Effect of a high-fiber diet compared with a moderate-fiber diet on calcium and other mineral balances in subjects with type 2 diabetes

OBJECTIVE - High levels of dietary fiber, especially soluble fiber, are recommended to lower serum cholesterol levels and improve glycemic control in patients with type 2 diabetes. It is not clear, however, how high levels of fiber affect mineral balance. RESEARCH DESIGN AND METHODS - In a randomized crossover study, 13 patients with type 2 diabetes were fed a high-fiber (50 g total and 25 g soluble fiber) and a moderate-fiber (24 g total and 8 g soluble fiber) diet of the same energy, macronutrient, calcium, magnesium, and phosphorus content for 6 weeks each. Intestinal calcium absorption was determined by fecal recovery of ⁴⁷Ca. Stool weight and mineral content were assessed during 3 days, and 24-h urinary mineral content and serum chemistry were assessed over 5 days at the end of each phase. The results were compared by repeated-measures ANOVA. RESULTS - Compared with the moderate-fiber diet, the high-fiber diet increased stool weight (165 ± 53 vs. 216 ± 63 g/day, P = 0.02) and reduced 24-h urinary calcium (3.3 ± 1.7 vs. 2.4 ± 1.2 mmol/day, P = 0.003) and phosphorus (29.2 ± 5.5 vs. 26.0 ± 3.2 mmol/day, P = 0.003) excretion and serum calcium concentration (2.33 ± 0.06 vs. 2.29 ± 0.07 mmol/l, P = 0.04). Calcium absorption, stool calcium, magnesium, and phosphorus content and serum phosphorus concentration were not significantly different with the two diets. CONCLUSIONS - A high-fiber diet rich in soluble fiber has a small impact on calcium and phosphorus balance in subjects with type 2 diabetes. It may be prudent to ensure adequate intake of calcium and other minerals in individuals consuming a high-fiber diet.