The digestibility of retrograded starch was evaluated by an in vitro digestion test and a nutritional test using rats. Uncooked polished rice (Koshihikari from Niigata Prefecture in 2019)，commercially available gelatinized rice (Onisi Foods Co., Ltd.)，and retrograded rice were used as foods containing raw starch, gelatinized starch, and retrograded starch. The retrograded rice was prepared by spreading cooked polished rice on a stainless-steel tray, leaving it at 4˚C for 1 day, and then freeze-drying. The in vitro digestibility of each rice sample was 86％ in gelatinized rice, 72％ in retrograded rice and 37％ in uncooked polished rice. Eighteen male 4-week-old Wistar rats were divided into three groups, and each of them was fed a diet containing 12％ of protein and consisting mainly of uncooked polished rice, gelatinized rice or retrograded rice for 4 weeks. The weight gain, feed efficiency, serum total protein, albumin and total lipid concentration, and liver lipid concentration in the retrograded rice administration group were significantly lower than those in the other two groups. These results indicate that retrograded starch is not well digested in vivo as compared with gelatinized starch. In the retrograded rice-administered group, hemoglobin concentration, serum transferrin saturation, and liver iron concentration were significantly high, and it was estimated that low protein and energy status caused by retrograded rice administration changed iron metabolism. On the other hand, the uncooked polished rice administration group showed the same growth as the gelatinized rice administration group, which was different from the result of the in vitro digestion test. This result is coincident with previous reports but the mechanism by which rats can utilize raw starch is unknown.
