Which rationale best explains why vitamins are classified as micronutrients rather than macronutrients?

• A. Vitamins are water-soluble molecules that dissolve easily in beverages and therefore are only needed in trivial trace amounts, while macronutrients are insoluble and thus required in large amounts.
• B. Vitamins are poorly absorbed and therefore must be eaten in tiny amounts compared with macronutrients, which are fully absorbed and metabolized to provide bulk tissue components.
• C. Vitamins are always synthesized by the gut microbiome in sufficient quantities, so dietary intake is minor unlike macronutrients that cannot be synthesized and must be consumed in bulk.
• D. Vitamins perform catalytic or regulatory functions in small quantities and are required in milligram or microgram amounts, unlike macronutrients that provide bulk energy and are needed in gram quantities.

Answer: (D)