I did some additional digging and found some important studies. It seems there is conflicting and confusing information on the specific subject of aerobic exercise lowering levels of DHT, at least in respect to middle age to older men. If the Medscape published study is what we're following as gospel, then the results are interesting with regard to lowering DHT, but also keeping hair. According to them, in men tested between the ages of 40-70, regular aerobic exercise actually showed increased DHT levels by 14.5% over a year long span of regular exercise. Quoting them:
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Conclusions: A yearlong, moderate-intensity aerobic exercise program increased DHT and SHBG, but it had no effect on other androgens in middle-aged to older men."
The kicker here is that regular aerobic exercise also increases SHBG levels, and in one study, men that were bald or balding had low SHBG levels. So prospectively the two studies could cancel each other out with regard to hair loss.
The online conflict is that at Wikipedia a cited source link lead one contributor to write that only free testosterone is used to convert to Dihydrotestosterone, and since cardio exercise reduces free testosterone, that should mean DHT would be lowered as well according to him. I can't confirm or deny that free tesosterone is all that is used in conversion to DHT or how much of free testosterone is reduced by running for example. The conflicting information is in the treatment is here:
http://en.wikipedia.org/wiki/Baldness in the Treatment section, under Exercise :
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Regular aerobic exercise can help keep androgen levels (particularly free testosterone levels) naturally lower while maintaining overall health, lowering stress and increasing SHBG.
SHBG has been found to be significantly lowered in men with hair loss before the age of 30.
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The Medscape published study is here (I actually had to register to view the entire article which is 4 pages in total)
http://www.medscape.com/viewarticle/569299 To add further confusion, on the same initial bounce page of the study it points out that increased SHBG levels counteract increased testosterone here:
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Prospective, nonrandomized studies of resistance exercise over a few weeks either increased testosterone[50] or not,[26,42] whereas one study of daily aerobic exercise together with a low-fat diet increased SHBG, which could counteract the biological activity of testosterone."
And as mentioned before, according to the cited study at the reference section of the wikipedia article, lower SHBG levels were found in balding men.
It's important to point out here that nothing in the study monitored links or correlations between said DHT levels and hair, or specificity in the type of 5-alpha-reductase. So we can't conclusively say that exercising more means that we'd lose more hair. For all we know the DHT increase in the studies was higher in type I 5ar. There's also the SHBG dilemma that any argument against exercise would need to get around that I pointed out earlier, not to mention that regular aerobic exercise leads to lower stress levels, lower production of pro-inflammatory cytokines, and increased circulation.
I should also point out the Medscape study makes a direct correlation between decreased DHT levels and cardiovascular disease risk, which would somewhat ironically mean we'd still be better off running if we're regular Finasteride users if we'd like to keep our tickers going. It also stresses the importance of omitting saturated fats and trans fats from our diets as much as possible if we're lowering our DHT with drugs/supplements.
Additionally it's important to note here, that the study was specific about the age groups it was testing on - 40-70 yr old men. Certainly that would factor into the results of the studies, considering the body's level of free testosterone, and the way in which it is produced vs other hormones as we age.
Bottom Line:
So does aerobic exercise fight hair loss? We now know that regular aerobic exercise has been shown to increase SHBG levels, and in one study men with hair loss were shown to have low levels of SHBG. Conversely, we also know that in older men regular exercise actually increases DHT a little (14.5% increase in DHT for regular exercisers over a 12 month span vs a baseline 1.7% in non exercisers), but which type of 5ar is unknown. The real question is - Should the slight increase in DHT from exercise prevent you from doing it, especially given the known hair benefits from doing it (e.g. increase in SHBG , lower stress levels, lower inflammation, and better circulation)? Also consider that lowering DHT with things like Finasteride and Dutasteride could place you at a higher risk for heart disease. For me the answer is clear, aerobic exercise is something I must continue to do for a host of reasons I just mentioned, even if it means a slight increase in DHT, it also increases my SHBG, and gives me a slew of other hair growing benefits I'd rather not give up.