PORTLAND — Physicians fighting obesity have long told patients to eat their fruits and vegetables. Now they’re writing prescriptions for it.

Health care providers are offering vouchers worth $1 per day to members of low-income families in Maine and Massachusetts participating in a new program. The idea is to boost consumption of fresh fruits and vegetables by one serving a day from local farmers markets.

The program will measure how the fresh produce affects blood pressure, weight and Body Mass Index; blood-sugar levels; and weight gain in pregnant women.

It was created by Bridgeport, Conn.-based nonprofit Wholesome Wave. It’s being formally launched in Maine on Wednesday, five days after it began in Massachusetts.