Federal officials often say that health insurance will cost consumers less than expected under President Obama’s health care law. But they rarely mention one big reason: many insurers are significantly limiting the choices of doctors and hospitals available to consumers.

From California to Illinois to New Hampshire, and in many states in between, insurers are driving down premiums by restricting the number of providers who will treat patients in their new health plans.

When insurance marketplaces open on Oct. 1, most of those shopping for coverage will be low- and moderate-income people for whom price is paramount. To hold down costs, insurers say, they have created smaller networks of doctors and hospitals than are typically found in commercial insurance. And those health care providers will, in many cases, be paid less than what they have been receiving from commercial insurers.

Some consumer advocates and health care providers are increasingly concerned. Decades of experience with Medicaid, the program for low-income people, show that having an insurance card does not guarantee access to specialists or other providers.