Colds, flu and immune dysfunction; is exercise to blame?

Being on the receiving end of a cold can be a drag at best, with just a small dent in training…. or at the other end of the spectrum, influenza can bring us down for weeks, significantly knock-out both aerobic and anaerobic conditioning and potentially cause a post-viral syndrome in unsuspecting or often over-trained athletes.

We all know the times of year when we are more prone to colds and flu, early winter and spring and sometimes even during the summer. Respiratory viruses are passed on more easily in colder ambient temperatures and lower humidity and love to replicate in mucous membranes of the upper-respiratory tract.

But are endurances athletes really prone to getting sick more often than ‘normal’ sedentary folk?

We all know that reducing human contact within confined spaces help reduce the risk of exposure, but how do we know if or when we are susceptible? I will take a brief look at the possible reasons for susceptibility of infection (Upper Respiratory Tract Infections, URTIs), and how you may be able to avoid them, if at all!

Academic studies have demonstrated that immune function in exercising individuals has a J-shaped response curve to training load. In that this means the most benefit to immune function is from light to moderate exercise in healthy individuals compared to sedentary subjects as controls. Any greater intensity or volume in exercise does not necessarily benefit immune system health, as severe chronic exercise can in fact be detrimental to the risk of acquiring URTIs.

Before highlighting the possible biochemical mechanisms exercise has with exercise, it is interesting to note that a few studies have shown that pathogens are implemented in only 30% of self-reported URTI symptoms and cases in experienced endurance athletes vs controls studied. Surprisingly, URTIs are seldom due to infections in endurance athletes, therefore non-infectious causes should be considered to identify alternative mechanisms such as pollution, allergy and air-way inflammation which can cause similar symptomology.

Evidence has now emerged to demonstrate that Toll-like receptors (TLRs) on immune cells, which play an important role in the detection and recognition of microbial pathogens may be involved in the link between a sedentary lifestyle, inflammation and disease.

The effect of acute exercise?

Single sessions of prolonged exercise may have a temporary depressive effect on immune function. This typically occurs after extreme acute bouts of exercise, for instance a few weeks after marathon or ultra-distance event. Recovery of immune function can take anywhere between 3-24hrs, returning to pre-exercise values. Strenuous or prolonged exercise greater than 1.5hr affects T-Lymphocyte CD4+/CD8+ ratio which influences susceptibility for contracting an infection ‘window of opportunity’.

These events can be related to an increase in inflammatory cytokines, particularly InterLeukin-6 (IL-6, a major immune cell signalling molecule) from contracting muscle fibers and acute phase proteins including C-Reactive Proteins. The specific effects can influence hormonal changes in cortisol, growth hormone and prolactin which may account for why athletes appear to be more prone to acquire URTI and a distorted cell-mediated immunity.

More importantly, an acute inflammatory episode from intense exercise may be associated with a reduced risk of chronic disease, such as Alzheimers and Parkinsons. Although in contrast, there may be a link between chronic inflammation and the development of auto-immune disease.

…..and chronic exercise?

After chronic periods of very heavy training, aspects of both innate and adaptive immunity are depressed, athletes are not chronically deficient but have an increased sufficient risk of URTI. Regular chronic exercise may impair immune function temporarily and increase susceptibility to infection but may not be entirely detrimental to the athlete.

Both acute aerobic and chronic restistance exercise results in decreased monocyte cell-surface expression of TLRs and decreased inflammatory cytokines which together may contribute to the immunodepression and reported higher incidence of URTIs. This may be beneficial long term, by reducing immune activation and altering whole body inflammation, lowering the risk of developing chronic disease.

Any benefit of anti-oxidant supplements?

In a previous post I have explained how preventing or reducing inflammation through common anti-oxidant supplements may suppress adaptations to training. In the context of reduced risk of colds and flu chronic dosing of Vitamin C/E can reduce inflammatory cortisol by as much as 50%.

This may limit exercise induced depression of immune function at the expense of physical adaptations. IL-6 release from muscle can be inhibited by long-term anti-oxidant supplementation and could explain the reduced incidence of URTI in ultra-marathoners who supplement with Vit C/E. Further, it is thought that alteration of whole body chronic inflammation, lowers the risk of chronic disease long term through acute aerobic and chronic resistance sessions which decrease inflammatory cytokines and biomarkers of inflammation (lower C-Reactive Protein, adipokine production) over time through suppression and natural long-term anti-inflammatory action.

Can nutrition help?

Anti-oxidants can limit adaptation to training and so nutrition might affect transcription regulation of metabolic genes in skeletal muscle. Intake of carbohydrate can reduce muscle and blood IL-6, ACTH and cortisol suggesting that low carbohydrate intake is associated with higher incidence of URTI.

30-60g of carbohydrate during a 2.5hr bout of exercise prevents T-lymphocyte depletion and therefore a negative effect on exercise-induced fall in net white blood cell immune function. Total energy control and macro-nutrition with BCAA’s is important for optimal immune functioning and is a commonly understood part of protein supplementation for recovery from extreme exercise bouts. Other supplements are thought to boost immunity, polyphenols, probiotics, and bovine colostrum- although the dose-response nature of these have yet to be established.

Negative consequences of poor nutrition are known especially in athletes who are intentionally calorie deficient and overtly concerned with body composition. Manipulation of High/Low Carb vs High Fat to influence training adaptation response and restricting total calories may augment training adaptation under the right conditions, but can lead to metabolic stress and hence further inflammation.

Time to take action?

Avoidance of contracting URTIs may be difficult for recreational athletes, especially after a series of acute breakthrough sessions of intense exercise when there are more bugs around in the colder months. Adequate nutrition during and post-session, while also minimising exposure may be the best means of defence against becoming ill and minimising disruption to training.

Supplementation to support training and boost immune function should be carefully considered, as the merits of common ‘nutriceuticals’ may be more obscure than first thought. Rather than preventing colds and flu, adaptations to training may be restricted as recovery is unnatural with these supplements.

Ultimately the protective effects of exercise out-weight the inconvenience of infrequent episodes of URTIs, in so much that the risk of chronic diseases longer term may be much lower. It may well be that you can get the best of both worlds, with smarter training and improved recovery without increasing the risk of URTIs. If you do experience what you believe is a higher than average frequency of infection, then addressing some of the aspects pointed out in this post would be worthwhile, particularly a nutrient or calorific deficit.

You may further investigate your genetic predisposition to inflammatory responses from exercise through specific gene sequences CRP, GSTT1 and IL6 which are profiled here at DNAfit as part of a more complete picture of metabolic health for individual endurance athletes.