Fidelity reported a 53% increase in health savings accounts opened in 2012, raising the number of individual accounts administered by the company to 182,000.

As of Feb. 28, Fidelity managed $471 million in HSA assets, up 44% from a year earlier. Among the reasons for this growth: many employers have begun to offer a high-deductible health plan together with an HSA, along with a defined contribution retirement plan, as an integrated employee benefit.

“This permits employees to see their balances in both plans,” said William Applegate. “It encourages employees to view HSAs as a component of their retirement investing, and helps them understand how an HSA can complement a DC plan such as a 401(k). We also can provide guidance to plan participants, explaining how to take advantage of the HSA tax benefits.”

Fidelity emphasizes employees should first contribute enough to their retirement plan to get a full employer match, if one is offered. That’s an immediate, risk-free, return on invested dollars.

“Then,” Applegate said, “the next dollars should go into an HSA, where employees can get triple tax advantages.” HSA contributions, investment earnings, and distributions used for qualified health care expenses all escape federal income tax. HSA contributions this year may be as high as $7,450, for families with HSA holders 55 or older. “Many employers contribute to an employee’s HSA, which adds value,” Applegate added. “After contributing as much as possible to an HSA, employees can make unmatched 401(k) contributions.”

Some observers have worried that such an approach will cannibalize 401(k) contributions, resulting in smaller retirement funds, but Fidelity data show that hasn’t been the case. For the past two years, Fidelity has analyzed the savings behaviors of employees who save in both a Fidelity HSA and a DC plan.

On average, employees with both plans tend to save more of their annual salaries (8.5% average annual deferral rate) in their DC accounts than employees with just a DC plan (8.1%). Applegate attributed the higher total savings rate to several factors, including an early adoption bias (participants in relatively new HSAs may be better advised and more engaged in retirement planning) and the fact that HSAs are paired with high-deductible health insurance, which has lower premiums.

So far, most HSA investment dollars go into cash. Applegate said that participants may eventually move on to other HSA investment options, such as stock, bond, and asset allocation funds, once their HSAs hold enough cash to meet medical emergencies.