Which vitamin cannot be synthesized by the body from a provitamin?

Which vitamin cannot be synthesized by the body from a provitamin?

• A. Thiamin
• B. Niacin
• C. Riboflavin
• D. Pantothenic acid

Answer: (A)

Vitamins vary in whether the body can make them from other compounds. Thiamin cannot be formed in humans from any precursor or provitamin, so it must come directly from the diet. This makes it the correct choice because other vitamins have known endogenous synthesis routes from other substances (for example, niacin can be produced from the amino acid tryptophan), so they don’t rely solely on dietary intake. In practice, pantothenic acid and riboflavin don’t have reliable, substantial body-synthesis pathways from provitamins that would meet daily needs in the same way niacin can, which is why thiamin is the best answer here.

Vitamins vary in whether the body can make them from other compounds. Thiamin cannot be formed in humans from any precursor or provitamin, so it must come directly from the diet. This makes it the correct choice because other vitamins have known endogenous synthesis routes from other substances (for example, niacin can be produced from the amino acid tryptophan), so they don’t rely solely on dietary intake. In practice, pantothenic acid and riboflavin don’t have reliable, substantial body-synthesis pathways from provitamins that would meet daily needs in the same way niacin can, which is why thiamin is the best answer here.