system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is a vitamin? And how did we determine which ones are necessary?
gpt
A vitamin is:

* A nutrient, that being a chemical substance that your body does not internally produce (or not enough of it) that you need to consume to stay alive and healthy. 

* A micronutrient that your body only needs small quantities of. This excludes the macronutrients, nutrients you need in large quantities. These are the things can get energy from, such as sugars, carbohydrates, fats, and proteins. As well as water and dietary fibre. 

* An organic compound, rather than a chemical elements which we call minerals. This excludes iron, magnesium, sodium, potassium, etc. Organic compound meaning made of carbon chains, but again excluding the macronutrients like carbohydrates. 


Anything that meets these requirements is a vitamin. Note it's not the same for all species, some can produce a chemical substance on their own without the need to consume it. Vitamin C for example is a required vitamin for humans, but not for all species. Some vitamins you can produce given the right conditions, but you can also consume if you are not producing enough. Vitamin D for example our bodies can make when exposed to sunlight, but given skin colour, climate, and life style you might not produce enough and needs to be consumed for ideal health. 


As for how we determined which organic compounds were required in small quantities, it started with the diseases linked to deficiencies in a certain type of food or diseases cured by consuming a certain type of food. Vitamin deficiencies. Likely the most famous example, scurvy from vitamin C (ascorbic acid) deficiency. It was soon discovered that citrus fruits would help prevent scurvy, which was a common disease among sailors taking long voyages where they had minimal fruit and vegetable consumption. 


Then as chemistry progressed in the 20th century, we began to investigate these foods and determine what the actual chemical composition was and what chemical, or vitamin, was in these foods that was necessary for humans to consume. First identified vitamin was vitamin A, which is a now a group of similar chemicals that provide the same benefit to humans. Next was vitamin B, then C. Eventually, we determined there was actually a vast amount of chemicals that humans needed to consume, so we started grouping them by function rather than continuing down the alphabet in the order discovered. The B vitamins for example are all vitamins that play a role in cell metabolism.  Which is the role the second vitamin identified, thiamine, or B1, has in humans.


Some vitamins were discovered and named, and then turned out to either not be a required chemical for human consumption, not a vitamin, or renamed as a number variant of an existing vitamin as it performs a similar role. For example, there is no vitamins F it J anymore. F for example was certain fatty acids, which again by the previous requirements is a fat which while an organic computer is not a vitamin.