Dry Eye Study Concerns
Regarding the news story Women with Dry Eye are Singing a New Tuna in the November issue, I would like to clarify some issues with the study that was the source of this article.

While we are aware that omega-3 fatty acids are critical in cellular development, they cannot produce series one anti-inflammatory prostaglandins (PGE1), which lead to mucous-specific tissue. The statement high intake of omega-6 fatty acids ... may increase the incidence of dry eyes is just wrong. If there is adequate eicosapentaenoic acid (EPA) in the diet to block the delta-5-desaturase, omega-6 could never cause dry eyes. The article confuses omega-6 fatty acids with trans fatty acids, which are not the same.

The study that the news story quoted states that the consumption of omega-6 in the food frequency questionnaire was mostly linoleic acid (LA), primarily from margarine, butter, mayonnaise, salad dressing, peanuts and cooking oils used both for frying and baking. All these omega-6 oils are normally hydrogenated or contain saturated trans fats and are rarely considered a good diet choice.

However, the food frequency questionnaire primarily looked for far healthier long-chain omega-3 fatty acids from seafood sources, including tuna, mackerel, salmon, sardines, shrimp, lobster, scallops and coldwater fish. The trans fats are the bad guys, and these are not omega-6 fatty acids. There is a common misconception that omega-3 fatty acids are responsible for eye moisture.

We are all likely guilty of evaluating studies based on the title, abstract and conclusion. However, if the time is taken to review the specifics of the study, many times one will find the deficiencies of the protocol and thus re-evaluate the weight given to a flawed conclusion.

Jeffrey Anshel, O.D.
Encinitas, Calif.

The authors respond:
Dr. Anshel uses strong language but largely unsubstantiated reasoning to suggest that the conclusions regarding the association between dietary intake of essential fatty acids and the risk of dry eye syndrome drawn from our study of more than 32,000 women are flawed. Although he misquotes our conclusions, we appreciate the opportunity to re-emphasize that one cannot generalize regarding the roles of all omega-6 fatty acids because the biology is complex and there are distinct differences in the biological effects of mediators derived from different omega-6 fatty acids.

Indeed, we discuss these issues at some length in the paper, which was peer-reviewed and published in the American Journal of Clinical Nutrition.1 Rather than review the biology again here, we would encourage those interested in the topic to read the paper.

Because our study was observational, we examined what people were actually eating. Using validated methodology, we calculated the amounts of individual omega-6 and omega-3 fatty acids in the reported diets. We included all omega-6 fatty acids in our assessment.

The types of omega-6 fatty acids consumed in large quantities in the typical American diet tend to result in much higher amounts of pro-inflammatory eicosanoids (locally acting hormone-like mediators involved in the control of inflammatory and immune responses). For example, prostaglandin E2, relative to prostaglandin E1, has several anti-inflammatory actions and appears to be an important stimulator of aqueous tear secretion.2,3 There is no evidence to substantiate a role for trans fatty acid consumption in dry eye.

As with any epidemiological study, findings must be interpreted in light of the weight of available evidence and alternative explanations considered, as detailed in the discussion of the paper.1 Despite the rhetoric, there appears to be common ground here with Dr. Anshel, who seems to agree that high consumption of the omega-6 fatty acids found in meats and commonly used food oils (e.g., corn oil) are unlikely to be beneficial and that an adequate amount of omega-3 fatty acids from fish or plant sources is needed. 

Biljana Miljanovic, M.D., M.Sc., M.P.H.
Debra A. Schaumberg, Sc.D., O.D., M.P.H.
Boston, Mass.

1. Miljanovic B, Trivedi KA, Dana MR, et al. Relation between dietary n-3 and n-6 fatty acids and clinically diagnosed dry eye syndrome in women. Am J Clin Nutr 2005 Oct;82(4):887-93.
2. Calder PC, Zurier RB. Polyunsaturated fatty acids and rheumatoid arthritis. Curr Opin Clin Nutr Metab Care 2001 Mar;4(2):115-21.
3. Pholpramool C. Secretory effect of prostaglandins on the rabbit lacrimal gland in vivo. Prostaglandins Med 1979 Sep;3(3):185-92.

Vol. No: 143:02Issue: 2/15/2006