Health plans that say mental illnesses can't be treated like physical ailments are relying on an "outdated argument" without merit that does not reflect "three decades of scientific research," 52 House members wrote in a recent letter to regulators. The comments come as a landmark law requiring parity in how health plans cover physical and mental illness went into effect Thursday.

"We encourage agencies involved with the implementation of [the parity and healthcare reform] laws to fully evaluate and reject clams by insurance companies seeking to weaken the consumer protections intended by Congress," the letter says. The letter was spearheaded by Rep. Paul Tonko (D-N.Y.) and co-signed by 49 House Democrats and Rep. Vernon Ehlers (R-Mich.); it's addressed to Kathleen Sebelius, Hilda Solis and Timothy Geithner — the secretaries of Health and Human Services, Labor and Treasury, respectively.

The Mental Health Parity and Addiction Equity Act, named after former Sens. Paul Wellstone (D-Minn.) and Pete Domenici (R-N.M.), was signed into law in 2008. It requires that large-group health plans that offer mental health and substance use disorder coverage do so on parity with their medical and surgical coverage. The parity requirements apply to six categories: inpatient in-network, inpatient out-of-network, outpatient in-network, outpatient out-of-network, emergency care and prescription drugs; for example, a plan can no longer offer different deductibles or out-of-network benefits for medical and mental health coverage.

The law applies to plan years starting Thursday, but some actuaries say most health plans aren't ready.