UnitedHealthCare offers a narrow network plan where patients receive all of their primary care services from clinics owned by the company's subsidiary, Optum. The company says that plan costs about 20 percent less than typical plans.

Insurers argue the discounted or free services will save them in the long-run by boosting patient health and catching medical issues before they become more serious and more costly.

Other medical providers are of course less than thrilled at the prospect of increased competition. Perhaps more worrisome than insurers setting up their own clinics is them buying up established practices. Those practices include doctors who may have traditionally referred patients to other specialists or hospitals but may now be directed by their corporate parent company to send them elsewhere.

At this point, however, no insurers are yet close to owning all of the medical services that its members need.

"Health plans want to exert pressure on provider systems, but they don't have a product without providers in it, so they're moving carefully," Sam Glick, a health care consultant from Oliver Wyman, tells the Wall Street Journal.

While the trend towards narrow networks is undeniable for both individual and employer plans, so far it looks like employers are more hesitant to embrace plans that only cover certain services through a provider owned by the insurer. Individuals shopping for plans on the Obamacare marketplace are more prone to pick the cheapest plan available.

Read more:

• Insurers entering physician practice buying frenzy
• UnitedHealth buying up doctors to defend against competition
• Employers increasingly cutting insurers out of health system contracts