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Sports nutrition: how your diet affects your immune system when you exercise
Exercise is not always kind to the human immune system
When sedentary mice begin to exercise regularly, their immune systems become significantly stronger(1). Unfortunately, exercise is not always quite so kind to the human immune system.Acute exercise, in particular, is no friend to your lymphocytes and neutrophils – the white blood cells particularly involved in fighting off infection. For example, research reveals that marathon runners are about six times more likely to come down with a respiratory tract infection during the week after a race than sedentary controls. As far as long-term effects are concerned, the news is not necessarily better: two important studies have found no difference in natural immunity between well-trained and untrained human subjects(2,3). Two other longitudinal investigations found that exercise had no significant impact on natural killer cell activity in healthy elderly subjects(4) and in individuals with rheumatoid arthritis(5). Natural killer (NK) cells are special white blood cells that actively destroy tumour cells and certain virally-infected tissues.
Fortunately, some cross-sectional studies have detected enhanced natural immunity in elite cyclists(6) and also in runners(7) compared with sedentary control subjects. Additionally, research has uncovered augmented resting levels of natural killer cell activity in elderly women after 16 weeks of training(8) and in mildly obese women after 15 weeks of moderate training(9). Why do some human studies link exercise with improved immune function while others do not? Certainly body composition and diet could be confounding factors.
How body fat affects immunity
For example, percentage body fat appears to have an inverse correlation with NK cell activity (the fatter you are, the more sluggish are your NK cells) and increased body fatness in general appears to have a depressive effect on the immune system(10). There is also evidence that immune system function is stimulated by a low-fat diet(11). Thus, it is possible that the increased NK cell activity reported in some athletes is not a direct function of their training but is primarily determined by diet and body composition.
In a bid to tease out the effects of diet on immune system function in individuals undergoing regular physical training, Dr B K Pedersen and colleagues in Copenhagen recently asked 20 healthy young males (average age 27) to train systematically for seven weeks(12). The subjects, all matched in terms of training history and normal diet, were randomly assigned to one of two diet groups, one consuming a carbohydrate-rich diet during the trial period and the other filling up on fat-laden fare. A further 20 non-training males, also split into carbohydrate and fat sub-groups, served as controls.
The active subjects trained three times a week for the first four weeks, then four times a week over the last three weeks of the study. Exercise intensity varied from 50 to 85% of VO2max. The carbohydrate-rich diet was 65% carbohydrate, 15% protein and 20% fat, while the fat-filled diet was 62% fat, 17% protein, and 21%. Energy intake from protein was deliberately kept as similar as possible for the two regimes, so that fat and carbohydrate intake would be the key variables.
What effect did training have on the 20 subjects’ immune systems over the seven-week period? In addition to increasing VO2max by about 11%, the workouts apparently nearly doubled the concentrations of a particular class of important NK cells (specifically, CD16, CD56 and CD3 cells) when the two groups of trainees were analysed as a whole. However, NK cell activity was unaffected by the training, and the levels of other key white blood cells, including neutrophils and lymphocyctes, were also unchanged.What about diet? As it turned out, NK cell activity increased in the group on the carbohydrate-rich diet (from 16 to 27%) but decreased in the group on the fat-rich diet from (26 to 20%) in response to the training. Statistically, the effect of training on unstimulated NK cell activity was significantly different between the groups, with the carb-loaders getting a ‘killer boost’ and the fat folks suffering a fall-off in killing capacity.
As the Danish researchers put it, the composition of an athlete’s diet can influence natural immunity, and the ingestion of a fat-rich diet during training may be detrimental to the immune system. These conclusions are basically in agreement with previous research(13). ‘Carbohydrate loading’ has also fared well in research investigating the acute – rather than long-term – effects of exercise on the immune system. Specifically, it is associated with fewer perturbations in blood immune-cell counts during strenuous exercise than ingestion of carbohydrate at lower rates(14).
This new study must be interpreted with caution, however. For one thing, the mechanisms by which carbs might boost (and fats might depress) natural killer activity are unknown, and the clinical relevance of the Danish findings are uncertain: is a jump in NK cell activity from 16 to 27% enough to ward off a significant number of illnesses, and is a drop from 26 to 20% enough to promote disease? The workings of the immune system are incredibly complicated, and disturbances in the numbers and activities of particular immune system sub-groups may not automatically change the way the overall system functions.
In addition, fats are not monolithic entities, and it may well be that certain fats have more detrimental effects on the immune system than others(15). Omega-3 fatty acids, for example, have been observed to have some positive effects on immune system modulation in comparison with certain ‘n-6’ fats; and it is reasonable to assume that increased consumption of omega-3 fats would have a quite different impact on the immune system than extra platters of beef tallow!
Finally, it is safe to hope that few athletes consume a diet in which 62% of calories are derived from fat with only 21% from carbs, as with the Danish ‘guinea pigs’. What is not yet clear is whether more modest elevations of fat consumption might also cause a significant downturn in natural killer cell activity.
1. Journal of Sports Medicine and Physical Fitness, vol 34, pp 83-90, 1994
2. Journal of Clinical Immunology, vol 5, pp 321-328, 1985
3. International Journal of Sports Medicine, vol 16, pp 329-333, 1995
4. Medicine and Science in Sports and Exercise, vol 25, pp 823-831, 1993
5. Neurobiol Aging, vol 12, pp 47-53, 1991
6. Scandinavian Journal of Medicine and Science in Sports, vol 1, pp 163-166, 1991
7. Medicine and Science in Sports and Exercise, vol 27, pp 986-992, 1995
8. Gerontology, vol 35, pp 66-71, 1989
9. International Journal of Sports Medicine, Vol. 11, pp. 467-473, 1990
10. Nutr Rev, vol 52, pp 37-50, 1994
11. American Journal of Clinical Nutrition, vol. 50, pp 861-867, 1989
12. European Journal of Applied Physiology, vol. 82, pp 98-102, 2000
13. Nutrition Research, vol 9, pp 956-975, 1989
14. Journal of Applied Physiology, vol 84, pp 1252-1259, 1998
15. American Journal of Clinical Nutrition, Vol. 59, pp 572-577, 1994