Aug 20, 2020
Enhancing Athletes’ Immune System Through Nutrition, Does It Work?
Carli Silvia, MS, RD, CSCS

Whether it’s running faster, lifting heavier, or achieving optimal body composition, athletes are always seeking ways to improve performance. Recently, particularly with the events surrounding COVID-19, many have begun to reconsider a major contributor to performance that is often overlooked: the immune system. 

The immune system is a specialized structure of cells (natural killer cells, macrophages, T-cells, and B-cells to name a few) and organs in the human body that, when functioning properly, protect us against harmful bacteria, viruses, and infections. Strategies to maintain a strong immune system are particularly relevant to athletes as prolonged and intense training (>90 min) has been observed to transiently decrease immune function by anywhere from 15-70%, making athletes more prone to upper respiratory tract infections (URI).

immune
Photo: Dan Gold / Unsplash Media

So, what can we do from a dietary standpoint to bolster our immune weaponry? Are there any “immune-boosting” foods or supplements we can add to our diet to achieve that goal? 

As you might suspect, when it comes to science, there is almost never a universal answer. The good news is that most of these factors can be controlled, and food is one of them. 

Foods are composed of the three macronutrients — carbohydrates, protein, and fat. For many reasons, registered dietitians (RD) often promote the importance of a diet that is balanced in macronutrient consumption, and, as it turns out, meeting macronutrient needs plays a pivotal role in the maintenance of a healthy immune system. Immunoglobulins (or antibodies), which are a vital part of the immune system, are made up of proteins, and carbohydrate availability before and after exercise is associated with a decreased stress hormone response and lessened suppression of immune cells. Some of the most robust evidence is linked to well-known nutrients like Vitamin D, Vitamin C, probiotics, and polyphenols.

Vitamin D

Beyond its established beneficial role on calcium in bone homeostasis, the fat-soluble Vitamin D seems to also have inductive effects on cells able to stimulate an immune response, also called antigenic cells. Multiple studies have associated Vitamin D deficiency to increased incidence of URI and a consequent decrease in performance – even when factoring in variables such as age, sex, and body mass. Meeting Vitamin D requirements is not hard to accomplish in the summertime by safely being exposed to sunlight; however, if deficient, daily supplementation with 1,000 IU/day of Vitamin D is recommended. Vitamin D is also contained in fatty fish (such as salmon, tuna, and mackerel), and in small amounts in beef liver, cheese, and egg yolks. Also of note, almost all milk produced in the U.S. is fortified with Vitamin D. 

Vitamin C

Vitamin C acts as an enhancer for immune cells in the process of microbial killing, and it also seems to augment proliferation and differentiation of T-cells and B-cells, which are responsible for making antibodies. Studies on athletic populations have consistently found that, while Vitamin C does not wipe out the chance of getting a cold, it might alleviate its symptoms and duration. In a dosage of 0.25-1 g/day, Vitamin C appears to also have a protective effect against URI in endurance and ultra-endurance athletes undergoing bouts of extreme physical stress. Vitamin C is a water-soluble vitamin. This means that any excess intake through mega-dose supplementation will be excreted through urine and therefore wasted. In extreme amounts, people may even see negative side effects like diarrhea, abdominal cramps, and kidney stones. Most of the population can meet daily Vitamin C needs by simply including foods that are naturally high in Vitamin C, such as bell peppers, broccoli, Brussels sprouts, papaya, guava, oranges, strawberries, and pineapple.

Probiotics 

By regulating immune homeostasis through an immense army of commensal bacteria, the gut is our first, and largest, line of defense against invading pathogens. Probiotics are live microorganisms that can increase the number of beneficial bacteria in the gut and potentially modulate immune function. For athletes, factors affecting gut microbiota composition are mainly correlated to the amount and intensity of exercise, quantity of protein consumed, and body composition. Some common symptoms associated with decreased gut immune function include abdominal cramps, acid reflux, nausea, vomiting, diarrhea, and URI. Probiotics are contained in foods such as yogurt, kefir, tempeh, kombucha. For athletes that are considering adding a probiotic to their regimen, it is suggested to refer to an RD for recommendations on the most appropriate strain(s), quantity, and mode of intake.  

Polyphenols

Consistent and abundant intake of fruits and vegetables in an athlete’s diet grants numerous benefits thanks to the provision of fiber, vitamins, and minerals. Polyphenols, the natural chemical compounds that give fruits and vegetables their bright colors, also bring added health benefits. Although further research on humans is required at this time, there is some evidence for the positive role played by flavonoids, which makes up nearly 50% of the polyphenol family, on immunomodulatory components of the immune system like natural killer cells, T-cells, and macrophages. However, all levels of athletes are advised to consume a wide variety of polyphenol-rich foods (such as fruits, vegetables, nuts, spices). A few ideas to incorporate these nutrients in an athlete’s diet include a post-workout smoothie made with berries, sprinkling curcumin on salad, snacking on walnuts, almonds, and pecans, pouring a cup of cherry or pomegranate juice, or enjoying a serving of dark chocolate. 

Conclusion

Gender, genetics, age, and how we manage sleep, stress, exercise, and nutrition all affect our immune system’s performance. For example, if an athlete’s perceived ability to handle stress is poor, it will negatively influence immunoregulation via the hypothalamus (center of emotional control) and the adrenal system (which secretes cortisol – the aptly named “stress hormone”). Furthermore, it is well known that chronic sleep disturbances impair cognitive function, mood, nutrient metabolism, perceived exertion, and performance in athletes. Preliminary studies on the relationship between sleep and immune response show that < 6 hours of sleep with sleep efficiency below 92% are associated with a 4-5-fold increase in the risk of developing URI. Tips like ensuring 7-9 hours of sleep in a dark and silent environment, napping during the day when possible, and decreasing screen time in the hour before bedtime can help improve sleep quality and maintain immune health.

According to current findings, below is some practical advice that athletes should follow to keep their immune system strong and capable:

  1. Match energy intake to energy expenditure
  2. Intake adequate protein on a range of 1.2-1.7 g/kg body mass depending on training and performance goals
  3. Ensure carbohydrate intake of about 60g/hour during intense training or competition
  4. Consider Vitamin D supplementation when deficiency is present
  5. Consider Vitamin C supplementation during periods of heightened physiological stress and infection risk
  6. Intake a variety of colorful fruits and vegetables to provide a multitude of micronutrients



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