The antioxidants in dark grape juice, noted for their health benefits in fighting heart disease, may have a downside, according to new research.
In cell studies, scientists with the US Department of Agriculture and Cornell University found that polyphenols in purple (also called red) grape juice can inhibit the uptake of iron, which could increase the risk of iron-deficiency anaemia.
The findings still need to be confirmed in human studies, warned the study's lead author, Dr Raymond P. Glahn, a physiologist/nutritionist with the USDA's Plant, Soil and Nutrition Laboratory, at Cornell. The study, which appears in the 6 November issue of the Journal of Agricultural and Food Chemistry, is the first comparative analysis among juices for iron uptake ability.
Iron-deficiency anaemia is the number one nutritional disorder in the world, and a leading cause for concern in the UK, as highlighted at a recent conference. Iron fortification of cereals, formulas and other foods, has remedied the problem among many US children, according to nutrition experts, although there are still some who do not get enough iron, noted Glahn. Iron-deficiency anaemia can lead to mental, physical and behavioural impairment, particularly in infants and toddlers, said the researchers.
Glahn did not however advocate removing dark grape juice from children's diets, but rather limiting the amount. "Since we don't know how much grape juice you have to drink to have an effect, I recommend alternating between dark and light juices. Don't just drink dark juices all the time," said Glahn.
"We're not saying, 'Don't drink grape juice,'" he added. "We're saying, 'Here are some conditions that inhibit iron availability.'"
The study looked at the effect of various juices - red grape, white grape, prune, pear, orange, apple and grapefruit - on the ability of intestinal cells to absorb iron. Using a novel laboratory model comprised of human intestinal cells, the researchers simulated conditions of digestion (including digestive enzymes and acidity) to compare the juices.
Dark grape juice reduced iron availability by 67 per cent, while prune juice produced a 31 per cent reduction. Light-coloured juices, on the other hand, actually had the opposite effect; they increased iron uptake. Pear juice produced the highest uptake levels, followed by apple, orange, grapefruit and white grape juices.
Iron is particularly important for infants and children, as the mineral is essential for normal physical and mental development, whereas adult males and post-menopausal women generally get sufficient amounts of iron. Symptoms of anaemia include excessive tiredness, decreased work and school performance, and decreased immune function, among others.