Kids who shun their spinach and leafy greens may be doing more than going against the conventional wisdom of Popeye: They just might be shortchanging their growing lungs.

Preventive medicine researchers at the Keck School of Medicine found that children who get low amounts of magnesium and potassium in their diet tend to have lower lung function, a measure of how well their lungs work. Investigators reported their findings in the Jan. 15 issue of the American Journal of Epidemiology.

Magnesium particularly seems to be a problem, the researchers noted, since most children studied did not reach their recommended daily allowance for the mineral.

“Magnesium is at the center 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 to 25 percent 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 kids’ respiratory health.

The team tested lung function by having each child take a deep breath, then measuring how much and how fast kids 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 percent 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 percent less air than boys who got more magnesium from their diet. Researchers are unsure of the reasons for disparities between genders.

The mineral potassium, too, was associated with lung function, Gilliland said.

Among girls, those who ate less potassium could blow out about 2 percent less air than girls with a higher potassium 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. Less than 14 percent of boys and 12 percent 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 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 yogurt 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.

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 national studies have shown such intake is usually low). Further study is needed to get more detailed data.

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 conclude. Added Gilliland: “And if kids have asthma, especially, they may benefit from eating more whole grains and vegetables.”

Frank D. Gilliland, Kiros T. Berhane, Yu-Fen Li, Deborah H. Kim and Helene G. Margolis, “Dietary Magnesium, Potassium, Sodium, and Children’s Lung Function.” American Journal of Epidemiology, Vol. 155, No. 2, Jan. 15, 2002.