Cows Don't Drink Milk: Feeding excess protein without supplying enough calories to meet energy needs is likely to result in the body burning the excess protein for energy instead of using it for tissue growth, repair, and maintenance. Food contains approximately 20 common amino acids, 9 of which are considered essential, or indispensable, for humans; i. Hormones released from the pancreas regulate the overall metabolism of glucose. The rest of the unused sugars are converted into lipids and stored in adipose tissue.
D-fructose rotates plane-polarised light to the left, hence the name. Archived from the original on JAN] — Similar structures search, synonyms, formulas, resource links, and other chemical information".
Archived from the original on 10 December On an analytic property of alcoholic and lactic fermentations, and on their application to the study of sugars , Annales de Chimie et de Physique , On page , Dubrunfaut relates the discovery and properties of fructose. Organic Chemistry; pages 52 and Parker and son, London, England. Journal of the American College of Cardiology. Advances in Nutrition Bethesda, Md.
Archived PDF from the original on Archived PDF from the original on 19 March Retrieved 1 April Origin and meaning of fructose". Online Etymology Dictionary, Douglas Harper.
Archived from the original on 25 December Retrieved 24 December Experimental Perspectives, 4th Edition. Archived from the original on December 20, Fructose and the Maillard reaction".
Journal of Clinical Nutrition. The American Journal of Clinical Nutrition. Archived from the original on 14 April Retrieved 7 February Kirk-Othmer Encyclopedia of Chemical Technology. Archived from the original on 8 February Australian Journal of Dairy Technology. Upper Saddle River, NJ: Development of a model". Essential Ingredients for Cooking and Eating Well. Diet and Nutrition Series; pages — American Journal of Clinical Nutrition.
Archived from the original on 3 March US Food and Drug Administration. Archived from the original on 25 January Retrieved 18 December What it is and what it ain't". Journal of the American Dietetic Association.
Comparison with sucrose and its constituent monosaccharides". Carbohydrate Digestion and Absorption. Metabolism of Fatty Acids, Acylglycerols, and Sphingolipids. A Systematic Review and Meta-analysis". Retrieved 23 February Fructose and galactose metabolic intermediates. Uridine diphosphate galactose Uridine diphosphate glucose.
Inborn error of carbohydrate metabolism: Congenital alactasia Sucrose intolerance. Glucose-galactose malabsorption Inborn errors of renal tubular transport Renal glycosuria Fructose malabsorption. Essential fructosuria Fructose intolerance. Glucosephosphate dehydrogenase deficiency Transaldolase deficiency 6-phosphogluconate dehydrogenase deficiency. Hyperoxaluria Primary hyperoxaluria Pentosuria Aldolase A deficiency.
Hepatic fructokinase Aldolase B Triokinase. Sorbitol dehydrogenase Aldose reductase. Aldose Ketose Furanose Pyranose. Anomer Cyclohexane conformation Mutarotation. Aldotriose Glyceraldehyde Ketotriose Dihydroxyacetone. Aldotetroses Erythrose Threose Ketotetrose Erythrulose. Octoses Nonoses Neuraminic acid. Sugar as food commodity. Sweeteners made from sugar cane and sugar beets are sources of sucrose. These products include table sugar, brown sugar, cane sugar, cane juice and beet sugar.
Maple syrup and molasses are also sources of sucrose. The main dietary source of glucose is corn syrup, but the starches in corn and potatoes also contain glucose. Fruits, vegetables and honey are mainly comprised of fructose. Fruit juice contains sucrose, glucose and fructose in varying proportions. Glucose is less sweet than sucrose, so it takes a greater volume of corn syrup than cane sugar to provide the same sweetening power when added to foods and beverages.
To boost the sweetness of glucose-based corn syrup, a product was developed that replaces some of the glucose molecules with much sweeter fructose molecules: This ubiquitous industrial sweetener is a common ingredient in many processed foods.
Blood glucose level refers to the concentration of glucose circulating in the blood. Sugars consumed are broken down into the simplest carbohydrate molecules. Under normal conditions, the level of glucose in the blood triggers the pancreas to release insulin.
Insulin causes muscle cells to take in glucose for fuel. Excess glucose is stored in muscles as glycogen or converted to lipids to be stored in adipose fatty tissue. Closely monitoring diet and sugar intake can go a long way toward controlling Type 2 diabetes. Eating or drinking something sweet requires cutting back on other carbohydrates.
The glycemic index assigns values to foods that correspond to the speed that sugars are absorbed by the body. High-glycemic foods such as bread and fruit juice raise blood sugar quickly. Basal or resting energy expenditure is correlated primarily with lean body mass fat-free mass and essential fat, excluding storage fat , which is the metabolically active tissue in the body.
At rest, organs such as the liver, brain, heart, and kidney have the highest metabolic activity and, therefore, the highest need for energy, while muscle and bone require less energy, and body fat even less. Besides body composition, other factors affecting basal metabolism include age, sex, body temperature, and thyroid hormone levels.
The basal metabolic rate BMR , a precisely defined measure of the energy expenditure necessary to support life, is determined under controlled and standardized conditions—shortly after awakening in the morning, at least 12 hours after the last meal, and with a comfortable room temperature. Because of practical considerations, the BMR is rarely measured; the resting energy expenditure REE is determined under less stringent conditions, with the individual resting comfortably about 2 to 4 hours after a meal.
Energy expenditure can be assessed by direct calorimetry, or measurement of heat dissipated from the body, which employs apparatuses such as water-cooled garments or insulated chambers large enough to accommodate a person.
However, energy expenditure is usually measured by the less cumbersome techniques of indirect calorimetry, in which heat produced by the body is calculated from measurements of oxygen inhaled, carbon dioxide exhaled, and urinary nitrogen excreted. The BMR in kilocalories per day can be roughly estimated using the following formula: The energy costs of various activities have been measured see table.
While resting may require as little as 1 kilocalorie per minute, strenuous work may demand 10 times that much. Mental activity, though it may seem taxing, has no appreciable effect on energy requirement. A kg pound man, whose REE over the course of a day might be 1, kilocalories, could expend a total of 2, kilocalories on a very sedentary day and up to 4, kilocalories on a very active day.
A kg pound woman, whose daily resting energy expenditure might be 1, kilocalories, could use from 1, to more than 3, total kilocalories, depending on level of activity. The law of conservation of energy applies: If one takes in more energy than is expended, over time one will gain weight ; insufficient energy intake results in weight loss, as the body taps its energy stores to provide for immediate needs.
Adipose tissue is mostly fat about 87 percent , but it also contains some protein and water. In order to lose grams one pound of adipose tissue, an energy deficit of about 3, kilocalories The human body consists of materials similar to those found in foods; however, the relative proportions differ, according to genetic dictates as well as to the unique life experience of the individual.
The body of a healthy lean man is composed of roughly 62 percent water, 16 percent fat, 16 percent protein, 6 percent minerals, and less than 1 percent carbohydrate, along with very small amounts of vitamins and other miscellaneous substances. Females usually carry more fat about 22 percent in a healthy lean woman and slightly less of the other components than do males of comparable weight. Tissues in the body are continuously being broken down catabolism and built up anabolism at varying rates.
For example, the epithelial cells lining the digestive tract are replaced at a dizzying speed of every three or four days, while the life span of red blood cells is days, and connective tissue is renewed over the course of several years. Although estimates of the percentage of body fat can be made by direct inspection, this approach is imprecise. Body fat can be measured indirectly using fairly precise but costly methods, such as underwater weighing, total 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.
The composition of the body tends to change in somewhat predictable ways over the course of a lifetime—during the growing years, in pregnancy and lactation, and as one ages—with corresponding changes in nutrient needs during different phases of the life cycle see the section Nutrition throughout the life cycle. Regular physical exercise can help attenuate the age-related loss of lean tissue and increase in body fat. The six classes of nutrients found in foods are carbohydrates, lipids mostly fats and oils , proteins, vitamins, minerals, and water.
Carbohydrates, lipids, and proteins constitute the bulk of the diet, amounting together to about grams just over one pound per day in actual weight. These macronutrients provide raw materials for tissue building and maintenance as well as fuel to run the myriad of physiological and metabolic activities that sustain life. In contrast are the micronutrients , which are not themselves energy sources but facilitate metabolic processes throughout the body: These nutrients are discussed in this section.
Although they are separated into categories for purposes of discussion, one should keep in mind that nutrients work in collaboration with each other in the body, not as isolated entities. Carbohydrates , which are composed of carbon , hydrogen , and oxygen , are the major supplier of energy to the body, providing 4 kilocalories per gram.
In most carbohydrates, the elements hydrogen and oxygen are present in the same 2: The simple carbohydrate glucose is the principal fuel used by the brain and nervous system and by red blood cells. Muscle and other body cells can also use glucose for energy, although fat is often used for this purpose. Because a steady supply of glucose is so critical to cells, blood glucose is maintained within a relatively narrow range through the action of various hormones, mainly insulin , which directs the flow of glucose into cells, and glucagon and epinephrine , which retrieve glucose from storage.
The body stores a small amount of glucose as glycogen , a complex branched form of carbohydrate , in liver and muscle tissue, and this can be broken down to glucose and used as an energy source during short periods a few hours of fasting or during times of intense physical activity or stress.
If blood glucose falls below normal hypoglycemia , weakness and dizziness may result. Elevated blood glucose hyperglycemia , as can occur in diabetes , is also dangerous and cannot be left untreated.
Glucose can be made in the body from most types of carbohydrate and from protein, although protein is usually an expensive source of energy. Some minimal amount of carbohydrate is required in the diet—at least 50 to grams a day. This not only spares protein but also ensures that fats are completely metabolized and prevents a condition known as ketosis , the accumulation of products of fat breakdown, called ketones , in the body.
Although there are great variations in the quantity and type of carbohydrates eaten throughout the world, most diets contain more than enough. The simplest carbohydrates are sugars, which give many foods their sweet taste but at the same time provide food for bacteria in the mouth, thus contributing to dental decay. Sugars in the diet are monosaccharides , which contain one sugar or saccharide unit, and disaccharides , which contain two saccharide units linked together.
Monosaccharides of nutritional importance are glucose, fructose , and galactose ; disaccharides include sucrose table sugar , lactose milk sugar , and maltose. A slightly more complex type of carbohydrate is the oligosaccharide e. Larger and more complex storage forms of carbohydrate are the polysaccharides , which consist of long chains of glucose units. Starch , the most important polysaccharide in the human diet—found in grains, legumes, potatoes, and other vegetables—is made up of mainly straight glucose chains amylose or mainly branching chains amylopectin.
Finally, nondigestible polysaccharides known as dietary fibre are found in plant foods such as grains, fruits, vegetables, legumes, seeds, and nuts. In order to be utilized by the body, all complex carbohydrates must be broken down into simple sugars, which, in turn, must be broken down into monosaccharides—a feat, accomplished by enzymes, that starts in the mouth and ends in the small intestine , where most absorption takes place.
Each dissacharide is split into single units by a specific enzyme; for example, the enzyme lactase breaks down lactose into its constituent monosaccharides, glucose and galactose. This inherited trait, called lactose intolerance , results in gastrointestinal discomfort and diarrhea if too much lactose is consumed.
Those who have retained the ability to digest dairy products efficiently in adulthood are primarily of northern European ancestry. Dietary fibre , the structural parts of plants, cannot be digested by the human intestine because the necessary enzymes are lacking. Even though these nondigestible compounds pass through the gut unchanged except for a small percentage that is fermented by bacteria in the large intestine , they nevertheless contribute to good health.
Insoluble fibre does not dissolve in water and provides bulk, or roughage, that helps with bowel function regularity and accelerates the exit from the body of potentially carcinogenic or otherwise harmful substances in food. Types of insoluble fibre are cellulose , most hemicelluloses , and lignin a phenolic polymer, not a carbohydrate. Major food sources of insoluble fibre are whole grain breads and cereals, wheat bran, and vegetables.
Soluble fibre , which dissolves or swells in water, slows down the transit time of food through the gut an undesirable effect but also helps lower blood cholesterol levels a desirable effect. Types of soluble fibre are gums, pectins , some hemicelluloses, and mucilages; fruits especially citrus fruits and apples , oats, barley, and legumes are major food sources.
Both soluble and insoluble fibre help delay glucose absorption, thus ensuring a slower and more even supply of blood glucose. Dietary fibre is thought to provide important protection against some gastrointestinal diseases and to reduce the risk of other chronic diseases as well. Lipids also contain carbon, hydrogen, and oxygen but in a different configuration, having considerably fewer oxygen atoms than are found in carbohydrates.
Lipids are soluble in organic solvents such as acetone or ether and insoluble in water, a property that is readily seen when an oil-and-vinegar salad dressing separates quickly upon standing.
The lipids of nutritional importance are triglycerides fats and oils , phospholipids e. Lipids in the diet transport the four fat-soluble vitamins vitamins A, D, E, and K and assist in their absorption in the small intestine. They also carry with them substances that impart sensory appeal and palatability to food and provide satiety value, the feeling of being full and satisfied after eating a meal. Fats in the diet are a more concentrated form of energy than carbohydrates and have an energy yield of 9 kilocalories per gram.
Adipose fatty tissue in the fat depots of the body serves as an energy reserve as well as helping to insulate the body and cushion the internal organs. The major lipids in food and stored in the body as fat are the triglycerides, which consist of three fatty acids attached to a backbone of glycerol an alcohol.
They are classified as saturated or unsaturated according to their chemical structure. A point of unsaturation indicates a double bond between two carbon atoms, rather than the full complement of hydrogen atoms that is present in saturated fatty acids.
A monounsaturated fatty acid has one point of unsaturation, while a polyunsaturated fatty acid has two or more. The common fatty acids in foods are listed in the table. Fatty acids found in the human diet and in body tissues range from a chain length of 4 carbons to 22 or more, each chain having an even number of carbon atoms. Of particular importance for humans are the carbon polyunsaturated fatty acids alpha-linolenic acid an omega-3 fatty acid and linoleic acid an omega-6 fatty acid ; these are known as essential fatty acids because they are required in small amounts in the diet.
The omega designations also referred to as n-3 and n-6 indicate the location of the first double bond from the methyl end of the fatty acid. Other fatty acids can be synthesized in the body and are therefore not essential in the diet. About a tablespoon daily of an ordinary vegetable oil such as safflower or corn oil or a varied diet that includes grains, nuts , seeds , and vegetables can fulfill the essential fatty acid requirement.
Essential fatty acids are needed for the formation of cell membranes and the synthesis of hormone -like compounds called eicosanoids e. The consumption of fish once or twice a week provides an additional source of omega-3 fatty acids that appears to be healthful. A fat consisting largely of saturated fatty acids, especially long-chain fatty acids, tends to be solid at room temperature; if unsaturated fatty acids predominate, the fat is liquid at room temperature. Fats and oils usually contain mixtures of fatty acids, although the type of fatty acid in greatest concentration typically gives the food its characteristics.
Butter and other animal fats are primarily saturated; olive and canola oils, monounsaturated; and fish, corn , safflower , soybean, and sunflower oils, polyunsaturated.
Although plant oils tend to be largely unsaturated, there are notable exceptions, such as coconut fat , which is highly saturated but nevertheless semiliquid at room temperature because its fatty acids are of medium chain length 8 to 14 carbons long. Saturated fats tend to be more stable than unsaturated ones. The food industry takes advantage of this property during hydrogenation , in which hydrogen molecules are added to a point of unsaturation, thereby making the fatty acid more stable and resistant to rancidity oxidation as well as more solid and spreadable as in margarine.
However, a result of the hydrogenation process is a change in the shape of some unsaturated fatty acids from a configuration known as cis to that known as trans. Trans -fatty acids, which behave more like saturated fatty acids, may also have undesirable health consequences. A phospholipid is similar to a triglyceride except that it contains a phosphate group and a nitrogen -containing compound such as choline instead of one of the fatty acids.
In food, phospholipids are natural emulsifiers , allowing fat and water to mix, and they are used as food additives for this purpose. In the body, phospholipids allow fats to be suspended in fluids such as blood , and they enable lipids to move across cell membranes from one watery compartment to another. The phospholipid lecithin is plentiful in foods such as egg yolks, liver, wheat germ, and peanuts.
However, the liver is able to synthesize all the lecithin the body needs if sufficient choline is present in the diet. Sterols are unique among lipids in that they have a multiple-ring structure. The well-known sterol cholesterol is found only in foods of animal origin—meat, egg yolk , fish , poultry , and dairy products.
There are a number of sterols in shellfish but not as much cholesterol as was once thought. Cholesterol is essential to the structure of cell membranes and is also used to make other important sterols in the body, among them the sex hormones, adrenal hormones , bile acids, and vitamin D. However, cholesterol can be synthesized in the liver , so there is no need to consume it in the diet.
Cholesterol-containing deposits may build up in the walls of arteries, leading to a condition known as atherosclerosis , which contributes to myocardial infarction heart attack and stroke.