The relation between dietary intake and glaucoma: a systematic review

Methodological quality of the review: Low confidence

Author: Ramdas WD

Region: Not reported

Sector: Glaucoma

Sub-sector: Risk factor

Equity focus: None specified

Review type: Other review

Quantitative synthesis method: Narrative synthesis

Qualitative synthesis method: Not applicable

Background:

Most medical practitioners have regular contact with patients who have glaucoma. A common question patients with (or at risk for) glaucoma ask their ophthalmologist is what can they do by themselves next to just taking their daily ophthalmic medication. Patients often look for alternatives. Several studies suggested that nutrition might have an effect on their IOP or glaucoma. With respect to this, a few pathophysiological pathways implicated in glaucoma can be highlighted.

Objectives:

To present an overview of the current scientific literature on nutrients (minerals and trace elements, nutrition with antioxidative properties (such as carotenoids and flavonoids) and omega-fatty acids) that are known to have one or more of the above-described properties and may affect glaucoma, and to determine the association of these nutrients with glaucoma.

Main findings:

A total of 46 articles were included in the review. Most covered open-angle glaucoma (OAG) and were cross-sectional case-control studies.

Most of these articles studied the effect of nutrients on OAG. Many trace elements have been investigated in the literature, but the most interesting are selenium and iron,  both of which may increase the risk of glaucoma. Investigated nutrients with antioxidative properties and omega-fatty acids included glutathione, nitric oxide, carotenoids, flavonoids, and omega-3 and omega-6 fatty acids. Of these, glutathione, nitric oxide, and flavonoids had a significant protective effect on glaucoma.

In conclusion, the author notes that intake of selenium and iron may increase the risk of glaucoma, though, only few studies have been done on this topic. Nitric oxide present in other dark green leafy vegetables seems to have a beneficial effect on glaucoma. However, the evidence for an association of dietary intake with glaucoma is still not strong. More (longitudinal and randomized clinical trials) studies are required to make the presented findings clinically applicable.

Methodology: 

The author conducted a search on PubMed for articles published prior to September 2017. The search terms used in PubMed included (“minerals[All Fields] AND glaucoma[All Fields]”), (“diet[All Fields] AND glaucoma[All Fields]”), (“trace elements[All Fields] AND glaucoma[All Fields]”), (“carotenoids[ All Fields] AND glaucoma[All Fields]”), (“flavonoids[All Fields] AND glaucoma[All Fields]”) and (“fatty acids [All Fields]AND glaucoma[All Fields]”). The included entries had to have an available abstract, which had to be in either English or German. Furthermore, investigations had to be performed in humans (or human tissue). Studies were only eligible if they were peer-reviewed. In addition, references lists from all identified studies were screened for further relevant studies.

The author notes that quality of included studies was assessed with a tool described elsewhere. The variables examined included the methods for selecting study participants, methods for measuring exposure (nutrients) and outcome variable (glaucoma), design-specific sources of bias, statistical methods (excluding control of confounding) and conflict of interest. Studies on the dietary effect of nutrition had to use (semi-quantitative) food frequency questionnaires or a 24-hr recall and/or a detailed interview.

The author used a narrative synthesis approach as the number of studies reporting an effect estimate of the specific nutrients on glaucoma was too low.

Applicability/external validity:

According to the current review, a patient with OAG should avoid selenium and iron (for example, red meat) and use glutathione and nitric oxide-rich foods (for example, dark green leafy vegetables). However, randomized clinical trials are lacking.

Geographic focus

Geographic focus of included studies was not reported.

Summary of quality assessment:

Overall, there is low confidence in the conclusions about the effects of this study as important limitations were identified. The author did not conduct a thorough search of the literature to ensure all relevant studies were identified and included. The author only conducted a search on one database and reviewed the references of identified studies, where language bias was not avoided.  In addition, the author did not use rigorous methods to sift studies for inclusion and extract data of included studies.