therapy. Knowledge about the association between malnutrition and TB is prevalent in some cultures, and may reduce diagnostic delay and improve adherence to treatment. Although blood levels of some micronutrients may be low in people starting treatment for active tuberculosis, a Cochrane review of thirty-five included trials concluded that there is insufficient research to know whether the routine provision of free food or energy supplements improves tuberculosis treatment outcomes. However, nutritional supplementation probably improves weight gain in some settings. Vitamin D and tuberculosis epidemiology 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. Concerns that tuberculosis treatment itself decreases vitamin D levels appear not to be an issue in clinical practice. Genetic differences in the vitamin D receptor in West African, Gujarati and Chinese populations have been noted to affect susceptibility to tuberculosis, but there is no data available in any population that shows vitamin D supplementation (that is, giving extra vitamin D to people with normal vitamin D levels) has any effect on susceptibility to TB. Vitamin D and tuberculosis treatment Giving vitamin D to TB patients who are vitamin D deficient may be beneficial in a proportion of patients. When taken as a group, vitamin D supplementation appears to have no benefit when using sputum culture conversion as an endpoint, and giving vitamin D supplements to TB patients who have normal vitamin D levels does not provide any benefit from the point of view of TB. In a subset of patients with the tt genotype of the TaqI vitamin D receptor and who are vitamin D deficient, vitamin D supplementation appears to hasten sputum culture conversion. There are no studies of vitamin D using the gold standard outcome of relapse, so the true benefit of vitamin D is not at present known. It was noted as early as the mid-19th century that cod liver oil (which is rich in vitamin D)