Question: I am training for my first marathon, but my friends are telling me I should stop running because marathon will ruin my joints forever. Is that true?
Not true! You should be staying active for your health, and marathon running is a wonderful sport for your health.  As long as you are not currently injured, you should be waking and running as much as you can. “Marathon will damage your joints” is a big lie. We know that weight bearing exercises like walking/running only strengthen the joint units (including muscle/ligaments/cartilages) and PROTECTIVE of your joints in lower extremities.
The current recommendation within the US medical community is as follows: We recommend weekly minimum 150 minutes of moderate exercise (brisk walking), or minimum 75 minutes of vigorous exercise (jogging). We actually suggest you exceed those minutes, as we have enough medical evidence that the more you move, the healthier you would become. (1)
Here is one of the latest studies published by NIH in 2020 (Saint-Maurice et al. JAMA 2020). They gave accelerometers (like a Fitbit) to almost 5000 people and tracked their steps and how they did for 10 years. The results were pretty clear; if you were sedentary, you didn’t do so well (more death, in all cause mortality), while the more active you were, you were less likely to die. (2)

A separate study published by Harvard Medical School in 2019 (Lee et al, JAMA 2019) also gave accelerometers for 18,000 women and found basically the same thing; as little as 4,400 steps a day was beneficial to keep you healthy and make you live longer, while the more steps taken were more beneficial. (3)
Interestingly, both of these two studies did not find “step intensity” to be a factor in positive health benefits, meaning the walkers did as well as the runners. Big thumbs up to the walkers & the walking/running groups!
There are many more studies indicating the same results already published in Japan, where they invented and marketed a pedometer or “Manpo-kei (10000 step counter)” in 1965, after the FIRST Tokyo Olympic in 1964.

The bottom line: We often talk about “Motion is lotion” and “health is wealth” in our medical community, and for a good reason. I suggest that you keep running/walking and complete that marathon goal. I’ve completed over a dozen marathons, and I love everything about it — training, camaraderie, the sense of accomplishment — and I keep coming back. You are doing a wonderful thing for your health, and the latest medical evidence backs you up on that point. 

References:

1. Physical Activity Guidelines for Americans 2018, US Dept of Health and Human Services.
2. Saint-Maurice et al, Association of Daily Step Count and Step Intensity with Mortality Among US Adults. JAMA 2020.
3. Lee et al. Association of Step Volume and Intensity with All-Cause Mortality in Older Women. JAMA 2019.

Dr. Suzuki is a Boston marathon qualifier/finisher and an Ironman finisher. He is a foot and ankle surgeon, an attending staff of the Cedars-Sinai Medical Center. He welcomes your running or health-related questions via email (Kazu.Suzuki@cshs.org), which may be featured in our weekly emails and in our website. 
(Disclaimer: This article is for your information only; it is NOT meant to be a substitute for a proper diagnosis and medical care by your own doctor.)