A vitamin D level of between 50 and 125 nanomoles per liter is considered sufficient, states the National Institutes of Health. At a blood serum concentration below 50 nanomoles per liter, adults develop bone weakness, and children can develop rickets.
Above 125 nanomoles per liter, the blood becomes flooded with excess calcium, notes the National Institutes of Health.
There are three sources of vitamin D, claims the NIH: sun exposure, food and supplements. Vitamin D is present in only a few food sources, including seafood such as salmon and cod liver oil, lists Nutrition Data.
An individual's vitamin D level is commonly measured with a blood test called 25-hydroxyvitamin D, according to the Vitamin D Council. Individuals should consult with a doctor to test their vitamin D level and, if necessary, determine a safe amount of daily vitamin D supplementation.