Imagine an investment with the stock market’s upside potential, but the downside protection of a bank CD. You can gain but not lose. Interested?

That’s the marketing angle of the fixed index annuity (FIA), an insurance-based retirement vehicle that’s grown sharply in popularity during this era of low interest rates and ever-increasing market highs. Sales of FIAs are on track to hit $60 billion this year, according to industry estimates — double the $30 billion in sales racked up in 2009.

But FIA sales pitches — often presented at free “dinner seminars” hosted by persuasive agents — sometimes obscure the impact of both fees and restrictions on earnings, according to government regulators and some industry insiders. “It is important to ask your agent to explicitly define how the product works,” warns Tim Gannon an executive at Fidelity Investments Life Insurance Company. “[You want to] know up front about any factors that could put a drag on your potential return.”