Beta -carotene is a precursor for which vitamin?

Question

BlackTom AI · Accepted Answer

Beta-carotene is a provitamin that the body converts into vitamin A, so the correct answer is b. Vitamin A.

1. Relevant knowledge points and concepts
- Beta-carotene is a carotenoid found in plants (e.g., carrots, sweet potatoes) and is classified as a provitamin A compound.
- Enzymatic cleavage of beta-carotene yields retinal (an aldehyde form) which can be reduced to retinol (vitamin A alcohol) or oxidized to retinoic acid (active signaling form).
- Vitamin A (retinol/retinal/retinoic acid) is essential for vision (11-cis-retinal in the visual cycle), epithelial integrity, and growth.

2. Step-by-step reasoning process
- After ingestion, beta-carotene is absorbed in the intestine and taken up by enterocytes and the liver.
- The enzyme beta-carotene 15,15'-monooxygenase cleaves beta-carotene centrally to produce two molecules of all-trans-retinal.
- All-trans-retinal is then either reduced to retinol (storage/transport form) or converted to retinoic acid (regulatory form), thereby supplying vitamin A activity.

3. Intermediate steps (biochemical conversion)
- Beta-carotene --(beta-carotene 15,15'-monooxygenase)--> 2 all-trans-retinal
- All-trans-retinal --(alcohol dehydrogenase)--> all-trans-retinol (vitamin A)
- Alternatively: all-trans-retinal --(aldehyde dehydrogenase)--> retinoic acid

4. Why the correct answer is correct
- Because beta-carotene is directly converted by a specific enzyme into retinal/retinol, it functions as a dietary source (provitamin) of vitamin A. This conversion is well-established biochemically and physiologically.

5. Why the other options are incorrect
- Vitamin C: a water-soluble vitamin (ascorbic acid) not derived from carotenoids; humans obtain it from fruits/vegetables and cannot synthesize it from beta-carotene.
- Vitamin E: refers to tocopherols/tocotrienols, distinct lipid-soluble antioxidants produced by plants, not produced from beta-carotene.
- Vitamin D: synthesized from 7-dehydrocholesterol in skin via UV-B light; unrelated to carotenoid metabolism.

6. Conclusion
- Therefore, beta-carotene is a precursor for vitamin A; the correct choice is b. Vitamin A.

Beta -carotene is a precursor for which vitamin?单项选择题

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