What the Nurses’ Health Study has to tell us about maintaining eye health
Loss of sight is a major cause of disability in the USA and greatly affects quality of life. By 2050, the number of Americans aged over 65 will have doubled, and the prevalence of the main causes of age-related vision changes (cataracts, AMD – age-related macular degeneration, glaucoma) will increase significantly. Not only has considerable progress been made in treating such diseases, but we have also significantly increased our understanding of what mechanisms are involved, what the important risk factors are and what may help for prevention.
The Nurses’ Health Study (NHS) data was used in order to better understand how genetic factors and lifestyle might influence the occurrence of agerelated disorders; the studies using these data from the period 1976 – 2016 are reviewed. This extensive and substantial long-term prospective study offered an incredible opportunity for epidemiological research into eye diseases. To use this large cohort’s data, we used methods that would enable a valid assessment of the risk factor relationships involved in these sight disorders.
First, we initially relied on nurses’ self-report of having reported receiving a medical diagnosis (of eye disease) in the first step of identifying cases; given their medical training as health professionals, the self-reports of being diagnosed with eye diseases by a physician were considered reliable. Second, for each eye disease, we developed precise definitions that maximised data specificity, thus avoiding bias, and for some diseases confirmed the self-reports using medical records with those definitions. Lastly, we included only those reporting having had eye examinations, in order to be able to perform sufficiently sensitive, valid statistical analyses.
If left untreated, cataracts are the leading cause of blindness worldwide. Cataracts represent an opacification of the crystalline lens (the eye’s main lens, which performs one of sight’s primary functions, known as accommodation; without this, objects would appear blurred). The only treatment is surgery (removal of the opacified crystalline lens and its replacement with an intraocular implant to restore clarity). The mechanisms underlying cataract formation are not clear. Crystalline lenses are normally transparent, but they become opaque as a result of protein aggregation and precipitation. There are three types of cataract, depending on the location of the opacity—nuclear, cortical, and posterior subcapsular—each of which could have a different cause. We conducted two types of studies for cataract; one by survey and one by in-person eye exams in a subset of participants. For the survey based studies, we defined cataract as the report of cataract surgery among those over 45 years of age, and in the in-person exam study, cataract was defined as the gradual opacification of the lens, confirmed by an eye examination in Boston, for subjects over 50 years of age.
Age, cumulative ultraviolet light exposure, and smoking are now considered established risk factors for cataracts: 20% are thought to be due to smoking, which induces oxidative stress and is associated with lower levels of antioxidants in the blood. In the NHS study, we observed that smoking 65 or more packs-years was associated with a 1.5 to 1.8 times greater risk of cataract extraction and that there was a dose/ response effect in smokers. Furthermore, we observed an inverse relationship with stopping smoking: former smokers who had stopped smoking for 25 years or more had a 20% lower risk of cataract extraction, especially for those who had smoked two or more packs per day, although the risk did not fall to the same level as that of never smokers. In prevention terms, this provides support for the importance of never smoking or stopping smoking.
- Role of antioxidants and fruit and vegetables
Antioxidants, such as vitamins C and E, are natural defences against oxidative stress. Vitamin C is present in the lens at more than 50 times the concentration found in plasma, and in the NHS study, vitamin C and E supplements used for 10 years or more, especially among non- smokers and women under 60 years old, was associated with a 20 to 30% lower risk of lens surgery.
We evaluated the influence of carotenoids on the development of cataracts. High intake of vitamin A, lutein or zeaxanthin (in which spinach and kale are particularly rich) were associated with a 20 to 30% lower risk of cataract extraction. However, trials using supplements of various vitamins have not demonstrated convincing protective effects. Nonetheless, the US National Eye Institute recommends eating “green leafy vegetables, fruit, and other foods with antioxidants” as part of a lifestyle that may help to lower cataract risk. We were also able to confirm that type 2 diabetes and obesity were risk factors.
Age-related Macula Degeneration (AMD):
AMD is a deterioration of part of the retina (macula), which can result in loss of central vision whilst the peripheral vision is maintained. It is of multifactorial origin and affects people aged over 50, with 15% of 85-year-olds being affected in the USA. It has a number of stages, starting with the accumulation of drusen (whitish deposits that can be seen at the back of the eye) in the macula (age-related maculopathy, or ARM, ‘dry’ type) before developing into the late degenerative type (atrophic or ‘wet’ AMD) which results in loss of central vision.
Age, family history, and smoking are all risk factors for AMD. In addition to genetic factors, our study confirmed an association with smoking.
As for dietary factors, we observed that higher consumption of F&V containing significant amounts of lutein or zeaxanthin was associated with 25 to 35% lower AMD risk. We also found that the consumption of carbohydrates with a high glycaemic index was strongly associated with higher risk of AMD.
The most common type This eye disease primarily affects people aged 40 or over, owing to an increase in intraocular pressure causing damage to the optic nerve (which sends visual information to the brain) and thereby diminishing the field of vision. Intraocular pressure is the currently the only known modifiable factor. The NHS study supported the role of type 2 diabetes in increasing intraocular pressure (82% higher risk). The relationship between antioxidants and glaucoma is not well known, and the NHS study did not reveal a relationship with smoking or the consumption of fats or antioxidants. However, we were able to demonstrate an association between consuming large amounts of nitrates (a source of nitric oxide, NO, itself a powerful vasodilator), which abound in leafy green vegetables, and around a 20% lower risk of primary open-angle glaucoma.
In conclusion, as well as regular eye exams, our study has made a major contribution to our understanding of what approaches may be important to prevent age-related eye disease for age-related eye diseases, namely:
- refraining from cigarette smoking or stop smoking
- keeping a healthy diet a weight and lifestyle to avoid diabetes
- eating a diet containing plenty of fruit and vegetables (particularly those high in carotenoids and green leafy vegetables).
Based on: Jae H. Kang et al. Contribution of the Nurses’ Health Study to the Epidemiology of Cataract, Age-Related Macular Degeneration, and Glaucoma . Am J Public Health. 2016 September; 106(9): 1684–1689.