1 Foundations of Nutrition Science
Learn how nutrients support the body, how digestion and metabolism process food, and how researchers assess diets and evaluate diet–health relationships.
What examines
draws on biology, chemistry, physiology, and population research to examine how food and its components affect the body, health, and disease. Food supplies energy and essential materials needed for growth, tissue maintenance, and normal body functions. Nutritional needs and health effects vary with factors such as age, body size, activity, health, and life stage.
and energy
Nutrients are commonly grouped as , which are needed in relatively large amounts, and , which are needed in smaller amounts. Water is essential but provides no energy.
Carbohydrates, protein, and fat
Carbohydrates are a major energy source and include sugars, starches, and fiber. Grains, fruits, starchy vegetables, legumes, and milk provide carbohydrates. Fiber, found in foods such as beans, vegetables, fruit, nuts, and whole grains, supports bowel function; some fiber is fermented by gut microbes.
Protein supplies amino acids used to build and repair tissues and to make enzymes, hormones, and other body components. Sources include beans, lentils, soy foods, nuts, seeds, eggs, dairy foods, fish, poultry, and meat. Some amino acids are essential: the body cannot make enough of them, so food must supply them.
Fat provides concentrated energy and essential fatty acids, forms part of cell membranes, and helps the body absorb vitamins A, D, E, and K. Sources include oils, nuts, seeds, avocados, dairy foods, and fatty fish. The types of fat in a diet matter as well as the total amount.
Using common food-energy factors, carbohydrate and protein provide about kilocalories per gram, fat about , and alcohol about . Alcohol provides energy but is not an essential nutrient. Vitamins, minerals, and water provide no calories.
and reference values
Vitamins are organic compounds that support processes including energy , vision, immunity, and tissue maintenance. Minerals are elements used in functions such as building bones, carrying oxygen, maintaining fluid balance, and enabling nerve and muscle activity.
Examples illustrate the variety of roles and food sources:
Vitamin A supports vision and normal growth. Sources include eggs and dairy foods, as well as orange and dark-green vegetables containing provitamin A carotenoids.
Vitamin C is needed to make collagen. Sources include citrus fruit, berries, peppers, and broccoli.
Iron is part of hemoglobin, which carries oxygen in blood. Meat and seafood provide heme iron; beans, nuts, and fortified grains provide nonheme iron.
Calcium helps build and maintain bones. Sources include dairy foods and some fortified foods.
A varied diet can supply many nutrients, but requirements differ among people. Inadequate intake and, for some nutrients, excessive intake can both be harmful. are reference values used to plan and assess intakes; they include the Recommended Dietary Allowance (RDA), Adequate Intake (AI), and Tolerable Upper Intake Level (UL).
, absorption, and
uses physical processes and digestive juices to break food into components small enough to absorb. Chewing and saliva begin in the mouth. The stomach mixes food with acid and enzymes. In the small intestine, enzymes from the pancreas and intestine complete much of the breakdown, while bile from the liver helps with fat .
Most nutrients are absorbed through the small-intestinal wall into the blood or lymph and then transported for use or storage. The large intestine absorbs water, and gut microbes act on some material that remains undigested.
is the network of chemical reactions by which the body uses, transforms, and stores absorbed nutrients. Cells may oxidize nutrients to release energy, use their components to build or repair tissues, or store them for later. For example, carbohydrates can be converted to glucose, which may be used immediately or stored as glycogen. The body can also store surplus energy largely as fat. Metabolic pathways are regulated by signals including hormones and respond to the body's needs and circumstances.
and nutritional adequacy
describes the relationship between energy consumed and energy expended over time. Expenditure includes energy used for basic functions at rest, physical activity, and the and processing of food.
When intake and expenditure are similar over time, body energy stores tend to be relatively stable. Sustained intake above expenditure tends to increase stored energy; sustained intake below expenditure tends to reduce it. These are broad patterns, not exact predictions: needs and responses vary, and naturally changes during growth, pregnancy, and other life stages.
is only one part of nutrition. Foods also differ in nutrient content, fiber, and other components, so diets with similar calories need not have the same nutritional composition. A person can consume enough energy while still having an inadequate intake of a particular nutrient.
Methods for assessing diets
estimates what a person or population eats and drinks. Common methods include:
24-hour recall: An interview about foods and amounts consumed on the previous day. Repeating recalls on different days can better reflect usual intake.
Food record or diary: Foods and drinks are recorded as they are consumed over a specified period.
Food-frequency questionnaire: Asks how often selected foods are usually eaten over a longer period.
Biomarkers and clinical measures: Blood or urine measurements, physical examination, and growth or body-composition measures can complement reported intake.
Each method has limitations. People may forget, misestimate portions, change what they eat while recording it, or have difficulty describing mixed dishes. A single day may not represent usual intake. Biomarkers can provide complementary evidence, but they do not directly measure every aspect of diet. Researchers choose methods to fit the question and interpret estimates with measurement error in mind.
Research designs in nutrition
Nutrition research asks whether dietary exposures, such as a nutrient, food, or overall eating pattern, are related to health outcomes. Different study designs address different questions.
measure diets and health as they occur. Cohort studies follow groups over time; cross-sectional studies examine exposures and outcomes at one point. These designs can study long-term patterns and outcomes, but associations may be influenced by confounding factors such as activity, smoking, income, or other parts of the diet.
assign participants to dietary interventions and compare outcomes. Randomization can strengthen causal inference, but trials may be limited by duration, adherence, cost, and how well participants represent the wider population.
Laboratory and mechanistic studies investigate how nutrients or dietary patterns might affect biological processes. Their findings can help explain results from human studies, but a mechanism alone does not establish a health effect in people.
assess findings across multiple studies, considering their quality, consistency, and limitations.
Interpreting diet–health evidence
A reported association does not by itself prove that a food or nutrient caused an outcome. Confidence grows when well-designed studies with different strengths produce consistent findings and when alternative explanations and limitations are considered.
Diet is complex, and accurately measuring long-term intake is difficult. Nutrition conclusions are therefore best understood in light of the total evidence. supply energy and building materials, while and water support essential body functions; , absorption, and make nutrients available for use or storage. Assessment methods and research designs help investigate diet–health relationships, but imperfect measurement and the many factors that affect health require careful interpretation.