I am 38 and I noticed over the last few years that the limbal rings around my irises were getting lighter. I just thought this was a sign of aging. But after going keto, about 1 month in, mine started darkening again! I found a few articles saying the lightening could be a sign of degenerative disease.
http://journals.sagepub.com/doi/full/10.1177/147470491100900201