Diet, nutraceuticals and the tear film

Highlights

• Hyperlipidemia and a high dietary ratio of omega 6 to omega 3 are risk factors for dry eye.

• Vitamin A supplementation in the setting of malnutrition is of clear benefit to the tear film health.

• Supplementation with nutraceuticals containing antioxidant and omega-3 or omega-6 appears to improve tear film health.

• The impact of these nutraceuticals on tear lipidome, proteome has yet to be fully investigated.

• More research is required to determine the exact dosage, composition and indications for these nutraceuticals.

Abstract

Nutrition disorders and their correlates such as obesity are increasingly prevalent worldwide. A number of studies to date have suggested numerous potential associations between diet and tear film health; this paper will provide a summary of the available literature. The tear film is characterized through its protein and lipid content and through clinical measurements of characteristics such as osmolarity, volume and stability. Malnutrition, protein and vitamin-A deficiencies are extremely deleterious to tear film health and supplementation with oral vitamin A in this setting is of clear benefit. The relative impact of diet on tear film within what would be considered normal ranges of consumption is less clear. A number of population studies have suggested that hyperlipidemia and a diet low in omega-3 fatty acids are risks factor for dry eye disease. Numerous studies have investigated the effectiveness of oral supplementation with antioxidants, omega-3 (e.g. fish oil and linseed oil) and omega-6 (e.g. evening primrose oil) fatty acids in the last 10 years. Taken together, these suggest a small benefit of oral supplementation on tear film volume, stability and decreased ocular symptoms in patients previously diagnosed with diseases involving the ocular surface (e.g. Sjögren’s syndrome, meibomian gland dysfunction, dry eye disease) and contact lens wearers suffering from dry eye. More research is required to determine the exact composition, dosage and indications for their use and to fully characterize how these nutraceuticals modulate the tear film.

Introduction

The world we live in offers stark contrasts between 868 million people suffering from food insecurity and malnutrition and a growing obesity problem (Stevens et al., 2012). Nutrition disorders play a major role in a number of highly prevalent diseases including coronary heart disease, stroke, hypertension, atherosclerosis, some forms of cancer, type 2 diabetes, osteoporosis, dental caries, gall bladder disease and nutritional anemia (http://www.aihw.gov.au/risk-factors-nutrition/). Drastic changes have occurred in the human diet in the past 10 000 years. Most of these diet changes have occurred in the last 100 years, with the advent of industrial food production practices.

Changes associated with eating practices include a significant decrease in the amount of vitamins ingested, a significant increase in the amount of fat in the diet and more importantly, a shift in the source of dietary fats from omega-3 types fats to trans and omega-6 types of fats (Simopoulos, 2002). Long-chain omega polyunsaturated fatty acids are essential components of cell structures that cannot be fabricated by the human body and must be obtained from the diet (Fig. 1). They are involved in various cellular functions and metabolic processes including inflammation and neurotransmission. Fish oil is a source of the long-chain omega-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Vegetable sources of omega-3 fatty acids include seed oils such as linseed (also called flaxseed) oil. Such seed oils contain the fatty acid alpha-linolenic acid (ALA), which can be converted after ingestion to EPA.

Omega-6 fatty acids include linoleic (LA) and gamma-linoleic (GLA) acids which can metabolize into prostaglandin E1 (PGE1) – an anti-inflammatory mediator - or into arachidonic acid (AA), a precursor for the pro-inflammatory mediators prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) (James et al., 2000). As EPA and AA act competitively for the enzymes cyclooxygenase and 5-lipoxygenase, the modulation of inflammatory activity in the body is thought to be based on the balance of EPA/AA in the body (James et al., 2000). Synthesis of the longer chain omega-3 fatty acids within the body is competitively slowed by the omega-6 analogs. Thus, accumulation of long-chain omega-3 fatty acids in tissues is more effective when competing amounts of omega-6 analogs do not greatly exceed the amounts of omega-3. Omega-6 rich animal foods include meat, poultry and eggs whilst plant sources include evening primrose and borage (also called starflower) oils. The intermediate-chain omega-9 fatty acid oleic acid (non essential) synthesizes eicosatrienoic acid (ETA) and can also results in beneficial effects through decreased synthesis of LTB4 (James et al., 2000). Food sources of omega-9 fatty acids include olive oil.

Much attention has been paid on the link between retinal diseases and nutrition (e.g. age-related macular degeneration) (Evans and Lawrenson, 2012, Merle et al., 2013, The Age-Related Eye Disease Study 2 (AREDS2) Research Group, 2013). Less research has comparatively been conducted on associations between diet and the tear film. A comprehensive review of this topic was published more than 20 years ago (Caffery, 1991). An updated review therefore appears timely given the recent explosion in our knowledge and understanding of the tear film as summarized in this journal issue.