Does exercise weaken the immune system?

Research has shown that exercise can have a negative impact on the body’s immune system, and most importantly that the body’s ability to resist infection as part of its first line of defence can be compromised. This is not usually a huge problem as most sports build recovery time into training and competition schedules. But what happens when athletes are faced with a tournament situation or are approaching the end of a competitive season and the prospect of having to play two to three matches per week as a result of various league and cup ties? Immune function requires more than two days to return to optimal levels. If one bout of intense exercise can knock your body for six, just think what can happen when you have multiple efforts back to back.

Researchers based in Japan have recently looked at the effects of repeated rugby sevens matches on immune function, to help them evaluate the cumulative effects of consecutive bouts of high intensity exercise (‘Effects of rugby sevens matches on human neutrophil-related non-specific immunity’, British Journal of Sports Medicine2007; 41:13-18). Rugby is arguably one of the most intense contact sports, requiring a high level of physical preparation. Rugby sevens is played on a normal- sized pitch and as the name implies, each team consists of seven players. It is not unreasonable therefore to conclude that these players have a potentially higher exercise loading than their counterparts who compete in normal rugby matches.

The research team monitored members of the Japanese sevens squad during the course of one day, when they had to play two competitive matches with just four hours’ break in between each match. Blood samples were taken immediately before and after each match: blood lactate measurements clearly showed that the matches were intense enough to induce muscle damage and fatigue. Further blood analysis showed that the counts of neutrophils (the immune system’s white blood cells that do a lot of micro-damage clearance) had a tendency to increase after the first match, and increased significantly four hours later.

This is good news: the neutrophils were doing their job, increasing in number, running around the body engulfing micro-organisms and preventing infection. The problems started after the second match. The researchers found no significant changes in neutrophil counts, suggesting that the repetition of intense exercise had reduced the neutrophil inflammatory reaction, and that recovery from physical damage could be compromised.

What this study adds to the body of research in this area is that repeated bouts of intense exercise with incomplete immunological recovery may increase the risk of infection through a cumulative effect.

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Extreme sport, extreme reaction

Motocross is an extremely demanding sport. Irregular terrain and sharp turns increase the physical and physiological demands and can cause problems such as ‘arm-pump’, a muscular stress in the upper limbs resulting from the isometric and/or eccentric muscular contractions required to absorb the repeated shocks of motocross riding.

The consequent muscle oxidative damage contributes to a temporary loss of exercise capacity and as we have seen in the previous study this can have a negative impact on immune function. As we continue to seek more extreme and demanding sports we must understand what effect they will have on the body’s ability to cope and recover from these unusual stresses.

Researchers in Portugal have recently investigated the effect of an off-road motocross race on plasma levels of oxidative stress and damage (‘Effect of off-road competitive motocross race on plasma oxidative stress and damage markers’, British Journal of Sports Medicine2007; 41:101-105). They took 10 elite off-road motocross riders with international experience and got them to simulate a competitive race over a 1.5km motocross circuit. Blood and urine markers of oxidative stress and damage were measured immediately after the race and then one hour later. The data reinforced the heavy metabolic and hormonal demands imposed by motocross and confirmed that motocross racing results in an elevated level of plasma oxidative stress and damage.

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Stay strong, healthy and lean

Despite the fact that resistance workouts are an integral part of most athletes’ training regimes, and weight training is hugely popular as a basic keep fit activity, studies on the effect of exercise on immune response have almost exclusively looked at aerobic activities. What if all those hours spent in the gym pumping iron are actually undermining our health?

Researchers from Greece have recently sought to fill the knowledge gap (‘Resistance exercise does not affect the serum concentrations of cell adhesion molecules’, British Journal of Sports Medicine2007; 41:76-79)and investigated the potential impact of resistance training on serum concentrations of ‘cell adhesion molecules’ – the proteins on the surface of cells that govern the immune system’s response to inflammation. Fourteen healthy men completed a circuit training session comprising three sets of 10 resistance exercises, with 10 to 12 repetitions at 70% to 75% of 1RM. Blood samples were taken prior to exercising and then at the end of the first, second and final sets of the circuit.

The results showed that moderate to high intensity resistance training did not affect serum concentrations of cell adhesion molecules, suggesting that immune function was not compromised. So far, so good. But the researchers were also keen to see whether obesity altered the body’s response at all. Six of the 14 subjects were obese (body mass index greater than 30).

The study found no negative effect on immune function for obese individuals. This is even better news, in the context of government initiatives in many countries to use exercise as a way of helping to reduce overweight among the general population. Diet alone is rarely sufficient to help obese people shift their excess weight, and while low impact cardiovascular work certainly burns calories, resistance training gives you even more bangs for your buck. In my opinion it should feature in all weight-loss programmes. This study removes one more potential fear/barrier for the reluctant exerciser.

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