Is Cardio Useless Or Harmful? Will It Make You Fat?

Is Cardio Useless Or Harmful? Will It Make You Fat? 1
Lots of Cardio Makes Your Muscles Waste Away and Makes You Get Fat? Hmmm… what about “Ultramarathon Man” Dean Karnazes?

The “Too Much Cardio is Harmful” Controversy

I promised, in my recent post about shorter more intense training, to talk more about controversies over excess cardio. Much of the discussion is about too much volume of intensity, which can be harmful to the cardiovascular system, leading to conditions like AFIB. as I covered here and here. But there’s another level to the discussion, authors who argue that even lower intensity activity like jogging are useless or even harmful. You can find this all over the internet, just search for “cardio and muscle wasting” or “cardio makes you fat”. Some of it comes from pretty well known strength and conditioning coaches so I’m not inclined to dismiss it. The argument is not that all cardio is useless, high intensity interval training is usually recommended, but steady state cardio is considered at best time-inefficient, or even useless or harmful.

This debate is relevant for the large percentage of people exercising that are interested in body transformation (fat loss along with retaining or gaining muscle). Even people who think their main objective is losing weight probably really want body transformation. For this purpose, those who are not fans of cardio argue that resistance training is the most important type of exercise. Cardio at a higher intensity, such as high intensity interval training, may play a useful role, but many say that the typical cardio you see people doing, such as jogging, is low or medium intensity steady state cardio and is not useful, at least for body transformation.

I actually agree with the priority order of resistance training first, followed closely by high intensity cardio. I avoid medium intensity steady state cardio (MISS) as I’ve discussed here. My main goal in doing low intensity steady state (LISS) is enjoyment and relaxation, but I also feel it has value for fat loss. So whether or not LISS cardio is a good idea is the main point of contention. There are two main arguments to support the view that it is not useful or even harmful.

The potential for muscle wasting leads some to claim that “cardio makes you fat”, because losing muscle slows your metabolism, so you burn less calories and get fat. But as we’ll see, significant muscle loss only occurs when doing a lot of cardio, so I think this claim is exaggerated.

What The Science Says

There are two main pieces of scientific evidence. The first is that if you do cardio training concurrently with strength training, you will gain less muscle than if you do strength training alone: Cardio can interfere with muscle gain. It does not prevent it, it slows it down [1]. There is a really good discussion (with references) here about concurrent cardio and resistance training, which gives tips on how to minimize the interference. In that discussion, I learned the interesting fact that the interference effect is worse for running than low-impact cardio modes like cycling, because the eccentric contractions of running causes microdamage to muscles.

The second piece of evidence is that long cardio events, like running, cause levels of cortisol (sometimes referred to as the “stress hormone”) to rise, for example cortisol levels rise significantly over baseline values during a marathon and remain so for a few hours afterwards [2]. Cortisol, in excess, has a catabolic effect and can lead to muscle loss [3]. So chronically doing an excess amount of cardio could lead to muscle loss. But excess seems to be routinely doing at least the level of training endurance athletes like marathon runners do, which can lead to longer term elevation of cortisol [4]. It is a little more complicated however, because the intensity level matters. I suspect many of the runners with elevated cortisol levels may do too much MISS training. Evidence of this is a study showing ultramarathoners not to have elevated cortisol runners [5]. They run longer distances than marathon runners, so if it were simply a case of “too much cardio”, their cortisol levels should be worse. The difference is that ultrarunners typically train at lower intensities than marathoners. And the average runner doing 30 to 60 minutes a day of LISS cardio almost certainly does not have chronically elevated cortisol.

So the two claims made above are supported by science, but only for excess cardio, and the amount that constitutes excess is quite a bit.

Common-sense Arguments

Another argument that you’ll typically see compares sprinters and marathoner runners, and asks “which body type you’d rather have”? Here is the great Usain Bolt compared to the current world record holder in the marathon, the amazing Eliud Kipchoge, who also informally ran a marathon is less than 2 hours. The attempt does not count as a world record because of the way it was paced. By the way I was saddened to hear recently that Usain Bolt has come down with the coronavirus, and I wish him a complete and speedy recovery:

Is Cardio Useless Or Harmful? Will It Make You Fat? 2

The rebuttal to the sprinter vs. marathoner argument is that there is genetics involved here, as well as “selection bias“. Eluid was more slender to begin with, which made him more likely to become a marathoner. Usain was bigger and more muscular to begin with, which made him more likely to become a sprinter. But I’ve seen at least one counter example: In Mark Sisson’s book Primal Blueprint, he describes two young identical twins from Germany. One decides to become a bodybuilder, and the other an endurance runner, and the former ends up much more muscular and the latter more slender. I must admit I sometimes motivate myself to try harder at resistance training or high intensity training by telling myself “if you want to look like a sprinter, train like a sprinter”.

Is it that cardio is bad for you, or neglecting resistance training is bad for you?

A lot of people have a single sport they love, and other physical activities are more of a chore for them. If your cup of tea is something like running, hiking, biking, you may want to spend a lot of your leisure time doing what you enjoy and not spend enough time strength training. Note that all the examples I gave are leg-dominated activities. It helps if your sport is something that uses upper body muscles a lot, like triathlon, swimming, rowing, kayaking, etc. For this reason I have intentionally cultivated enjoyable activities that use the upper body, including canoe paddling, kayaking, and cross country skiing.

Is Cardio Useless Or Harmful? Will It Make You Fat? 3

But even that is not enough, resistance training is still vitally important for health as we age. A good example of an endurance athlete that includes strength training is ultramarathoner Dean Karnazes, who has achieved crazy things like running 50 marathons in 50 states on 50 consecutive days. As seen in the photo at the top, “all that cardio” has not made his muscles waste away or made him get fat. But I’m sure the strength training helped a lot. The only specific cases of muscle wasting I’ve heard about are from people during ultra-endurance events where refeeding was difficult. For example, Scott Jurek lost almost 20 lbs (about 9 kgs) when he set the record for the Appalachian trail, but that was exercising for many hours for over 46 days, and he talks about the problems taking in enough calories in his book North. Since he was lean to begin with, this had to include a lot of muscle loss. I also recently read Mimi Anderson’s fascinating account Beyond Impossible: From Reluctant Runner to Guinness World Record Breaker, of her setting the Guinness world record for running JOGLE (running the length of Great Britain north to south- John O’Groats to Land’s End). She lost a stone (14 lbs or 6.4 Kg). most of it muscle, during the 12-day run. But she again talks in detail of having trouble taking in (or keeping down) enough calories. In contrast, Dean Karnazes is renowned for his ability to eat on the run. In his book Ultramarathon Man, he described running through the night and having a pizza delivery guy meet him with a pizza. How do you eat an entire pizza while running? You role it up like a burrito!

What about athletes whose sport is leg-dominant, but who still need considerable upper body strength to excel? A good example is cycling sprinters who do stage races like the Tour de France. They need plenty of stamina to get over several mountain passes, just to stay in the pack so they’ll be in contention for the final sprint. But they need upper body strength for the sprint, so in addition to hours of cardio per day of training, they do a lot of resistance training. Consider Thor Hushovd of Norway, nicknamed the “God of Thunder”:

Is Cardio Useless Or Harmful? Will It Make You Fat? 4

All the cardio he does clearly has not caused him to lose muscle mass (or get fat).

Excess cortisol and the cardio-strength interference effect are not the only health negatives claimed for cardio. Other claims are inflammation and free radical production (discussed here). But again I think these claims would only be valid for doing cardio in excess or doing too high a volume of moderate to high intensity training.

I recently cut back on my volume of cardio because of the bad air we were experiencing. But lately the air’s been fine again, so I’ve gone back to my enjoyable long rides and hikes. But I do them at LISS pace, And I am sticking with making sure I give my strength training and high intensity work its proper priority, even if it is in shorter sessions.


  1. Wilson, J, et al, “Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises”, J Strength Cond Res, 2012 ( ).
  2. Cook, N, et al, , “Changes in adrenal and testicular activity monitored by salivary sampling in males throughout marathon runs”, Eur J Appl Physiol Occup Physiol, 1986.
  3. Tataranni P, et al, “Effects of glucocorticoids on energy metabolism and food intake in humans”,  Am J Physiol, 1996.
  4. Skoluda, N, et al, “Elevated hair cortisol concentrations in endurance athletes”, Psychoneuroendocrinology, 2012.
  5. Deneen, W, and Jones, A, “Cortisol and Alpha-amylase changes during an Ultra-Running Event”, Int J Exerc Sci, 2017.