Inulin and fructo-oligosaccharides differ in their ability to enhance the density of cancellous and cortical bone in the axial and peripheral skeleton of growing rats.

Although dietary fructans improve calcium absorption and bone mineral content (BMC) in rats, their effect on calcium bioavailability and bone density may vary with their degree of polymerization. Therefore, for a 3-month period, growing rats received either a control diet or a diet enriched with either oligofructose (OLF) or inulin. At sacrifice, body weight, lean body mass and appendicular bone length were similar in the 3 groups. Rats fed fructans had a similar increase in cecal wall weight (30%), but the relative increase in cecal levels of calbinding-9 K was 2 in the OLF group and 4 in the inulin group. Further, the significant decrease in serum levels of type I collagen C-telopeptide was greater in the inulin group (30%) than in the OLF group (16%). The increase in whole-body bone mineral content (BMC) as measured by DXA was greater in the inulin group than in the OLF group but DXA detected an increase in the BMC of excised femurs only in the inulin group. In contrast, pQCT conducted ex vivo detected a significant increase in the area and mineral density (BMD) of the cancellous bone of both the proximal tibia and vertebra in rats fed fructans and the effect of inulin was greater (P < 0.01) than that of OLF (P < 0.05). Further, inulin but not OLF significantly enhanced the BMD of the cortical bone in both appendicular and peripheral sites (P < 0.01) as well as the polar stress/strain index of femurs (P < 0.01). These observations suggest that, although both inulin and OLF both have a positive effect on BMD, the greatest effect of inulin is related to the higher capacity of this fructan to reduce bone resorption. The different anti-resorptive capacity of the 2 fructans might be related to their different impact on calcium absorption and bioavailability since the increase in cecal amounts of calbindin-9 K, a protein known to play an important role in calcium absorption, was greater in rat fed inulin than in rats fed OLF. Although cecal wall hyperplasia may be of concern, it remains to establish whether the positive effect of fructans observed on calcium absorption in humans is also associated with a positive effect on bone mass and/or mineral density.