Staying Well: The link between Nutrition and the Immune System

By Candace Chemtob, B.S. and M.S. in Human Nutrition

Staying well is a priority for most athletes, as performance is sure to suffer when one is fighting off a cold or an upper respiratory tract infection. To stay well and fight off pathogens (germs), the immune system works on several different levels. The first level of defense is to maintain physical barriers to keep the pathogens out, such as the skin. The next level of defense is to attack the pathogens on a cellular level (i.e., white blood cells, phagocytes, and other cells). The immune system’s ability to fight off infections is strongly linked to nutritional status and this relationship is widely known. In fact, the most common cause of immunodeficiency worldwide is malnutrition.

Protein energy malnutrition negatively impacts the immune system by increasing susceptibility to infection, and the devastating effects are seen throughout the third world. To a lesser degree, on the home front, athletes who voluntarily restrict food intake are also at risk for protein energy malnutrition, but to a lesser degree. Chronically consuming less energy (calories) than expended, as might be the case in aesthetic sports such as gymnastics and sports with weight limits, or athletes trying to lose weight, can impair the immune system. A malnourished body cannot produce enough new immune cells to effectively attack pathogens. Additionally, the metabolic pathways needed to mount an adequate defense, such as antioxidant and inflammatory processes, are depressed in the nutritionally depleted.

More recently, attention has been focused on the effect of strenuous athletic training on the immune system. Research has shown that the immune system can be suppressed during periods of intensive training in some, not all, athletes. This suppression is transient and the results are not dramatic, though usually resulting in minor illnesses, such as upper respiratory infections. To put this in perspective, the effect on ones’ overall health may be trivial, but a minor illness can have a negative impact on athletic performance.

There is a direct link between the immune system and the nervous and hormonal systems. Prolonged periods of intensive training lead to an increase in “stress hormones”, such as adrenaline, epinephrine, and cortisol. Stress hormones serve an important purpose and allow an athlete to achieve peak performance by mobilizing fuels for muscle cells, heightening alertness, and allowing for increased oxygen uptake. During stress, the immune system is not a priority and resources are diverted to other organ systems with a pressing need. The result is a short lived suppression of the immune system.

Carbohydrates have been found to down-regulate the stress response in athletes. Immune cells are carb lovers! Their preferred fuel is glucose. For this reason, athletes can minimize the effects of intense training on the immune system by consuming a high carbohydrate diet and ingesting carbohydrates during and after prolonged exercise. The recommendation is to consume 30 to 60g carbohydrate per hour during prolonged workouts and then another 1.0 to 1.5g carbohydrates per kg (for a 150 lbs. person, this is 70 to 100g carbohydrates) within two hours. Despite the effect of carbs on the stress response, there is not strong scientific data to confirm that an increased carb intake will reduce the incidence of upper respiratory tract infections in athletes. Since the current sports nutrition recommendations are to eat a high carb diet and to increase carb intake before, during and after exercise to improve overall athletic performance, coupled with the potential protective effect on your immune system, there is no downside to following these guidelines!

On a cellular level, vitamin and mineral status also plays a role in maintaining optimal immune function. The role of “anti-oxidant” micronutrients such as Vitamin C, Vitamin A, Vitamin E, iron and zinc on the immune system have been examined and well publicized. In general, the mechanism by which anti-oxidants may boost the immune system is as follows.

During exercise, oxygen uptake can be increased by 10-20 fold. Oxygen is required in the metabolic pathways that convert the fuel (derived from food) into usable energy (mainly ATP). In addition to yielding energy, these pathways also create a by-product referred to as reactive oxygen species (ROS). ROS are associated with suppression of the immune system, fatigue, muscle damage, and the pathogenesis of some chronic diseases. Our bodies produce antioxidants, in concert with those provided from our diet, whose job is to limit exposure to these reactive oxygen species by “neutralizing” with them.

The most famous advocate for Vitamin C was Nobel Prize winner Linus Pauling who proposed that Vitamin C could boost the immune system and help fight off the common cold. Decades later, these well-publicized recommendations remain controversial as research has not consistently supported this claim. The recommended intake (DRI) for Vitamin C is 60 (adult females) to 75 (adult males) mg per day. Despite the lack of concrete evidence, “mega doses” of 500 to 1000mg Vitamin C per day have been recommended by many health professionals and can be found at every pharmacy. The maximum recommended daily dose (referred to as Upper Limit) for Vitamin C is 2000mg per day. Given that an intake of 500 to 1000mg per day might provide a benefit and does not pose a health risk, this would seem advisable during periods of intense training. This logic, however, is not applicable to the other anti-oxidants (Vitamins A and E) which, in “mega” doses, have been found to have the unintended effect of suppressing the immune system.

To keep yourself well, and perform your best on the court, follow some simple rules that will keep your immune system defenses in fighting shape.

1. Do not restrict caloric intake during periods of intense training;
2. Eat a high carbohydrate diet, paying close attention to intake during and after training and/or competition; and
3. Eat a well balanced diet, with a minimum of 2 cups of fruit and 3 cups of vegetables per day to meet your needs for vitamin, minerals, and antioxidants.

With a flurry of research on the immune system and it’s relationship to nutritional status, we can expect further recommendations in the near future. Until then, train hard, and stay well!