Macronutrients constitute the bulk of the diet and supply energy and many essential nutrients. Carbohydrates, proteins (including essential amino acids), fats (including essential fatty acids), macrominerals, and water are macronutrients. Carbohydrates, fats, and proteins are interchangeable as sources of energy; fats yield 9 kcal/g (37.8 kJ/g); proteins and carbohydrates yield 4 kcal/g (16.8 kJ/g).


Dietary carbohydrates are broken down into glucose. Carbohydrates increase blood glucose levels, supplying energy. Simple carbohydrates are composed of small molecules, generally monosaccharides or disaccharides, which increase blood glucose levels rapidly. Complex carbohydrates are composed of larger molecules, which are broken down into monosaccharides. Complex carbohydrates increase blood glucose levels more slowly but for a longer time. Glucose and sucrose are simple carbohydrates; starches and fiber are complex carbohydrates.

An important consideration in all dietary planning is the glycemic index of the food. The glycemic index measures how rapidly consumption of a carbohydrate increases plasma glucose levels. Values range from 1 (the slowest increase) to 100 (the fastest increase, equivalent to pure glucose).However, the actual rate of increase also depends on what foods are consumed with the carbohydrate.

Carbohydrates with a high glycemic index may increase plasma glucose to high levels rapidly. It is hypothesized that, as a result, insulin levels increase, inducing hypoglycemia and hunger, which tends to lead to consumption of excess calories and weight gain.


Dietary proteins are broken down into peptides and amino acids. Proteins are required for tissue maintenance, replacement, function, and growth. However, if the body is not getting enough calories from dietary sources or tissue stores (particularly of fat), protein may be used for energy. As the body uses dietary protein for tissue production, there is a net gain of protein (positive nitrogen balance). During catabolic states (eg, starvation, infections, burns), more protein may be used (because body tissues are broken down) than is absorbed, resulting in a net loss of protein (negative nitrogen balance).

The daily dietary protein requirement decreases from 2.2 g/kg in 3-mo-old infants to 1.2 g/kg in 5-yr-old children and to 0.8 g/kg in adults.


Fats are broken down into fatty acids and glycerol. Fats are required for tissue growth and hormone production. Saturated fatty acids, common in animal fats, tend to be solid at room temperature. Except for palm and coconut oils, fats derived from plants tend to be liquid at room temperature; these fats contain high levels of monounsaturated fatty acids or polyunsaturated fatty acids (PUFAs).Partial hydrogenation of unsaturated fatty acids (as occurs during food manufacturing) produces trans fatty acids, which are solid or semisolid at room temperature. Essential fatty acids (EFAs) are linoleic acid, an ?-6 (n-6) fatty acid, and linolenic acid, an ?-3 (n-3) fatty acid. Other ?-6 acids (eg, arachidonic acid) and other ?-3 fatty acids (eg, eicosapentaenoic acid, docosahexaenoic acid) are required by the body but can be synthesized from EFAs.


Na, Cl, K, Ca, P, and Mg are required in relatively large amounts per day


  • Milk and milk products, meat, fish, eggs, cereals, beans, fruits, vegetables
  • Bone and tooth formation, blood coagulation, neuromuscular irritability, muscle contractility, myocardial conduction


  • Many foods, mainly animal products but some vegetables; similar to Na
  • Acid-base balance, osmotic pressure, blood pH, kidney function


  • Many foods, including whole and skim milk, bananas, prunes, raisins, and meats
  • Muscle activity, nerve transmission, intracellular acid-base balance, water retention


  • Green leaves, nuts, cereals, grains, seafood
  • Bone and tooth formation, nerve conduction, muscle contraction, enzyme activation


  • Many foods, including beef, pork, sardines, cheese, green olives, corn bread, potato chips
  • Acid-base balance, osmotic pressure, blood pH, muscle contractility, nerve transmission, maintenance of cell membrane gradients


  • Milk, cheese, meat, poultry, fish, cereals, nuts, legumes
  • Bone and tooth formation, acid-base balance, energy production


Water is considered a macronutrient because it is required in amounts of 1 mL/kcal (0.24 mL/kJ) of energy expended, or about 2500 mL/day. Needs vary with fever, physical activity, and changes in climate and humidity.


Vitamins and minerals required in minute amounts (trace minerals) are micronutrients.Water-soluble vitamins are vitamin C (ascorbic acid) and 8 members of the vitamin B complex: biotin, folate, niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), vitamin B6 (pyridoxine), and vitamin B12 (cobalamin)

Fat-soluble vitamins are vitamins A (retinol), D (cholecalciferol and ergocalciferol), E (a-tocopherol), and K (phylloquinone and menaquinone). Only vitamins A, E, and B12 are stored to any significant extent in the body; the other vitamins must be consumed regularly to maintain tissue health.

Essential trace minerals include chromium, copper, iodine, iron, manganese, molybdenum, selenium, and zinc. Except for chromium, each of these is incorporated into enzymes or hormones required in metabolism.

Other Dietary Substances Fiber

Fiber occurs in various forms (eg, cellulose, hemicellulose, pectin, gums). It increases GI motility, prevents constipation, and helps control diverticular disease. Fiber is thought to accelerate the elimination of cancer-causing substances produced by bacteria in the large intestine. Soluble fiber (present in fruits, vegetables, oats, barley, and legumes) reduces the postprandial increase in blood glucose and insulin and can reduce cholesterol levels.

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