Can intense exercise make you sick?

June 7, 2017

One of the primary indicators of an individual who is overtraining (or better known as under-recovering) can be illness. It has been widely accepted that intense exercise can increase the risk of illness due to a depressed immune system. What does this mean for most people, however? We hear of extreme training regimens from top level athletes and believe that they can be applied to everyone, when that is simply not the case.


Rather than thinking about pushing an individual to an overtrained state, I believe the more apt explanation should surround the lack of sufficient recovery. It turns out, the body is quite capable at accepting intense bouts of exercise stimulus, problems arise, however, when there is insufficient recovery. So what is sufficient recovery, and how do we achieve this? Well, it turns out there are a number of factors at play, which will be discussed in this blog, but also keep in mind that how each of us recovers is largely individual.


As a CrossFit coach, and someone who programs workouts for the "masses" all the time, I have to carefully consider the work to rest ratio of everything we do and ensure that not only are workouts challenging, engaging, and fun to do, but also do not push us to our absolute limits all the time. Working hard and being intense is important, but certainly not something that can be done consistently all the time.


This very topic is examined in Journal of Applied Physiology:


Recovery of the Immune System After Exercise




The notion that prolonged, intense exercise causes an “open window” of immunodepression during recovery after exercise is well accepted. Repeated exercise bouts or intensified training without sufficient recovery may increase the risk of illness. However, except for salivary IgA, clear and consistent markers of this immunodepression remain elusive. Exercise increases circulating neutrophil and monocyte counts and reduces circulating lymphocyte count during recovery. This lymphopenia results from preferential egress of lymphocyte subtypes with potent effector functions [e.g., natural killer (NK) cells, γδ T cells, and CD8+ T cells]. These lymphocytes most likely translocate to peripheral sites of potential antigen encounter (e.g., lungs and gut). This redeployment of effector lymphocytes is an integral part of the physiological stress response to exercise. Current knowledge about changes in immune function during recovery from exercise is derived from assessment at the cell population level of isolated cells ex vivo or in blood. This assessment can be biased by large changes in the distribution of immune cells between blood and peripheral tissues during and after exercise. Some evidence suggests that reduced immune cell function in vitro may coincide with changes in vivo and rates of illness after exercise, but more work is required to substantiate this notion. Among the various nutritional strategies and physical therapies that athletes use to recover from exercise, carbohydrate supplementation is the most effective for minimizing immune disturbances during exercise recovery. Sleep is an important aspect of recovery, but more research is needed to determine how sleep disruption influences the immune system of athletes.


We know that physical stress causes a stimulation and therefore chemical response (inflammation) within the body. Inflammation and damage to our tissues, caused by exercise, is therefore viewed in essentially the same way as a physical trauma would be (blunt force, for example). Something is wrong, and the body does its best to try and fix it. These inflammatory markers invade the necessary tissues to signal repair, which can include muscular regeneration as well as vascular adaptations.


Monocytes mobilized by exercise are likely to infiltrate skeletal muscle and differentiate into tissue-resident macrophages that facilitate repair and regeneration, particularly following arduous bouts of exercise that cause significant skeletal muscle damage (85). Monocytes with effector phenotypes are also preferentially redeployed after exercise. The CD14+/CD16+ “proinflammatory” monocytes are preferentially mobilized over their CD14+/CD16− counterparts (109). Monocyte expression of pathogen recognition receptors [e.g., toll-like receptors (TLRs)] tends to decrease in response to moderate-intensity exercise (109). Conversely, prolonged, intense exercise (60-km cycling time trial) increases TLR2 and TLR4 expression on monocytes, which may indicate a heightened proinflammatory state (11). A recent study showed that acute exercise mobilizes angiogenic T cells, which may facilitate vascular remodeling during exercise recovery (53). Exercise is also known to mobilize hematopoietic stem cells, which may participate in skeletal muscle repair and regeneration after exercise (25, 49). It has been suggested that exercise may have a role as an adjuvant to mobilize stem cells in donors for hematopoietic stem cell transplantation (25).


If inflammatory markers are primarily focusing on tissue damage created during intense bouts of exercise, then the immune system itself is then depressed and less likely to be as efficient at dealing with foreign invaders. So, prolonged or consistent bouts of intense exercise can increase the risk of illness in athletes.


One thing to note, this review also discusses dietary interventions to aid in reducing the effects of immunodepression. It posits that consuming carbohydrates shortly before and during intense bouts of training can aid in boosting the immunity.


The immunomodulatory effects of carbohydrate may depend on the timing of carbohydrate intake. The ingestion of a glucose solution 15 min, but not 75 min, before 1-h high-intensity cycling prevented immunoendocrine perturbations (50). The lack of an effect of carbohydrates ingested 75 min preexercise was potentially associated with an insulin-induced decrease in the plasma glucose concentration before exercise, which, in turn, might have enhanced immunoendocrine responses (50). Carbohydrate ingestion during either the first or the second of two 90-min bouts of cycling on the same day better maintained plasma glucose and attenuated plasma stress hormone responses to the second bout (59). By contrast, carbohydrate ingestion during the 2-h recovery period between these exercise bouts had no such effects (59). These findings suggest beneficial effects of a timely carbohydrate supplementation (i.e., shortly before and/or during exercise) on immune responses to exercise. This may be particularly relevant with more prolonged and/or intense exercise protocols and when the recovery duration between two consecutive exercise bouts is short.


This is especially important to note for those individuals who have been recommended to reduce their carbohydrate consumption due to various reasons such as an apparent increased rate of weight loss. Low carbohydrate diets, coupled with intense exercise exacerbate these problems, and increase your risk of illness.


Some studies have investigated the effects of dietary carbohydrate intake on immune responses to consecutive days of exercise intended to deplete muscle glycogen (9, 10, 34, 65). A higher carbohydrate intake consistently attenuated certain components of immunodepression well into the recovery period (i.e., ≥2 h postexercise) after the second exercise session (10, 34, 65). Athletic training often involves conditions of low carbohydrate availability, e.g., due to abbreviated recovery periods and/or as part of a “train low-compete high” training regime (41, 42). These investigations therefore have particular practical implications. Compared with a higher carbohydrate intake (8 g·kg−1·day−1), very low carbohydrate intake (0.5 g·kg−1·day−1) leads to greater perturbation in leukocyte subsets during recovery from exercise (65). These effects may be related to sustained elevation of plasma cortisol concentration (65). Bishop et al. observed that compared with a low-carbohydrate diet (1.1 g·kg−1·day−1), a high-carbohydrate diet (8.4 g·kg−1·day−1) for 3 days after glycogen-lowering cycling attenuated plasma cortisol and cytokine concentrations and circulating total leukocyte and neutrophil counts following subsequent exercise (10).


Higher than "normal" amounts of protein - or at least what most would probably consider to be higher than normal, also aid to reduce the effects of this immunodepression from intense exercise. But readers of my blog probably already knew that.


Recognizing the importance of protein for immunocompetence (15), there are benefits of postexercise protein ingestion (18, 19, 69) or a diet high in protein (128) on immune responses to exercise. On the basis of previous results indicating that exercise-induced lymphocyte trafficking was impaired during high-intensity training, Witard et al. examined whether a high-protein diet can restore these impaired immune responses (128). Consuming a high-protein diet (3 g·kg−1·day−1) helped to minimize exercise-induced changes in lymphocyte distribution during a period of intense training (128). Interestingly, an energy- and carbohydrate-matched normal protein diet (1.5 g·kg−1·day−1) failed to provide the same benefit (128). The high-protein diet was also associated with fewer self-reported upper respiratory illnesses (128). Another study demonstrated that protein and leucine supplementation for 1–3 h postexercise during 6 days of high-intensity training enhanced neutrophil respiratory burst activity after the last exercise session (69).


Consuming a carbohydrate-protein solution immediately, but not 1 h, after exercise prevents a decrease in neutrophil degranulation during the postexercise recovery period (19).


To summarize, one of the most important factors in preventing overtraining is just the cognizance of work to rest. Sure, intense exercise is great, but is not necessary all the time. Now also realize that intensity is a relative term. What is intense for one individual may be a "walk in the park" for someone else. Your current training status, how much stimulus your body can handle, your age, your activity level outside of the gym, etc. all play a part in how intense you can push yourself in the gym.


Having a coach that can discuss and assess these aspects, as well as design and implement a training program for you that is challenging, fun, engaging, and rewarding is also extremely important. Oftentimes individuals get stuck with the mindset that more must be better, and that working harder must be the path that is taken in order to reach your goals.


Beyond having a balanced training program with sufficient rest, and as this review points out, other strategies such as nutritional interventions can go a long way to help keep your immune system working as good as possible to prevent illness and overtraining....I mean under-recovering.



Tyler Robbins B.Sc., CSCS
Director of Fitness
Head of CrossFit

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