The inflammatory response in endurance athletes - by Melody Evans

Endurance athletes, including cyclists, runners, swimmers and triathletes, are required to exercise intensely and/or for prolonged periods each week when training for competitions.

Whilst moderate exercise appears to help prevent upper respiratory tract infections (URTIs), there is a correlation between an increase in training volume and intensity and the occurrence of URTIs, particularly after training and competition, due to a temporary suppression of the immune system – the ‘open window’ theory (4). This can undesirably disrupt the athlete’s training schedule and competitions. Several mechanisms may explain this 'open window' and this article explores ways that we can measure and mitigate the risk of immunosuppression in athletes.

Physiological stress 

Endurance exercise can induce oxidative stress due to an increased production of reactive oxygen species (ROS). Oxidative stress can cause skeletal muscle damage, fatigue, pain and delayed onset muscle soreness, and impair performance and recovery, caused by an inflammatory reaction to protect against tissue damage. Ultra-endurance events, including ultra-marathons and ironman triathlons, with very high training loads, have been associated with an acute reduction in antioxidant capacity and increased oxidative stress markers (2), justifying the need for athletes to monitor their training loads and support antioxidant capacity.

During exercise, increases in adrenaline and cortisol are associated with an increase in leukocytes and neutrophils (immune cells that are responsible for defending against viral and bacterial infections), but high-volume training can suppress these cells. Leukocyte and neutrophil counts tend to be substantially lower in endurance cyclists and triathletes (3), which could be due to their migration to the site of damaged skeletal muscle (1), meaning they are not available to fight off infections.

Psychological stress 

High levels of stress can lead to immunosuppression, so while elite endurance athletes are susceptible to increased risk of infections due to their high training load, recreational endurance athletes can be too because of juggling their training schedule with the demands of family and work. This highlights the importance of supporting athletes to manage their stress loads more effectively. Psychological stress also needs to be considered when preparing for competitions, which can involve long-haul travel (increasing exposure to infectious agents) and poor sleep.

Heart Rate Variability (HRV) is a useful non-invasive tool to monitor the nervous system’s response to stress and to regulate training load, if required. A high HRV indicates more resilience to stress and greater cardiovascular fitness, while a low HRV has been linked to depression and anxiety.

How can nutrition help?

It is important that the athlete’s macronutrient requirements are met. Sufficient dietary carbohydrate intake can support plasma glucose levels, which in-turn may reduce the release of cortisol, and consequently decrease the levels of stress placed on the immune system. Conventional sports nutrition guidelines suggest that 30 to 60 grams of carbohydrates should be consumed per hour of exercise, and that endurance athletes require a daily consumption of 1.2g to 1.6g of protein per kg of body mass.

A diet rich in micronutrients from fresh fruit and vegetables (organic, where possible) is important to mitigate oxidative stress and to regulate the inflammatory response. Some micronutrients and their benefits are listed below:

  • - Phytonutrients such as catechins found in cocoa, green tea and grapes have anti-inflammatory action and can protect cartilage, while quercetin found in apples, green tea and berries has antioxidant properties and can improve lean body mass, basal metabolic rate and total energy expenditure in athletes.
  • - Key antioxidants:
    • - Vitamin A: important for eye health, protects mucus integrity and supports reproduction
    • - Vitamin C: important for collagen production, protein metabolism and supports adrenal function
    • - Vitamin E: involved in the production of smooth muscle cells and regulates gene expression
    • - Copper: required for immune function, bone and connective tissue health, and supports the cardiovascular system
    • - Zinc: vital for cell growth and replication, and immunity, and supports carbohydrate metabolism
    • - Selenium: supports thyroid function

Other important nutrients:

  • - Vitamin D3 is important for antimicrobial activity and is involved in immune cell regulation.
  • - Iron is necessary for immune cell production, oxygen transport and energy production. Vegan and vegetarian athletes need to combine vitamin C with plant-based iron to increase absorption.
  • - The omega-6 to omega-3 ratio should be monitored to ensure an adequate intake of anti-inflammatory omega-3 fatty acids.
  • - Considering that around 70 per cent of the immune system is contained within the gut, with 80 per cent of secretory IgA in the gut-associated lymphoid tissue,  probiotics may reduce the risk of URTIs and gastrointestinal illness during stressful periods of training and competition.
  • - Glutamine can be deficient in overtrained athletes and can be significantly reduced following intense training efforts, which can compromise lymphocyte immune cell function.

As well as ensuring that the athlete’s nutritional needs are met, lifestyle support is also important to ensure that the athlete is managing stress levels, training load, personal hygiene and sleep, to provide the body and mind with the tools to cope with the psychological and physiological demands of training and competition. These factors should help reduce the risk of URTIs and tissue damage, and promote optimal performance and recovery for training and competitions.


  1. Cruz G et al (2016). Leukocyte populations are associated with heart rate variability after a triathlon. J Hum Kinet. 54(1):55-63.
  1. Elkington L et al (2015). Inflammation and immune function. Antioxidants in Sports Nutrition. CRC Press/Taylor & Frances.
  1. Horn PL et al (2010). Lower white blood cell counts in elite athletes training for highly aerobic sports. Eur J Appl Physiol. 110(5):925-932.
  1. Moreira A et al (2009). Does exercise increase the risk of upper respiratory tract infections? Br Med Bull. 90:111-131.