Ten reasons not to run marathons:

10.. Marathon running damages the liver and gall bladder and alters biochemical markers adversely. HDL is lowered, LDL is increased, Red blood cell counts and white blood cell counts fall. The liver is damaged and gall bladder function is decreased. Testosterone decreases.

9.. Marathon running causes acute and severe muscle damage. Repetitive injury causes infiltration of collagen (connective tissue) into muscle fibers.

8.. Marathon running induces kidney disfunction (renal abnormalities).

7.. Marathon running causes acute microthrombosis in the vascular system.

6.. Marathon running elevates markers of cancer.

5.. Marathon running damages your brain.

4.. Marathons damage your heart.

3.. Endurance athletes have more spine degeneration

Reasons 1 and 2 weren't peer-reviewed studies.

To be fair, I think most of what's studied is with regard to people who haven't trained adequately for the level that they push themselves at. If you took 10 hours to do a marathon (which is far slower than I usually walk), I don't think there'd be a problem. Conversely, if you've really trained well for a marathon, you might be able to do it in under 3 hours without hurting yourself. The problem isn't aerobic endurance sports - it's pushing yourself so hard for so long.

I would refer you to Dr. Kurt Harris and Mark Sisson for really excellent articles on this topic:

http://www.psychologytoday.com/blog/p-nu/201103/cardio-may-cause-heart-disease-part-i

http://www.marksdailyapple.com/case-against-cardio/#axzz1iy0tkKmN

No, there's certainly no consensus, and most recent work suggests that it is beneficial. The difficulty you will have is in determining the control group. You need a long-term comparison between people who do the right amount of good exercise, and those that do these exercises but also train for marathons most days, while isolating the effects of diet (which in recent years will have contained all manner of crap as 'fuel').

We can look at the historical precedent - there's no evidence that 'chronic cardio' was routine or beneficial in the past (in fact for a long time the consensus was clear that it was detrimental. We can look at evolution -the sort of training conducted by high-end endurance athletes would not confer a survival advantage. We can look at modern-day HG - none of their regular activities resemble intense endurance activity. We can look at the science - increasing stress without any health advantage over that found with much less exercise. And then there's the many other correlative factors, diet, sleep, lifestyle that go with being a professional endurance athlete.

If you find a study that claims we know for sure, I'd be highly sceptical.

"The mortality of 396 Finnish champion skiers born from 1845 to 1910 was followed up to the end of 1967. Their median life expectation was 73.0 years. During the study the median life expectation of the general male population increased, but in 1956-60 it still was 2.8 years shorter than that of the skiers" http://journals.lww.com/acsm-msse/Abstract/1974/06010/Longevity_of_endurance_skiers.9.aspx

"It appears that elite endurance (aerobic) athletes and mixed-sports (aerobic and anaerobic) athletes survive longer than the general population, as indicated by lower mortality and higher longevity. Lower cardiovascular disease mortality is likely the primary reason for their better survival rates. On the other hand, there are inconsistent results among studies of power (anaerobic) athletes. When elite athletes engaging in various sports are analysed together, their mortality is lower than that of the general population. In conclusion, long-term vigorous exercise training is associated with increased survival rates of specific groups of athletes." http://www.sciencedirect.com/science/article/pii/S1440244009001145

I think this is yet another instance in which the underlying assumption is that it's the same for all of us and I don't buy that. I have no doubt endurance activities are risky for some, neutral for some, and beneficial for at least a few.

I'm NOT saying endurance activities are right for everyone, perhaps only a small sub-section, but if they come naturally to you they can be important for your body and mind. Not everyone thrives on high intensity either. You might go out for track but I'd go for cross-country (and with no encouragement from me, both my son and grandson made that choice.) Our species conquered all types of terrain/environments and we don't all have the same talents and needs.

I am only n=1, of course, but here I am at 65 with noticeably more flexibility and strength than many of my peers. I take no prescription drugs either and if I have any serious health conditions they haven't made themselves obvious.

As a young child I was already doing what would be considered endurance walking plus barn work. In my teens I was still working in barns and walking about 2 hours every day. In my 20s and 30s I enjoyed long-distance hiking, 15-20 miles at least once per week and every day on vacations. In my 40s I was riding my bike 78 miles every Saturday with shorter rides and line dancing/aerobics during the week. I did a 2-day 150-mile biking event and outperformed many younger riders. Etc., etc.

Unlike "normal" people, it always took me 40 minutes to an hour to fully warm up and then I was good for 6 hours or more of hiking/biking/horse riding. I didn't have to follow an insane training program, either. If I was fit enough to go for an hour I could go all day; I'm convinced I have nomadic genes among others.

It's true I have significant wear (translation aches and pains) in all joints including my spine, but it's also true that I walk with a bounce like someone 10-20 years younger. The aerobics turned out to be a bad idea because my joints loosened too much, but my muscles thrived on mild lifting and I still carry groceries and laundry around without difficulty.

Here's the thing: I never smoked and never drank heavily so the greatest risk to my health has been binge eating of highly processed foods. Exercise always steered me toward healthier food so I'd say it was a good thing and short, intense activities just weren't right for me.

I agree that no one should attempt such feats without sufficient training. But does anyone perhaps think that this may be due to the poor diet of endurance athletes? They are told to 'carbo-load' and eat bread, pasta etc. and then refuel during training and races with Gatorade and sugar gels etc. I think Sports Nutrition, like a lot of nutrition 'knowledge' is a bit outdated and perhaps this is what causes the health problems over the long term? Endurance athletes in my experience, seem very results-driven.