> The authors do, however, acknowledge some major limitations. Among them, it was impossible to differentiate between active sun-exposure habits and a healthy lifestyle, and they did not have access to exercise data.
> Whether the positive effect of sun exposure demonstrated in this observational study is mediated by vitamin D, another mechanism related to ultraviolet radiation, or by unmeasured bias cannot be determined.
Generally the way this is explained is that a base tan provides ~4 SPF of protection, which is far less than the recommended 15-30 SPF of sunscreens. I believe I read once that 4 SPF is only good for maybe 15 minutes of protection.
What I haven't seen explained (and what I want to learn) is how the body's "sun timer" resets. If I'm in the sun for 15 minutes and then go in the shade, how long does it take before I can spend another 15 minutes in the sun? If I'm hiking in a forest, I wouldn't be surprised if 15 minutes of protection is enough to completely avoid sunburn, since you're constantly going back and forth between sun and shade.
Another thing I want to clarify is mutation risk vs sunburn. Does a base tan protect you from mutations at all? I'm not actually sure.