June 27, 2007

Long-term vision for vitamin C and diabetic eye health?

By Stephen Daniells

27/06/2007 - Long-term supplementation with vitamin C may one day help prevent diabetic retinopathy, the most common diabetic eye disease, if preliminary results from animals can be reproduced in humans.

The incidence of diabetic retinopathy among persons with diabetes mellitus increased from 6.9 per cent to 17.4 per cent between 1991 and 1999, showing the potential opportunities for preventative approaches, according to a 2003 study carried out by researchers at Duke University in Durham, North Carolina and commissioned by Kemin Foods.

The new study, published in the Elsevier journal, Microvascular Research, compared the effects of vitamin C supplementation on the development of diabetes retinopathy and found significant improvements in markers of eye health.

"Vitamin supplementation suppressed leukocyte adhesion and thus endothelial dysfunction, associated with increase in iris blood flow perfusion in diabetes. The antioxidant vitamin C may be a therapeutic agent for preventing diabetic retinopathy," wrote lead author Amporn Jariyapongskul from Srinakharinvirot University in Bangkok.

However, a leading independent expert has warned that the research is too preliminary to draw any firm conclusions as this stage.

According to the US National Eye Institute, diabetic retinopathy is the most common eye disease for diabetics and a leading cause of adult blindness. It can be caused by the swelling and leaking of blood vessels or the growth of abnormal new blood vessels on the surface of the retina.

Jariyapongskul and co-workers divided male Wistar-Furth rats into three groups; one group acted as control (health rats), while the other two groups had diabetes induced by a single intravenous injection of streptozotocin (STZ, 55 mg per kg body weight). The STZ rats were further divided into two groups, with half the animals receiving vitamin C supplements (ascorbic acid, 1 g/l) added into the drinking water.

Levels of blood glucose, tissue lipid peroxidation and plasma vitamin C were measured at eight, 12, 24 and 36 weeks after STZ injection. Rats in the STZ group not supplemented with vitamin C were found to have significantly higher blood glucose levels and levels of tissue lipid peroxidation, as well as lower vitamin C levels, which improved by vitamin C supplementation.

STZ caused reductions in blood flow in the iris of the animals eyes, and gradually increased over the weeks following the injection. In the STZ rats who received vitamin C supplementation, however, this blood flow was significantly increased compared to the STZ rats, while the adhesion of leukocytes to the endothelium (cells that line the walls of blood vessels) was decreased at 24 and 36 weeks.

"The present results have shown a good correlation among significant decreases in blood glucose, significant increase in plasma vitamin C and significant decrease in the number of adherent leukocyte in 24 and 36 weeks after supplementation with vitamin C in diabetic rat," stated the researchers.

"In diabetes with hyperglycemia, reactive oxygen species are continuously formed throughout the duration of diabetic state. In the present study, vitamin C was continuously supplemented to decrease blood glucose in diabetic rats, which might improve ascorbic acid transportation into the endothelial cells," they continued.

"Therefore, a balance of reactive oxygen species and antioxidant system was fully recovered, especially in 36 weeks of STZ-vitC rats," they concluded.

Commenting independently on the research, Dr Iain Frame, research manager at British charity Diabetes UK told NutraIngredients.com that the research is still at an early stage and the results are too preliminary to draw meaningful conclusions.

"The experiments were conducted in an experimental animal model with a relatively large dose of Vitamin C. As the authors admit, much more work needs to be done on the effects of Vitamin C on the cells in question in normal and diabetic animals to determine whether this approach would make any difference to the prevention of retinopathy," said Dr. Frame.

"We would certainly not encourage people to start taking vitamin C supplements based on the findings of this research," he added.

An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.

In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132 billion, with $92 billion being direct costs from medication, according to 2002 American Diabetes Association figures.

Source: Microvascular Research (Elsevier)
July 2007, Volume 74, Issue 1, Pages 32-38
"Long-term effects of oral vitamin C supplementation on the endothelial dysfunction in the iris microvessels of diabetic rats"
Authors: A. Jariyapongskul, T. Rungjaroen, N. Kasetsuwan, S. Patumraj, J. Seki and H. Niimi

***********************************

Reader comments:

"If this study is correct and applies to humans, diabetics will need huge amounts of vitamin C. Wistar rats can make their own vitamin C - and usually make huge amounts per kilogram body weight. Humans do not manufacture their own vitamin C. If the rats were protected they obviously needed vitamin C over and above what they could make themselves. Rats and mice are never appropriate models for human conditions, particularly when investigating vitamin C, unless they have been genetically altered. The researchers should have used guinea pigs or primates."
- Aileen Burford-Mason, PhD., DRS Consulting, Toronto

Source: www.nutraingredients.com