Micronutrients are vitamins and minerals that are required in very small amounts by organisms for sustaining life, growth and reproduction. This contrasts with macronutrients, which are required in large amounts.
Vitamins are organic compounds that are manufactured by plants and animals and which can be broken down by heat, acids or air, whereas minerals (in the context of nutrition but not in geology or other contexts) are elements that are locked in compounds that naturally exist in soil or water and cannot be easily broken down. When humans eat, they consume the vitamins that plants and animals created and the minerals they absorbed.
Some vitamins are water-soluble, including the B vitamins and vitamin C. Because they are not easily stored in the human body and excess amounts are flushed out in urine, they must be replenished at frequent intervals. Others, including A, D, E, and K, are fat-soluble and are most efficiently absorbed when consumed together with a fat. They are retained in the liver and fatty tissues for future use.
Of the approximately 94 elements that occur naturally on the earth, roughly 60 are found in the human body, most in very tiny amounts. This includes the six macronutrient elements, whose approximate shares of body mass range from 45 percent for oxygen to 0.78 percent for phosphorous and account for about 99 percent of total body mass. Of the remaining 54 elements, which account for the remaining one percent of body mass, at least 13 are clearly essential, the largest of which are potassium and sulfur at about 0.14 percent each, followed by sodium, chlorine, magnesium, iron, zinc, copper, iodine, selenium, manganese, molybdenum and cobalt. Moreover, silicon and boron are strongly suspected as also being essential but there may not yet be conclusive proof, while several others might likewise be essential but the evidence is still weak or contradictory, including fluorine, bromine, tin, nickel, chromium, arsenic, lithium and vanadium.
There has been increasing concern about a diminishing of micronutrients in the food supply resulting both from the continued loss of soil from erosion and from their depletion from the soil that remains. This depletion is the result of policies aimed at increasing crop yields, the use of land for the continuous and long-term growing of crops, and the widespread replacement of micronutrient-rich organic fertilizers with mineral fertilizers.