While correlation doesn't necessarily = causation .... We have seen whole foods and those closer to the body's way of absorption be better then synthetic supplements in the case of Vitamin A, Vitamin E, heme iron, folic acid, Vitamin D3, etc.
Average daily calcium and iron intake was determined from the amount of supplemental calcium/iron intake (including antacids, multivitamins, and prescription and nonprescription supplements) reported during a 30-day period. "These data were then divided into quintiles for logistic regression models," Dr. Wang explained.
Models accounted for the confounders of age, sex, ethnicity, education, income, health-related behaviors, general health condition, and comorbidities. The main outcome was the association between self-reported glaucoma and iron and calcium supplementation.
Of the 3848 subjects enrolled, 248 were subsequently found to have NTG. Associations between glaucoma incidence and dietary supplements were observed for the highest quartile of calcium intake (at least 800 mg/day). "These individuals have a 2.4 times greater risk of developing glaucoma than those who do not take supplements" (95% confidence interval [CI], 1.25 to 4.76), reported Dr. Wang.
Elevated risk for disease was also seen for iron supplementation, with the top quartile (at least 18 mg iron/day) demonstrating a statistically significant association between high supplement intake and NTG, compared with control subjects (odds ratio [OR], 3.80; 95% CI, 1.79 to 8.06).
"Most striking was the relationship for the concurrent use of calcium and iron in the top quartiles," said Dr. Wang, with high amounts of both supplements taken together demonstrating the greatest risk (OR, 7.24; 95% CI, 2.42 to 21.62).
She noted that the study had several limitations, one being that dietary sources of calcium and iron (e.g., kale, spinach) were not explored. However, given these results, confirmatory studies are being considered.
Interplay of Supplements: What About Magnesium?
Commenting on the study, Robert Ritch, MD, chief of glaucoma service and surgeon director at the New York Eye and Ear Infirmary in New York City, said that "in middle-aged people or older individuals, magnesium deficiency is not uncommon."
"There are some groups now — Dr. Josef Flammer's group at the University of Basel [Switzerland] for one — that are using magnesium supplementation in normal-tension glaucoma; I've started to use it in some patients as well," he noted.
One recent study that looked at a 300 mg oral dose of magnesium citrate in patients with NTG found a statistically significant improvement in mean deviation on visual field after 1 month of use, compared with patients who did not receive any magnesium (Eur J Ophthalmol. 2010;20:131-135). However, those study results revealed nothing about the mechanism of action of magnesium in improving vision scores.
Dr. Ritch suggested that if the data exist, associations between calcium, iron, and magnesium should be investigated, particularly with an eye toward [determining] the effect of magnesium on calcium metabolism.
The study was supported by the National Eye Institute, That Man May See, Research to Prevent Blindness, and a grant from the National Institutes of Health to the Clinical and Translational Science Institute at the University of California San Francisco. The study authors and Dr. Ritch have disclosed no relevant financial relationships.