Conventional wisdom holds that being physically fit equates to better health and a stronger resistance to colds and the flu. But what about athletes involved in strenuous training and competitions?
Investigators seeking to answer this question studied 150 ultra-marathon runners competing in a 90-kilometer race in South Africa. Surprisingly, a whopping one-third of athletes developed upper respiratory tract infections within two weeks after the race. In another study, involving 1,828 runners competing in the Los Angeles Marathon, almost 13% reported coming down with a cold within a week of the race. Among athletes competing in the Western States Endurance Run, approximately one in four reported symptoms of an upper respiratory tract infection in the two weeks following the race.
Why would there be a spike in colds after these types of events? Well, it turns out that strenuous bouts of exhaustive exercise temporarily suppress immune function. The prevailing thinking is that the physical stress of prolonged exercise increases the circulating concentrations of stress hormones like adrenaline and cortisol. While in the short term these hormones help meet the immediate physical demands the athlete faces, they have the side effects of temporarily suppressing the immune system. Unfortunately, for many athletes, these effects seem to be just enough to allow a head or chest cold to set in.
So, how do you reap the benefits of strenuous training or competition without temporarily compromising your immune system? The answer may lie in what you eat.
Understanding the connection between nutrition and your immune system requires some background on how the immune system functions. The job of your immune system is to protect you against foreign elements like pathogenic bacteria or viruses. The immune system has two arms that work together to keep you healthy and help you recover if you do get sick.
The first arm, or line of defense, essentially blocks the entry or penetration of foreign elements into the body. Your skin, the acid in your stomach, mucosal secretions, enzymes in saliva and digestive fluids, and cells in your blood called macrophages all serve to prevent foreign invaders from taking hold in your body and causing infections. But, if that first line of defense fails, and a foreign invader does gain a foothold, a second line of defense immediately comes into play. This second arm of the immune system involves the rapid replication of immune cells and related factors designed to target and destroy the specific invader.
The key to avoiding the immunosuppressive effects of heavy exercise is to ensure that your diet is rich in nutrients and dietary factors, which play important roles in supporting immune function. Protein in the diet is important because many components of the immune system are made up of protein. In addition, a variety of vitamins and minerals are needed to support the rapid replication of immune cells that are critical to warding off infectious agents.
The strongest evidence in favor of a link between diet and immune function comes from studying malnutrition. It is well established that among individuals suffering from protein-calorie malnutrition (where calorie and/or protein intakes are inadequate), immune function is weakened, and rates of infection are high. With malnutrition, there just aren’t enough calories, protein, or critical vitamins and minerals for the immune system to mount an effective defense of the body.
Serious athletes involved in strenuous training are obviously not malnourished, but they are in a state where physical demands are pushing the body’s limits. Interestingly, scientists have found that in these types of athletes, even a few weeks of dieting to lose weight can impair the function of their immune cells. So, as a general rule, to insure that your immune system is able to function at its best, make sure that periods of heavy training and strenuous competition don’t overlap with periods of dieting to lose weight.
While calories and protein are certainly mainstays of a diet to ensure a strong immune system for athletes, the role of carbohydrates has also intrigued investigators. Carbohydrates are of particular interest as a strategy to reduce the stress hormone response to exercise. The thinking is that if the rise in stress hormones associated with strenuous exercise can be blunted, the negative impact on immune function may be substantially reduced as well. Studies show that athletes eating low carbohydrate diets who engage in prolonged strenuous exercise show sharp increases in circulating levels of stress hormones. Scientists also found that as stress hormone levels rise, the number and activity level of key cells involved in immune function decline. Other research shows that consuming carbs during exercise reduces the rise in stress hormones seen with strenuous exertion and helps to offset the suppressive effect on immune function. Does this translate to fewer colds? No one knows for sure, but given the already well-established benefits of carbohydrate consumption for extending endurance performance, it’s gratifying to think that carbs may be offering an immune system boost as well.
Scientists have also studied a virtual smorgasbord of vitamins, minerals, herbs, and related dietary factors to see if any can impact exercise-induced immuno-suppression. Unfortunately, to date there is no strong evidence that any specific nutrient or herb can offset the effects of strenuous exercise on immune function. Numerous studies have looked at one or two biomarkers of immune function, sometimes showing evidence of benefit, but few studies have actually looked at the effect of these dietary factors on rates of upper respiratory tract infections.
The notable exception is vitamin C. In two different studies, ultra-marathon runners taking 500-600 mg of vitamin C for a few weeks before and a few days after a 90-km ultra-marathon had lower rates of upper respiratory tract infections compared to those taking a placebo. Unfortunately, other investigators were unable to find beneficial effects. Although definitive proof is lacking, taking a daily 500 mg vitamin C supplement for a few weeks before and the week after competing in a marathon, triathlon, or ultra-marathon might be a measure to consider. Finally, a reasonable argument can be made for a balanced once-a-day type multivitamin/mineral supplement to help ensure that athletes obtain adequate amounts of the micronutrients needed to support immune function.
In summary, while being physically fit helps strengthen the immune system, strenuous bouts of exhaustive exercise can suppress immune function and substantially increase the risk of developing an upper respiratory tract infection. What can you do to help avoid the post-marathon sniffles? Make sure you’re not compounding the physical demands of the event itself by scrimping on calories before hand. Consume carbs during the event – they not only increase endurance, they may reduce the immuno-suppression associated with strenuous exercise. Also consider routinely taking a balanced multivitamin/mineral supplement to ensure that your diet always has an adequate supply of the nutrients needed to support immune function. Finally, a little extra vitamin C a week or two before and after the event may give your immune system an added boost.
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