The findings could influence dietary guidelines set by national authorities for the nutrient, which has been the question of some debate after trials showed it may pose risk to smokers in high quantities.
Neither Europe's Scientific Committee for Food, nor the US' Food and Nutrition Board have yet issued guidelines on safe intake levels, although an expert group in the UK recently concluded that not more than 7mg should be consumed in supplements.
However Guangwen Tang and colleagues at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University showed that men who ate a diet that contained 100 grams (about half a cup) of pureed carrots absorbed less beta carotene than had previously been thought.
The conversion factor for beta carotene had been put at 12 micrograms for one retinol activity equivalent, but speaking at the Experimental Biology meeting in Washington last week (abstract 131.10), researchers said this number increased to 15 micrograms of beta carotene for one retinol activity equivalent, meaning more food needs to be consumed to get adequate vitamin A.
Beta-carotene is a precursor to vitamin A, which is needed for healthy eyesight and the immune system. The vitamin can aid the immune system in fighting certain infections and inflammations, such as measles and infections caused by some food-poisoning organisms.
This conversion factor became even more significant when the researchers tested their theory with the beta-carotene in spinach. They gave 300 grams of pureed spinach to the same study volunteers five months later. Like the carrots, the beta-carotene in spinach was also not as well absorbed as thought, and even less well absorbed than from carrots.
For spinach, like carrots, the old beta-carotene to vitamin A conversion factor was 12 units for one unit of vitamin A, but the researchers now contend that the conversion factor for beta-carotene in spinach is 22 units to one.