Breast milk has less protein than formula, a new study has claimed.
It has been believed that formula-fed babies, who tend to be bigger, are "programmed" to store fat and so have a higher risk of childhood obesity.
Now, an international study of 1,000 babies, published in the American Journal of Clinical Nutrition, has suggested that protein levels in formula should fall.
The study, which was carried out in Belgium, Italy, Germany, Poland and Spain, included babies born between 2002 and 2004.
Parents were recruited to take part in the first few weeks of their babies' lives, reports The BBC.
To reach the conclusion, a third were given a low protein content formula milk, a third had a formula with a higher level of protein, while the rest were breast-fed during their first year.
In order to qualify as breast-fed, kids had to be either exclusively given breast milk, or have a maximum of three bottles per week.
Then the infants were followed up to the age of two with regular weight, height and body mass index measurements taken.
At the age of two, there was no difference in height between the groups, but the high protein group were the heaviest.
The researchers suggest lower protein intakes in infancy might protect against later obesity.
The children are being followed up further to see whether those given the lower protein formulas have a reduced risk of obesity later on.
Professor Berthold Koletzko, from the University of Munich, Germany, and who led the study, said: "These results from the EU Childhood Obesity Programme underline the importance of promoting and supporting breastfeeding because of the long-term benefits it brings.
"They also highlight the importance of the continual development and improvement in the composition of infant formula. Limiting the protein content of infant and follow-on formula can normalise early growth and might contribute greatly to reducing the long-term risk of childhood overweight and obesity."