Consumption and plasma levels of three micronutrients had no clear association with diabetic retinopathy, according to a meta-analysis involving 4,100 patients.
The studies included in the analysis showed no association between vitamin E and retinopathy and inconsistent results for vitamin C and magnesium.
Because of differences in study design and methods, the analysis could not exclude an association. So future studies should include serial measurement of micronutrient intake and plasma levels, employing standardized tests and definitions to evaluate possible associations, the investigators concluded.
“It is a very attractive proposition that what one eats, rather than a medication, might reduce the risk of diabetic complications,” Amanda I. Adler, MD, PhD, of Addenbrooke’s Hospital in Cambridge, England, said in a statement.
“If [better-designed] studies showed apparent protection against diabetic retinopathy, then a randomized clinical trial could determine precisely how a person with diabetes might, or might not, alter his intake of any of these nutrients,” she added.
Laboratory studies have produced a circumstantial case for a beneficial effect of micronutrients against diabetic retinopathy. Vitamins C and E suppress production of vascular endothelial growth factor, which stimulates retinal neovascularization, the authors wrote.
Hyperglycemia may indirectly increase adhesion of monocytes to retinal endothelial cells by expression of intercellular cell adhesion molecule-1, which vitamins C and E may inhibit.
Hyperglycemia also increases protein kinase C-dependent endothelial dysfunction, and some evidence suggests vitamin C may decrease activation of protein kinase C, the authors continued. Increased peripheral intracellular concentration of free calcium impairs insulin action and glucose uptake, worsening hyperglycemia. Magnesium, meanwhile, acts as a calcium antagonist.
Observational evidence to support laboratory findings has been inconsistent and sometimes conflicting. In an effort to sort out the findings, Adler and colleagues performed a systematic review and meta-analysis of epidemiologic studies that examined associations between micronutrients and diabetic retinopathy.
For the meta-analysis, the investigators identified 15 studies suitable for inclusion, yielding these principal findings:
- Hospital-based studies showed an inverse association between plasma levels of vitamin C and retinopathy, whereas population-based studies showed no association.
- None of the studies that evaluated vitamin E demonstrated an association with retinopathy.
- One prospective analysis showed an association between low magnesium levels and progression of retinopathy, but cross-sectional studies produced inconsistent results.
“The evidence suggests that dietary intake or plasma levels of vitamins C and E and magnesium do not seem to be associated with diabetic retinopathy,” the authors wrote in conclusion. “Because of differences in study designs and measurement of micronutrients, incomplete ascertainment of retinopathy, and residual confounding, these findings require confirmation.”
Lee CTC, et al “Micronutrients and diabetic retinopathy. A systematic review” Ophthalmology 2010; DOI: 10.11016/j.ophtha.2009.06.040.