Oct. 8 (HealthDay News) -- New moms who breast-feed may need exercise -- including cardiovascular activity and strength training -- to fight off a loss of bone density caused by lower levels of calcium, research shows.

A new study published in the October issue of Medicine & Science in Sports & Exercise found that new mothers who didn't exercise lost about 7 percent of the bone mineral density in their lower spine after giving birth. The findings come from an analysis of 20 women whose bone density was tested between four and 20 weeks post-partum.

"During lactation, women transfer around 200 milligrams of calcium per day from their own stores to their breast milk," researcher Cheryl Lovelady said in a news release from the American College of Sports Medicine.

"Calcium is critically linked to bone density and health, and this depletion can result in loss of bone mineral density," she added. "When mothers wean their infants, bone mineral density usually returns to normal levels. We proposed that weight-bearing exercise would minimize bone losses during lactation and decrease the risk of osteoporosis later in life."

Exercise decreased the level of bone loss during breast-feeding, the researchers found. A combination of strength training and aerobic exercise for three days a week resulted in less loss of bone mineral density: 4.8 percent compared to 7 percent among those who didn't exercise.

Women who performed weight-bearing exercise also lowered their percentage of body fat and increased their strength, the study authors noted.

"Women in our study found themselves overall healthier and stronger after completing the post-partum exercise program, which lasted just 16 weeks," Lovelady said. "Moreover, implementing this exercise into daily life can help entire families get active and improve their overall health."

SOURCE: American College of Sports Medicine, news release, Sept. 28, 2009