Growth in overall health-care spending is slowing, but middle-class families’ share of the tab is getting larger, squeezing households already feeling stretched financially.

Overall, health-care spending across the economy reached 18.2% of gross domestic product as of June, up from 13.3% in 2000, according to Altarum Institute, a health research group.

However, the mix of who pays has evolved. The government has taken on a larger share in recent years as more people age into Medicare, and the Affordable Care Act expanded Medicaid and provided subsidies for low-income people buying insurance on state exchanges. Middle-class households are finding more of their health-care costs are coming out of their own pockets.

David Cutler, a Harvard health-care economist, said this may be “a story of three Americas.” One group, the rich, can afford health care easily. The poor can access public assistance. But for lower middle- to middle-income Americans, “the income struggles and the health-care struggles together are a really potent issue,” he said.