Getting at least 30 minutes of moderate-intensity cardiovascular exercise daily continues to be the standard recommendation for improving overall health and longevity. This is why it's common for most to select running, cycling, swimming, or any other form of endurance training as their predominant form of exercise. It also appears to be standard practice to set a goal of completing a marathon or triathlon in order to stay motivated. If some cardiovascular exercise is good, then more must be better, right?
Wrong. Exercise becomes damaging when it's excessive. Unfortunately, when your method for getting fit is moderate-intensity cardiovascular training or steady-state endurance exercise, that excessive line is crossed more frequently than not. Don't get me wrong, there are benefits to regular exercise and daily movement, but as far as heart health goes, marathoners may be no better than the guy on the couch, and as far as longevity goes, they may be worse off.
The human body is extremely adaptable, which means diminishing returns in progress are inevitable unless a unique or more challenging stimulus is repeatedly introduced. Those selecting running, cycling, or swimming as their method for "getting fit" must continuously go farther or train harder or more frequently in order to experience any benefit from exercise. Five miles last week becomes 8 miles this week, and quickly reaches 30-40miles/week for those with aspirations of completing a marathon or triathlon. As the endurance athlete seeks more miles and higher speeds they put additional stress on their body, which results in excessive free radical production, cortisol secretion, lactate accumulation, and inflammation.
Nearly every type of workout – aerobic or anaerobic, high-intensity or low-intensity, isometric or isokinetic – produces free radicals (or reactive oxygen species), although the amount generated, and whether there's corresponding oxidative damage, depends on the workout design and delivery (mode, intensity, duration). A model developed in 1992 by M.B. Reid suggests that free radicals are generated faster during strenuous exercise than any buffering agent can handle. Above the optimal threshold, antioxidants are outnumbered and harmful oxidative stress prevails. This leads to muscle dysfunction and muscle loss, along with damage to proteins, lipids, and even DNA.
Some argue that higher levels of reactive oxygen species (free radicals) from exercise are beneficial because they increase the body's internal production of antioxidants. However, science has demonstrated that there's a breakeven point where the accumulation of free radicals overburdens any antioxidant defense. Sadly, most that choose cardio as their method for staying healthy or getting fit consistently surpass this point.
The oxygen requirement during exercise is a determining factor in the number of free radicals generated. Consistent movement for greater than 45-60 seconds is predominantly aerobic, meaning oxygen is required to produce energy (ATP). Conversely, short and intermittent (or anaerobic) exercise does not use oxygen to produce energy. Not only does this suggest higher free radical production during aerobic training, but unlike the anaerobic energy system, the oxidative stress (or cell damage) takes place inside the mitochondria. Since mitochondria are the dominant producers of free radicals, skeletal muscle has one of the highest concentrations of mitochondria, and muscle represents the largest organ in the human body, this is a BIG problem.
The oxygen demands during aerobic exercise produce considerable damage within muscle cells that leads to eventual cell death. Essentially, the muscle cells are "oxidized," and once destroyed they unfortunately can't be replaced. Research from as early as 1987 has suggested that free radical damage from long and frequent cardio workouts is especially detrimental to cardiac and skeletal muscle. As Dr. James O'Keefe discusses, endurance training causes "structural cardiovascular changes" and "elevations of cardiac biomarkers" that appear to return to normal in the short term, but when taken on as a regular activity results in "patchy myocardial fibrosis... an increased susceptibility to atrial and ventricular arrhythmias, coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening."
Dr. O'Keefe and other researchers have suggested that it's common to see extreme variations (5-fold) in atrial fibrillation when elite level endurance athletes are compared to non-runners, and other studies have found troubling medical anomalies such as:
- Impaired Cardiac Contractile Function
- Decline in Peak Systolic Tissue Velocity
- Cardio Myocyte Damage
- Myocardial Fibrosis
- Cardiac Arrhythmias
- Poor Left Ventricle Function
In April of 2014, The Journal of the Missouri State Medical Association published research showing that "long-term male marathon runners may have paradoxically increased coronary artery plaque volume." And in another study, the researchers compared a group of sedentary men to men that competed in at least one marathon annually for 25 years. Compared to the inactive group, the runners had nearly double the total plaque and calcified plaque volumes, and almost 1.5 times the non-calcified plaque volume.
Not surprisingly, the marathoners in the study from Missouri State had lower resting heart rates, BMI (Body Mass Index), and triglyceride levels than the sedentary group. The fact that "all looks good on the outside," is potentially the most frightening thing. This can be seen in the cardiovascular health of ultra-endurance athletes and cardio kings and queens who continuously put their bodies through a pounding. These guys and girls aren't just running farther than everyone else, they're running more consistently and faster.
Generally, many (including me) have idolized these individuals as we couldn't envision ourselves doing one marathon, let alone two in a row on a Saturday afternoon. However, as the evidence suggests, duration and intensity have a profound effect on free radical accumulation. Despite the natural increase in antioxidant production, the adjustment is short-lived and serious damage ensues over time. This resulting heart damage may have played a part in the early (or near) death of several famous ultra-endurance and marathon runners:
- Micah True (Caballo Blanco): One of the ultra runners featured in the popular book, Born to Run, died in 2012 at 58 years old of Phidippides cardiomyopathy – an enlarged heart from chronic excessive endurance exercise.
- Alberto Salazer: Won three New York City Marathons and one Boston Marathon between 1980 and 1982 but had a near fatal heart attack at 49 years of age.
- Jim Fixx: The man credited for popularizing jogging and author of the best-selling book, The Complete Book of Running, died of a heart attack at 52.
One study, from the European Heart Journal, looked at marathon runners, triathletes, alpine cyclists, and ultra triathletes who competed in races lasting 3, 5, 8, and 11 hours respectively. Dysfunction in the right ventricle after the race was least in the marathon runners (3 hours) and highest in the ultra triathletes (11 hours). Although it's been suggested that sudden death during marathon training only occurs in 1 in 100,000 people, the majority of those fatalities are from a cardiovascular event. As Dr. O'Keefe writes:
"If we went out for a run right now and you ran hard... by 60 minutes something starts happening... the free radicals blossom, and it starts burning the heart. It starts searing and inflaming the insides of your coronary arteries."
If that weren't bad enough, excessive free radical accumulation and resulting oxidative damage increases your risk of degenerative disease and accelerates aging. Anyone with a goal of living a long and disease-free life should avoid instances that promote excess free radical production, as the damage that ensues is at the root of many chronic diseases like cancer, heart disease, Alzheimer's, Parkinson's, and many more.
Although the free radical theory of aging is still considered a hypothesis, it's been proven that DNA damage to mitochondria increases our disease risk. Telomeres are found at the ends of chromosomes that protect DNA and the length of these tiny caps can determine our rate of aging. One analysis of skeletal muscle from a 90-year-old man revealed that only 5% of his mitochondrial DNA was full length, while that of a 5-year-old boy was almost completely intact. Our telomeres shorten during normal cell division, but if they get too short, chromosomes get damaged, cells stop dividing, and our ability to repair tissue is inhibited.
Numerous studies have found that short telomeres are associated with older cells and an increased risk of mortality and disease, and longer telomeres are associated with younger cells and a higher resistance to disease. The exact cause of telomere shortening is still up for debate, but the leading hypothesis points to chronic stress. The researchers believe that excess exposure to stress overwhelms anti-oxidant protection, resulting in cell damage – specifically to DNA and the telomeric region. Not only does oxidative stress cause DNA damage, but it appears to disrupt the enzyme responsible for telomere elongation (telomerase), meaning any chance of future repair and growth is inhibited.
Another harmful byproduct generated during aerobic exercise is cortisol. Similar to free radical accumulation, its concentration is determined by intensity and duration. When our bodies are under stress, cortisol helps to increase the concentration of glucose in our blood so there's readily-available energy for our muscles to utilize. Cortisol secretion is a natural response to stress and it's a good thing when released infrequently and for short periods as it helps the body deal with the threat to homeostasis. However, when we're exposed to chronic and consistently elevated cortisol for extended periods of time, we experience long-term consequences.
Unfortunately, prolonged endurance training causes the body to release an abundant amount of cortisol. Research from 1976 in The Journal of Applied Physiology showed no increase in cortisol secretion after 10 minutes (at 75% intensity), but cortisol doubled after 30 minutes. Another study, this one from 2011, analyzed the cortisol levels in 304 amateur endurance athletes and the average additional secretion above the control (in white in the graph below), was 42%!
Athletes who ran more kilometers per week, trained for more hours, or took part in more competitions over the year exhibited higher hair cortisol levels.
Intensity seems to play just as important a role, as 80% exercise intensity for 1 hour produces high cortisol levels while exercise at 40% intensity for 1 hour actually lowers it. With an activity like walking, cortisol is removed faster than it can be secreted, yet, as individuals looking to get fit, we're consistently told to train harder, run farther, and burn more calories.
Likewise, when cortisol is elevated, Testosterone is inhibited, meaning that consistently elevated cortisol lowers Testosterone. Cortisol increases steadily throughout a workout, while Testosterone levels peak at 20-30 minutes. That means the longer the exercise bout, the more unfavorable the Testosterone-to-Cortisol ratio (T:C). A better T:C ratio promotes muscle growth and tissue repair, while a higher proportion of cortisol leads to muscle and tissue loss.
In a nutshell, cortisol burns muscle (catabolic) and Testosterone builds muscle (anabolic), and unfortunately the increases in cortisol from endurance training leads to the former. Other than muscle loss, chronically elevated cortisol leads to injuries, sickness, and inflammation in the brain, reproductive system, intestinal tract, and heart. The elevated inflammatory markers experienced after aerobic training are much higher than those tested after alternative forms of exercise. The long-term effects of chronically elevated cortisol have nearly as detrimental an effect as oxidative stress with respect to disease, showing associations with the metabolic syndrome, diabetes, heart disease, depression, and in fact all causes of mortality.
Lactic acid is another problem associated with endurance training. Large amounts of it are produced during exercise that's beyond a certain intensity or duration, which increases oxygen and acidity (lowers pH) inside and outside muscle cells. This accumulation of lactate depends on a balance between production by the working muscles and removal by the liver and other tissues. If exercise is continuous, lactate production persists while removal declines.
Lactic acid is of relevance to health and longevity because it lowers pH. The act of simply running for a few minutes drops our normal pH of 7.4 to 7.0. Continuing or repeating the same activity can lower it to 6.8, which is considered the lowest tolerable, survival pH. Many mistakenly blame food for putting our bodies into an acidic state, yet conveniently forget that their 2-hour run that same morning results in an acidic environment with a higher likelihood of causing damage.
To handle an acidic meal, the kidneys regulate pH by excreting more or less bicarbonate. This buffering system (to bring pH up) is hampered during exercise as it can take several hours to initiate. Unlike acidic food, which only affects the pH in urine, exercise lowers pH in extracellular fluid and blood. This lactate build-up not only adds to the stress put on our cells, but arterial pH disturbance alone has been associated with life-threatening rhythmic disturbances of the heart. As written in 2002 in The Journal of Internal Medicine:
"Although acids and bases are present in foods, the major threat to bodily fluid pH is acids formed in the metabolic processes."
If you're looking to reduce your risk of Alzheimer's and dementia, heart disease, and diabetes, daily low-intensity movement will cut your risk in half without increasing stress (walking, in fact, reduces stress) or promoting oxidation. Just 30 minutes of walking 5 times per week has been shown to reduce death risk by 50%!
If performance is your goal, you're better off doing High-Intensity Interval Training (HIIT). Short sprint intervals produced equal aerobic improvements (VO2 max, lactate threshold, aerobic power) and better fat loss when compared to moderate intensity jogging, and that was with 1/18th the time commitment!
If you're in search of a six-pack, your time is better spent lifting weights and eating right. When you work out to build muscle (not burn calories), you burn more energy throughout the day. Any new muscle needs energy just to exist, which means an increase in the number of calories burned, even while sedentary. Unlike resistance exercise, aerobic training does not produce significant positive changes in muscle size or strength, only producing favorable increases in endurance capacity. A strong, muscular physique is not only more aesthetically pleasing, but research suggests that strength and muscle mass are the two most important biomarkers for health and longevity.
And lastly, if you love running... I suggest finding a new hobby. I love eating chocolate and drinking wine, but that doesn't mean I'm consuming them 5 times a week for 3 hours at a time. In all seriousness, anything you love about running (endorphins, alone time, camaraderie, competition) can be experienced elsewhere, while potentially increasing your lifespan instead of knowingly shortening it.