Can within-workout carbs and supplemental zinc keep you from getting ill?
A heavy training schedule can improve an athlete's sport-specific fitness dramatically, but it can also have an unwanted effect: hard training can actually suppress your immune system, putting you at greater risk of developing an opportunistic infection which could decrease your quality and quantity of training and even force you to miss a key competition (1).
Improper nutrition can compound the negative effect of strenuous exercise on the immune system. For example, athletes who fail to include enough carbohydrate in their overall diets suffer from larger increases in blood levels of immune-system-suppressing 'stress hormones', and they experience increased perturbations in various indices of immune-system function, compared with athletes following a carbohydrate-rich diet(2). In addition, dietary deficiencies of protein and various vitamins and minerals (especially iron, zinc, and B vitamins) have also been linked with immunosuppression. When extreme training is combined with poor nutrition, it becomes increasingly difficult for athletes to avoid illness.
Naturally, athletes can increase their chances of staying healthy by following a diet which is adequate for carbohydrate, protein, and fat, as well as vitamins, minerals, fibre, and antioxidants. Sports-nutrition experts have also suggested an additional strategy for staying well: taking in carbohydrates during intense or prolonged workouts.
The athletic world generally views the consumption of carbs during workouts as a performance-enhancing strategy, but the evidence supporting the notion that such carbs are also immune-system boosters is actually fairly strong. As it turns out, glucose (a six-carbon carbohydrate which is so prevalent in the blood that it is often called 'blood sugar') is an extremely important fuel for immune-system cells, including specialised white-blood cells called lymphocytes, neutrophils, and macrophages. Test-tube studies reveal that increases in white-blood-cell numbers are totally dependent on glucose concentrations; if glucose levels are low, white cells multiply at very modest rates(3). This may be one reason why athletes whose diets are deficient in carbohydrate are at higher risk of suppression of antibody production, lymphocyte proliferation, and natural-killer-cell activity (natural killer cells are key components of the immune system; it is believed that they have the capacity to directly attack and destroy virally infected cells, as well as malignant tissue).
Consumption of carbohydrate during a tough or long workout also modifies
the commonly observed post-workout spike in a key 'stress hormone' called cortisol which possesses known immunosuppressive effects. For example, in a high-quality, randomised, double-blind, placebo-controlled investigation, 30 experienced marathon runners who imbibed 750 ml (about 25 ounces) of a 6% carbohydrate sports drink right before a 150-minute treadmill run at 77% of VO2max and then consumed 250 ml (approximately eight ounces) of the same beverage every 15 minutes during the two and one-half hours of exercise had significantly lower blood-cortisol levels after the exertion was over, compared with athletes who ingested a similar quantity of placebo beverage(4). In this study, there was a direct, inverse relationship between plasma cortisol concentrations and blood glucose: the higher the glucose level, the lower the cortisol.
The inflammatory cascade
In addition, carbohydrate ingestion during exercise reduced the interleukin-1 receptor antagonist response to exercise, compared with placebo drinking (a good thing, since interleukin-1 heightens immune-system activity). Finally, swallowing the carb drink led to a significantly smaller increase in interleukin-6 levels post-exercise, compared with drinking the placebo. This could potentially be advantageous to athletes, since interleukin-6 is considered to be an inducer of the 'inflammatory cascade', a physiological process in which tissues become sore and inflamed and associated blood vessels become leakier, filling tissue spaces with fluid.
Other studies have shown that within-workout carbohydrate consumption prevents the commonly observed, exercise-induced fall in neutrophil function(5). It also helps maintain T-lymphocyte proliferation after a workout is over(6): in a study again involving 30 well-trained marathon runners, the placebo group exhibited a 45% decrease in T-cell function immediately after a long run, whereas the carbohydrate group experienced no such shortfall.
The amount of carbohydrate required to produce these salubrious effects is not definitively known, but it is likely that the consumption of about 300 ml (10 ounces) of carb-containing sports drink 10 minutes before a strenuous or prolonged workout begins and then 150 to 180 ml (five to six ounces) of the same fluid every 15 minutes or so during the training session will likely do the trick. Incidentally, 'carb-containing sports drinks' are defined as beverages which contain a carbohydrate concentration (in grams per 100 ml) of between 5% and 8%. A drink below 5% is probably too weak, and a drink at 8% or above might increase your risk of gastrointestinal upset (such a drink will be better suited for relaxed sipping during recovery â€“ after the workout is over). Note, too, that easy workouts lasting just 30 to 45 minutes probably have minimal effects on the immune system, making within-workout carb consumption much less important. As a general rule, workouts lasting more than an hour or particularly intense sessions lasting 40 to 50 minutes or more are probably the ones which call for within-workout, immune-system boosting with carbs.
The perils of zinc deficiency
Naturally, carbohydrate is not the only nutrient in the immune-system spotlight. Over the past five years or so, athletes and sports nutritionists have also become increasingly interested in the possible effects of below-par zinc intakes on immune system function, especially since an increasing number of studies have revealed that zinc exerts a modulatory effect on the immune system. Zinc is essential for the normal development of the immune system, and in the human body over 100 different enzymes are zinc-dependent, including the key enzymes involved in DNA transcription and the synthesis of important proteins. Proven zinc deficiencies can cause a number of severe, adverse effects on the immune system, including atrophy of lymphoid tissue, decreased responses to pathogens and cancer-causing agents, and drop-offs in interleukin-1 production (7).
A potential concern for the serious athlete is that zinc is lost from the body in sweat(8), with chronic, prolonged sweating or high rates of sweat production (during repeated long or intense workouts, for example) potentially increasing the risk of zinc deficiency. It is well-known that male and female athletes involved in serious training generally have lower plasma zinc levels, compared with sedentary individuals(9). So should you begin taking zinc supplements to decrease your chances of getting ill? Somewhat paradoxically, in a recent study in which male runners supplemented their diets for almost a week with 25 mg of zinc and 1.5 mg of copper (each taken twice a day), the supplementation actually exaggerated the exercise-induced suppression of T-lymphocyte proliferation(10). In another study, a daily intake of 300 mg of zinc (150 mg taken twice a day) by 11 males over a six-week period led to a reduced T-lymphocyte proliferative response to the introduction of cancer-causing agents and also hampered phagocytic activity by neutrophils (ie, the ability of neutrophils to 'eat' and destroy pathogenic organisms)(11).
Zinc-rich foods are the answer
Based on this kind of evidence, sports nutritionists have concluded that megadosing with zinc is not a good idea; just as too much high-quality training can actually bring down an athlete's performance capacity, intakes of excessive amounts of nutrients â€“ even those which are essential for proper immune-system function â€“ can harm immune-system performance. Rather, it makes sense for athletes to include many zinc-rich foods in their diets (things like poultry, meat, fish, and dairy products). Vegetarians are often instructed to ingest a 10- to 20-mg supplement of zinc each day, but â€“ given the concerns about the negative effects of oversupplementation with zinc â€“ it may be prudent to take about 10 to 12 mg (the RDA is 10 mg for females and 12 mg for males)(2).
So, what are the bottom lines concerning carbohydrate, zinc, and your immune system? During periods of heavy training, you would be wise to follow a high-carbohydrate diet, from the standpoints of both performance and good health. On days when you train for approximately two hours, the recommended daily intake of carbohydrate is eight to 10 grams of carbs per kilogram of your body weight(12). If you weigh 70 kilos (154 pounds), this would amount to 560 to 700 grams of carbohydrate, or 2240 to 2800 calories. If you engage in strenuous or prolonged workouts, it also makes sense for you to consume carbohydrate-containing drinks during your training sessions; this practice will boost the quality of the workout and will also moderate the immunosuppressive effects of rugged exertion. Finally, make certain that your consumption of zinc is adequate, but don't overdo it: too much zinc can actually make immune performance sink.
(1) 'Infectious Episodes in Runners before and After the Los Angeles Marathon', Journal of Sports Medicine and Physical Fitness, Vol. 30, pp. 316-328, 1990
(2) 'Elite Athlete Immunology: Importance of Nutrition', International Journal of Sports Medicine, Vol. 21 Supplement 1, pp. S44-S50, 2000
(3) 'Role and Regulation of Glucose in Proliferating Cells', JNCI, Vol. 62, pp. 3-8, 1979
(4) 'Carbohydrate and the Cytokine Response to 2.5 H of Running', Journal of Applied Physiology, Vol. 82, pp. 1662-1667, 1997
(5) 'The Effects of Carbohydrate Supplementation on Neutrophil Degranulation Responses to Prolonged Cycling', International Journal of Sports Medicine, Vol. 21, p. S73, 2000
(6) 'Carbohydrate Supplementation and the Lymphocyte Proliferative Response to Long Endurance Running', International Journal of Sports Medicine, Vol. 19, pp. 574-580, 1998
(7) 'Zinc, Copper, and Iron Nutriture and Immunity', Journal of Nutrition, Vol. 122, pp. 604-609, 1992
(8) 'The Effects of Intermittent High Intensity Exercise on Blood Zinc Status and Urinary Zinc Excretion', Journal of Sports Science, Vol. 16, pp. 59-60, 1998
(9) 'Serum Zinc in Athletes in Training', International Journal of Sports Medicine, Vol. 2, pp. 135-138, 1981
(10) 'Exercise-Induced Changes in Immune Function: Effects of Zinc Supplementation', Journal of Applied Physiology, Vol. 76, pp. 2298-2303, 1994
(11) 'Excessive Intake of Zinc Impairs Immune Responses', Journal of the American Medical Association, Vol. 252, pp. 1443-1446, 1984
(12) 'Nutritional Practices of Athletes: Are They Sub-Optimal?', Journal of Sports Science, Vol. 13, pp. S75-S81, 1995