Guar smoothens postprandial glucose peaks and reduces serum cholesterol. Both properties are advantageous for diabetic patients and can be realized, in part, by adding fiber to the diet. Depending on the blood glucose concentration, a small portion is incorporated into hemoglobin (HbA1), building a largely irreversible complex, making it a long-term indicator of blood glucose status. HbA1 may increase from a normal value of about 5% to as much as 20% of total hemoglobin in diabetics. This study tested the efficacy of two guar preparations in diabetic patients over 3 months by measuring HbA1, cholesterol and triglycerides. Forty diabetic patients with HbA1 > 10% ingested either 3 x 4 g daily of a new guar preparation (GU-052, Steigerwald, Darmstadt, Germany) or 3 x 5 g daily of Glucotard (Boehringer, Mannheim, Germany) for 12 weeks. Before and after 30, 60 and 90 days of treatment, HbA1, cholesterol and triglycerides were determined. After 90 days of treatment, GU-052 caused a reduction in HbA1 from 12.6 +/- 2.6% to 10.5 +/- 1.5% (-15.6 +/- 9.0%) in the GU-052 group and from 12.0 +/- 2.6% to 10.9 +/- 1.82% (-8.6 +/- 7.2%) in the Glucotard group. Cholesterol was reduced from 269 +/- 28 mg/100 ml on day 0 to 227 +/- 18 mg/100 ml on day 90 in the GU-052 group and from 261 +/- 40 to 235 +/- 26 mg/100 ml in the Glucotard group. Both GU-052 and Glucotard had little effect on plasma triglycerides. Fewer unpleasant side effects were reported for the GU-052 preparation than for Glucotard.(ABSTRACT TRUNCATED AT 250 WORDS)