"Effect of SNPs in the Beta-carotene 15, 15'-Monooxygenase (BCMO1) Enzyme on Retinol Formation and Beta-carotene Plasma Responses"
Here's the study summary:
Since studies show large inter-individual variation–up to 8-fold!–in ability to convert carotenoids to retinol, and 44% of Western (i.e. European ancestry) individuals carry a single nucleotide polymorphism that results in reduced ability to convert carotenoids ot retinol (via the enzyme BCMO1), the authors are concerned that "A high percentage of the Western population may therefore not be able to achieve adequate vitamin A intake if dietary β-carotene is a major source of their vitamin A intake."
Lietz et al. (2012) found that single nucleotide polymorphism can reduce carotenoid conversion by ~50-60%, and these polymorphisms affect ~70% Asians, ~30% of Europeans, and ~20% of Africans:
The distribution of these SNPs is of evolutionary interest as they are more common among those of us with exclusively Neanderthal DNA, descended from lineages that developed in cold northern habitats where plant foods were less available compared to Africa.
Hence the hypothesis of the clinical trial:
"The investigators hypothesize that the current maximum recommended intake of 7 mg of β-carotene per day cannot overcome the low convertor phenotype in BCMO1 to fulfill vitamin A requirements in these people."
However this has never been tested directly, until this study. The study has been completed, but no results have been posted.
I searched PubMed for a published report of the results, and found nothing.
One limit of the study is the authors are testing only one daily dose of ß-carotene, 7 mg. That's about the amount in one (45 g) carrot. A single cup of carrot juice contains about 22 mg ß-carotene. A single sweet potato contains 13 mg. A cup of cooked kale provides about 11 mg. Hence, as I noted in Powered By Plants, a plant-based diet can easily provide 30-40 mg ß-carotene daily.
This study showed that subjects absorbed ~65% of ß-carotene provided by cooked, pureed carrots and ~41% from raw, chopped carrots when consumed with 40 g sunflower oil. Thus we may be able to assume about 50% absorption of ß-carotene from plant foods when consumed with fat in meals.
This means people would need to consume whole foods containing at least ~14 mg ß-carotene to get the 7 mg tested in this study.
Not everyone eating a plant-based diet consumes that much ß-carotene daily.
This study goes in the direction of testing whether some people may require animal-source retinol vitamin A in their diets, but they will probably have to test larger doses of ß-carotene, depending on how inefficient the "low responders" to ß-carotene are.
I await the publication of the results.