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The Evidence Continues to Mount Against Chronic Cardio

Posted By Mark Sisson On July 9, 2013 @ 8:00 am In Fitness,Health | 202 Comments

It’s been awhile since I did a post on chronic cardio [7]. I had a good string of them [8] going several years ago, and I thought I’d done a good job explaining why I was so opposed to excessive endurance training [9]. Despite my attempts to clarify [10], though, I still receive a lot of questions and comments about cardio. People just have a tough time divorcing themselves from the notion that cardio – as much as you can cram into your schedule – is the key to health and fitness. I don’t blame them, really. It’s conventional wisdom [11], after all, and it’s what I thought for years and years. Clearly, another post is needed.

Evidence against chronic cardio continues to mount, so there’s a lot to cover. But before we get to all the research, I have a few thoughts about the heart.

Here’s the thing about the heart: being an involuntary muscle, it has no say in the matter. It pretty much feels nothing, too. It’s along for the ride. Just like the liver, kidneys, pancreas, thyroid, adrenals, etc., the heart responds to biochemical signals. It’s a demand organ. Minor changes in blood chemistry (epinephrine, cortisol [12], insulin, lactic acid, hemoglobin-depleted RBC’s, to name a few) cause it to respond by beating faster or slower, forcefully or not, to keep pace with the muscles’ (and other organs’) demand for oxygen and fuel. During exercise, it’s the brain that starts this whole process with a (usually) conscious decision: “I think I’ll run to that tree.” That thought prompts the muscles of the legs to start moving faster and the arms to pump. The new, increased demand for oxygen and added fuel (over and above normal resting metabolism) signals the heart to start to fulfill the demand, to pump harder and faster. It’s obliged to do so. Period. No choice. That’s also why it’s always a bit behind schedule: it takes more than a few seconds to ramp itself up once the action begins and a few seconds or minutes (or hours, in the case of an over-trainer [13]) to ramp down, once it’s over.

The problem with chronic cardio is that we can force our brains to override some of the tiredness (no pain, no gain, pal) and discomfort in the legs – and to a certain extent even the lungs – and keep doing these hard endurance workouts incessantly day in and day out. The ostensible limiting factor is the ability to burn fat [14] or, at the very least, the amount of glycogen [15] still left in our muscles. That’s what eventually brings us to a halt, frequently because we have willed ourselves to keep going through the wall at all costs. But the heart is often over-worked in this scenario, just trying to keep up with that “inhuman” (and inhumane) desire to run, cycle, or swim further and faster in pursuit of…what? A medal? A ribbon? Bragging rights? It can’t say no. It attempts to do as we bid it. And because the heart feels little-to-no pain – unless, perhaps, it feels the REAL pain of a heart attack – it very often suffers silently as a result without us ever knowing. The walls of the heart start to hypertrophy [16] over time the same way a biceps muscle does when you do curls. But do a few too many curls and your biceps will get sore quickly. Force yourself to do a few more and you could even tear something and be out of contention for a few weeks. We know when to stop before that bicep tears.

Cardiac muscle doesn’t tear that way when over-worked, but it does enlarge and thicken with chronic overuse. In some – most – people the thickening is probably not life-threatening, but in some cases, as with dozens of world class athletes I have personally known, this thickening can cause all manner of issues later in life. Atrial fibrillation has become a mild epidemic in my generation of life-long aerobicizers; several of my friends have had pacemakers or defibrillators implanted before the age of 40 to head-off those sporadic life-threatening cardiac enervation problems. A few more friends have lost significant cardiac function and a few have died.

But don’t take my word for it. The silent epidemic of heart issues among endurance athletes is getting serious attention in the research community. Let’s take a look at some of the latest research.

Cardiac Arrhythmias

Cardiac arrhythmias are abnormal electric activities of the heart. An arrhythmia can describe a heart that beats too fast, too slowly, too irregularly, or too “fluttery.” An arrhythmia doesn’t always indicate or foretell heart trouble, but it’s a common risk factor. One of the more common varieties is atrial fibrillation (AF), which describes a fast, irregular heartbeat. AF is strongly linked to stroke and cognitive decline [17].

Endurance athletes are at a greater risk for atrial fibrillations [18] than the general, non-running public. One recent study of cross country skiers even found that the best athletes, the top performers, were more likely to have cardiac arrhythmias than the rest. Moderate exercisers, meanwhile, are at a lower risk for AF [19] than the general, non-running public. A recent comprehensive study offers several potential explanations [20] for the increased risk:

  • Increased fibrosis (scar tissue formation) in the heart.
  • Myocardial injury to the heart, as evidenced by post-training elevated cardiac biomarkers typically used to diagnose injury. Probably not a big deal so long as you recover fully from your training, but most cardio junkies can’t wait that long to log more miles.
  • Excessive amounts of inflammatory markers brought on by training. These markers have been linked to AF.

Endurance-related AF usually starts off infrequent. The older you get and the more miles you log, the more entrenched and regular your atrial fibrillation may get. Some studies [21] found that around 40% of athletes with AF eventually progress to persistent AF, where it’s happening on a regular basis. That’s the troubling kind of AF that may presage serious cardiovascular problems, like stroke.

Atherosclerosis

It’s totally counterintuitive to think that endurance athletes are at risk for arterial plaque. “You mean to tell me that the wispy greybeard whizzing past my house in short shorts every evening could have clogged arteries? No way.” Maybe, just maybe.

A 2011 study [22] found evidence of carotid and peripheral atherosclerosis in a group of marathoners. Although there was no control group of non-runners in that study, another study compared the arteries of marathon runners to a control group of sedentary non-marathoners. Marathoners had more calcified plaque in their coronary arteries [23], which has been linked to stroke and dementia [24]. The tricky thing about these cases is that endurance athletes with atherosclerosis don’t evince the regular signs. Whereas your typical sedentary guy with extensive atherosclerosis will probably have all the hallmarks (metabolic syndrome, abdominal obesity, hypertension, etc.), marathon runners with atherosclerosis don’t fit the traditional cardiovascular risk profile [25].

It might be time to add “trains for endurance athletics” to the list of risk factors.

Oxidative Stress/Overtraining

It’s no secret that endurance training induces oxidative stress on the athlete [26]. That’s how we get better – by encountering a stressor, being broken down a bit, and then recovering stronger than before so that the next time we encounter the stressor, we’ll be better than the last time. Whether we’re talking strength training, marathon running, cycling, gymnastics, martial arts, or even studying for a trigonometry class, we have to challenge our physiology to get better, and challenges to the physiology mean oxidative stress. Problems arise when we don’t let up, when we keep the intensity elevated and the days off few and far between. We’re constantly in that post-workout state, and it starts to look like chronic oxidative stress for all intents and purposes. Even if our times are improving, we’re not truly recovering. It’s a two steps forward, one step back kind of thing.

So. Those are just a few of the reasons I am no fan of chronic cardio [9] (and don’t get me started on the bad backs, osteoarthritis [27], hip and knee replacements and chronic tendonitis among my former elite endurance peers). A strong will can be a great thing for survival, for business and for relationships, but it can also get you in trouble if you don’t pay attention to your training load.

Having said all that, I am still a big fan of weights, of brief, intermittent interval training and I am all for doing a fair amount of mixed low-level cardio, the kind that doesn’t overstress the heart or involve so much repetitive joint motion that it causes chronic injury. That makes sense in an ancestral context. You’re expending energy at a high rate, but you’re not going long enough that it becomes a liability. Or, if you’re going long, you’re taking it easy enough that you have the energy to make it back home, possibly carrying food.

I’m not even against a long training run or ride once in a while, provided you are trained, rested and allow enough recovery afterwards. I’m even OK with running marathons [28] occasionally or jumping into a short triathlon now and then. As a species, we obviously have the capacity to go long and relatively hard every now and again. It’s the chronic, day-in, day-out long, hard stuff that is counter-productive. If you did that twenty thousand years ago, when your next meal – and that of your entire family/tribe – was on the line, when calories were somewhat precious, when you didn’t have an air-conditioned caravan of trainers, massage therapists, and coolers filled with electrolyte drinks [29] following along after you, you’d be foolish. You simply wouldn’t do it.

That we can run marathons (and do other stupid things) and know that we’ll get out alive is a luxury of modern living. There are so many other less damaging ways to achieve what I would call high-level adaptive fitness by using a variety of training methods, all of which can be cardio-protective and joint strengthening when done the right way in at the appropriate times. Heck, when it comes to hypertension, blood lipids, and type 2 diabetes, walking is just as effective as running [30] - without the potential downsides. Everyone can walk [31]. Everyone thinks they can run, but running is a skill that must be learned. To run with poor form is to welcome injury, doubly so if you’re running an excessive amount. And all this will be addressed in detail in my forthcoming book, Primal Endurance. For now, use your brain and listen to your body [32].

My point, of course, is that the human organism is made for short, intense bursts of activity laid atop a foundation of frequent slow moving. We aren’t “supposed” to run as hard as we can for two or three hours. We’re not supposed to run with the express purpose of “burning calories.” We can certainly choose to do those activities, and we’ll become adapted (or perhaps inured) to them, and they may even make us “fit,” but they’re not the healthiest, most efficient path to fitness. Chronic cardio is the meandering, roundabout trail that will get you there with a ton of bruises, scratches, a tick or two, and a sprained ankle. Oh, and you might get eaten by a bear along the way.

Your choice.

Thanks for reading, folks. I’d love to hear your thoughts on cardio, both chronic and otherwise. Let me know in the comment section!


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[7] chronic cardio: http://www.marksdailyapple.com/chronic-cardio-2/

[8] good string of them: http://www.marksdailyapple.com/did-humans-evolve-to-be-long-distance-runners/

[9] opposed to excessive endurance training: http://www.marksdailyapple.com/case-against-cardio/

[10] attempts to clarify: http://www.marksdailyapple.com/why-you-shouldnt-burn-more-than-4000-calories-a-week-through-exercise/

[11] conventional wisdom: http://www.marksdailyapple.com/the-definitive-guide-to-conventional-wisdom/

[12] cortisol: http://www.marksdailyapple.com/cortisol/

[13] over-trainer: http://www.marksdailyapple.com/overtraining/

[14] ability to burn fat: http://www.marksdailyapple.com/what-does-it-mean-to-be-fat-adapted/

[15] glycogen: http://www.marksdailyapple.com/carb-refeeding-and-weight-loss/

[16] hypertrophy: http://en.wikipedia.org/wiki/Hypertrophy

[17] stroke and cognitive decline: http://www.medicalnewstoday.com/articles/261531.php

[18] greater risk for atrial fibrillations: http://www.ncbi.nlm.nih.gov/pubmed/19633305

[19] are at a lower risk for AF: http://circ.ahajournals.org/content/118/8/800.abstract?ijkey=678b3f685d0f4ac69f65328d5f174ec5dd8a744c&keytype2=tf_ipsecsha

[20] several potential explanations: http://bjsm.bmj.com/content/46/Suppl_1/i37.full#ref-22

[21] studies: http://europace.oxfordjournals.org/content/10/1/15.abstract?ijkey=798f639a6665055e11fc8f511b6ef3320800a34a&keytype2=tf_ipsecsha

[22] study: http://www.ncbi.nlm.nih.gov/pubmed/21200345

[23] Marathoners had more calcified plaque in their coronary arteries: http://www.healio.com/cardiology/chd-prevention/news/online/%7Bcdd889ca-bfa9-4ca7-9c95-25ad09d999ee%7D/endurance-runners-at-risk-for-calcified-coronary-plaque

[24] linked to stroke and dementia: http://newsroom.heart.org/news/calcified-plaque-in-arteries-may-213920

[25] marathon runners with atherosclerosis don’t fit the traditional cardiovascular risk profile: http://www.ncbi.nlm.nih.gov/pubmed/18426850

[26] endurance training induces oxidative stress on the athlete: http://www.ncbi.nlm.nih.gov/pubmed/16941810

[27] osteoarthritis: http://www.marksdailyapple.com/arthritis-diet/

[28] running marathons: http://www.marksdailyapple.com/dear-mark-marathon-fuel-muscle-cramps-and-another-dr-oz-miracle/

[29] electrolyte drinks: http://www.marksdailyapple.com/how-to-fuel-a-marathon/

[30] walking is just as effective as running: http://www.ncbi.nlm.nih.gov/pubmed/23559628

[31] Everyone can walk: http://www.marksdailyapple.com/urban-hiking/

[32] listen to your body: http://www.marksdailyapple.com/when-listening-to-your-body-doesnt-work/

[33] Primal Blueprint Books: http://www.primalblueprint.com/category/Books_and_Media?utm_source=mda_sidebar&utm_medium=banner&utm_campaign=sidebar_like_article_books

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