Getting moving is one of the best things we can do for our bodies. The benefits are almost endless: improved insulin resistance & cardiovascular health, good for our muscles & bones (especially as we age), improved cognition, mood and coordination. The list goes on. And since it seems the majority of the UK are not doing enough of it, the general public health message is do more! But what about the effects of exercise on our immune system?
Exercise has a profound, yet much less appreciated effect on our immune system – the complex, highly organised constellation of cells and molecules spread throughout our body that provide potent multi-layer defence from infection, damage repair and cancer surveillance. Our immune system is complex, not simply an ‘on’ or ‘off’ switch that can be ‘boosted’ with the right combo of nutrients & supplements. It is responsive to our actions and our environment, detecting and adapting to changes in metabolism, nutrition and movement, informing different behaviours for the many different immune cell types. Some immune cells are ‘effectors’ seeking out and fighting signs of danger with inflammation – a natural response to injury or illness inducing heat swelling, pain and redness, killing dangerous microbes and simultaneously stimulating bodily repair and regeneration. But inflammation, by design, is only ever meant to be an acute, short-term assault, kept in check by peace-keeping pro-resolvers called regulatory immune cells which extinguish unruly inflammation and keep our bodies in a status quo.
The bulk of the new research on the long term benefits of exercise reinforce a well-established theory called the “J-shaped curve” whereby the risk both of catching an infection and of having a particularly severe form of the infection drops if you exercise moderately on a regular basis. When we do get sick, the immune system mounts an inflammatory response but then needs to switch to a regulatory response so that in fighting the infection, it doesn’t actually cause us too much collateral damage in the meantime. Partaking in moderate regular exercise not only improves the effectiveness of immune cells at killing any invading infection but actually hastens this shift back to regulation! So regular exercise might actually prop up your immune response and lessen the duration of mild infections.
But exercise it not just helpful for infections. The immune system can be triggered incongruously or inappropriately regulated, igniting unruly ‘silent’ inflammation that sits below the threshold of perception. Silent inflammation is a sign of immunological malfunction and, if left unattended, can become a shared mechanism of progression in many chronic diseases ranging from diabetes and cardiovascular disease to dementia, autoimmunity, allergies, various cancers and even depression. Regular moderate-intensity exercise can combat this low-grade inflammation. In fact, the exercise induced anti-inflammatory reaction occurs our evolution with movement; regular bursts of moderate-to-high intensity exercise have formed part of our daily lives for most of the past 200,000 years, acting as a natural cleansing mechanism from any lingering inflammation. Study after study (epidemiological and clinical alike) shows that regular exercise programs generally reduce markers of inflammation (like C-reactive protein) over the long-term. With support from over 80 meta-analyses covering 22 different chronic disease states, medically directed exercise therapy is gradually becoming a mainstay component for treatment of chronic disease. Though still currently as much an art as a science, it is also an area of intense research as we gradually uncover how to optimise exercise prescriptions for different clinical conditions.
So overall, we know that regular exercise actually helps us deal with infections better and lower markers of inflammation, helping us ward off our risk for lifestyle related disease. So far, so clear? But like anything, exercise isn’t just a limitless good – there is a point with everything where ‘healthy’ shades into ‘too much’ and you can’t actually improve your health by doing too much. So if we dig a little deeper, a complex relationship between exercise and the immune system, seemingly fraught with inconsistencies, emerges. Despite inflammation being a key part of our immune armoury designed to protect us, the majority of sports and exercise actually increases inflammation in the short term (acute). This is because exercise ultimately constitutes a physiological stress on the body.
To some degree this is a good type of stress and necessary if you hope to get anything tangible out of a workout – by stressing our body, we encourage adaptation, strengthening our physiological systems through hypertrophy and improved work capacity. This is followed by a subsequent acute anti-inflammatory return to normal, rebuilding and refortifying its tissues to deal with future demands. Much of this is mediated by cortisol, a known immunosuppressive. It is part of our body’s healing machinery, driving repair to the short-term damage inflicted by intense work-out changes, but consequently temporarily blunting our infection defence. Although transient, it can be enough to make us susceptible to infection in the hours following our training. The longer the intensity, the longer the temporary period of suppression with greater than 90 minutes of heavy exercise suppressing the function of immune cells in the 48 hours that follow, meaning it is considered to be an ‘open window’ of infection.
So how can we reap the health benefits of exercise without tipping the balance into chronic over exercising?
The key is in duration and frequency. Exercise stress is proportional to the intensity and duration and can be exacerbated by things like insufficient nutrition. So if you keep stringing together spikes in inflammation without sufficient recovery from the previous one, they start to overlap leaving you open to infection & sliding towards detrimental chronic inflammation. From an immune perspective, if a pattern of over training continues without adequate rest and recovery, it can manifest as a maladaptation coined ‘overtraining’. Coexistent overtraining causes transient suppression of immune cell activity that can last for several hours or days elevating risk of respiratory infections. Adequate vitamin D and polyphenols have been explored to help with such overtraining effects but ultimately there is no short-cut to taking adequate rest days.
So through the lens of the immune system, some regular exercise is good, more is not always better, and too much is very bad. This can be hard to hear for dedicated gym-goers who are used to the no-excuses, just-do-it model of training every day whether they feel like it or not. But the evidence is clear: this simply isn’t good for your health or performance. The tipping point will be different for each person but train your intuition to inform your workouts to minimise those workout-induced lurgies and let your body adapt to increments in training. Or if you’re constantly feeling beat-down by your workouts, or if you’ve been getting sick a lot lately, take some rest instead. But, ultimately now and again everyone gets sick. So should you sweat it out or rest and recover? Clearly there is a difference between just simply moving your body (like going for a walk) and working out in the gym. Listen to your body when you feel the early stirrings of sickness and be prudent in your choices. And remember, working out in the cold can exacerbate symptoms and common-cold inducing rhinovirus can replicate more robustly at cooler temperatures.
Let’s now look at a few other immunologically important fitness angles:
Both exercise and nutrition exert separate influences on the immune system but greatest when working synergistically. The immune system needs adequate macro and micro-nutrients to do its job properly. When fighting an infection, the immune system places huge metabolic demands on your whole body, triaging nutrients amongst all the other vital body functions so adequate nutrition is vital. Energy intake must therefore equal expenditure overall. What about supplements? Many supplements claiming ergogenic properties that bolster or boost immunity generally fail to show consistent or meaningful positive effects when subjected to rigorous scientific testing. This is because supplementation is generally only beneficial when correcting a deficiency. For example, supplementing those with compromised immunity such as the elderly or clinically unwell. Overall you cannot ‘compensate’ for overtraining by just eating more/different foods. Conversely, recently some concerns have been raised about the potential negative effects on training adaptation and supplementing with high dose anti-oxidants. Vitamin D is an obvious exception to this rule with evidence now supporting supplementation for the general public.
White adipose tissue (otherwise known as the fat that sits as a scaffold under our skin, muscles and organs) is an immunologically active organ. Stuffed full of regulatory immune cells which turn down inflammation, this balance is derailed when we become overweight. Fat is also home to specialised immune memory cells — immune cells that play a critical role in recognizing and responding to infectious agents the body has previously encountered—so when we drop body fat too low, we risk losing this important facet of immunological protection from infection. So by maintaining a healthy weight, we can utilise our body fat stores to optimise our infection fighting potential
Living for longevity
From around age 20 our thymus gland starts to shrink and with is so does the number of immune white blood cells called T Lymphocytes. These are master controllers of our immune system. Recently it was discovered that healthy active muscle cells produce a molecular messenger called interleukin-7 which prevents the extent of Thymus gland shrinkage, therefore helping us to maintain a healthy number of T lymphocytes. In fact, using this as a measure, active octogenarians had better immune systems than sedentary twentysomethings. So you can delay inevitable age induced decline in your immune system through exercise. Don’t worry if you have spent most of you adult life sedentary, many of the subjects involved in the study hadn’t picked up regular exercise until their late 30s/40s.
Natural Killer cells are white blood cells and part of our innate immune system. They play a key role in a first line defence, which constantly surveys our body detecting and eliminating virus infected cells or ‘abnormal’ cells (these are cells which may go on to become cancerous). Exercise induces increases in NK cells and their activity in the body which is part of the explanation as to why exercise benefits us and lowers cancer risk. Deeper research in this area also shows that the role of NK cells extends to preventing poor-lifestyle associated unhealthy metabolic status (even in normal weight individuals), and diet-induced obesity. Conversely, being underweight is also associated with a reduced NK cell function supporting the importance of maintaining a healthy weight.
Although the direct causal relationship between exercise and gut microbial composition is not clear, prolonged excessive exercise, but not moderate levels, has a deleterious influence on the gut barrier, inducing a transient ‘leakiness’ whereby bacterial components in our gut can wrongly end up in our bloodstream. Luckily there are ways to negate this since a diet full of plant-based prebiotic foods can fuel the generation of short chain fatty acids which act like molecular sticky tape, sealing up our guts again.