Identifying modifiable risk factors for cataract is a public health priority. The number of Americans affected by cataracts is expected to rise dramatically in the next 20 years, and cataract surgery is one of the costliest outpatient surgeries in the U.S. Body Mass Index (BMI) appears to be one modifiable - and independent - risk factor. BMI, calculated by dividing weight in kilograms by height in meters squared, generally indicates whether weight falls within a healthy range. A BMI below 18.5 indicates underweight, 18.5-24.9 is normal, 25-30 is overweight, and > 30 signifies obesity.
While a number of studies report that variations in BMI influence cataract risk, a lack of consensus as to the direction and nature of the risk has hampered application of the findings for risk intervention. A study using data from the large U.S. Nurses' Healthy and Health Professionals Follow-up Studies, for example, found that being overweight or obese raises the risk for all cataract types (1), while the Salisbury Eye Evaluation Project (SEE Study) reported that being underweight or overweight affects the risk for certain types of cataract (2).
Graphs courtesy of Archives of Ophthalmology
Many factors likely contribute to the differences in findings including the use of different criteria for defining cataract or relying on self-reported anthropometric data rather than measurements conducted by trained personnel. The results of a newly published and well-conducted study support the assertion that BMI is an independent risk factor for cataract and - similar to the findings from the SEE Study - suggest that there is some trade-off between nuclear and cortical opacities (3).
The Shihpai Eye Study
The new study included 1360 men and women 65 years and older, with 806 of the participants diagnosed as having age-related cataract of any type. LOC System III was used to grade cataracts, and weight and height were measured by intensively trained interviewers. Potentially confounding factors such as smoking, age, sex, socio-economic status, history of hypertension and presence of diabetes were evaluated. A U-shaped relationship between BMI and nuclear opacity was observed, indicating that risk was lower in those with normal weight, but higher in the overweight and obese (fig. 1). In contrast, a reverse U shape association was seen for cortical opacity (fig. 2). Women were found to have a higher BMI and greater prevalence of cataract.
While the exact mechanism by which BMI influences cataractogenesis is not clear, some theorize that glucose intolerance and insulin resistance - even in the absence of clinical diabetes - play a role for those who are overweight. The increased risk in thinner subjects has been interpreted by some as an indicator of under-nutrition or poorer quality of diet.
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