Molly Martin, assistant professor of sociology and demography at Pennsylvania State University, and Adam Lippert, a graduate student, said low-income mothers -- who are often the food managers in the household -- might skip meals and eat cheaper but less nutritious foods, to ensure their children are fed.
The sociologists found food-insecure mothers were more likely than fathers who face food shortages, as well as food-insecure, childless women and men, to be obese or overweight.
The researchers analyzed data of 7,931 participants in the Panel Study of Income Dynamics -- data on weight and food insecurity in 1999.
The study, published in Social Science and Medicine, found food-insecure mothers gain more weight over time, compared to all food-insecure men and food-insecure women not caring for children.
"To make sure mothers can provide for their children, they may eat only once a day, for example, or they may eat erratically, which are all behaviors that are obesity risks," Lippert said in a statement.
Besides causing weight gain, irregular eating can also cause a metabolic response that retains fat, the researchers said.
Women who have given birth tend to gain weight, but the researchers determined having children did not explain the weight gain for the food-insecure mothers.