Vitamins are organic compounds needed in small amounts for normal metabolism. Unlike macronutrients, they provide no calories but are essential for life.

Fat-soluble vitamins (A, D, E, K):

• Stored in body fat and liver
• Can accumulate to toxic levels if over-supplemented
• Require dietary fat for absorption

Water-soluble vitamins (B complex, C):

• Not stored — excess excreted in urine
• Need regular dietary intake
• Less risk of toxicity

Critical deficiencies to know:

• Vitamin D — epidemic in modern indoor populations. Bone health, immunity, mood.
• Vitamin B12 — critical for vegans. Nerve function, red blood cells.
• Vitamin C — immune function, collagen synthesis.
• Folate (B9) — critical in pregnancy. Prevents neural tube defects.

Reference:

Wikipedia: Vitamin

https://en.wikipedia.org/wiki/Vitamin