Despite the presence of the "de novo" pathway, the salvage reactions are essential in humans; a lack of niacin in the diet causes the vitamin deficiency disease pellagra.
Common health problems included smallpox, diphtheria, typhoid, pellagra, tuberculosis and diabetes, with diagnosis depending on active symptoms, physical findings, and sputum examinations.
Although pellagra can still be seen in developing countries, fortification of food with niacin played a huge role in eliminating the prevalence of the disease.