Nearly a quarter, or 23 percent, of U.S. adults who were insured all last year lacked adequate protection from big medical bills based on their income, according to Commonwealth Fund research.

The nonprofit foundation estimates that about 31 million people between the ages of 19 and 64 were underinsured due in part to the out-of-pocket expenses they have to pay for care. That includes deductibles, or payments a patient has to make before most coverage begins.

Insurers and employers who offer coverage have been raising deductibles for years as they try to contain premiums, or the price of insurance coverage. That means more of the bill for care is being passed along to patients. Some benefits advisers say this approach can help lower health care costs because it compels patients to shop for better deals on care, and insurers and employers are providing tools for patients to do that.