Vitamins are defined organic compounds that cannot be synthesised in sufficient quantities by the body and thus must be obtained though diet. Vitamins have hundereds of biological functions within our bodies and many vitamins carry out diverse roles.
For example vitamin C is an antioxidant and can protect against free radicals, but it is essential to the formation of collagen (the body's chief structural protein). Another example would be Vitamin D which plays a massive role in mineral balance and calcium homeostasis, but also plays a huge role in immunity.
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