In the News:
Vitamin C & Retinal Cell Function, Metformin & Vitamin B12 Depletion, Meta-analysis: lutein & AMD

Background: Role of GABA in Retina

GABA (gamma-aminobutyric acid) is the main inhibitory neurotransmitter in the central nervous system (CNS). GABA receptors modulate the rapid communication between brain cells by acting as an inhibitory ‘brake’ on excitatory nerve cells.

There are different types of GABA receptors: The ‘a’ type, for example mediates fast inhibition, while the ‘c’ receptor mediates slow and sustained responses. GABAc receptors are expressed in many regions of the brain, with prominent distribution on retinal neurons. These receptors are thought to play important roles in retinal signal processing.

Vitamin C Needed for Retinal Neuron Function

Researchers at Oregon Health and Science University report this month that the function of both retinal GABAa and c receptors is regulated by vitamin C (1).

According to the authors, “the cells need to be bathed in relatively high doses of vitamin C inside and out” for proper receptor function. Conversely, the receptors failed to function in the absence of the vitamin.

Vitamin C appears not only to play an indispensible role in retinal nerve cells, it may also be important for brain function. “Because the retina is part of the CNS, this suggests there’s likely an important role for vitamin C throughout our brains, to a degree we had not realized before”, explained the lead investigator. Vitamin C stores in the brain are known to be the last to be depleted during times of vitamin C deprivation.

The findings may also have implications for other conditions involving malfunctioning nerve cells in the retina and brain, such as glaucoma and epilepsy, the investigators theorize, and it “may be that a vitamin C-rich diet could be neuroprotective for the retina”.

Metformin and Vitamin B12 Depletion

There’s been increasing concern over the risk of vitamin B12 deficiency in type 2 diabetics taking the common diabetes medication metformin (2) (e.g. Glucophage®). According to some studies, 10-30% of people taking this drug show evidence of decreased B12 absorption. Since people over 50 are generally less efficient at absorbing B12, this drug-nutrient interaction could exacerbate levels that are already low. Vitamin B12 depletion is more common in people who’ve been taking metformin the longest, and/or at high doses.

Clinicians suggest that patients who’ve used the medication for several years or who take a high dose, should have their B12 levels tested. Checking blood levels of B12 should always be done with signs of deficiency, especially those related to nerve problems such as numbness, pain or tingling in the hands or feet.

Lutein Reduces Risk of Late Stage AMD

Lutein and zeaxanthin are thought to decrease the incidence of AMD; however, findings have not always been consistent. A team of Chinese researchers from Peking University recently conducted a systematic literature review and meta-analysis to evaluate the relationship between dietary intake of lutein and zeaxanthin and AMD risk (3).

Six longitudinal cohort studies met the strict inclusion criteria, and their results were pooled for analysis. The researchers excluded studies that examined the use of supplemental carotenoids.

Comparing the highest with lowest category of intake showed that higher intakes of these carotenoids were associated with a 26% reduction in the risk of late-stage AMD, and a risk reduction of 32% for neovascular AMD. Little effect of lutein and zeaxanthin in the early stages of this disease was found.

References

1. Calero CI, et al. Allosteric modulation of retinal GABA receptors by ascorbic acid. J of Neurosci 26:9672-82, 2011.
2. Kos E, et al. The effect of metformin therapy on vitamin D and B12 levels in patients with Diabetes Mellitus Type 2. Endocr Pract [Epub ahead of print], September, 2011.
3. Ma L, et al. Lutein and zeaxanthin intake and the risk of age-related macular degeneration: a systematic review and meta-analysis. Br J Nutr [Epub ahead of print], Sept 8:1-10, 2011.