Nutrients may be particularly beneficial for eye health given the causative role of the combined effects of oxidative stress and inflammation in ocular disease aetiology. The eye is innately vulnerable to chronic inflammation and to the formation of free radicals, due to the high levels of metabolic activity at the retina, the plentiful amounts of readily oxidisable polyunsaturated fatty acids and the eye’s almost continuous irradiation with sunlight.
Glaucoma is the most common cause of irreversible blindness worldwide and with the rapidly growing and ageing global population, its prevalence continues to rise inexorably. Glaucoma is a classic example of a condition in which oxidative stress and inflammation combine to create a vicious circle of ocular insult that leads to progressive retinal damage.
Current treatment strategies for glaucoma concentrate solely on reducing intraocular pressure (IOP). Elevated IOP is only one of many causative factors and it is not surprising, therefore, that glaucoma often progresses despite “successful” IOP reduction.
Nutritional supplementation could be beneficial in glaucoma management through direct neuroprotective effects on the retina and optic nerve, or more indirectly due to an additional IOP-lowering effect or ocular blood flow enhancement effect. Numerous candidate nutrients have been identified, and insights into the role of nutritional supplementation in glaucoma are increasing. Our evolving understanding of nutrient effects on the eye can provide the basis for novel therapeutic targets for both the prevention and treatment of glaucoma.
Perhaps one of the most researched natural supplements in glaucoma is ginkgo biloba extract (GBE). Five clinical trials of GBE have been conducted in glaucoma, most of which have demonstrated some beneficial effect, such as improvement of visual field indices or increased ocular blood flow. Several randomised clinical trials have been conducted using combination nutritional supplements, including vitamins A, B, C and E, essential fatty acids, carotenoids lutein and zeaxanthin, and trace metals, to evaluate a possible synergistic effect. Some of these nutrients offer multiple benefits, for example, the carotenoids lutein and zeaxanthin exert potent antioxidant and anti-inflammatory effects but can also alleviate the symptoms of glare that are often experienced in glaucoma. Another example of a multifunctional compound is forskolin, a triterpene found in the roots of the Indian Coleus plant. It has been shown to decrease IOP, presumably by reducing aqueous humour production in the ciliary epithelium and may also offer a neuroprotective effect through stimulation of neurotrophin activity that promotes neuron survival.
Nutritional supplementation may also be useful in addressing a common disease (an unrelated complaint of glaucoma patients), the symptoms of dry eye. It’s typically a result from the long-term use of IOP-lowering drops with preservatives. Compounds studied for their ability to reduce ocular discomfort and support normal tear film composition include antioxidant vitamins, polyunsaturated fatty acids (PUFAs), trace metals, and Korean red ginseng.
Some nutrients, such as GBE, are already being used in anti-glaucoma therapy. Other, less researched nutrients are also commercially available (more than 50 nutritional supplements are currently marketed for glaucoma related benefits) and ready for incorporation into glaucoma patient treatment regimes.
A simple recommendation to change dietary habits or to start taking an appropriate nutritional supplement may reduce a patient’s risk of developing glaucoma, may limit disease progression or indeed alleviate associated symptoms such as glare and dry eye. Overall, while there is a strong theoretical rationale and some promising initial clinical evidence for the beneficial effect of dietary fortification as an adjunct treatment for glaucoma, the evidence is far from conclusive. Appropriate dietary guidance should be an important aspect of glaucoma management, but there is a need for high quality future clinical trials that explore the specific linkages between nutrient intake, their biological effects in the human body and clinical outcomes specific to glaucoma.