Ditto for what CrisDopher said above, and my own take:
Running a marathon is indeed a stress on the body. Your son's doc said (quoting you):
1. "...said it isn't healthy." It is well-established in the CF community that exercise is beneficial for lung function improvement. While I concede that anaerobic exercise of HIIT *may* have additional benefits for lung function over aerobic exercise *of the same duration* (there are molecular mechanisms related to the shear stress of phasic air flow in the lungs), any exercise, including aerobic is better than no exercise.
2. "It is harmful to the joints, there are many injuries..." There's a grain of truth to this, but probably not what the doc thinks. A recent study showed that there is no change in incidence of osteoarthritis between runners and sedentary people. However, injury, including to joints, is a common problem for marathon runners. The key is to take it slow...increase training runs by no more than 10% per week, include at least one rest day per week, and ice often. That, and make darn sure you've been fitted for the correct type of shoe.
3. "...and how the body metabolizes glucose, etc after a certain amount of time changes." Indeed it does. It all has to do w/ glycogen stores. Unless you're a Kenyan running a sub-2:30 marathon, your body's glycogen stores will be depleted after about 2-3 hours, not enough time to run a marathon. Thus, part of standard marathon training nowadays is lower intensity training to teach your body to metabolize fat while running. Humans naturally metabolize fat during low intensity exercise (hiking, walking), but to get fat metabolism to occur while running takes time...part of why marathon training programs are so long and why the long runs are supposed to be at comfortable pace (you should be able to speak 1-2 sentences at a time to a training partner). Other key thing is to ensure that you eat during the longer sessions (bars, gels, etc), and make darn sure that you eat 150-250 calories within 30min of finishing your training session.
4. However, one thing for CFers to think about as well is salt. Depending on when/where your son wants to do the marathon, he could lose a signficant amount of salt, and thus it is imperative that he practice salt replenishment during training, esp the long runs, and esp during the race. Experimenting with my own body, I estimate that I lose ~1g of salt per hour of exercise in the heat, and probably about 0.25-0.5g/hr when it's cool. Thus, your son needs to be taking in salt during race/training. Salted pretzels are great, many gels contain a fair amount of salt (though not enough I think), bananas do ok, and there's also salt tablets (they're very much like enzymes, so we should have no problem w/ them
).
My creds: 2 half marathons, 1 full marathon, and 12 triathlons (2 of them Half-Ironmans). PM me if you have more questions. Tell your son to go for it.
