Vitamin D deficiency is associated with type 1 and type 2 diabetes. As some evidence suggests that vitamin D may have insulin sensitising effects, these associations have been interpreted as providing evidence that higher plasma concentrations of the active vitamin D metabolite 25-hydroxyvitamin D, may be protective of diabetes. However they may be other interpretation of this association. For example, diabetes increases the risk of nephropathy due to the renal complications that arise from severe blood sugar disorders. The renal pathology associated with diabetes include proteinuria, or high levels of protein in the urine. This essentially results from a breakdown of the reabsorption mechanism that is required to prevent excessive excretion of protein following filtration of plasma through the bowman’s capsule. The vitamin D binding protein is normally returned to circulation via an endocytotic process in the proximal tubule of the nephron, and this allows the plasma levels of vitamin D binding protein to be maintained.
As vitamin D is lipid soluble, it attaches to the vitamin D binding for transport around the circulation. The vitamin D binding protein may also be required for the conversion of 25-hydroxyvitamin D to 1,25 dihydroxyvitamin D, a process that occurs in the kidney. The implication here is that by increasing the excretion of proteins in the urine, including the vitamin D binding protein, diabetics may be more likely to suffer from lower levels of plasma 25-hydroxyvitamin D and 1, 25-dihydroxyvitamin D. In addition, those with type 2 diabetes may be obese, or overweight, and this may further lower vitamin D plasma concentrations by sequestering vitamin D in adipose tissue. Fibre is protective of blood sugar disorders and it is increasingly being evidenced that high fibre diets are able to modify blood sugar levels downwards, and that this in turn is able to reverse or limit the damage caused by glucose intolerance or diabetes. High fibre diets may therefore improve plasma vitamin D levels.
Data on the effects of fibre on vitamin D levels is not extensive, but some animal studies have investigated the effects of dietary fibre on vitamin D levels in rats. For example, in one study1, researchers fed diabetic Zucker rats or control rats a rat diet with or without additional resistant starch (dietary fibre). The results showed that the resistant starch was effective at attenuating hyperglycaemia and prevent the urinary excretion of vitamin D binding protein (and other proteins including albumin) in the diabetic rats, when compared to the control diet. In the diabetic rats fed a control diet, the excretion of 25-hydroxyvitamin D and 1, 25 dihydroxyvitamin D were significantly higher, and the plasma levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D significantly lower, compared to the rats fed resistant starch. When histological examination of the rats was performed, it was observed that the resistant starch fed diabetic rats have 21 % less kidney damage compared to the control diet diabetic rats.
Dr Robert Barrington’s Comments: The inverse association between plasma levels of 25-hydroxyvitamin D and diabetes in humans may be more complex than previously considered. Although vitamin D does have some insulin sensitising effects, the low plasma levels of vitamin D in diabetics may be due to accelerated excretion of vitamin D and its binding protein as a result of kidney damage. The ability of fibre to protect the kidney from damage and to increase levels of vitamin D are further benefits of high fibre diets that have not been extensively reported.
RdB
