Endurance exercise and cardiovascular health: a dose-dependent relationship – Ian Craig

Based on the widely accepted assumption that endurance training is good for cardiovascular health, Ian Craig actually sees the opposite scenario within his ‘serious recreational’ athletes. This article therefore attempts to present the other side of the story.

In recent decades, it has been globally accepted that endurance exercise is good for us for multiple reasons, most notably cardiovascular health. As a young exercise physiology student many years ago, the American College of Sports Medicine guidelines were gospel to me and all other students in my field of study: regular aerobic exercise supports cardiovascular health and reduces the rate of mortality and morbidity (1). From an evolutionary perspective, humans are designed to be ambulatory, so the advice made common sense. Apparently, Hippocrates (460–377 BC) wrote: “In order to remain healthy, the entire day should be devoted exclusively to ways and means of increasing one’s strength and staying healthy, and the best way to do so is through physical exercise.”

Benefits of regular aerobic exercise

Regular exercise has been shown to reduce type 2 diabetes, some cancers, the incidence of falls and osteoporotic fractures, depression, weight management, and cognitive function, plus it enhances the quality of life and decreases mortality (2). In addition, Table 1, adapted from Nystoriak & Bhatnagar (3), who wrote an extremely comprehensive review paper on this topic, provides a list of physiological variables that are positively associated with regular cardiovascular exercise.

Table 1 – Physiological improvements associated with physical activity (3)

To extend the research support for regular physical activity, as a young student I was fortunate enough to meet the eminent Professor Steven Blair, and see him present on two different continents – his long-term epidemiological research through the fabled Cooper Institute in Dallas was at the time revolutionary in the way that we thought of exercise and its health benefits. He and his team demonstrated that an individual’s fitness level was a more important predictor of mortality and morbidity than established risk factors such as smoking, high blood pressure, high cholesterol, and diabetes (4). This data has been supported by many other researchers (e.g. 5).

The focus then, and still now, was on increasing public health physical activity to benefit health, reducing morbidity and mortality, and lessening the impact on healthcare systems. The physical recommendations for improved cardiovascular health, such as by the Surgeon General are actually fairly modest (6). Mortality risk reduction appears with even small bouts of daily exercise, such as 15 minutes, and has been shown to peak at 50–60 minutes of vigorous exercise (7). Additionally, in older patients with coronary heart disease, it has been shown that 30-minute exercise sessions produce less oxidative stress and improve arterial elasticity compared to a sedentary state, whereas 60-minute sessions may worsen oxidative stress and increase vascular stiffness (8).

The marathon boom and cardiovascular health

It was in the 1970/80s when we saw the beginning of the marathon boom – using American statistics, approximately 25,000 participants finished a marathon in 1976, which increased to 143,000 in 1980, and by 1990, the number had swelled to 224,000 (9). More recent statistics suggest that in 2014, 550,600 people completed a marathon in America (10). Around the time of the running boom, there was a well-known health advocate of running and participation in events such as marathons – his name was Jim Fixx and he authored a best seller called The Complete Book of Running. He started running at age 35 to lose a lot of weight and to improve his cardiovascular health after a previously very unhealthy lifestyle. Sadly, but ironically, Mr Fixx died of a heart attack in 1984 aged 52, while out running. This untimely death was put down to congenital reasons at the time, but thankfully the running boom, and other fitness crazes continued to exponentially expand.

When I first studied exercise physiology, I was a good middle-distance runner, and like many young athletes, had my sights firmly set on an Olympic place. My training was daily, and included high-intensity interval training two to three times per week, one long run, a few shorter runs, and at least two weight training sessions per week. My assumption at the time, just like other keen athletes (elite or recreational), was that if the exercise guidelines were formulating a positive correlation with cardiovascular health, then peak health must surely relate to peak fitness. And to this day, this is still the general assumption of everybody who participates in regular endurance activities. However, how would your attitude towards training be influenced if I told you that large amounts of endurance training could (over time) potentially be bad for your heart?

It has taken me several years of retirement after running, plus countless clinical interactions with my active clients, to think a little differently about this general assumption. Such has been the strength of my belief about the positive power of exercise, that it has taken several challenges to shake this belief. So, what if we extend our endurance exercise scenario from regular moderate activity (for health) to those individuals who year in and year out push their body physically for participation in endurance events? In my previous Johannesburg clinic, I spent eight years working with countless marathon, ultra-marathon and long-distance trail runners, multistage race cyclists, open water swimmers, and long-course triathletes, most notably Ironman participants. What I was observing was not a picture of cardiovascular health – on a daily basis I would meet ‘fit’ individuals with significant endocrine and immune dysfunction, often accompanied by signs of metabolic syndrome, including hyperlipidaemia, insulin resistance, hypertension, atrial fibrillation, plus overt signs of oxidative stress and inflammation.

Exercise and cardiovascular health – the other side of the story

I decided that it was time to investigate the other side of the story with regard to the relationship between endurance exercise and cardiovascular health. It didn’t take me very long to find a few well-written, but potentially disturbing articles – if you are an avid endurance athlete anyway.

I started with an excellent 2012 review article by O’Keefe et al (11), who focused more on competing endurance athletes. They noted that in contrast to the fairly modest exercise needs for improved cardiovascular health, highly trained endurance athletes often perform strenuous aerobic exercise for several hours daily, sometimes accumulating workloads of 200 to 300 metabolic equivalent (MET) hours per week – this figure contrasts with generally recommended ranges of 10 to 20 MET hours per week for health and weight management.

O’Keefe et al cited several studies, which I then viewed individually. For example; Breuckmann et al (12) conducted MRI evaluations of 102 apparently healthy runners (aged 50-72), who had completed at least five marathons in the past three years. Evidence of patchy myocardial scarring was noted in around 12 per cent of these runners, which represented a three-fold increased incidence compared to age-matched controls. Even more noteworthy was a similar study that found pathologic myocardial scarring in 6 out of 12 asymptomatic men who were lifelong veteran endurance athletes, but not in younger endurance athletes and age-matched controls (13); however, it was a small study. Additionally, a German study observed a greater atherosclerotic burden in 108 middle-aged marathon runners compared to matched non-runner controls, as documented by higher coronary artery calcium (CAC) scores (14). What’s more, during follow-up, it was ascertained that the adverse cardiovascular event rates in the marathoners were equivalent to those in a population with established coronary heart disease (CHD)!

These cardiovascular observations have also been supported by more recent studies, showing that participants who completed more than 2000 MET-minutes per week (MET-min/wk) had a higher prevalence of CAC and atherosclerotic plaques than individuals who exercised less than 1000 MET-min/wk (15). The most active group, however, had a more benign composition of plaques, suggesting that there might still be more to learn about this cardiovascular health and exercise story. Additionally, the extraordinary 25-year epidemiological CARDIA study of 3175 individuals (16) revealed that Caucasian men who participated in three times the recommended physical activity guidelines over a 25-year period had 27 per cent higher odds of developing coronary subclinical atherosclerosis by middle age. The authors could not explain the lack of observed links in females and African Americans.

O’Keefe and colleagues (11) also reviewed several other aspects of cardiovascular health, but I’ll mention just one more here. Citing 15 papers, they noted that high-intensity endurance training (such as marathon and ultra-marathon running or professional cycling) over a prolonged period of time has been associated with as much as a five-fold increase in the prevalence of atrial fibrillation; what’s more, they noted that left atrial hypertrophy, which is commonplace in competitive endurance athletes, may be a predictor of atrial fibrillation.

Reasons for compromised cardiovascular health

Although scientists still have to reach a consensus when it comes to the cardiovascular implications of high-volume endurance exercise, I feel that there is enough evidence to voice concern. Let’s therefore look at why these cardiovascular changes may be happening in athletes. It is well established that cardiovascular training transiently increases heart rate, blood pressure, cardiac output, and stretches the heart chambers: cardiac output may increase from approximately 5 litres/min at rest up to about 25 litres/min during vigorous training (17). Therefore, in individuals taking part in long-term endurance training and racing, these adaptive cardiovascular responses may, in some cases, occur for several hours every day.

Additionally, although regular aerobic exercise is known to improve total oxidative capacity and glutathione peroxidase activity (18), heavy and sustained endurance training generates large quantities of free radicals that may outstrip the buffering capacity of the athlete’s body, leaving them susceptible to oxidative stress and transient cardiomyocyte dysfunction (19). The cardiac geometric dimensions are restored post-exercise, but with such a recurrent stretch of the chambers, combined with adaptive immune and inflammatory responses that secrete pro-collagen, some individuals may be prone to the development of chronic structural changes, myocardial scarring, and even arrhythmias over the very long term (11). In line with these physiological observations, serologic markers of cardiac damage, including cardiac troponin, creatine kinase MB, and B-type natriuretic peptide, have been documented to increase in up to 50 per cent of participants during and after marathon running (11).

To further illustrate the physiological risks of long-term intensive endurance training, Figure 1 is reproduced from the review of O’Keefe et al (11).

Figure 1 – Possible pathophysiology associated with heavy endurance training (11).

Contextualisation of this cardiovascular information

Now let’s use my favourite word: contextualisation. Just like any physiological observations, they have to be related to the context of the individual athlete. In other words, because even the hardest training athletes still have 20+ hours in the day when they are not training, we need to look at the rest of their lives. Here are some of the many factors that influence cardiovascular health:

• Food – dietary sources of antioxidants and anti-inflammatories are vital for an athlete’s cardiovascular health and exercise recovery potential.
• Sleep – you don’t have to look too far on Pubmed to find a strong link between sleep deprivation and cardiovascular disease risk (e.g. 20).
• Stress – even lay people are aware of the link between stress and cardiovascular wellness, and there is a whole scientific book written on the subject, with a focus on acute stress, chronic stress, mechanisms and treatments (21). As an aside note – hard training is known as a ‘physiological stressor’.
• Environmental toxicity – a recent systematic review and meta-analysis established a clear relationship between toxic (heavy) metal exposure and cardiovascular risk (22). Of course, there are many other toxins that we are now exposed to on a daily basis.
• Genetics – genetic connection with cardiovascular risk is extremely well-established (e.g. 23). However, as functional practitioners, we are more interested in genetic expression (epigenetics) and how the combination of all of the above factors influences a person’s cardiovascular risk.

With these observations in mind, the people who concern me most, with regard to whether heavy endurance training adversely affects their cardiovascular health, are those who may be described as ‘serious recreational’ athletes: they hold down work and family commitments, often miss sleep to get in their training, they compromise on nutrition because they don’t have time to feed themselves properly, plus they do most of their training within the confines of a polluted city – an exact description of many of my historic clients! It’s not just the long-distance athletes who worry me though: the current trends of high-intensity interval training (HIIT) style workouts, most notably CrossFit, represent an intense load on the cardiovascular system. By increasing peripheral vascular resistance (afterload of the heart) via strong muscular contraction, while simultaneously pushing up aerobic demands of the heart and circulation during intensive circuits and supersets, the cardiovascular demands on a participant can become incredibly high, albeit for a shorter period of time.

Conclusions

I would like to finish with an observation that has been made within the type of literature that I’ve reviewed for this article. According to Laddu et al (16), recent studies of frequency and dose of physical activity have proposed a U-shaped dose-response curve: sedentary behaviour and high exercise doses (above 150 minutes of moderate-to-vigorous intensity exercise per week) increase cardiovascular disease risk, while moderate volumes and intensities of exercise improve cardiovascular health. This U-shaped curve would concur with already established exercise-health relationships, such as the immune system (24).

Scientists have not agreed on what constitutes ‘moderate’ activity, but from my perspective, all the above-mentioned components of a person’s life need to be contextualised. I for one know that I can comfortably cope with more exercise when I am relaxed, well fed, watered, and recuperated, compared to when life is full and stressful. Unlike my younger self, my body now tells me when certain types of training at certain times doesn’t feel right. In other words, my ‘moderate’ exercise dose (the optimum point of the inverted-U) shifts based on what else is happening in my life. It is this awareness that I try to impart to my clients, but that task often feels like trying to roll a very heavy stone up a steep hill – but maybe this article will help!