Babies who are breastfed gain less weight over their first year of life compared to babies fed either breast milk or formula from a bottle, according to a US study.

The report, in the Archives of Pediatrics & Adolescent Medicine, is one piece of a growing body of evidence that breastfeeding appears to be the best choice for a newborn and protect against obesity later in life.

Lead author Ruowei Li of the United States Centers for Disease Control and Prevention said the difference may come down to how much a role babies play in deciding when to stop feeding instead of mothers or fathers forcing them to finish a bottle.

“If the babies are fed by the breast, the baby plays a very active role, because they are the ones who decide when to suckle and when to stop,” she said.

Li and her fellow researchers followed about 1,900 babies from across the United States who were born in the mid-2000s.

Through a series of surveys sent to their mothers, the researchers asked for, among other things, babies' weights at different ages and how often women breastfed, pumped their breast milk or used formula.

Babies who were fed from a bottle, either with only breast milk or only formula, gained about 85 grams (three ounces) more per month compared to those who were solely breastfed.

After that, the findings became a bit more complicated.

When mothers combined breastfeeding and bottle feeding with human milk only, babies didn't gain any extra weight. But if the babies were fed both breast milk and formula, they grew similarly to babies who were solely breast-fed.

It's not clear why those babies fed a combination of breast milk by bottle as well as formula may not have gained additional weight, the researchers said.

But “the key message out of this study is that breastfeeding really is the first feeding choice for the babies,” said Li, adding that supplementing breastfeeding with breast milk from a bottle is a good second option.

The American Academy of Pediatrics recommends that mothers exclusively breastfeed for six months and continue breastfeeding as foods are introduced until at least 12 months.

But experts acknowledged that care needs to be taken not to put mothers under pressure about breastfeeding, acknowledging that it just may not be possible for a number of reasons.

“There were millions of babies raised on formula well before the obesity epidemic started,” said Jeffrey Wright, a pediatrician from the University of Washington School of Medicine, who wrote an editorial linked to the study

“Each family should weigh the benefits they see against the hassles they take to get there, and the father should be involved in that discussion.”