In the news: Vitamin K and Retinal Health

The results of a new observational study from Belgium and the Netherlands, highlight a possible role for vitamin K in promoting retinal health.

Matrix Gla Protein and Vitamin K

Matrix Gla protein is a small, vitamin K-dependent protein that binds to calcium ions. It is expressed in vascular smooth muscle and endothelial cells, where it acts as an inhibitor of vascular mineralization.

Once activated by vitamin K (referred to as active matrix Gla protein or MGP), it becomes a potent, locally acting inhibitor of calcification in large arteries and protects against macrovascular complications and arterial stiffening. MGP is also found in the kidneys and micro-vessels of the heart, where it contributes to maintaining organ function.

In addition, MGP is abundantly produced in the eye. In ocular tissues it helps protect the structural integrity of the trabecular meshwork, the sclera, and retinal ganglion cells. Animal studies have demonstrated that MGP is expressed in retinal vessels as well.

Inactive matrix Gla is a biomarker for poor vitamin K status. It is also a risk factor for cardio-vascular disease. A number of studies have shown that the diameters of micro-vessels in the retina can offer valuable information. Narrower retinal arteriolar caliber and smaller arteriovenous ratio, for instance, predict the incidence of diabetes and coronary heart disease. 

Against this backdrop, researchers from the University of Leuven in Belgium assessed whether levels of circulating inactive matrix Gla might predict retinal microvascular diameters in a longitudinal cohort study (1).

Study Highlights

The study included 935 randomly recruited men and women who were participants of the large Flemish Study on Environment, Genes and Heath Outcomes. The researchers measured plasma levels of inactive matrix Gla protein, a marker of inadequate vitamin K, at baseline (1996-2010) using an ELISA-based assay.

Retinal microvascular diameters, which included central retinal arteriolar and venular equivalent (CRAE and CRVE), and arteriole-to-venule ratio (AVR), were measured at follow-up (2008-2015) employing retinal photography and IVAN software.

CRAE (p= 0.005) and AVR (p= 0.08) at follow-up decreased across tertiles of inactive matrix Gla protein. The key finding is that, even with multiple adjustments, smaller microvascular diameters were associated with higher circulating baseline levels of inactive matrix Gla protein.

According to the authors, these findings are in line with studies showing expression of MGP in capillaries and small arterioles, and in particular in the glaucoma-relevant tissues of the eye and the retina’s large network of arterioles, capillaries, and venules.

Comments

This is the first longitudinal population-health study to assess the association of retinal microvascular diameters and circulating levels of inactive Gla. The researchers call for further studies to clarify underlying molecular pathways and to substantiate their theory that vitamin K supplementation might promote ocular health and prevent glaucoma-induced optic nerve damage.

In the meantime, consuming adequate vitamin K is important for bone and cardiovascular health, as well as proper blood clotting. (Avoiding fluctuations in vitamin K intake is necessary for those taking coumadin, who should follow the advice of their physician). The best food sources of vitamin K are leafy green vegetables, followed by cruciferous vegetables. Fish, meat, egg and cereals contain small amounts.

References

1. Wei FF, et al. Inactive matrix Gla protein is a novel circulating biomarker predicting retinal arteriolar narrowing in humans. Sci Rep. 8(1):15088, 2018.