Advocates are asking regulators to carefully scrutinize two massive health insurance mergers to make sure they won't harm some of the most vulnerable people in the health care system: mentally ill people, who already struggle to get access to treatment in many places.

The proposed mergers would turn the top five insurers in the country into the top three, combining Aetna with Humana and Cigna with Anthem. The billion dollar deals will have uncertain effects on consumers, which have been a point of debate. Some argue that larger insurers will be able to take advantage of economies of scale to negotiate better prices with hospitals, while others worry that bigger companies will drive up premiums for consumers and reduce choice, particularly in certain geographic areas.

What gets easily left out of that conversation -- and many conversations about health care -- is mental health. Although the Affordable Care Act put in place a parity requirement that care for depression or schizoprenia be covered, advocates worry about inequalities that could be exacerbated as insurers grow larger and face less competition.

"There’s a gaping hole between the need for mental health care and substance abuse care and the ability for patients to get that care," said Rep. Joseph P. Kennedy III (D--Mass.), who sent a letter this week to antitrust officials requesting careful scrutiny of the deals. "My concern is with that continued consolidation in the marketplace, that could get worse and not better."

The American Psychiatric Association also sent a letter to antitrust officials this fall, outlining the ways existing problems in access to mental health care might get worse with fewer, larger insurers. For example, a problem documented by several mental health advocates is the phenomenon of "phantom networks," in which a list of doctors that provide services include many who are not taking new patients, are not reachable, or were listed by insurers as accepting a plan when they did not.

A 2014 study by the Mental Health Association of Maryland, for example, examined psychiatrists listed as in-network for plans sold through Maryland Health Connections and found that only 14 percent of the doctors accepted new patients and could schedule an appointment within the next 45 days. More than half were unreachable.

"Creating mega-insurers threatens to exacerbate the 'phantom network' problem by eliminating competition on both the consumer and provider ends of the service chain," the American Psychiatric Association wrote in its letter.

Another issue Kennedy is concerned about is that despite parity requirements, the bigger companies could reimburse psychiatrists at lower rates than other doctors, providing an incentive for mental health professionals to not accept insurance.

There is also concern that insurers may deny mental health claims more often than other medical procedures or surgeries. In March, New York Attorney General Eric T. Schneiderman reached a settlement with a company that administers behavioral health coverage, ValueOptions, to pay a $900,000 penalty for rejecting mental health and substance abuse claims far more often. Schneiderman's investigation found that the company denied addiction recovery claims four times as often as was typical of other medical claims at insurers and behavioral health claims twice as often.

"My area of concern is really just asking our federal regulators to take a good, hard look at our insurance market," Kennedy said. "And as it consolidates to a big three, that are left, what has already been spotty access to mental health care and insurance is not adversely affected by these mergers going through."