in athletes. However, the impact of deficiency on immune function remains inconclusive. Zinc supplementation has been shown to prevent and shorten the duration of symptoms of the common cold in the general population. Transient supplementation is recommended for athletes during intensive training periods coupled with the psychosocial stress of competition, especially if the athlete has a history of recurrent infections.
Magnesium deficiency is linked to signs of inflammation, such as increased levels of inflammatory mediators and immune cell activation. According to the Consensus Statement, supplementation is recommended only for magnesium-deficient athletes (< 0.75 mmol/l and symptoms must be present for diagnosis).
Even so, I find that most athletes, especially endurance-based and CrossFit athletes, can benefit from magnesium supplementation due to the high demands of their training and sub-optimum dietary magnesium intakes. Most of them exhibit the classic signs of muscle cramps, muscle weakness and lack of energy, despite not being tested for magnesium-deficiency. A little bit of magnesium support in the recovery process as a preventative measure is not a bad idea.
Iron deficiency is more prevalent in female athletes due to heavy losses through menstruation. Iron supplementation is recommended for athletes with iron deficiency (serum ferritin level < 30 ug/l), except during periods of infection as pathogenic microorganisms might benefit from the iron. Athletes should combine iron supplementation with vitamin C in order to increase absorption and also be carefully monitored for prophylaxis.
Evidence suggests that vitamin D plays a vital role in both innate and acquired immunity by modulating gene expression. Many immune system cells contain the vitamin D receptor and also express the enzyme responsible for activating vitamin D. A few studies in athletes have shown a link between low vitamin D plasma levels and greater URTI incidences, as well as greater symptom-severity scores. It is recommended that athletes monitor their vitamin D concentrations and aim for > 75 nmol/L in terms of immune health. During summer months, athletes should take advantage of sun exposure (without getting sunburnt) and consider a supplement of up to 4000IU/day in winter if vitamin D levels fall below 75nmol/l.
The Consensus Statement also covers antioxidants, herbal supplements, probiotics and bovine colostrum. A very worthwhile read!
To conclude, the Consensus Statement states that an athlete’s diet must provide an adequate quantity and quality of nutrients to support a healthy and functioning immune system. If this, is in place, further supplementation probably won’t be needed. But this depends on the individual and context. An athlete may sometimes require supplementation – e.g. during periods of restricted eating to make weight for a competition.
That said, most athletes do not get enough from their food. Plus, they always have outside stressors which increase their likelihood of insufficiency and/or deficiencies. So well-chosen, individualized and good-quality supplements (with the guidance of a qualified practitioner who is aware of anti-doping rules) can benefit an athlete’s health, performance and recovery in many cases.