Children who fail to consume enough magnesium and potassium in their diets may be damaging the function of their lungs, according to recent research from the US.
Preventive medicine researchers at the Keck School of Medicine at the University of Southern California found that children who do not eat enough leafy green vegetables such as spinach tend to have lower lung function, a measure of how well their lungs work.
Writing in the American Journal of Epidemiology, the researchers said that getting enough magnesium was a particular problem, since most children studied did not reach their recommended daily allowance for the mineral.
"Magnesium is at the centre of so many processes important to the body - energy metabolism, immune function and muscle and nerve function, for example," said Frank D. Gilliland, associate professor of preventive medicine and lead author of the study. "Yet only about 15 per cent of children eat the recommended amount of magnesium."
Researchers looked at the diets and pulmonary function of 2,566 children ages 11 to 19 living in a dozen Southern California communities in the late 1990s. The work was part of the USC-led Children's Health Study, an extensive investigation into the respiratory health of children.
The team tested lung function by having each child take a deep breath, then measuring how much and how fast they could blow out the air. The more they could exhale, the higher the level of lung function.
Scientists suspect that children with decreased lung function might be more susceptible to respiratory disease and more likely to have chronic respiratory problems as adults.
In girls, those with low magnesium intake blew out air about 8 per cent slower than girls with higher intake; and in girls with asthma, the deficit was even larger. Boys with low magnesium intake could blow out nearly 3 per cent less air than boys who got more magnesium from their diet. Researchers are unsure of the reasons for disparities between genders.
As for potassium, girls who ate less of the mineral could blow out about 2 per cent less air than girls with a higher intake.
The recommended daily allowance for magnesium during adolescence - when the body most needs magnesium - is 410 milligrams (mg) a day for boys and 360 mg a day for girls. Fewer than 14 per cent of boys and 12 per cent of girls in the study had adequate intake. Potassium intake also was higher in boys than girls, but researchers found that potassium intake for both boys and girls was within the recommended range (2,000 to 3,500 mg a day).
Nutritionists find that people who eat a variety of foods tend to get enough potassium. Foods high in potassium include fruits and vegetables such as oranges, bananas, potatoes, squash, avocados and tomatoes. Meats, beans and yoghurt also contain potassium.
Green vegetables such as spinach offer the greatest concentration of magnesium in the diet. Nuts, seeds and whole grains also are good sources of the mineral. The researchers found that children mostly got their magnesium from meats, milk and Mexican food such as tacos.
"The magnesium content in refined food is generally low," Gilliland said. "Because the consumption of processed and refined foods has increased in the US diet, magnesium intake has decreased and is now more likely to be inadequate."
Researchers caution that their study did not take into account mineral content of local drinking water or magnesium contained in any vitamins children may have been taking (though US national studies have shown such intake is usually low). Further study is needed to get more detailed data.
But scientists know magnesium and potassium participate in several biochemical and physiologic processes; they directly affect lung function and indirectly influence respiratory symptoms. Magnesium tends to relax muscles in the airway, for example.
Based on growing evidence that low magnesium intake is associated with adverse outcomes in children, consideration should be given to developing public health interventions to increase magnesium intake, the authors concluded.