Hartford, Conn. ? Amid a growing obesity epidemic in the United States, an insurance company has started giving customers another reason to slim down by being one of the first in the nation to offer discounts to customers who keep a low body-mass index.

The program by Phoenix Cos. Inc. offers discounts up to 20 percent on life insurance policies to customers whose BMI is verified by a doctor to be 19 to 25.

BMI is a ratio of body fat that takes height and weight into account. The Centers for Disease Control and Prevention defines obesity as a BMI of 30 or more; people between 25 and 30 are considered overweight.

“We tried to come up with a program that accounts for factors such as strokes, and help those who maintain healthy weight, lifestyle, what they eat and go to the gym,” said Joe Kelleher, senior vice president and chief operating officer of The Phoenix. “We thought we’d be able to reward those people.”

Customers who qualify for the program can start seeing reduced rates after five years if their BMI remains in the 19-to-25 range. Customers will see their premiums drop by 5 percent for every five years that they keep a healthy BMI ratio, up to a maximum of 20 percent after 20 years.

The plan comes as U.S. obesity rates have risen to an all-time high. Nearly one-third, or 32 percent, of adult Americans, are considered obese, the federal government says.

Insurance companies prize healthy customers because they live longer. Insurers make more revenue from healthy customers who pay monthly premiums well into their 70s than from customers who die of natural causes years earlier.