But people who start off with higher vitamin A tend to be better at making more of the vitamin from the plant beta-carotene, found the researchers from the Agricultural Research Service (ARS).
This points to the considerable challenge for health officials trying to raise vitamin A levels in people with a deficiency.
Vitamin A deficiency is the most serious nutritional deficiency problem worldwide, causing blindness in up to 500,000 children each year, according to World Health Organisation figures.
Yet although scientists are working on creating plants with enhanced levels of beta-carotene - which is converted by the body to vitamin A - to combat this problem, it has been difficult to assess how successful plant sources of beta-carotene actually are at improving vitamin A status.
"Vitamin A is tricky to measure as it stays at the same level until you start to become deficient," explained Janet Novotny, physiologist at the ARS.
"Therefore it is difficult to show how much beta-carotene is being converted to vitamin A," she told NutraIngredients.com.
Novotny and colleagues at the ARS Beltsville Human Nutrition Research Center (BHNRC) used a stable isotope to tag carotenoids in kale plants.
The plants were grown in a chamber supplied with a rare form of carbon dioxide where the carbon molecules were heavier than usual. When the plants formed beta-carotene, the nutrient was made with these heavier carbon molecules, which could then be easily tracked in blood samples taken from seven healthy volunteers fed with the kale.
The vitamin A made by the body from this tagged beta-carotene was also visible, according to the researchers' report in the September issue of the Journal of Lipid Research (vol 46, issue 9, pp1896-903).
"We don't yet have a specific number for the rate of absorption but it helps us to understand more about how the body makes vitamin A," said Novotny.
The work also revealed lutein was much better absorbed than beta-carotene, despite being structurally very similar.
Further, those people who absorbed lutein well, were also good absorbers of beta-carotene, suggesting there may be a shared mechanism.