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2 substances of diverse structure that are treated as a group only because, as nutrients, they are required in relatively small amounts). There are also a few known conditional nutrients, that is, essential components of cellular biochemistry that are not normally considered to be nutrients because they are made internally in adequate amounts, but that become nutrients under stressful conditions of increased need or diminished sup- ply. Vitamin D, for example, would not be considered a nutrient if adequate exposure of the skin to sunlight were consistently assured. Most nutrients were recognized in the nineteenth century, but the vitamins and some trace minerals did not become known as fundamental parts in the machinery of all living things until the early twentieth century. The discovery of vitamins and some of the trace minerals originally came about through the recognition of deficiency diseases, including beriberi, scurvy, pellagra, and rickets, which arise because of specific nutritional deficiencies that can be cured or prevented by supplying the needed nutrients. Different species of mammals have some distinctive nutritional needs. Guinea pigs, monkeys, and humans, for example, require an exogenous supply of ascorbic acid (vitamin C) to maintain life and health, whereas most animals, including rats, do not. Ascorbic acid, however, is an essential part of the metabolic machinery of animals that do not need an exogenous supply. Rat tissues, for example, are relatively rich in ascorbic acid; un- like guinea pigs, these animals are genetically endowed with biochemical mechanisms for producing ascorbic acid from carbohydrates. Nutritional Requirements The amounts of nutrients adequate for health are not well known for several reasons, including uncertainties in how to define adequacy, substantial individual variations due to genetic differences, and environmental influences. Even less is known about alterations in nutritional needs caused by various disease conditions, the use of pharmaceuticals, and injury. More is known about many aspects of the nutrition of livestock, laboratory animals, and pets than is known about human nutri- tion, and considerably greater care and expertise are exercised with prized animals than with humans, in part because animal diets can be more easily controlled. Early workers in human nutrition focused on the minimum amounts needed to prevent or cure acute deficiency diseases, such as scurvy and beriberi. Since that time, the Recommended Dietary Allowances and Adequate Intakes (RDAs and AIs, re- spectively; collectively called Dietary Reference Intakes) in the United States and similar recommendations in other countries include consideration of biochemical criteria of adequacy (Fig. 2). They also include approximate adjustments for age, sex, and pregnancy and lactation, along with rough estimates for some other sources of individual variation. However, statistical data needed to adequately assess individual variations are not yet available for any nutrient. Interests have shifted toward what may be more nearly optimal nutritional intakes based on the amounts needed to promote health (not merely to avoid disease or biochemical deficiency), longevity, and resistance to chronic disorders, including cardio- vascular diseases, cancer, hypertension, and diabetes. Increas- ing evidence indicates that nutrients also protect against environmental pollutants and some human birth defects that formerly were not believed to be nutrition-related. Animal studies have long shown that diets adequate for youths and adults may not be adequate for good reproduction, and that deficiencies or imbalances of virtually all nutrients can cause myriad physical and mental defects. Nutrition (continued) + ward ' s science Fig. 2: "Nutrition facts" labels found on food products advise and inform consumers about the food's nutritional value. The United States Food and Drug Administration (FDA) has designed a new label to reflect the latest scientific information; this label went into effect in 2018. (Credit: United States Food and Drug Administration)