Vitamins are nutrients required for proper metabolic function that cannot be sufficiently synthesized by the body and therefore must be obtained from external sources such as diet or resident symbiotic microbes. Vitamins may have both widespread and organ-specific functions, during development and throughout adult life. The letter descriptor for vitamins (Vitamin A, etc) typically refers to a group of inactive provitamin forms or “vitamers” that are metabolized into active form within the body. There are 13 accepted vitamins in human metabolism, each with distinct sources and functions. Vitamins are either water soluble or fat soluble. Water soluble vitamins are easily excreted and therefore must be replenished more often. Fat-soluble vitamins are stored in tissues and do not require daily intake, while sustained increased intake can lead to hypervitaminosis as concentrations increase to toxic levels.
Bioavailability of Vitamins
It is unclear exactly what percentage of vitamins ingested in supplement form are available for absorption and metabolic action. One obvious factor is the fat solubility of many vitamins, which are often ingested as a pill with water, but in reality determining the “bioavailability” of vitamins ingested from supplements is very complex. The two major factors affecting bioavailability of dietary supplements are: the ease of molecule release into solution and the presence of interfering inactive ingredients. Additionally, a multitude of physiological factors (affected by age, sex and other variables) also have a large effect on the relative bioavailability of ingested vitamins. For example, inherent homeostatic (equilibrium) regulation of existing nutrients may prevent utilization of further ingested nutrients while shorter digestive time correlates with improved nutrient absorption. The bioavailability of certain nutrients may also be affected by the size of the overall “load” of food or supplements ingested, and nutrients are much better absorbed when intake levels are spread out over a long period of time.
Variation also often exists in the actual amount of vitamin present within commercially sold supplements. Manufacturers are required by US regulations to include greater amounts of listed vitamins within supplements than is listed on the label, in order to account for degradation and potential disparities among batches. While most vitamins are sold at 30-100% in excess of the listed amount, examples have been reported showing actual values as low as 20% below listed amount and as high as 2.5 times the listed amount.
The risk of vitamin deficiency varies for each vitamin depending on the individual metabolic requirements and how they are stored in the body. Vitamins A, D and B12 are stored in large amounts by the liver, preventing deficiency conditions for several months up to a few years. At the other extreme, niacin is very poorly stored and reserves may only last a few weeks. Common vitamin deficiency diseases in humans include beriberi for thiamine, scurvy for Vitamin C, pellagra for niacin and rickets for Vitamin D, however deficiency diseases are rare in developed nations both due to the abundance of sufficient amounts of food and additional nutrient fortification of the food supply.