Waiting longer to start infants on solid food could make for slimmer adults, new research shows.

“The later you introduce complementary feeding to an infant, within the range of 2 to 6 months, the smaller is the risk that the infant will be overweight as adult,” Dr. Kim Fleischer Michaelson of the University of Copenhagen, one of the researchers on the study, told Reuters Health via E-mail.

Some studies have suggested that breastfeeding protects against obesity, although the data are not yet conclusive. The World Health Organization and American Academy of Pediatrics currently recommend that babies be breastfed exclusively until six months of age, and advise against introducing solid foods until then. But many parents start their children on solid foods earlier.

Michaelson and her team looked at a sample of 5,068 men and women born in Copenhagen between 1959 and 1961. At that time, Michaelson noted, parents were instructed to start giving their infant solid foods between four to six months of age, but many started sooner.

Half of the study participants were breastfed for at least two and a half months, while half started eating solid food at three and a half months of age or later. Seventeen percent of the babies started spoon feeding before two months of age, while 46 percent didn’t start until they were four months old or later.

At age one, babies who were breastfed for longer had lower body mass indexes - BMIs, standard measures used to gauge how fat or thin a person is. However, there was no association between duration of nursing and BMI in later childhood, adolescence or adulthood.

The researchers did find that the age at which parents introduced various types of foods to an infant did seem to have an effect on BMI in adulthood. “In our study the risk of overweight at age 42 years was reduced by 5 percent to 10 percent for each month introduction of complementary foods was delayed,” Michaelson explained.

“I think it is best to wait with complementary foods until the infant is about 6 months old,” she added. “But there is no reason to be too rigid about the age. Some infants will need complementary foods before the age of 6 months, but they should not get them before four months of age.”

SOURCE: American Journal of Clinical Nutrition, online December 24, 2009.