Running and Your Immune System
by Pete Pfitzinger, M.S.
Since you are reading Running Times, you most likely take your training seriously. You love the challenge of racing, and running improves your cardiovascular system and quality of life. But does it make you more susceptible to illness? The human immune system is highly complex. Many types of inter-related cells are involved in protection against disease and infection. In addition, factors such as psychological stress and diet influence your resistance to infection, so it is difficult to isolate the effect of training. Some aspects, however, of the relationship between running and immune function are well established. First, there is ample evidence that moderate-intensity running up to an hour or so strengthens the immune system of runners who are otherwise healthy. Second, there is increasing data that high-intensity running of long duration temporarily reduces the function of some types of protective cells, creating an open window during which runners are at increased risk of infection. Third, there is evidence that substantially increasing the overall intensity or volume of training may lower runners’ resistance until the body has time to adapt to the increased training load. How hard and how long you need to run to significantly increase your risk of infection depends on the strength of your immune system. The types of running most likely to lead to temporary immune suppression are races of 10 miles or greater, long interval sessions or unusually taxing long runs. The dip in immune function generally lasts for a few hours, and may not completely rebound for two to three days. For example, marathon runners were found to be six times more likely to have an upper respiratory tract infection (common cold) during the week after a marathon compared to other runners who did not race. Similarly, after racing a half marathon the number of natural killer cells (a key component of the immune system) was dramatically reduced 30 minutes after the race, and did not fully recover for 24 hours. Another study found decreases in the ratio of immune “helper” cells to “suppressor” cells when runners added several long interval sessions to their training or greatly increased their weekly mileage.
Reducing Immune System Suppression
The simplest way to stay well is to follow your mother’s advice: wash your hands, get a good night’s sleep, avoid sick people and eat a healthy diet. Let’s look closer at nutritional concerns. Increasing evidence indicates that immune function is reduced by carbohydrate depletion. Two hypotheses for this link are: 1) reduced blood glucose levels lead to the release of stress hormones which alter immune function; and 2) when immune cells run low on glucose they cannot do their job as well. Depleting your carbohydrate stores, however, also provides the stimulus for your muscles and liver to stockpile more glycogen (the storage form of carbohydrate). So, on the one hand, becoming carbohydrate depleted provides positive training adaptations, while on the other hand it appears to lower your defenses. To obtain the positive training stimulus to increase glycogen storage while minimizing immune system suppression, replace carbohydrates as soon as possible after your long carbohydrate-depleting efforts. Antioxidant supplementation (e.g. vitamins C, E, beta carotene and selenium) may help maintain immune function after strenuous exercise by neutralizing free radicals. The results of studies have been mixed, however, with some showing fewer post-exercise infections after antioxidant supplementation and others showing no benefit. It is best to get your antioxidants from a diet high in fruits and vegetables, which likely also provide other immune-strengthening substances. If your diet is poor, then a standard antioxidant supplement may help, but as with any supplementation more is not better (e.g. excess intakes of vitamin E and zinc actually reduce immune function). Diets deficient in protein, iron, zinc, vitamin B6 and vitamin B12 also weaken the immune system. Similarly, low-fat or high-fat diets tend to reduce immune function, as do diets deficient in calories. Fortunately, all of these potential pitfalls are covered by a basic healthy diet.
What Are the Training Implications?
1) After a race or grueling workout you should not do another hard session until your immune system recovers. Depending on how hard the session was, you should take one or two easy days. Although this advice is logical, it is sometimes hard to adhere to, if, like many runners, you tend to race or do a hard workout on Saturday and follow that up with a long run on Sunday. If you must do back-to-back hard sessions, take in plenty of carbohydrates after Saturday’s effort to ensure you are not depleted going into Sunday and replace your carbohydrate stores as soon as possible after that run. Then take it easy Monday and Tuesday. 2) Do not suddenly increase the overall intensity of your training or your mileage. The immune system typically adapts fairly quickly to increased training loads, however, so a reasonable strategy is to increase either your mileage or your training intensity (but not both) moderately for a week or two and then increase again. The more moderate approach may require patience but will reduce immune system suppression, reduce your likelihood of injury and is more likely to lead to positive training adaptations.
Two-time Olympian Pete Pfitzinger is an exercise physiologist.