While we know that breastfeeding has many health benefits for mothers and babies, the studies have been a bit fuzzy when it comes to the link between breastfeeding and preventing obesity in children. Some studies show a clear link, but in others that link is less clear. A new study published in the journal Pediatrics may help explain the fuzziness. It showed that what really helped prevent obesity was getting breast milk directly from the breast.

That’s not to say that drinking expressed breast milk from a bottle isn’t healthy. After all, it’s the food that was explicitly designed for infants — and in the study, babies that got breast milk from a bottle did have lower rates of obesity at 12 months. Some of that benefit is thought to be related to the microbiome that breast milk helps create. Babies who drink breast milk are more likely to have certain bacteria in their digestive tracts that help prevent obesity.

But the babies that had the lowest risk of obesity in the study were those that got only breast milk directly from the breast for the first three months of life. Why would that be?

To be able to breastfeed directly from the breast for three months, you have to be able to be with your baby constantly for three months. Mothers who can do that either have access to paid maternity leave or have enough resources to take an unpaid leave — or to stay at home with their babies and not work outside the home at all. Studies have shown that mothers who breastfeed longer are more likely to have higher incomes, more education, and private insurance.

These, then, are mothers who are also more likely to have access to and be able to afford healthy foods, to live in areas where there are safe places to exercise — and to be able to pay for sports and other forms of exercise as their children grow. It’s not just about how these babies are fed, but also about the context in which they are born and raised.

The way in which they are fed, though, is important. Babies who feed directly from the breast are less likely to be overfed. When they are full, they stop sucking, or switch to a “comfort” kind of sucking that doesn’t produce milk. When babies are fed from bottles, parents and caregivers are more likely to push them to finish the bottle; feeding becomes a bit less about appetite and more about volume and schedule.

Learning to eat only when you are hungry and stop when you are full is a really good skill when it comes to preventing obesity. That’s why the American Academy of Pediatrics has encouraged parents to learn and use “responsive feeding,” that is, responding to the cues of babies and children of both hunger and being full. The motto is, “You provide, your child decides.”

What this study helps us see is that the link between breastfeeding and obesity prevention is part of a bigger picture we need to pay attention to if we want to fight the obesity epidemic. It shows us that we need to: