I was considering taking part in marathon, but recently I came across this article. To summarize, the author claims that long distance running, and actually any cardio done for hours at or above 65 percent of your max heart rate, is harmful particularly for females as it devastates their metabolism.

Studies—both clinical and observational—make a compelling case that too much cardio can impair the production of the thyroid hormone T3, its effectiveness and metabolism[1-11], particularly when accompanied by caloric restriction, an all too common practice.


When T3 levels are normal, the body burns enough energy to stay warm, and muscles function at moderate efficiency. When there’s too much thyroid hormone (hyperthyroidism), the body goes into a state where weight gain is almost impossible. Too little T3 (hypothyroidism), and the body accumulates body fat with ease, almost regardless of physical activity level. Women inadvertently put themselves into a hypothyroid condition when they perform so much steady-state cardio.

Then he also makes a point that too much running causes muscle loss that in turn might be detrimental to general health.

A couple of years ago I did 3 half-marathons, and I must admit, although I trained a lot I remained chunky. Shortly after my runs I switched to non-mixing diet as well as removed all processed food from my diet and without any regular physical activity I lost 8kg. I concluded that it was the wrong diet that didn't help me lose weight while running. But after reading the above article it makes me think that maybe it was due to running and metabolism?

My question is: are the claims that running ruins metabolism true? I am aware that there are many myths around that females should not do plenty of sports, but this author is making a strong claim about cardio while promoting other activities in the gym like weightlifting (throughout his website, not explicitly in the article). Another thing is that I was considering marathon mainly as a challenge and a test for my endurance as I am not interested in losing weight anymore. But then the muscle loss might occur which is definitely not my goal. If the metabolism and muscle loss claims true, then running doesn't seem healthy to me anymore - is it correct or am I missing something?

  • 3
    I think the point is that "run endlessly" is a lousy way to "get fit" if your goal is "get fit." If you enjoy running and want to be a marathoner, there are certainly far more harmful things you could be doing with your time.
    – Affe
    May 8, 2013 at 18:27
  • The article is a bunch of pseudo-scientific crap with a ton of "related" articles to hoodwink people into believability. If you even read just the summaries, they admit they aren't sure what causes the drop, some aren't even evaluating hormone levels but rather the size of the thyroid itself (With no concurrent hormone changes), and a few even say that it's an energy conservation strategy by the body. Very few support the author's claims in any way.
    – JohnP
    Aug 27, 2013 at 17:50
  • Thanks for your comments @JohnP. After reading the article again I see your points. The article is full of author's emotions rather than objective facts;)
    – Rabbit
    Sep 16, 2013 at 10:49

4 Answers 4


Your question is in some ways quite broad, but in the narrow sense: yes, long distance slow paced steady-state cardio like marathon running absolutely reduces muscle mass, and has a distinctly diminished return on investment after you've been doing it for a while. That is, if you work yourself up to running X miles every week, staying at X miles each week will eventually not do much good for losing weight, since your body will adapt to the activity. There's more nuance there, but it's not wrong.

I'm not familiar with the ways in which this might be particularly harmful to women, but I am quite aware that the training goals of many women are to lose fat or have a particular body type. Long, slow, steady-state cardio helps work towards that goal, but works a lot better when mixed with a lot more short high-intensty interval training and heavy weight lifting.

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    My training goal is actually to prove myself that I can survive marathon... and I don't have excess fat to lose. As for the body type - I wouldn't like to lose my muscle mass but I see that long-distance running doesn't support muscular figure. Now I understand why it is called endurance training - you not only do lots of running but also spend extra hours in the gym not to lose your muscles and strength:)
    – Rabbit
    May 9, 2013 at 17:42

So, to the extant that endurance training makes you lose muscle mass, it will likely be noticeable as its happening. If you WANTED to do distance running, it might make sense to combine it with a basic strength workout once or twice a week. That way, as you lose strength, at least you'll be able to see it happening, and track its progress.

This answer is totally from my personal experience, this i what I've done when I wanted to do some heavy endurance activity for a while (road cycling, rock climbing). I would lose strength through a summer / fall season of these activities, but keeping a minimal strength program going helped minimize the loss, and maintained a foundation to re-gain strength in the winter when I was indoors more.

All of this is going to be sensitive to the particulars of your situation (exactly how much you're trying to run, exactly how strong you naturally are, and what level you're trying to maintain). A lot of these effects (how different activities affect your strength and energy levels) are pretty noticeable, and if you don't like what you're seeing, you can always revise your training plan. You may be better off thinking less about what happens to athletes in the abstract, and what you actually see happening to you as you train.

  • Good point on trying rather than thinking, however, after reading the claims about the thyroid hormone I thought that maybe it's better to think first before embarking on a time-consuming training plan that could cause some harm. But overall I see that no matter what, strength workout is important. To be a good-looking (muscular) marathoner, one would need to devote lots of time to both, endurance and strength training.
    – Rabbit
    May 9, 2013 at 10:03
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    :) I didn't mean not to think... I just meant, if you really wanted to do distance running for its own sake as a sport (not for health reasons) and you had concern about losing muscle mass, there were ways to manage that.
    – DavidR
    May 9, 2013 at 16:26

The article you cited suggests that "too much" cardio is bad for you, but the actual number he gives is 20 hours or more.

Assuming you take one day off per week, that is 3-4 hours of cardio every day. The only people I know who come anywhere near this figure are either pro athletes, or one friend training for an ironman. Even they don't maintain 20 hours a week of straight cardio on a regular basis. All include strength training in their regimes, and are careful about diet and monitoring their health.

If all you want is to do run a marathon, most of the training plans max out around 8-10 hours of running a week towards the end, assuming a 12 minute mile (google marathon training). The Jeff Galloway one maxes out around 6 hours.

If you are still worried, consider how many women do run marathons each year, without "devastating" their health. Just pay attention to your body and your health as you go. If you think something is wrong, cut back on the exercise, and see a doctor. Don't let one alarmist article stop you before you start.

http://www.runnersworld.com/training-plans/marathon-training-plans?page=single http://www.jeffgalloway.com/training/marathon.html

  • Actually, if you read the article carefully, it suggest only that "Some physique coaches prescribe 20-plus hours per week of pre-contest cardio for women, which essentially amounts to a part-time job". This means that they are specifically doing this for a contest of some sort (such as the fitness competitions). The rest of the article is carefully cherry picked pubmed articles for "validity and credibility".
    – JohnP
    Aug 27, 2013 at 17:43

I seriously doubt that long distance running can ruin your metabolism, especially when we're talking about 3 half marathons.

Actually women have a lower abandon rate than men at ultramarathons, double, triple or deca ironmans or similar endurance races. There are a lot of examples for this - you can check out Chris McDougall best-seller "Born To Run" http://www.ted.com/talks/christopher_mcdougall_are_we_born_to_run.html

But if you expect to lose weight only by running (unless you run hundred or more miles a week), you might be waiting quite some time for this to happen :). You need to combine this with a healthy life - I don't like diet as I don't believe in diets - restricted and calculated. I think you should eat natural (as much as possible) food, dropping sugar, burned fats and maintaining a well balanced intake of most of the basic nutrients.

Myself I started long distance running less than two years ago and meantime I ran 15 marathons (of which mostly mountain ones, a 70km ultra) and several half's. I personally feel that my metabolism has but improved.

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