Researchers from The Children's Hospital of Philadelphia, the University of Pennsylvania and the University of Iowa studied 653 adults, ranging in age from 20 to 32.

The subjects, all of whom were white, had been measured as newborns while participating in infant formula studies in Iowa. Those who had gained weight more rapidly during their first week were significantly more likely to be overweight decades later.

Potential Targets for Obesity Prevention

"Our main finding was that rapid weight gain during the first week of life in this population of healthy, European-American, formula-fed infants was associated with being overweight two to three decades later," said lead author Nicolas Stettler, M.D., M.S.C.E., a pediatric nutrition specialist at The Children’s Hospital of Philadelphia.

"It suggests that there may be a critical period in that first week during which the body's physiology may be programmed to develop chronic disease throughout life," Dr. Stettler added.

"Our findings also point toward new potential targets for preventing obesity," he said. "If these results are confirmed by other studies, they may lead to interventions in newborns to help prevent long-term development of obesity."

Such interventions have not yet been developed, however, and it is premature to make recommendations about specific targets for optimum weight gain, Dr. Stettler explained.

"Normal weight gain is desirable for infants," he said. "Babies double their birth weight during the first four to six months. During the first week of life, however, a too-rapid gain in weight may increase the risk of future weight problems."

After adjusting for other factors, Dr. Stettler's team found that each additional 100 grams of weight gained during the first eight days increased a baby's risk of becoming an overweight adult by about 10 percent.

Exclusive Breastfeeding for Six Months

The current research builds on several previous studies by Dr. Stettler and his collaborators focusing on rapid weight gain in infancy. They found that rapid weight gain during the first four months increased the risk of being overweight at age seven.

They also found a higher rate of obesity at age 20 among African Americans who gained weight rapidly during the first four months.

The current study revealed an association between rapid weight gain in the first four months (112 days) and being overweight in adulthood. However, the first week of life may be particularly sensitive. Why that first week of life plays such a critical role in affecting lifelong physiology remains an open question.

"Animal studies have shown that overfeeding in the first few days of life leads to long-term obesity, possibly from programming in the developing brain or the endocrine system," said Dr. Stettler. "We don't know how this effect may occur in humans. However, obesity is increasing in prevalence worldwide, and we hope our research may help contribute to ways to intervene in newborns to improve their lifelong health."

Given that participants in the current study all received infant formula, Dr. Stettler says, it may be relevant that exclusive breastfeeding during early infancy is known to be associated with a slower rate of weight gain, and possibly with a lower risk of overweight in childhood and adolescence.

"For a variety of health reasons, the American Association of Pediatrics recommends exclusive breastfeeding during a baby's first six months of life," says Dr. Stettler. "Although we cannot yet make specific recommendations about targets for newborn weight gain, we can certainly endorse breastfeeding."