Here is my own personal observations on the subject and from that you will see why I am so interested in the subject.
I have been more or less constantly hiking or cycling since 2007. Every year I have hiked between 5,000 and 7,000 km and have done some serious cycling and a bit of paddling as well between trails. The maximum down time between serious long outdoor activities has been 3 months twice, when I had gone home to Germany to visit friends. I hike at normal thruhiker pace at about 20 - 22 miles per day. When cycling I average about 100 day. I have been doing this for four years straight now and I start wondering what the effects on my body will be if I continue much longer. So far I could observe the following:
Nutrition: I have lost the hiker hunger. On previous long-distance hikes the hiker hunger kicks in after about 3 weeks on the trail and I could just pig out. I have lost these cravings almost completely. When on a long and demanding hike I still eat a lot, but AYCE buffets have lost much of their appeal. Also, I don't suffer from the yoyo effect any more, probably because I just don't have enough long down time to seriously gain weight.
Another problem for me that has not been mentioned here is the long-term effect of water treatment. I have been using Aquamira for 4 years straight now and wonder if this could have a carcinogen effect on the long run.
Physical effects: I have never had any serious orthopedic issues - no stress fractures, no knee problems, no shin splints, no tendonitis, no nothing. Maybe about 3 blisters in 4 years. But now my hip starts hurting a bit, not only when I am hiking, but also when I have down time, but it is not very painful or serious. I will have to look into this problem more closely.
Whenever I go back to Germany I have done standard testing of blood and urine for cholesterol, diabetis and the like. The results have always been perfect. Despite the fact that I eat about 300 gr of chocolate every day on the trail even my blood sugar is ok. Apparently the malnutrition on the trail (too much sugar, crappy processed food etc.) has not had any ill effect on me (yet). My test result are so good, that last time my doctor thanked me for coming in - she said she hardly ever sees patients as healthy as I am. What a compliment!
Psychological effects: This is actually a big issue. I do love what I am doing (elsewise I would not have been doing it for 4 years straight), but on the long run it is difficult to deal with loneliness. Sometimes I don't talk to people for a whole week - just because nobody is on the trail. On the other hand I seem to look younger the more I hike. When I ask people to guess my age now I am generally estimated about 7 to 10 years younger than I actually am. (I am 43 years now). When I was still pursuing a "normal" life, people would normally guess my age more or less correct. Therefore I guess that all the hiking must have some positive influence.
I find it very difficult to deal with doctors now because they just cannot comprehend the lifestyle and deal with me like with a "normal" patient. I tried to talk to my orthopedist about the "wear and tear" problem, but I did not get the impression that he really grasped the concept of long-distance hiking. This is one of the reasons why I was hoping for some input here.
Christine aka German Tourist