Athletes beware! Strenuous exercise can raise your risk of infection.
Post-exercise immunosuppression (PEIS) is a physiological response to large training loads or strenuous exercise, which generally happens due to a rise in stress hormones such as adrenaline and cortisol.
Load and intensity
Research affirms that intense exercise causes immunosuppression. And exercising during the incubation period of an infection can worsens symptoms once an upper-respiratory-tract infection (URTI) becomes full-blown.
Interestingly, moderate-intensity exercise seems to improve immune function and potentially reduces the risk and severity of respiratory viral infections.
As such, it is important to understand how the immune system functions following exercise-induced stress and how to recognise the signs that may indicate an imminent infection. You can then decide how hard or how much to exercise in an effort to support your immune system.
The immune response
Your immune system comprises a complex array of different elements that work synergistically to recognise, attack and destroy viral and bacterial infections. Your immune response depends on two systems:
- The innate immune system: Includes the barriers that attempt to prevent foreign substances from entering and infecting the body. These include physical, chemical and cellular barriers.
- The acquired immune system: Includes the different immune cells that work together to help the body fight infections.
Suppressed immune function
When athletes endure chronic training loads, infection risks rise, particularly in the upper respiratory tract, because stress hormones impair immune cell function.
Lymphocyte levels in the blood can also fall below resting levels following strenuous exercise, which seems to inhibit innate immunity by reducing mucosal protection.
Consequently, athletes who train hard become more susceptible to infections, especially during flu season.
There are numerous ways that athletes can support their immune system during intense or high-volume training blocks in an effort to mitigate PEIS.
Among these, proper nutrition and adequate recovery are perhaps the most important to minimising the risk of time off from training due to illness.
From a diet perspective, studies show that training in a glycogen-depleted state after spending several days on a low-carbohydrate diet (less than 10% of dietary intake from carbohydrates), raises stress hormone levels in comparison with normal or high-carbohydrate dietary conditions.
However, these studies also found that athletes can manipulate their carbohydrate intake to alleviate PEIS. High-carbohydrate diets can reduce the stress hormone response and consuming carbs during exercise reduces stress hormone production which, in turn, seems to alleviate PEIS.
For individuals who choose to train hard in a glycogen-depleted state, essential vitamins and mineral supplements can help to boost immunity.
Vitamin D plays a role in various bodily systems, including both innate and adaptive immune responses. The so-called ‘sunshine’ vitamin enhances innate cellular immunity by stimulating immune cell expression and helps to maintain cell integrity. Supplements that boost vitamin D levels and address any deficiency can help to reduce the incidence and severity of viral infections and URTIs.
Vitamin E is a potent antioxidant. Together with selenium, these nutrients help to keep rising stress hormone levels in check and form major components of the body’s antioxidant defence system and immune cell response. Studies show that supplementing with selenium and vitamin E can increase resistance to respiratory infections.
A zinc deficiency can increase pro-inflammatory cytokines and alters cell barrier function, making them more susceptible to infection. This trace mineral also has anti-viral properties, as do omega-3 fatty acids, which can also inhibit flu virus replication.
And vitamin C is a vital nutrient needed to fight infections. Vitamin C acts as an antioxidant, and immune cells called leukocytes require significant amounts to fight infections. This makes vitamin C a conditionally essential nutrient during illness, which is where supplements can really help. Consuming additional vitamin C can actually reduce an infection’s duration and severity.
Additional nutrients worth considering in your supplement plan include magnesium, as a deficiency is associated with decreased immune cell activity, and iron because anaemia, which is not uncommon in endurance athletes, can weaken your immune system.
Supplement the response
Circulating glutamine levels also tend to drop after exercise. As glutamine stimulates the activity of certain immune cells, infection risks can rise without adequate glutamine replenishment.
Studies also show that many immune cells have an unusually high capacity to utilise glutamine, but are unable to synthesise it in its original state and, therefore, require additional sources from our diets and supplements.
And whey protein contains beneficial compounds, including lactoferrin and immunoglobulins, which have immune-enhancing bioactive properties and can help to boost your immune system following intense training.
Gut health matters
An explosion of research into the immuno-stimulatory properties of probiotics has also emerged over the last decade.
Studies on their effects on PEIS found that probiotics caused a faster rise in circulating natural immune killer cells to normal pre-exercise resting levels following intense training. These results indicate that probiotics may help to reverse PEIS.