OAK FOREST, Ill. — The idea is simple: Load fresh fruits and vegetables into a refrigerator truck and drive it to a health clinic, then have a doctor write a “prescription” for food to improve the diets of low-income people with diabetes and high blood pressure. U.S. food banks — the organizations on the front lines of fighting hunger — increasingly are promoting “food as medicine” strategies designed to address, not exacerbate, the high rate of chronic health problems among the poor. One-third of households using food banks are feeding a family member with diabetes, and 58 percent have a family member with high blood pressure, according to a recent survey by Feeding America, a national network of 200 food banks.