Vitamins are organic substances that are necessary for effective cell functioning, development and growth. Regardless of whether you consume vitamins from foods or in supplement form, the amount of the nutrient the body will be able to access will vary with its bioavailability, defined as the extent to which nutrients can be absorbed and utilized by the body.
Factors Influencing Bioavailability
The relative bioavailability of a vitamin is affected by a variety of factors including the mechanics of its digestion and absorption, how the nutrient is distributed throughout the bloodstream and the efficacy with which it enters specific body tissues and fluids.
Certain components in foods can also affect the bioavailability of vitamins. Compounds called oxalates, for example, bind with calcium in the digestive tract. The body will then excrete the oxalate-calcium complex, limiting the amount of calcium that can be absorbed and used in body tissues.
You can estimate bioavailability either by quantifying the amount of the nutrient that is found in body tissues and fluids, or by evaluating the growth or enzyme activity that is dependent on the particular vitamin. Because of the complexity involved in measuring bioavailability, researchers have begun developing techniques that allow for more accurate estimates of bioavailability, including liquid chromatography to differentiate between individual nutrients and tracing isotopes of a particular vitamin as it makes its way throughout the body.
Vitamins A, D, E and K are fat-soluble vitamins, which require fat for absorption and are stored in the body's lipid stores. You can increase the bioavailability of these vitamins by consuming them with dietary fats such as olive and canola oils, peanut butter or cheese. Storing foods that are sensitive to light and heat in dry, cool places will also ensure that vitamins are maximally available in the body.