Infant formula could change gut bacteria, contribute to childhood obesity: Study The study looked at infants who were exclusively breastfed and formula fed.

What an infant eats might have effects on later weight -- and it likely starts in the gut, according to a new study.

A new study shows that babies who were breastfed had different bacteria environments, or microbiomes, in their guts –- and lower obesity levels as they grew -– than babies who were primarily fed formula.

Obesity begins early, research has shown, and breastmilk is known to lower a baby’s risk of obesity as an adult.

"Breastmilk is a very specialized food –- not just for babies, but also for their gut bacteria. Breastmilk contains oligosaccharides, which are complex sugars that feed specific gut bacteria," Dr. Meghan Azad, lead researcher and assistant professor in the Department of Pediatrics & Child Health and Community Health Sciences at the University of Manitoba, told ABC News.

The study looks at a theory on why this happens: That "good" bacteria in babies' digestive systems affects how they burn and store fat, as well as how they use energy.

For the study, published in the Journal of Pediatrics, researchers in Canada looked at data from the Canadian Healthy Infant Longitudinal Development, or CHILD, focusing on the first year of life for more than 1,000 infants from four different sites.

Researchers in this study wanted to know if only breastfeeding, breastfeeding plus some early foods, or formula feeding alone affected the type of bacteria found in the infants' guts at two ages: 3 to 4 months and 12 months.

Mothers reported on breastfeeding and when formula and complementary food was introduced to the infant. Other factors --infant sex, birth weight, antibiotic use, maternal smoking status, race, education level, pet ownership, diet and pre-pregnancy body mass index -- were reported, and babies’ stool samples were taken at 3 to 4 months and 12 months to test for the variety of gut bacteria.

Most mothers in the study group were white and delivered vaginally, which is known to help set up babies' digestive systems, and about 40 percent were overweight or obese.

Surprisingly for a Western nation, 96 percent of mothers were breastfeeding after birth. However, at 3 months, only 54 percent of infants were solely breastfed. Another 30 percent of the babies were partially breastfed and 16 percent were fed formula alone.

Weight differences began to show between the different groups of babies at 3 months. Of the formula-fed babies, 33 percent were overweight or at risk of being overweight, while 19 percent of exclusively breastfed babies were overweight or at risk.

The differences are likely connected to what's happening in the gut, researchers said.

Gut microbes, especially in infants and children, help develop the digestive tract and immune system. These "new friends" to the growing infant are affected by the type of delivery, either vaginal or Cesarean, whether the baby or mother gets antibiotics and, most importantly, what the baby is fed and when solid foods are introduced.

These microbes can "train the immune system to prevent allergies," the researchers said. "They also help us digest and extract energy from food, which can influence weight gain."

For infants, changes begin in the gut each time foods are introduced. The first happens in breastfeeding, which adds a helpful bacteria called Bifidobacterium into the infant’s gut that helps digest complex sugars called oligosaccharides. The second change happens when solid foods are introduced and the baby is weaned, which creates a more adult complex of microbes, usually adding Bacteroidetes and Firmicutes. Those changes continue until the child reaches about age 4.

A more adult complex of microbes at an earlier age is not beneficial to babies in terms of weight -- and that's what the study showed. At age 3 to 4 months, babies who were fed formula showed the most adult-like diversity of microbes, partially breastfed babies had lower diversity of microbes and exclusively breastfed babies had the lowest mix of adult-like microbes.

The helpful bacteria continued to be more prevalent at 12 months for babies who were breastfed. Bifidobacteriaceae, Veillonellaceae and Proteobacteria were thriving in infants who were still breastfeeding and low in those who had never been breastfed at 12 months.

The study also showed that mothers don't have to be "perfect" and always exclusively breastfeed. Many of the babies who were breastfed -- 31 percent -- were given some formula as infants. This brief addition of formula caused a decrease in the amount of Bifidobacteriaceae, at 3 to 4 months, but did not increase the babies' chances of being overweight at 12 months, if they continued to be breastfed.

The research emphasizes that breastmilk has "many important bioactive components that influence appetite and weight gain, including growth factors and hormones, which are not present in infant formulas."

Formula feeding appears to cause changes to the gut microbes, according to the study, which can cause a baby to be overweight, whereas introducing other complementary foods with solids does not.

It isn't just the formula itself that can lead to possible childhood obesity, Azad notes, it could also be the difference in the feeding process for fomula versus breastfeeding.

"Maternal-infant bonding, which may be reduced with bottle feeding, and self-regulation, which may not be optimal in formula fed babies who do not learn to stop eating when they are full," could also contribute to the weight gain, she said.

Early infancy remains an important period for developing the gut -- and feeding choices and methods may have effects on weight later in life.