Season and location are factors too. In the winter in the SF bay area, it's very unlikely your skin can make enough vitamin D. After one winter here, I my D l

Season and location are factors too. In the winter in the SF bay area, it's very unlikely your skin can make enough vitamin D. After one winter here, I my D levels were at 20 ng/ml -- way below what many believe is the sufficient level of 30 ng/ml.