Efficacy study of two dietary supplements, one based on resistant dextrin from wheat starch, fibers from Citrus spp. (pectin), and fibers from oat (Avena sativa L.), one based on resistant dextrin from wheat starch, fibers from carob (Ceratonia siliqua L.), and fibers from oat (Avena sativa L.), on intestinal function in subjects with primary functional constipation: single-center, placebo-controlled, randomized clinical study, parallel arms, double-blind.