Vitamins are classified as either water-soluble or fat-soluble. Water-soluble vitamins include vitamin C and the B-complex vitamins. They are absorbed through the intestinal wall directly into the bloodstream. Although some tissues are able to hold limited amounts of water-soluble vitamins, they usually are not stored in the body. Deficiency symptoms are apt to develop quickly when intake is inadequate; therefore, a daily intake of prenatal vitamins is recommended.
However, because water-soluble vitamins are not stored, amounts consumed in excess of need are excreted in the urine. Toxicities are not likely, although mega doses of certain water-soluble vitamins can be harmful. Vitamins A, D, E, and K, the fat-soluble vitamins, are absorbed with fat into the lymphatic circulation. Like fat, they must be attached to a protein to be transported through the blood. Secondary deficiencies of the fat-soluble vitamins can occur anytime fat digestion or absorption is altered, such as during malabsorption syndromes and pancreatic and biliary disease.
The body store excesses of the fat-soluble vitamins mostly in the liver and adipose tissue. Because they are stored, a daily intake is not imperative and deficiency symptoms may take weeks, months, or years to develop. Excessive intake, particularly of vitamins A and D, are toxic. Many studies have attempted to evaluate the relationship of vitamin intake to disease ranging from cardiovascular disease, osteoporosis, and neural tube defect.