The process by which nutrients found in food are converted into bodily tissues and provide energy for all of the physical and mental activities that comprise a person's existence.
Human nutrition is a multidisciplinary area that encompasses physiology, biochemistry, molecular biology, anthropology, and psychology, which investigate how cultural traditions, attitudes, and beliefs affect dietary choices. Political science and economics are further impacted by human nutrition as the global society acknowledges and addresses the demise and suffering brought on by starvation. The ultimate objective of nutritional science is to avoid traditional nutritional deficiency disorders like pellagra and kwashiorkor while also promoting optimum health and lowering the risk of chronic illnesses like cancer and cardiovascular disease.
Find out what foods include the proteins and other nutrients needed for a balanced, healthful diet.
Find out what foods include the proteins and other nutrients needed for a balanced, healthful diet.
Discover the wholesome foods that make up a balanced and healthful diet. View all of the article's videos.
The main topics of human nutrition are covered in this article, including suggested dietary recommendations, energy production and balance, and necessary nutrients. See nutritional illness to comprehensively treat health issues resulting from inadequate nutrition. Nutrition describes how food is used by all living organisms, whereas metabolism describes particular metabolic processes.
Food utilization by the body
Kilocalories and calories: the energy source
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What exactly constitutes a full and nutrient-dense breakfast?
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Imagine the human body as an engine that releases the energy found in the food it breaks down. This energy is used in part for the secretory activities and the mechanical work that the muscles undertake, and in part for the tasks required to keep the body's structure and functions intact. Work-related heat generation is linked to regulated heat loss, which maintains body temperature within a certain range. But unlike other engines, the human body is always disassembling (catabolizing) and reassembling (anabolizing) its constituent elements. Foods offer the energy required for the chemical processes involved as well as the nutrients required for the production of the new substance.
India, grains, and spices in sacks. (food, market, seller, Indian)
Britannica Test
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How to interpret food labels' nutritional information
How to interpret food labels' nutritional information
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As energy sources, protein, fat, and carbohydrates are mostly interchangeable. Food's energy content is often expressed in kilocalories or calories. A kilocalorie is 1,000 gram-calories, often known as little calories, which is a unit of measurement for heat energy. Kilocalories are often referred to as "calories" in everyday speech, meaning that a diet of 2,000 calories really contains 2,000 kilocalories of potential energy. The quantity of heat energy needed to elevate one kilogram of water from 14.5 to 15.5 °C at one atmosphere of pressure is equal to one kilocalorie. The joule is another commonly used unit of energy that expresses energy as mechanical labor. The energy required to move one kilogram one meter with a force of one newton is equal to one joule. It is more common for the comparatively greater energy levels in the human diet to be expressed in megajoules (one megajoule equals 106 joules) or kilojoules (1 kilojoule = 103 joules). 4.184 kilojoules make up one kilocalorie.
proteins
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By measuring the amount of heat produced when food is burnt (oxidized) in a bomb calorimeter, the amount of energy contained in food may be found immediately. However the human body is not a calorimeter, and during digestion and metabolism, some potential energy is wasted. The three energy-producing nutrients have corrected physiological values for their temperatures of combustion, which are adjusted to whole numbers: 4 kcal (17 kilojoules) for carbohydrates, 4 kcal (17 kilojoules) for proteins, and 9 kcal (38 kilojoules) for fats per gram. Although it is not necessary in the diet, beverage alcohol, or ethyl alcohol, also produces energy—7 kilocalories (29 kilojoules) per gram. Water, vitamins, minerals, and other dietary ingredients don't contain any energy, yet many of them help the body release energy.
If the gram quantities of energy-yielding compounds (non-fiber carbohydrate, fat, protein, and alcohol) in a well-digested diet are known, the quantity of energy it provides may be approximated. A piece of white bread, for instance, has 67 kilocalories (280 kilojoules) of energy and contains 12 grams of carbohydrates, 2 grams of protein, and 1 gram of fat. Food labels and food composition tables (see table) provide helpful information for assessing a person's diet's nutrient and energy consumption. The majority of meals have a variety of nutrients that offer you energy in addition to vitamins, minerals, water, and other ingredients. Two noteworthy exceptions are vegetable oil and table sugar, which are essentially pure fat and carbohydrate (sucrose), respectively.
Protein makes approximately 8–16 percent of the energy in diets throughout much of the globe, however, the quantities of fat and carbohydrates in various societies vary greatly. About 12 to 15 percent of energy comes from protein, 30 to 40 percent from fat, and 50 to 60 percent from carbohydrates in more affluent regions. However, in many impoverished agricultural communities where grains make up the majority of the diet, carbohydrates account for an even higher proportion of energy, while protein and fat account for a smaller portion. The human body is very versatile and can function well on a broad range of diets. Nonetheless, some eating patterns are linked to certain health outcomes (see nutritional disease).
BMR and REE: balance of energy
Energy is required when one is not just physically active but also when one's body is at rest. Between 50 and 80 percent of the energy used each day, depending on physical activity level, goes towards basic metabolic processes (basal metabolism), which allow the body to perform various physiological and biosynthetic functions, such as the synthesis of new tissue in growing children and pregnant and lactating women, as well as breathing and pumping blood. The body consumes energy and creates heat during food digestion and subsequent processing. The thermic impact of food, sometimes referred to as diet-induced thermogenesis, is a process that contributes around 10% of daily energy expenditure. The exact percentage depends on the diet's composition and previous dietary behaviors. Another minor but significant part of energy expenditure is called adaptive thermogenesis, and it represents changes in metabolism brought on by variations in the surrounding temperature, hormone secretion, mental stress, or other variables. Physical activity, which encompasses both voluntary activities like exercise and other activities and involuntary ones like fidgeting, shivering, and maintaining posture, is the last and most variable component of energy expenditure. Twenty to forty percent of total energy expenditure is attributed to physical activity; this percentage may be even lower in very sedentary individuals and higher in highly active ones.The body's metabolically active tissue, lean body mass (mass devoid of fat and necessary fat but excluding stored fat), is largely connected with basal or resting energy expenditure. Organs with the greatest metabolic activity at rest, such as the liver, brain, heart, and kidney, also have the largest energy needs; in contrast, the needs for muscle, bone, and body fat are much lower. Age, sex, body temperature, and thyroid hormone levels are other variables that impact basal metabolism in addition to body composition.
A pleasant room temperature, at least 12 hours after the previous meal, and soon after waking up in the morning are the regulated and standardized settings under which the basal metabolic rate (BMR), a carefully defined measure of the energy expenditure required to maintain life, is measured. The resting energy expenditure (REE), which is calculated under less strict circumstances with the subject resting comfortably roughly two to four hours after a meal, is assessed less often than the body mass index (BMR) due to practical reasons. In actuality, the words are interchangeable and the difference between the BMR and REE is often little more than 10 percent, with the REE typically being somewhat greater.
Direct calorimetry, or the measuring of heat dissipated from the body, utilizes devices like water-cooled clothing or insulated rooms big enough to fit a person to determine how much energy is used. However, the less laborious methods of indirect calorimetry—which calculates heat generated by the body from measurements of oxygen breathed, carbon dioxide exhaled, and nitrogen voided in the urine—are often used to quantify energy expenditure. The following formula may be used to approximate the BMR (in kilocalories per day): BMR = 70 times (body weight in kilograms)3/4.
A table that shows the energy costs of different activities has been measured. Work that requires a lot of energy might need ten times as many calories per minute as relaxing. Despite its apparent drain, mental activity has no discernible impact on energy use. A male weighing 70 kg (154 pounds), with a potential daily REE of 1,750 kcal, may consume 2,400 kcal on a very inactive day and as much as 4,000 kcal on an extremely active day. A 55-kg (121-pound) woman, whose daily resting energy consumption may be 1,350 kilocalories, might need between 1,850 to more than 3,000 total kilocalories, depending on the amount of activity.
The rule of conservation of energy applies: If one takes in more energy than is spent, over time one will gain weight; inadequate energy intake leads to weight loss, as the body taps its energy reserves to compensate for urgent demands. Excess dietary energy is stored in tiny quantities as glycogen, a short-term storage form of carbohydrate in muscle and liver, and as fat, the body’s major energy reserve located in adipose tissue. Adipose tissue is predominantly fat (approximately 87 percent), although it also includes some protein and water. To shed 454 grams (one pound) of adipose tissue, an energy deficit of roughly 3,500 kilocalories (14.6 megajoules) is necessary.
Body mass, body fat, and body water
The human body comprises of components identical to those present in meals; however, the relative quantities change, according to genetic mandates as well as to the particular life experience of the person. The body of a healthy lean man is made of around 62 percent water, 16 percent fat, 16 percent protein, 6 percent minerals, and less than 1 percent carbohydrate, along with extremely minute quantities of vitamins and other miscellaneous elements. Females normally carry more fat (approximately 22 percent in a healthy lean woman) and somewhat less of the other components than do men of equivalent weight.The body’s various compartments—lean body mass, body fat, and body water—are continually responding to changes in the internal and external environment so that a condition of dynamic balance (homeostasis) is maintained. Tissues in the body are continually being torn down (catabolism) and built up (anabolism) at variable speeds. For example, the epithelial cells lining the digestive system are replaced at a dizzying pace every three or four days, whereas the life span of red blood cells is 120 days, and connective tissue is replenished over many years.
Although estimations of the proportion of body fat may be established by physical observation, this technique is imperfect. Body fat may be quantified indirectly using reasonably exact but expensive procedures, such as underwater weighing, whole-body potassium counting, and dual-energy X-ray absorptiometry (DXA). However, more practical, albeit less accurate, methods are often used, such as anthropometry, in which subcutaneous fat at various sites is measured using skinfold calipers; bioelectrical impedance, in which resistance to a low-intensity electrical current is used to estimate body fat; and near-infrared interactance, in which an infrared light aimed at the biceps is used to assess fat and protein interaction. Direct measuring of the body’s distinct compartments can only be conducted on cadavers.
The composition of the body tends to change in somewhat predictable ways throughout a lifetime—during the growing years, in pregnancy and lactation, and as one age—with corresponding changes in nutrient needs during different phases of the life cycle (see the section Nutrition throughout the life cycle). Regular physical activity may help attenuate the age-related loss of lean tissue and rise in body fat.
Essential nutrients
The six kinds of nutrients contained in diets include carbohydrates, lipids (primarily fats and oils), proteins, vitamins, minerals, and water. Carbohydrates, fats, and proteins represent the majority of the diet, totaling combined to around 500 grams (just over one pound) each day in actual weight. These macronutrients supply raw materials for tissue construction and maintenance as well as fuel to operate the plethora of physiological and metabolic operations that maintain life. In contrast, are the micronutrients, which are not their energy sources but aid metabolic processes throughout the body: vitamins, of which people require around 300 milligrams per day in the diet, and minerals, of which about 20 grams per day are needed. The fourth nutritional group is water, which supplies the medium in which all the body’s metabolic activities occur.A nutrient is called “essential” if it must be brought in from outside the body—in most instances, via food. (See table.) These nutrients are described in this section. Although they are split into groups for reasons of discussion, one should bear in mind that nutrients act in partnership with each other in the body, not as standalone entities.
Dietary Reference Intakes for certain nutrients for adults
required daily intake (required Dietary Allowance or Adequate Intake)
women men
macronutrients
Macronutrients
Carbohydrates
Carbohydrates, which are made of carbon, hydrogen, and oxygen, are the principal supply of energy to the body, delivering 4 kilocalories per gram. In most carbohydrates, the elements hydrogen and oxygen are present in the same 2:1 ratio as in water, hence “carbo” (for carbon) and “hydrate” (for water).Sugar
Red blood cells, the neurological system, and the brain all primarily require glucose, a simple carbohydrate. Although fat is often utilized for this purpose, glucose may also be used by muscle and other body cells as an energy source. Because cells depend so heavily on a constant supply of glucose, a variety of hormones work together to keep blood glucose levels within a very small range. Insulin, for example, controls the flow of glucose into cells, while glucagon and adrenaline pull glucose from storage. In the liver and muscle tissue, the body stores a tiny quantity of glucose as glycogen, a complex branching form of carbohydrate that may be converted to glucose and utilized as an energy source during brief fasts (a few hours) or periods of high physical activity or stress. Hypoglycemia, or low blood sugar, may cause weakness and lightheadedness. As with diabetes, hyperglycemia, or elevated blood glucose, is harmful and has to be managed.Ceviche. Ceviche (sebiche) from Peru. A raw fish meal topped with plantains, lime, cilantro, and chiles. Food and cuisine
The majority of carbohydrates and proteins, while often a costly source of energy, may be converted by the body into glucose. A minimum of 50 to 100 grams of carbohydrates per day must be included in the diet. This preserves protein while guaranteeing that fats are fully metabolized and avoiding the buildup of fat breakdown products in the body, or ketones, which is known as ketosis. Despite wide regional differences in the amount and kind of carbs consumed, most diets provide more than enough of them.
Additional sugars and starch
The most basic kind of carbs are sugars, which give many meals their sweet flavor while also acting as a food source for oral bacteria that cause tooth disease. Dietary sugars come in two forms: disaccharides, which include two connected saccharide units, and monosaccharides, which have one sugar or saccharide unit. Glucose, fructose, and galactose are monosaccharides that are important for nutrition; sucrose, or table sugar, lactose, or milk sugar, and maltose are disaccharides. The oligosaccharide (e.g., raffinose and stachyose), a little more complex kind of carbohydrate, has three to ten saccharide units; the gas-producing properties of these meals are caused by these molecules, which are abundant in beans and other legumes and are poorly digested by humans. Polysaccharides, which are larger and more intricate types of carbohydrates that are stored, are made up of extended chains of glucose units. The most significant polysaccharide in the human diet, starch is mostly composed of branching chains (amylopectin) or straight glucose chains (amylose), and it may be found in grains, legumes, potatoes, and other vegetables. Lastly, plant foods such as grains, fruits, vegetables, legumes, seeds, and nuts include nondigestible polysaccharides called dietary fiber.All complex carbohydrates must first be converted into simple sugars, which must then be converted into monosaccharides for the body to use them. This process is carried out by enzymes and begins in the mouth and finishes in the small intestine, which is where the majority of absorption occurs. A particular enzyme breaks down each disaccharide into its component monosaccharides, such as glucose and galactose, for example, lactase breaks down lactose into these two monosaccharides. Lactase activity decreases throughout infancy and adolescence in a large portion of the world's population, making lactose indigestible. If an excessive amount of lactose is taken, this hereditary condition known as lactose intolerance causes diarrhea and gastrointestinal distress. The majority of people who are still able to digest dairy products as well as adults are mostly from northern Europe.
dietary fibre
The structural components of plants, or dietary fiber, are indigestible by humans due to a lack of digestive enzymes. These indigestible substances support optimal health even though they are passed through the gut undigested (except a little portion that is fermented by bacteria in the large intestine). Insoluble fiber provides roughage, or bulk, that promotes regularity in bowel movements and speeds up the body's removal of chemicals found in food that may be carcinogenic or otherwise hazardous. It also does not dissolve in water. Cellulose, the majority of hemicelluloses, and lignin—a phenolic polymer rather than a carbohydrate—are examples of insoluble fiber types. Vegetables, wheat bran, and whole grain bread, and cereals are important dietary sources of insoluble fiber. In addition to slowing down the amount of time food travels through the stomach (an undesired impact), soluble fiber, which dissolves or swells in water, also lowers blood cholesterol levels (a positive effect). Gums, pectins, certain hemicelluloses, and mucilages are examples of soluble fiber types; fruits (particularly citrus fruits and apples), oats, barley, and legumes are important dietary sources of soluble fiber. Soluble and insoluble fiber contributes to the slowing down of glucose absorption, resulting in a more gradual and consistent release of blood glucose. It is believed that dietary fiber may protect against some gastrointestinal disorders and lower the chance of developing other chronic illnesses. Refer to nutritional disease.