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Nutrition and biochemistry were new concepts in the 20th century, when vitamins and their value in the human and animal diet were discovered. Previously, Koch and Pasteur’s germ theories had guided most scientific thinking about disease. At the turn of that century, the leading causes of mortality were infectious diseases - pneumonia, influenza, diphtheria, and tuberculosis - and the focus of medicine was on defeating and eradicating the “germs” causing illness and death.
The importance of nutrition in health was not generally recognized until nutrients were linked with deficiency diseases, which were very common until World War II. The early pioneers and “vitamin hunters” who found that vitamin deficiencies caused diseases such as scurvy, rickets, and pellagra were belittled and mocked. Even in the 1920s and 1930s, medical professionals in the US would not consider that a disease could be caused by diet.
Current thinking regarding correct nutritional intake is expressed in the dietary reference intakes (DRIs), a set of reference values used to assess and plan the nutrient needs of the general population. The DRIs include recommended dietary allowances (RDAs), average daily intake levels that are believed sufficient to prevent disease in most healthy people. These figures do not take into account nutritional needs arising from individual genetic variations; some people, for optimal health, could require a hundred times the RDA of a specific nutrient.