Researchers from South Korea, the US and Zimbabwe now believe this could become a useful complementary food for young children in much of the developing world, where vitamin A deficiency affects over 40% of children below the age of five and is responsible for approximately a third of under-five deaths (2.7 million).
Previous studies have shown that vitamin E and oils "enhance the bioavailability and bioconversion of β-carotene to vitamin A", thereby boosting kale's vitamin A content.
Based on this, researchers from Boston's Tufts University, South Korea's Konkuk University and Zimbabwe's National University of Science and Technology sought to investigate peanut butter's effect on the bioconversion of beta-carotene in kale to vitamin A in preschool children.
They assessed 37 children aged one to three years, 86% of whom had marginal vitamin A deficiency. They were assigned to two groups: in one, each child was given 50g of kale cooked with 33g of peanut butter.
In the other, each child was given the same amount of kale cooked with 16g of lard and a reference dose of 1mg of retinyl acetate.
Their blood samples were then collected and analysed, and it was found that both peanut butter and lard had increased the children's absorption of kale beta-carotene, as well as its conversion to vitamin A retinol, but that peanut butter was the preferential partner.
"This study showed that peanut butter enhances the vitamin A value of kale," they wrote.
"Peanut butter components may increase the bioavailability of vitamin A compared with lard, which contains mostly saturated and monounsaturated fatty acids. It could be that the type of fat (unsaturated fats in peanut butter) affects the bioavailability of vitamin A. More research is required in this area to fully understand the mechanisms."
Complementary, not complete
The researchers said this was the first study to show beta-carotene in kale being converted to vitamin A when consumed by children.
They added that "kale can be promoted as a complementary food for infants and toddlers who are often vulnerable" to vitamin A deficiency.
However, they concluded: "Kale β-carotene cannot be depended on as the sole source of vitamin A, but can be used as part of a dietary diversification strategy to complement vitamin A intake from other sources. It is noteworthy that kale is also a good source of lutein."
Source: Asia Pacific Journal of Clinical Nutrition
"Peanut butter increases the bioavailability and bioconversion of kale β-carotene to vitamin A"
Authors: Tawanda Muzhingi, et al.