Apple and Aetna both have something to gain from a new partnership that allows Aetna customers to get discounted Apple Watches through their insurance: Your data.

Aetna customers who opt to purchase Apple Watches through the partnership between the two companies will be sharing their health data with the tech giant and the insurance provider.

Aetna and Apple announced Tuesday that the insurance provider would be the first to subsidize purchases of the Apple Watch for its customers. Along with its Apple Watch subsidy, Aetna will develop iOS apps around medical care, medical billing, Aetna insurance plans and adherence to medication.

Those apps, and their pairing with iOS devices, will provide data to both Aetna and Apple, an Aetna spokesman confirmed. Apple declined to comment.

The data — at the aggregate, not individual level — will be related to "usage adoption" and "benefits achieved by participants," Aetna spokesman Ethan Slavin said. He declined to comment on what metrics would be included in those two categories. The data will be shared between both companies.

Usage adoption likely covers how Aetna customers are using their Apple Watches for health purposes. But benefits achieved could more closely track with particular health data, including health metrics like weight and activity or cholesterol and blood pressure.

That's useful information for an insurer to have — and more information than some consumers might want to share, even at an aggregate level.

Data like that could be used to adjust insurance premiums, push new products and services in a more tailored way to individuals and for medical purposes, said Roeen Roashan, a senior analyst for digital health at IHS Markit.

"Knowing how people sleep, breathe, stay active is good to know for providers," Roashan said.

There is some concern, Roashan noted, over whether the use of such trackers should be taken into account when considering a person's overall health. Some research has questioned the actual health benefits of fitness trackers

"The security concerns that may have actual health implications can become real if the use of a 'prescribed' device directly affects a diagnosis, a dosage of medicine or provides any guidance that has life or death implications," Roashan added.

The information is also useful for Apple, which is ramping up its entry into the health space.

The tech giant has already encouraged health data tracking through its CareKit and ResearchKit frameworks, which provide developers the ability to create iOS apps linked to research or care without building them from scratch. ResearchKit has helped researchers get participants to better track information about themselves for studies, while apps made through CareKit offer similar tracking benefits for users' own health.

Apple acquired Gliimpse, a personal health data startup with a similar focus, earlier this year. That was the company's first known acquisition in the digital health industry.

Apple has also promoted the Apple Watch as a health-related product, highlighting its water and sweat resistance for exercise at September's Apple event.

The US healthcare industry is worth at least $2.8 trillion, according to a report from Deloitte. If Apple can engrain itself further among the insurers, medical providers and consumers who make up those trillions, it opens up a huge market at a time when Apple is facing serious competition from other smartphone makers as well as questions about whether its days of growth are over.

For Aetna, the partnership also has some financial benefits. Encouraging fitness tracking and healthy behaviors can save the insurer money in the long run — even though customers who opt to purchase an Apple Watch won't be required to use the health apps in any way.

"Of course Apple will be shipping more Apple Watches, and their footprint in healthcare will grow, but it is Aetna's engagement with their member that will aid from this partnership," Roashan said.  

Other insurers have also introduced wearables to their customers, citing similar long-term benefits.

Cigna gives its customers a 20-to-50 percent discount on FitBits and MisFit wearables. UnitedHealthcare offers its customers at participating employers the Trio Tracker for free through a partnership with health technology company QualComm Life.

"If we can get people up and moving, we know that has the potential to reduce the prevalence of obesity and diabetes," UnitedHealthcare spokesman Will Shanley said. "It's a turnkey way to get wellness integrated into employers' day-to-day operations."

But no other insurers offer the Apple Watch, which does a lot more than track health and activity -- and carries a significantly heftier price tag than other wearables. Apple Watches range from $269 to more than $1,000, compared to around $100 for most wearables.

A Cigna spokesman said his company is in constant conversation with most companies working in health technology, which would likely include Apple. Shanley said he couldn't say which companies UnitedHealthcare has discussed the sector with.

The Apple Watch models available to Aetna customers will vary by employer, Slavin said. Aetna's 50,000 employees are the first to be provided with Apple Watches, in their case free of charge. In that scenario, the employees were able to choose from all of Apple's models.

For other Aetna customers, the insurer will subsidize a "significant portion" and customers can pay off the rest through monthly payroll deductions. Individuals, not their employers, will be the ones to purchase the Apple Watch. Aetna declined to disclose exactly how much of the Apple Watch it would pay for.