Diets with a high-fiber content have been shown to produce some beneficial effects on metabolic factors in subjects with NIDDM. However, some controversies still exist. In this report, the long-term effect of guar gum (Guarina) on both glycemic and blood lipid profiles was assessed in a randomized, double-blind and cross-over study on 16 (seven male and nine female) subjects with NIDDM. Each subject received placebo (P) and Guarina (G) treatment for two eight-week periods separated by a four-week period to facilitate wash-out. Fasting plasma glucose levels showed significant improvement during G treatment but not during P treatment (151.7 +/- 7.9 vs 168.6 +/- 12.2 mg/dl, p less than 0.01 by paired Student's t test). Hemoglobin Alc levels decreased significantly during G treatment but not during P treatment (6.9 +/- 0.2 vs 7.2 +/- 0.8%, p less than 0.001). Fasting insulin concentrations also showed significant lowering during G treatment but not during P treatment (18.3 +/- 2.1 vs 23.1 +/- 2.9 U/ml, p less than 0.005). Other variables, including serum total cholesterol, triglyceride, HDLc, LDLc, sodium, potassium, chloride, magnesium and calcium levels showed no significant changes during G or P treatment. Ten out of the 16 patients (62.5%) suffered from side effects; these included abdominal cramps (one case), diarrhea (seven cases) and skin itching (one case). In conclusion, guar gum effectively lowers fasting plasma glucose and HbAlc levels in subjects with NIDDM. Hyperinsulinemia could also be ameliorated. The effectiveness and side effects of guar gum treatment should be cautiously evaluated in each NIDDM subject.