BackgroundThe human body is composed of thousands of different chemicals. Amazingly, provided with enough calories, the body is capable of manufacturing nearly all of its needed nutrients. There are, however, some 45 or so nutrients that the human body is incapable of manufacturing. These nutritional elements are called essential because it is essential for humans to incorporate them into their diets. The essential nutrients can be divided into six general categories: water, proteins, carbohydrates, fats, vitamins, and minerals. Strictly speaking, fiber is not an essential nutrient, but it is also extremely important in prevention of certain diseases.
WaterApproximately 60% of the adult human body is composed of water. Nearly all of the life-sustaining chemical reactions require an aqueous (watery) environment. Water also functions as the environment in which water-soluble foodstuff is absorbed in the intestines and the waste products are eliminated in urine. Another essential role of water is to maintain body temperature through evaporation, as in sweating. Severe dehydration will result in cardiovascular collapse and death. On the other hand, water toxicity (too much water) is also possible, resulting in dilution of important electrolytes (mineral salts) that may lead to erratic heart rhythm and death. The estimated water requirement of an average adult is two liters per day.
Calorie SourcesThere are three sources of caloric energy: proteins, carbohydrates, and fats. Proteins make up the structural foundation of the cells, tissues, and organs. As a source of calories, proteins provide four Calories per gram. Proteins are made up of smaller building blocks called amino acids. Twenty amino acids make up all of the human proteins. Of these twenty amino acids, the body can produce only 12, but 8 must be incorporated into the diet. These amino acids are lysine, leucine, isoleucine, methionine, phenylalanine, threonine, tryptophan, and valine. In addition, two amino acids, histidine and cysteine, are essential to the newborn infants, and histidine may even be essential for adults. Any source of protein (meat, fish, poultry, cereals or legumes) is sufficient in providing the body with the required amino acids, although animal proteins are richer in essential amino acids than cereal or vegetable sources. Protein deficiency is probably more prevalent than most people would believe, even in the developed countries. Severe protein deficiency, rare in the developed countries, results in growth retardation, anemia, and swelling of the body in small children (a condition called Kwashiorkor) and muscle wasting and weakened immunity in adults. Protein toxicity (too much protein) results in accumulation of waste products (nitrogen) that are harmful to the kidneys. The recommended daily protein requirements are about 20-35 grams for children and 45-55 grams for adults. Pregnant- and breast-feeding women must increase their protein intake by 20-30 grams per day to meet their increased demand.
Carbohydrates are the main source of calories for the majority of people. Carbohydrates also contain four Calories per gram. Strictly speaking, carbohydrates are not essential in that the body is capable of making some carbohydrates, but if one never consumes carbohydrates and obtains his/her calories strictly from proteins and fats (this is practically impossible), then toxicity from the breakdown products of these nutrients can ensue. Excess carbohydrates, however, also leads to obesity. Carbohydrates are widely available in normal diets, even in the poorest countries, but the preferred sources of carbohydrates are cereals (wheat, rye, corn, etc.), fruits, and vegetables, which also contain adequate fiber and other nutrients. "Refined" carbohydrates like sugar and flour contain practically no fiber and very little of the other essential nutrients, but unfortunately, they comprise a large portion of the western diets. There are no specified daily requirements for carbohydrates, but about 100 grams of carbohydrates should be enough to prevent breakdown of the body stores of proteins and fats (for calorie needs).
Fats provide energy for the body and are among the main components of healthy cells; they contain more than twice the number of calories (nine Calories per gram) than proteins or carbohydrates. About 98% of the dietary fats are composed of triglycerides that are, in turn, made up of fatty acids. Three types of dietary fatty acids-saturated, monounsaturated, and polyunsaturated-are known. The saturated fatty acids have been shown to increase the levels of the "bad cholesterol" (LDL cholesterol). Monounsaturated and polyunsaturated fatty acids have been shown to have the opposite effect. Different foodstuff contain different proportions of the above fatty acids. As a rule of thumb, animal products have an abundance of the saturated fats, and vegetable oils are richer in the healthier mono- and polyunsaturates. For example, butter contains nearly 70% saturated fatty acids, 30% monounsaturates, and very little polyunsaturates. Sunflower oil, on the other hand, contains nearly 75% polyunsaturated fatty acids. The human body is capable of manufacturing saturated and monounsaturated fats, but not the polyunsaturates. The daily requirement of the polyunsaturated fats is very small, and any source of fat-animal or vegetable-will provide it. However, it is important to consume fatty foods and oils that are high in unsaturated- and low in saturated fatty acids in order to protect the cardiovascular system from the ravages of the bad (LDL) cholesterol.
VitaminsFourteen vitamins have been identified to play key roles in the metabolism of the healthy body. These vitamins include vitamins A, D, E, K, B1 (thiamine), B2 (riboflavin), B6 (pyridoxine), B12 (cyanocobalamine), C, biotin, folic acid, niacin, and pantothenic acid. With the rare exception of vitamin D, the human body is incapable of making vitamins, and they are thus essential. Vitamins function primarily as cofactors (helpers) in various metabolic (energy-producing) processes of the body. Deficiencies and toxicity states of all vitamins have been characterized and well documented. Because they have been covered extensively elsewhere (please see the article entitled "Vitamins") they will not be discussed in this article. Vitamins are required in very small amounts and can be obtained from a variety of food sources.
MineralsMinerals are inorganic (not made by living things) substances that are essential for the proper functioning of the body. Minerals are divided into two general categories: electrolytes and trace elements. Electrolytes must be consumed in relatively large quantities, but trace elements are needed in very small amounts. Electrolytes are the more important of the minerals and include sodium, potassium, chloride, calcium, magnesium, and phosphorous. Sodium is the primary electrolyte of the blood and the fluid that baths the tissues (extracellular fluid). Low blood sodium can result in confusion, seizures, erratic heart rhythm, and possibly death. Potassium is the main electrolyte present inside the cells. Potassium deficiency results in weakness, erratic heart rhythm, and death. Calcium and phosphorous are the main mineral components of the human bones and teeth. Calcium deficiency results in thinning of the bones (osteoporosis) that results in fractures. The recommended daily requirements of calcium are approximately 800 milligrams in children and 1000-1200 milligrams in adults. Chloride is almost always present in association with sodium (table salt is sodium chloride) and its deficiency is always related to sodium deficiency. Magnesium deficiency is relatively common, but only very severe deficienies result in heart-rhythm problems. Most electrolytes are widely abundant in nature and more than adequately present in a balanced diet. The best sources of calcium are the dairy foods.
The trace elements include iron, zinc, copper, manganese, fluoride, iodide, sulfur, molybdenum, and a few relatively unimportant minor elements. Iron is the most important of all the trace elements because it is essential in the structure of hemoglobin, the red blood cell molecule that carries oxygen to the tissues. Iron deficiency-mainly from prolonged or extensive bleeding, such as in women with heavy periods-causes anemia. Fluoride is crucial for healthy development of teeth in very young children and is added to majority of city water supplies. The deficiencies of most of the other minerals have never been demonstrated in humans, but animal studies have shown that these deficiencies are possible. Similar to electrolytes, trace elements are present in more than adequate amounts in a balance diet.
Although not absolutely essential, fiber in the diet plays a crucial role in the proper functioning of the gastrointestinal system. Absent or low dietary fiber may result in severe constipation and diverticular disease of the colon. Natural sources of fiber include fruits, vegetables, and cereals.