Exercise and Immune Function

• Regular exercise does not alter overall blood leukocyte counts.
• Exercise lowers neutrophil levels in the blood or inflamed areas, acting as an anti-inflammatory for people with chronic inflammation.
• Intense exercise reduces neutrophil respiratory bursts.
• Physically active people have lower monocyte drive inflammatory responses and reduced expression of certain immune markers.
• Exercise may increase dendritic cell numbers, influencing early immune regulation.
• Moderate exercise may boost NK cell function, but intense training can reduce it.
• Moderate training enhances immune function and reduces chronic inflammation.
• Excessive training suppresses immune functions, increasing the risk of infection.

Exercise Training Effects on Mucosal Immune Function - SIgA

• SIgA is a key defense protein in saliva.
• Moderate exercise has little affect on SIgA levels.
• Intense training lowers SIgA levels.
• Lower SIgA levels is linked to a higher risk of URTI.
• Stress and heavy training reduce SIgA levels due to hormone changes.
• Athletes with consistently low SIgA levels are more prone to infections.
• Mucosal immunity is maintained with moderate exercise.
• Mucosal immunity declines with heavy training due to reduced SIgA.

Exercise Training Effects on Acquired Immune Function - Hormones

• At rest (24+ hours after exercise) athletes and nonathletes have similar immune function.
• Moderate exercise does not harm immunity in the long term.
• Too much intense training without enough recovery can weaken the immune system, making athletes more prone to illness.
• Lower type 1 t cell number, reduced t cell response and decreased b cell immunoglobulin production can occur.
• High stress hormones and changes in inflammation levels can cause this.
• The result is a temporary weakening of cell-mediated immune responses, making athletes more vulnerable to illness during heavy training.

Intense Training Studies

• Short-term studies (1-3 weeks) and long-term studies (4-10 months) have examined how intense training affects the immune system in athletes.
• Some studies compare overtrained athletes with healthy ones or illness-prone athletes with those who rarely get sick.
• Athletes often increase training intensity for short periods, which can temporarily lower performance but later improve it after rest.
• Short periods of intense training can reduce immune function, affecting white blood cells (neutrophils, monocytes, and T-cells).
• In well-trained athletes, just 1-2 weeks of hard training can significantly weaken immune responses, including antibody production.
• Heavy training suppresses both innate and adaptive immunity but does not cause severe immune deficiency.
• This may increase the risk of common infections like colds or flu
• Long-term studies show that athletes like cyclists, swimmers, and football players have lower immune function during intense training.
• One study on American football players found more colds and infections during heavy training, linked to lower levels of a protective antibody (SIgA) in saliva.
• An elite Australian swimmers study found that intense training over months reduced immune markers and increased infection risk.
• In swimmers, lower pre-season SIgA levels predicted higher chances of getting sick later in the season.
• Over a long season, natural killer (NK) cell numbers dropped, but overall B and T-cell counts stayed the same.
• The ability of neutrophils to destroy harmful invaders was reduced after intense training, showing that the immune system was temporarily weakened.
• After a period of intensified training, the number of T-cells that produce interferon-gamma (IFN-γ) was lower at rest, indicating a weakened immune response due to heavy training.
• The ability of lymphocytes to multiply in response to threats decreased after both acute exercise and intensified training but this effect reversed after two weeks of light recovery training.

Biomarkers

• Immune strength varies due to genetics, age, gender, and lifestyle.
• Some people get sick more often, but it's unclear which immune factors cause this.
• Predicting infections using biomarkers could help athletes and military personnel.
• Low saliva antibody (SIgA) levels are linked to more colds in athletes, but studies are small.
• Blood immune markers and infection risk haven’t been studied much.
• A study tracked 80 endurance athletes for four months and found:
  • Those who got sick more often had higher training loads
  • Illness-prone athletes had 2.5 times more IL-4 and IL-10 (immune proteins) than healthy ones.
  • They also had lower saliva flow and lower SIgA levels.
• High IL-10 levels may weaken the immune response, making infections last longer.
• Saliva flow is important for immunity, likely due to antimicrobial proteins like lysozyme and lactoferrin.
• A study on distance runners found:
  • Illness-prone runners had lower IL-8, IL-10, and IL-1ra levels.
  • After exercise, their IL-10 and IL-1ra levels were lower, but IL-6 was much higher.
  • This suggests they have a weaker inflammatory response after exercise.
• Genetic differences in immune signaling (cytokines) may partly explain why some athletes get sick more often, but more research is needed.

Key Points

• Athletes and non-athletes have similar immune systems when resting.
• Training too hard for too long can weaken the immune system in well-trained athletes.
• Some athletes who get sick often have an unusual immune response to exercise.
• Low levels of a saliva antibody (IgA) make athletes more likely to get colds.
• Overtraining can lead to frequent illnesses and a weaker immune system, but immune changes don’t always predict overtraining.
• The immune system may weaken due to repeated intense workouts, stress hormones, and not enough recovery time.
• One theory suggests that an overreaction to a stress-related immune protein (IL-6) could play a role in overtraining syndrome.
• Some athletes feel flu-like symptoms after too much exercise, but this may be due to immune system signals rather than an actual infection.