Vitamin D deficiency is a risk factor for tuberculosis, and vitamin D deficiency appears to impair the body's ability to fight tuberculosis, but there is no clinical evidence to show that treating vitamin D deficiency prevents tuberculosis, although the available evidence is that it ought to. Reduced levels of vitamin D may explain the increased susceptibility of African-Americans to tuberculosis, and may also explain why phototherapy is effective for lupus vulgaris (tuberculosis of the skin) (a finding which won Niels Finsen the Nobel Prize in 1903), because skin exposed to sunlight naturally produces more vitamin D.