by The New York Times
Vitamins do not share a common chemistry, but they do share certain characteristics. They are all organic nutrients that are necessary in small amounts for normal body functioning and good health. Your diet or any supplements you take provide most vitamins. The body can manufacture only three vitamins (D, K, and the B vitamin biotin) from nondietary sources. Unlike carbohydrates, fats, and proteins, vitamins are not sources of energy. Instead, vitamins are chemical partners for the enzymes involved in the body’s metabolism, cell production, tissue repair, and other vital processes.
Vitamins are either fat soluble or water soluble. The fat-soluble vitamins, which include A, D, E, and K, are absorbed by the body using processes that closely parallel the absorption of fat. They are stored in the liver and used up by the body very slowly. The water-soluble vitamins include vitamin C and the B complex vitamins. The body uses these vitamins very quickly. Excess amounts are eliminated in urine.
Guidelines for Adequate Intake of Vitamins
The Recommended Dietary Allowance (RDA) for vitamins, set by the Food and Nutrition Board of the National Academy of Sciences-National Research Council, has been used for years as a guide for determining the amount of vitamins needed to prevent deficiency diseases. The RDA refers to an estimate of the average requirements of dietary components such as calories, vitamins, minerals, and proteins that are required to prevent deficiency. Different values apply to different groups based on gender and age.
The RDA is gradually being replaced by a new standard called the Dietary Reference Intake (DRI). The DRI represents a shift in nutritional emphasis — from preventing deficiencies to lowering risks of chronic diseases, such as cardiovascular disease. The DRI values comprise four categories:
- The recommended dietary allowance (RDA). This is the current rating on most vitamins.
- The estimated average requirement (EAR). This is the amount that will meet the nutritional requirements of 50% of the population.
- Adequate intake (AI). This is an amount that will be used if there is insufficient data to calculate the RDA.
- Tolerable upper intake level (UL). This is the maximum dose likely to be safe in 98% of the population.
Food and supplement labels now typically list the Daily Value (DV). This is the percentage of the amount of a nutrient that experts believe a person needs in their daily diet. On food labels it is usually based on one serving size for a person who takes in 2,000 calories a day.
Regulation of dietary supplements by the U.S. Food and Drug Administration (FDA) is a complex issue.
Labels on vitamins and other dietary supplements now include nutrient information and list all ingredients, including identifying parts of plants from which ingredients may be taken. Unlike the labels for drugs, however, labels for vitamins and supplements may not claim to prevent or treat any specific disease. Labels for vitamins and supplements include one of the following:
- Health claim — description of how the substance may reduce the risk of a health-related condition
- Nutrient claim — description of the amount of the nutrient in the product or
- Structure or function claim — description of how the product may affect organs or systems of the body, without claiming to prevent or treat specific disease
The quality of dietary supplements depends on the manufacturer and is not regulated by FDA. The U.S. government does not require that supplements be standardized, meaning that the amounts or quality of nutrients may vary depending on the batch. So, more expensive supplements are not necessarily better than the less expensive ones. Government regulations are in the process of catching up to the boom in the supplement industry. In the meantime, some companies voluntarily adhere to rigorous quality controls, while others do not.
The U.S. Pharmacopeia (USP), an independent organization that sets quality standards for drugs, has also implemented standards for vitamins. Consumers may look for the USP label on products of companies that adhere to these standards. USP verification means the following:
- What is in the bottle matches what is listed on the label.
- There are no harmful levels of contaminants.
- The supplement will be absorbed properly into the body.
- It has been produced according to good manufacturing standards.
The FDA does not require manufacturers to provide any scientific evidence that dietary supplements are safe and effective before a product is sold (unlike drugs, which must be proven both safe and effective through clinical trials). If a supplement causes side effects in people once it is for sale, the government may place restrictions on the supplement or withdraw it from the market. The FDA may also withdraw products from the market if their labels are misleading or false.
People Who Should Take Vitamin Supplements
About 30% of Americans take at least one vitamin or mineral supplement daily. However, studies evaluating the population as a whole found that there was no difference in mortality rate between those who took vitamin supplements and those who didn’t. Most people who have a healthy diet do not need vitamins, but there are some exceptions.
Pregnant and Breast-feeding Women. Women who are pregnant or who are breast-feeding generally need additional vitamins. Folic acid and vitamins B6 and B12 are particularly important. Women who are vegetarians must be sure to avoid vitamin B12 deficiencies, which can harm their offspring. Folic acid reduces the risk for neural tube defects and possibly facial abnormalities, such as cleft palate. Studies also show that low folate levels during pregnancy are associated with low birth weight, a risk factor for the development of cardiovascular disease in adulthood. A woman’s best approach is to take extra folic acid plus multivitamin supplements (which have additional benefits), starting them before becoming pregnant.