Human nutrition

Human nutrition is the study of how food provides the essential nutrients the body needs to stay alive and healthy. When nutrition is poor—often due to poverty, lack of food security, or insufficient knowledge about what the body requires—it becomes a long-term problem. Malnutrition and its effects are major causes of death, physical deformities, and disabilities around the world. For children, good nutrition is essential for both physical and mental growth, as well as for normal biological development.

The Recommended Dietary Allowances (RDAs) are science-based levels of essential nutrient intake that the Food and Nutrition Board considers enough for nearly all healthy individuals. First published in 1943 during World War II, the RDAs were created to set standards for optimal nutrition, with early editions offering daily recommendations for different age groups based on the best science of the time. The chair of the first committee later documented their history and evolution. Since then, the RDAs have been updated periodically, with the tenth edition being the most recent. Originally focused on national defense nutrition, the RDAs now guide food supply planning for groups, help interpret dietary intake data, set standards for food assistance programs, assess the nutritional adequacy of food supplies, shape nutrition education, assist in developing new food products, and inform food labeling guidelines. However, the data behind these nutrient estimates is often limited.

The United States and Canada use a system called Dietary Reference Intake (DRI), developed by the National Academy of Medicine. Established in 1997, the DRI expanded on the older RDAs to include a wider range of nutritional recommendations. These DRI values are different from the Reference Daily Intakes (RDIs) and Daily Values (%) found on food and supplement labels in the U.S. and Canada, which were originally based on 1968 RDAs but updated in 2016.

The United Kingdom and the European Food Safety Authority (EFSA) use Dietary Reference Values (DRVs). The UK's Department of Health introduced these in 1991 with a report on food energy and nutrients for the population. DRVs are divided into three types: Reference Nutrient Intake (RNI), covering the needs of 95% of people; Estimated Average Requirement (EAR), meeting the needs of 50%; and Lower Recommended Nutritional Intake (LRNI), addressing the needs of 5%. These categories help tailor dietary advice to different groups.

There are seven major classes of nutrients: carbohydrates, fats, fiber, minerals, proteins, vitamins, and water. They are grouped into macronutrients (needed in large amounts) and micronutrients (needed in small amounts). Carbohydrates, fats, and proteins are macronutrients that provide energy. Water and fiber are also macronutrients but do not provide energy. Micronutrients include minerals and vitamins.

Macronutrients (excluding fiber and water) supply structural material—amino acids for building proteins, and lipids for cell membranes and some signaling molecules—as well as energy. Some structural material can be used internally for energy, measured in joules or kilocalories (often called "Calories" with a capital C). Carbohydrates and proteins provide about 17 kJ (4 kcal) per gram, while fats provide 37 kJ (9 kcal) per gram. The actual energy the body gets depends on factors like absorption and digestive effort, which vary widely.

Vitamins, minerals, fiber, and water do not provide energy but are needed for other reasons. Fiber, a nondigestible material like cellulose, appears necessary for both mechanical and biochemical reasons, though the exact reasons are still unclear. For all ages, males generally need to consume more macronutrients than females. Intakes typically increase with age until the second or third decade of life.

Some nutrients, like fat-soluble vitamins, can be stored in the body, while others must be taken in regularly. Poor health can result from a lack of required nutrients, or, for some vitamins and minerals, from getting too much. Essential nutrients cannot be made by the body and must come from food. Carbohydrates and fats are made of carbon, hydrogen, and oxygen. Carbohydrates range from simple monosaccharides (like glucose, fructose, galactose) to complex polysaccharides (like starch, glycogen). Fats are triglycerides made of fatty acid monomers attached to a glycerol backbone. Some fatty acids are essential and cannot be made in the body. Protein molecules contain nitrogen in addition to carbon, oxygen, and hydrogen.