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?