I.E., get out in the sun to exercise, feel better (vitamin D, plus melanin - an important part of the innate immune system) therefore exercise more and get more

I.E., get out in the sun to exercise, feel better (vitamin D, plus melanin - an important part of the innate immune system) therefore exercise more and get more benefits. I can see that as a possibility. Still, if they didn't control for the amount of exercise, it's a pretty horrible study. Surely they did.