Thoughts of exercise usually suggest fitness, better health, recovery from injury or surgery, and superior athletic performance. Admittedly, for some the idea is unappealing; but seldom does exercise suggest a “clinical condition,” unless exercise has been specifically prescribed to improve one’s ill health or recover from injury or surgery. The health benefits of regular exercise in individuals of all ages and physical abilities have been well described for improved quality of life, mental health and cognition, cardiovascular and respiratory health, type 2 diabetes and impaired glucose tolerance, weight loss, bone health and other conditions. Physical fitness and its health benefits are best achieved through regular exercise. As described by the American College of Sports Medicine in a
position statement, exercise means planned, structured and repetitive activity such as jogging, an aerobics class, swimming, bicycling or other activities aimed at improving or maintaining fitness. In contrast, physical activity includes everyday activities such as walking, gardening or climbing the stairs. Physical fitness is the ability to carry out daily tasks with vigor, ample energy and alertness, without undue fatigue, and has measurable attributes such as muscle strength and endurance. A useful guide for older adults wishing to improve their fitness through different types of exercise is available from the U.S.
National Institute on Aging. One of the less well known benefits of regular moderate exercise is its effect on immune function. Most chronic diseases, including type 2 diabetes, heart disease and obesity, have underlying subclinical inflammation, which exacerbates their condition. Physically fit and active adults have
reduced inflammation, lower oxidative stress and a lower susceptibility to
respiratory tract infections compared with sedentary individuals. Moderate intensity exercise is also associated with a lower risk of
respiratory viral infections. On the other hand, intense training
depresses immune function in elite athletes and
increases susceptibility to upper respiratory tract infections—at least in marathon runners—which may impair athletic performance. This conclusion has been
challenged. In recognition of the effects of exercise on the immune system, the International Society of Exercise and Immunology published two consensus statements for athletes on
immune function and exercise and
maintaining immune health, acknowledging that exercise affects immune function, but the evidence for clinical effects remains inconclusive. The British Association of Sport and Exercise Sciences also released an
expert statement on exercise, immunity and infection. It remains unclear if there are
clinical implications or changes in host defense resulting from the altered immune responses in trained athletes.
Because immune function may be depressed by endurance training, it would be useful to know if exercise-induced impairments could be prevented and whether immune function could be enhanced or maintained with non-pharmacologic anti-inflammatory agents, such as long-chain omega-3 PUFAs (n-3 LC-PUFAs). These fatty acids were associated with improved pulmonary function, reduced inflammatory mediators and less frequent use of bronchodilator therapy in exercise-induced bronchoconstriction in
asthmatic individuals and elite athletes. In endurance cyclists, consumption of 2.4 g/day of n-3 LC-PUFAs for 6 weeks before and during a 3-day period of cycling with time trials was
not associated with changes in plasma cytokines and other immune markers or exercise performance before or after heavy exertion. Use of n-3 LC-PUFAs in athletes and untrained individuals has received little scientific scrutiny.
In this study, Patrick Gray and colleagues at the University of Aberdeen, U.K., examined the short-term effects of the consumption of 3 g/day of fish oil providing 1.6 g/day of EPA + DHA for 6 weeks on markers of immune function in 8 young men who were untrained, but recreationally active. A placebo control group of 8 men consumed 3 g/day of olive oil under the same experimental conditions. Prior to the trial, participants performed a maximal exercise test to determine the appropriate workload and provided a fasting blood sample. Workload was determined from the pretrial performance of cycling at 70 rpm with increasing workload of 30 watts every minute until exhaustion. During this exercise the investigators measured peak oxygen consumption throughout the test. After 6 weeks’ supplementation, the men cycled for 1 hour at 70 rpm at the workload associated with 70% of their peak oxygen consumption measured at baseline. Post-exercise blood samples were taken immediately after the exercise and at 1 and 3 hours thereafter. The investigators examined neutrophil function, natural killer cell cytotoxic activity, cytokines, cortisol and plasma EPA and DHA. Three hours after exercise, peripheral blood mononuclear cell interleukin-2 (IL-2) production and natural killer cell cytotoxic activity increased in the fish oil-treated men, but not the controls, while plasma cortisol levels fell in both groups. The increase in natural killer cell activity was due to an increase in cell number, not in activity per cell. In a different study among cyclists, natural killer cell activity was significantly
higher after the exercise, independent of carbohydrate consumption. In the present study, the findings might be related to the effect of n-3 LC-PUFAs, especially EPA, in reducing the production of prostaglandin E
2, which suppresses natural killer cell function. Increased natural killer cell activity in fish oil-treated men shortly might suggest improved resistance to infection in the post-exercise period. Fish oil supplementation might provide a strategy for reducing the immunodepression observed in endurance athletes, as noted in a recent
review. However, it remains to be established that the observed changes are linked to susceptibility to infection. Changes in immune function in athletes vary with the nature, intensity and duration of the exercise, with
moderate exercise having protective effects and repeated strenuous exercise resulting in immune dysfunction.
Others have reported that the consumption of EPA and DHA suppresses natural killer cell activity in healthy adults above 55 years of age. Long-term consumption of n-3 PUFA-enriched foods was also associated with
reduced numbers of natural killer cells in healthy adults 23 to 63 years of age. Natural killer cells from older adults also appear to be
less responsive to IL-2 and may exhibit other functional changes. However, in younger men, the consumption of up to 4 g/day of EPA for 12 weeks
tended to increase natural killer cell activity. These contrasting observations suggest that age, intense exercise and n-3 LC-PUFA consumption all affect natural killer cell activity. The increase in both natural killer cells and IL-2 production from mononuclear cells 3 hours after exercise may be related as IL-2
stimulates natural killer cell activity. Increased natural killer cell responsiveness to IL-2 was
also reported in the previous study of cyclists mentioned above.
It is also of interest that natural killer cell activity is stimulated in allergic airway inflammation. These cells express
receptors for resolvin E1, a potent mediator in resolving and ending inflammation in airway tissues. When natural killer cells were reduced, resolvin E1-mediated resolution of the inflammation was reduced. Resolvin E1 is
derived from EPA and its production is increased in the presence of EPA. Unanswered in this short-term study is whether increased consumption of n-3 LC-PUFAs along with moderate exercise is associated with the incidence of upper respiratory tract infections or protection against viruses in trained and untrained athletes. Supplementation with n-3 LC-PUFAs has been associated with fewer upper respiratory tract infections in
Indian school children, reduced
antibiotic consumption in adults with cystic fibrosis,
fewer episodes of illness, especially respiratory illness, in Thai school children and
less respiratory illness in U.S. toddlers. As immune function appears to be improved with
regular long-term exercise and with the consumption of n-3 LC-PUFAs, it remains to be established whether these fatty acids affect respiratory illness in athletes with more intense training. Gray P, Gabriel B, Thies F, Gray SR. Fish oil supplementation augments post-exercise immune function in young males.
Brain Behav Immun 2012;26:1265-1272. [
PubMed]
