JAMA Ophthalmology Journal Club Slides: Dietary Nitrates and Primary Open-Angle Glaucoma

1. JAMA Ophthalmology Journal Club Slides:Dietary Nitrates and PrimaryOpen-Angle Glaucoma Kang JH, Willett WC, Rosner BA, Buys E, Wiggs JL, Pasquale LR. Association of dietary nitrate intake with primary open-angle glaucoma: a prospective analysis from the Nurses’ Health Study and Health Professionals Follow-up Study. JAMA Ophthalmol. Published online January 14, 2016. doi:10.1001/jamaophthalmol.2015.5601.

2. Introduction • Primary open-angle glaucoma (POAG) is a major cause of blindness; however, the etiology of POAG is poorly understood. Nitric oxide signaling alterations in outflow facility and retinal blood flow autoregulation are implicated in POAG. Nitric oxide donation has emerged as a POAG therapeutic target. An exogenous source of nitric oxide is dietary nitrates. • Objective • To evaluate the association between dietary nitrate intake, derived mainly from green leafy vegetables, and POAG.

3. Methods • Study Design • Prospective cohort study from the Nurses’ Health Study (NHS; 1984-2012) and the Health Professionals Follow-up Study (HPFS;1986-2012). • Main exposure was updated dietary nitrate intake; information on diet and potential confounders was updated with validated questionnaires. • The main outcome was incidence of POAG and POAG subtypes. • Participants • 63 893 women in the NHS and 41 094 men in the HPFS. • Eligibility criteria: (1) aged ≥40 years; (2) free of glaucoma; (3) reported eye examinations; and (4) complete diet data (from 1984 in the NHS and from 1986 in the HPFS). • During follow-up, 1483 incident cases were confirmed with medical records and classified into subtypes defined by intraocular pressure (IOP) (≥22 or <22 mm Hg) or by visual field (VF) loss pattern at diagnosis (peripheral loss only or early paracentral loss).

4. Methods • Data Analysis • Relationship between dietary nitrate intake and incident POAG was evaluated with Cox proportional hazards models with time-varying covariates. • Cohort-specific and pooled multivariable rate ratios (MVRRs) and 95% CIs were estimated.

5. Methods • Limitations • Underascertainment of cases due to lack of standardized eye examinations for all participants (analyzed person-time data only from participants reporting eye examinations and underascertainment is unlikely to be related to dietary nitrate intake to bias results). • Misclassification of dietary nitrate intake due to lack of detailed information on soil conditions, storage, etc (would have biased results toward the null). • Confounding by other dietary factors (results were robust in models further adjusted for intake of 13 other nutrients, including various antioxidants, caffeine, and alcohol). • Lack of generalizability as most participants were white (needs confirmation in other study populations; however, 1 study among African American women found that higher kale/collard intake was inversely associated with POAG1). • First study to evaluate dietary nitrates and glaucoma (confirmation needed in other studies). 1Giaconi JA, Yu F, Stone KL, et al; Study of Osteoporotic Fractures Research Group. The association of consumption of fruits/vegetables with decreased risk of glaucoma among older African-American women in the Study of Osteoporotic Fractures. Am J Ophthalmol. 2012; 154(4):635-644.

6. Results • Compared with the lowest quintile of dietary nitrate intake (quintile 1: approximately 80 mg/d), the pooled MVRR for the highest quintile (quintile 5: approximately 240 mg/d) was 0.79 (95% CI, 0.66-0.93; Pfor trend = .02). • The dose response was stronger (Pfor heterogeneity = .01) for POAG with early paracentral VF loss (433 cases; quintile 5 vs quintile 1 MVRR = 0.56; 95% CI, 0.40-0.79; P for trend < .001) than for POAG with peripheral VF loss only (835 cases; quintile 5 vs quintile 1 MVRR = 0.85; 95% CI, 0.68-1.06; P for trend = .50). • The association did not differ (P for heterogeneity = .75) by POAG subtypes defined by IOP (997 cases with IOP ≥22 mm Hg: quintile 5 vs quintile 1 MVRR = 0.82; 95% CI, 0.67-1.01; P for trend = .11; 486 cases with IOP <22 mm Hg: quintile 5 vs quintile 1 MVRR = 0.71; 95% CI, 0.53-0.96; P for trend = .12).

7. Results POAG by Quintiles of Nitrate Intake in the NHS (1984-2012) and the HPFS (1986-2012)a aIntake calculated using cumulative average (ie, average of all available intake data from food frequency questionnaires completed before each 2-year period at risk). bPooled results were calculated using Dersimonian and Laird methods with random effects.

8. Results • Green leafy vegetables accounted for 56.7% of nitrate intake variation. Compared with consuming 0.31 servings/d, the MVRR for consuming ≥1.45 servings/d was 0.82 for all POAG (95% CI, 0.69-0.97; P for trend = .02) and 0.52 for POAG with paracentral VF loss (95% CI, 0.29-0.96; P for trend < .001). • Iceberg lettuce, a type of green leafy vegetable, accounted for 23.2% of the variance in total dietary nitrate. Compared with consuming 0.11 servings/d, the MVRR for consuming 0.86 servings/d was 0.89 for all POAG (95% CI, 0.75-1.06; P for trend = .06) and 0.69 for POAG with paracentral VF loss (95% CI, 0.49-0.97; P for trend = .001).

9. Comment • Greater intake of dietary nitrate and green leafy vegetables was associated with 20% to 30% lower POAG risk; the relationship was particularly strong (40%-50% lower risk) for POAG with early paracentral VF loss at diagnosis, for which ocular vascular dysregulation has been implicated. • Biological mechanism: alterations of the nitric oxide system may lead to dysregulation of ocular blood flow and elevated IOP, which may be etiologic factors for POAG. Although nitric oxide is mainly generated endogenously via the L-arginine/nitric oxide pathway, when there is hypoxia or when this pathway is compromised as in POAG, the nitrate-nitrite–nitric oxide pathway can be an alternative source of nitric oxide. The stronger inverse association with POAG with early paracentral VF loss is consistent with evidence that this subtype is more strongly associated with vascular dysregulation.

10. Comment • Consistency of findings with prior literature: these results are consistent with data from 2 cross-sectional studies. In all women (95 cases among 1155 total)2or only African American women (77 cases among 587 total)3 in the Study of Osteoporotic Fractures, the only vegetable that was consistently inversely associated with POAG was kale/collard greens: ≥1 serving/mo of kale/collard greens was significantly associated with 55% to 70% reduced odds of POAG. • However, because this was the first study to evaluate intake of dietary nitrate specifically in relation to POAG, confirmation in other studies is warranted. 2Coleman AL, Stone KL, Kodjebacheva G, et al; Study of Osteoporotic Fractures Research Group. Glaucoma risk and the consumption of fruits and vegetables among older women in the Study of Osteoporotic Fractures. Am J Ophthalmol. 2008;145(6):1081-1089. 3Giaconi JA, Yu F, Stone KL, et al; Study of Osteoporotic Fractures Research Group. The association of consumption of fruits/vegetables with decreased risk of glaucoma among older African American women in the Study of Osteoporotic Fractures. Am J Ophthalmol. 2012;154(4):635-644.

11. Contact Information • If you have questions, please contact the corresponding author: • Jae H. Kang, ScD, Channing Division of Network Medicine, Department of Medicine, Brigham & Women’s Hospital and Harvard Medical School, 181 Longwood Ave, Boston, MA 02115 (nhjhk@channing.harvard.edu). Funding/Support • This work was supported by grants UM1 CA186107, UM1 CA167552, EY09611, and EY015473 (Dr Pasquale) and grant R21 EY022766 (Dr Wiggs) from the National Institutes of Health and the Arthur Ashley Foundation, the Harvard Glaucoma Center of Excellence (Drs Pasquale and Wiggs), and a Harvard Medical Distinguished Ophthalmology Scholar Award (Dr Pasquale). Conflict of Interest Disclosures • All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.