Advice to Moms: Breast-Feed, Then Exercise

A hard workout, which can be good for a mother, can be bad for her baby if the mother breast-feeds soon after exercise, a study finds.

Mothers pass along immune protection in their milk, but levels of proteins that carry the protection drop sharply after the workouts, the researchers said.

“Nurse before exercise,” advised researcher Richard L. Gregory of Indiana University.

Gregory and his colleagues looked at immunoglobulin A proteins in breast milk. Their findings were published in the American College of Sports Medicine journal, Medicine and Science in Sports and Exercise.

The IgA proteins are made by disease-fighting lymphocyte cells in the small intestine as a response to foreign bacteria. Each version of the protein is targeted at a specific form of bacteria. So the mother, through her milk, shares her personal arsenal of antibodies with her baby, Gregory said.

“It provides protection for the infants during at least the first six months of life,” Gregory said. The infant’s own immune machinery is starting to produce its own immune proteins, but these don’t reach adult levels until at least 1 year of age, he said.

The proteins fight diseases that live in the lymph system, the saliva or the gut--among them salmonella, a potentially deadly form of food poisoning.

But IgA levels change with exercise, the study found. The researchers compared before-and-after milk samples from 17 healthy women who did treadmill exercise at an increasing intensity until the women were at their personal maximums.

Compared with milk taken at rest, milk after 10 minutes of exercise had 60% less IgA, the researchers found. And after 30 minutes, there was 73% less, they found.

But levels rose later, and were virtually the same an hour after exercise as they were when the women were at rest. “What’s striking is that it rebounded,” Gregory said.

The study indicates that women who do hard exercise give their babies a lot less immune protection if they breast-feed soon afterward, Gregory said.

It’s not known why immune proteins fall after exercise, but it may be that the woman’s body is trying hard to recover from the stress of exercise, Gregory said.

An occasional nursing after a hard workout may not leave the baby worse off, because the low IgA levels would be averaged with higher ones from other breast-feedings, Gregory said.

The immune benefit of breast milk is one reason health experts want mothers to nurse instead of feed their babies formula, said Dan Smith of the Harvard-affiliated Forsyth Dental Center in Boston.

“The concentration of IgA in breast milk is very high--much higher than in any other fluid in the body,” said Smith, who studies IgA in saliva as he searches for a vaccine against the bacteria that cause tooth decay.

The intensity of the treadmill workout was above the minimum needed for aerobic conditioning, so the findings may only apply to these hard workouts, not to moderate-to-vigorous ones, said David C. Nieman, a professor of health and exercise science at Appalachian State University in Boone, N.C.

“This is the first study done,” said Nieman, who studies the effect of exercise on the immune system. “The next logical step is to contrast this with moderate exertion.”