A vitamin is any organic compound that an organism requires in small amounts and must acquire through diet because it cannot synthesize it in sufficient quantities. For any species that does not require a specific organic compound, that compound is not considered to be a vitamin for that species.
Vitamins are distinct from macronutrients (i.e., carbohydrates, proteins and fats) both in the quantities required (micrograms or milligrams vs. grams) and in that they do not provide energy directly. Instead, they are essential for metabolic processes that extract energy from other nutrients.
Vitamins also differ from minerals, which are likewise essential micronutrients obtained through diet and perform specific functions. Minerals are inorganic and much more chemically stable than vitamins, which can break down or be lost during cooking or storage.
There are 13 known vitamins, all of which are required by humans and many other species. However, some species require fewer because they can synthesize some of these compounds internally. The smallest number required by any animal species is three or four, which is for certain ruminants, such as cattle and sheep, and some herbivores such as horses. These species rely heavily on their intestinal microbes to synthesize vitamins internally, especially the B-complex and vitamin K.
Whereas most animals can produce vitamin C internally, and it is thus not considered a vitamin for them, humans, along with most other primates, guinea pigs and a relatively few other species require it from external sources.
Vitamins are classified as being either fat-soluble or water-soluble. The former are A, D, E and K. The latter are C and the eight B vitamins (thiamine, riboflavin, niacin, pantothenic acid, biotin, B6, B12, and folate). Water-soluble vitamins are readily excreted in the urine, making toxicity from excess consumption of them rare. However, they need to be consumed more regularly than the fat-soluble vitamins because the body cannot store them in large amounts and symptoms of deficiency can appear quickly. On the other hand, fat-soluble vitamins are not readily eliminated from the body and thus can accumulate and cause harm if consumed in large amounts over time.
There are numerous other organic compounds similar to vitamins but which organisms produce internally in sufficient amounts. Referred to as "vitamin-like" compounds, they likewise perform vital functions, but they are not considered true vitamins because it is not necessary to obtain them from external sources.
Although there is much overlap in the vitamins required by humans and other animals, considerable variability exists regarding synthesizing capabilities, quantities required, and availability.
For example, dogs can produce some vitamin C internally, so their dietary requirement is much lower than for humans. They also usually require less vitamin A, in part because their bodies can store it more efficiently, whereas they need larger amounts of vitamin D per kilogram of body weight because they cannot synthesize it by exposure to sunlight. In addition, they consume the B vitamins more rapidly than humans because of their higher metabolic rate.
Plants also require the same organic compounds that are considered vitamins for humans and other animals. However, they produce all of them internally and therefore they are not considered vitamins for them. Thus, the use of vitamin supplements is not necessary, although they may help stressed or undernourished plants in some situations.
The levels of vitamins, as well as minerals and other nutrients, in food have been declining in a variety of food crop types for more than half a century. A major cause has been the spread of intensive agriculture, including the use of synthetic fertilizers and pesticides and higher yields from selective breeding. These factors tend to reduce nutrient concentration per unit of food because such plants grow faster but do not produce proportionally more vitamins or absorb proportionally more minerals from the soil.
Another factor is soil degradation, including the loss of soil biodiversity and declining organic matter. Increased levels of atmospheric carbon dioxide and higher temperatures have also been reducing the concentrations of vitamins and minerals, primarily by changing plant metabolism and nutrient uptake. While elevated atmospheric carbon dioxide is causing an increase in crop yields, this gain comes at the expense of nutrient density.