Depending on who you ask, cardio exercise is either the fountain of youth or the ticket to an early grave. The claim that many groups—particularly CrossFitters and paleo diet enthusiasts—make is that prolonged cardio exercise for 45-60 minutes is bad for you.
It allegedly results in a host of health problems like skyrocketing cortisol (a stress hormone) levels, excess inflammation and oxidative damage, and a state of being over-trained. Most startling is the claim that aerobic exercise doesn’t aid weight loss or fat burning. All of these issues result in an increased susceptibility to injuries, sickness, and a higher likelihood you’ll store fat instead of lose weight.
So, is cardio slowly killing you?
Of course it isn’t. As with any exercise, it can be overdone to the point of diminishing returns and negative health consequences. But claiming “cardio is bad for you” is misleading at best and at worst, encouraging a large group of folks who are intimidated by group exercise or the weight room to do nothing but spend more sedentary hours on the couch.
Opponents of endurance training explain that it increases stress hormones, inflammation, and often leads to over-training. The truth is that any exercise releases cortisol and results in a certain level of inflammation—and this is a good thing! These are the natural and normal stages of the adaptation process. Without them, you’d never increase your endurance, get stronger, or increase your speed.
Plus, anti-cardio’ers ignore study after study after study that consistently demonstrate how aerobic exercise reduces the markers of inflammation. The real problem results when inflammation and stress hormones are persistently released faster than your body can adapt to them and heal itself. As long as you are recovering properly and exercising at a comfortable effort on most days, over-training is the least of your concerns. And remember this: you can experience over-training from any type of exercise if you overdo it, whether that’s cycling, sprints, or deadlifts.
Similar to inflammation is oxidative stress—when the production of free radicals creates a toxic environment that damages cells—which happens during strenuous training. But this entire field of study is new, and oxidative stress is not clearly linked to aging or cell damage (see here). Interestingly, this study shows that the free radical production from strenuous exercises signals our bodies to produce more protective antioxidants! Adaptation at its finest.
The next bold claim that cardio-phobes consistently make is that running doesn’t aid weight loss because it decreases efficient fat metabolism. But the science reveals that aerobic training is the best form of exercise for weight loss. This study (and this one) show aerobic exercise burns more liver and visceral fat (intra-abdominal fat that surrounds your internal organs) than resistance training. Another study published in Medicine & Science in Sports & Exercise sums it up simply: Aerobic exercise is better than resistance training for weight loss.
What’s ignored is that cardio increases your appetite and many runners or other types of endurance athletes then think, “I ran for 40 minutes so now I deserve a bagel and a plate of spaghetti!” Diet is equally—if not more important—for weight loss. And when carbohydrates are consumed in excess—particularly simple carbs like sugar and white flour—weight gain will inevitably follow. Those carbs need to be burned with cardio, or else weight loss will plateau.
If you know a marathoner or Ironman triathlete, it’s clear that extreme forms of aerobic exercise aren’t the healthiest options for everyone. But those athletes are not exercising for health, they’re training for performance. Their goals are not weight reduction or reducing inflammation, but rather, to race as fast as possible.
What both camps need to understand is that any form of exercise can be extreme. Back-to-back tournaments of Ultimate Frisbee will spike cortisol and inflammation, as well as a hard 20-mile run or two hours of medicine ball exercises. Over-training is the result of systemic inflammation and under-recovery—not aerobic exercise.
In other words, you should avoid too many hard miles, too many CrossFit AMRAP workouts, and too many sets in the weight room. Keep on training for your endurance sport of choice—you’ll be far healthier than those who avoid cardio altogether.