Lipids

Lipid classes include fatty acids, fatty acid derivatives, and sterols. Fatty acids act primarily as metabolic fuel. The fatty acid derivatives are triglycerides, phospholipids and cholesteryl esters. Triglycerides are the transport form of lipids in the body, and also the form in which fatty acids are stored. Phospholipids the primary component of cell membranes, and cholesteryl esters are the storage form of cholesterol. The sterol class of lipids includes cholesterol, bile acids and hormones. Cholesterol is a component of cell membranes, bile acids function in fat digestion, and hormones function in metabolic regulation.

Fatty acids are an essential component of the diet, albeit at a low level of 10% of the total calories in the diet to assure that 6% comes from a combination of linoleic and linolenic acids, essential fatty acids. Fats impart many qualities to foods, including flavor, texture, aroma and appearance. Many of the compounds that provide flavor are fat soluble, and fat transports these substances to sensory cells in the mouth, providing taste and aroma. Fat also provides a creamy sensation in foods such as ice cream, and tenderness in meats which are marbled with fat. In baked goods, fats provide structure and tenderness. In response to consumer demand, food manufacturers have introduced thousands of fat-free and low fat foods to the marketplace, which has created the challenge of reducing food fat content while at the same time preserving the taste and textural qualities that fat provides.

Carbohydrates

Carbohydrates are classified as simple or complex. Simple carbohydrates include the monosaccharides (glucose, fructose and galactose) and disaccharides (maltose, lactose and sucrose). Glucose is the primary carbohydrate in the bloodstream; fructose is the primary sugar in fruits. Galactose rarely appears free in nature, but in combination with glucose to form the dissaccharide, lactose, which is the predominant sugar in milk. Sucrose (glucose + fructose) is table sugar, and maltose (glucose + glucose) is the primary disaccharide in beer. Complex carbohydrates are composed of many monosaccharides linked together to form starches and fibers. An anomalous group classified as complex by labeling law are the polysaccharides. These dissolve in the stomach and essentially act like simple carbohydrates except for their effects on intestinal osmolality which may influence bowel motility.

Fiber is an important non-nutrient constituent of foods such as cereals, grains, fruits and vegetables. Fibers can be classified as soluble or insoluble. Soluble fiber is present in apples, citrus, oats, barley and legumes, and delays gastric emptying time, delays glucose absorption, and stimulates colonic fermentation. Insoluble fiber, found in whole grain breads and cereals, wheat bran, and vegetables, accelerates GI transit time, delays glucose absorption, and increases fecal weight. Fiber has a number of actions in the colon. In addition to the binding of certain drugs, fiber can have effects on drug metabolism through the actions of colonic bacteria which can in turn be influenced by the fiber content of the diet. Studies have demonstrated that dietary differences correlate with differences in colonic bacteria species. Some of these species differences have functional implications for nutrient and hormone metabolism. Finally, fiber can affect transit time of nutrients in the colon which in turn can affect absorption of drugs through the colonic mucosa.

Vitamins and Minerals

Vitamins are organic compounds which largely serve as coenzyme factors. Their name derives from "vita" from the Latin for life and "amine" signifying a protein, since the early vitamin researchers assumed these factors must be proteins. In fact, the vitamins are defined operationally by their necessity for the maintenance of life. Some vitamins such as vitamins A and D are really steroid hormones unrelated to the many enzyme co-factors found among the vitamins.

The fat soluble vitamins are A, D, E, and K. Because they are stored by the body, fat soluble vitamins are less readily excreted than water soluble vitamins, and are needed in periodic doses. The water soluble vitamins include the B vitamins [B1(thiamin), B2 (riboflavin), B3 (niacin) B6(pyridoxine), folic acid and B12 (cyanocobalamin)] and vitamin C (ascorbic acid). These vitamins are absorbed directly into the blood stream and freely circulate. Since the kidneys detect excesses in the blood and excrete them into the urine, water soluble vitamins are needed in more frequent doses. In addition to these vitamins, fruits and vegetables provide a large number of phytochemicals, including carotenoids, isoflavones, terpenoids, isothiocyanate, organic sulfides, and polyphenols which have various physiological effects. The functions of many of these compounds have not been fully elucidated.

Vitamin D is different from all the other nutrients in that it can be synthesized from 7 dehydrocholesterol in the skin in the presence of sunlight. Ergosterol can also be ingested from plants and converted to active forms of vitamin D. The vitamin functions as a hormone, with the intestines, kidneys and bones as target organs. These organs respond to vitamin D by making calcium available for bone growth. Ultraviolet rays from the sun convert 7-dehydrocholesterol into previtamin D3. The liver hydroxylates the compound to 25-hydroxy vitamin D3, and within the kidney further hydroxylation yields either active (1,25 dihydroxy vitamin D3) or inactive (24, 25 dihydroxyvitamin D3) forms of the vitamin. This important branch point regulation is critical in regulation of calcium metabolism.

Vitamin B 12 illustrates an example of complex regulation of vitamin absorption. After ingestion, the vitamin B12 present in protein foods is released from the proteins by hydrochloric acid and pepsin in the stomach. Absorption of the vitamin requires another protein (intrinsic factor), secreted by parietal cells in the stomach, to bind to B12 and travel to the ileum where the complex is absorbed. Many vitamin B 12 deficiencies reflect inadequate absorption, rather than poor intake. Inadequate absorption can occur due to lack of hydrochloric acid, or lack of intrinsic factor. Many older adults develop atrophic gastritis, in which inadequate hydrochloric acid is produced, and vitamin B12 supplements are often recommended to this population for this reason. Vitamin B12 and folate are interdependent for activation. Since vitamin B12 is required to convert folate to its active form, the most obvious vitamin B12 deficiency symptom is anemia of folate deficiency.

Minerals are inorganic compounds and are classified as either major minerals or trace minerals, depending upon the amounts present in the body. The major minerals (calcium, phosphorus, potassium, sulfur, sodium, chloride and magnesium) are present in amounts larger than 5 grams. The major minerals provide structure to the body and help to maintain fluid balance. The trace minerals function primarily as coenzymes in energy metabolism. The trace minerals (iron, zinc, copper, manganese, iodine and selenium) are found in the human body in amounts less than 5 grams. Some foods contain binders, such as phytic acid and oxalic acid, which can form complexes with minerals and prevent their absorption.

Iron metabolism illustrates a carefully regulated system at the intracellular level. When there is deficient iron, the cell triggers production of transferrin receptors at the cell membrane to transport iron into the cell. When there is excess iron available, the cell triggers the production of ferritin, an iron-storage protein. The iron response element (IRE) is responsible for both of these environmental signals at the molecular genetic level. Most iron is recycled, but some is lost via sweat, skin and urine.