Methodological quality of the review: Medium confidence
Author: Zhou Y, Zhang Y, Shi K, Wang C.
Region: Japan, Singapore, Bangladesh, Australia, China, United Kingdom (UK), South Africa, Iran, India, Croatia, Samoa, Sweden, Korea and United States of America (USA).
Sector: Diabetic retinopathy
Sub-sector: Body max index
Equity focus: None specified
Review type: Other review
Quantitative synthesis method: Meta-analysis
Qualitative synthesis method: Not applicable
Diabetic retinopathy (DR) is the most common visual complication of diabetes, is a frequent cause of acquired blindness. Body mass index (BMI), the most commonly used index of body mass, is calculated by dividing the weight in kilograms by the square of the height in meters. It has been shown that being overweight and obesity are two risk factors for diabetes mellitus. Thus, overweight and obese people are more vulnerable to DR. However, the results from previous studies were equivocal, with some studies observing a decreased incidence of DR in higher BMI individuals, while other studies detected a null association between high BMI and the incidence of DR.
To evaluate the relationship between BMI and the risk of DR.
In total, authors included 27 studies in the meta-analysis: seven were conducted in Japan, five from Singapore, three from China, two from Korea and one each in UK, South Africa, Iran, India, Croatia, Samoa, Sweden and USA.
Authors report that when BMI was analyzed as a categorical variable, six studies compared the risk of DR between ‘overweight or obese BMI’ and ‘normal BMI.’ Based on the meta-analysis conducted by the authors, neither being overweight (odds ratio (OR)=0.89, 95% confidence interval (CI) 0.75–1.07; P=.21; I2=65%) nor obesity (OR=0.97, 95% CI 0.73–1.30; P=.86; I2=72%) were associated with an increased risk of DR when compared with normal weight.
When authors analyzed BMI as a continuous variable, 23 studies reported an association between BMI and the risk of DR: a higher BMI was not associated with an increased risk of DR (OR=0.99, 95% CI 0.97–1.01; P=.25; I2=79%).
Sensitivity analysis conducted by the authors, they noticed that the I2 values were still over 50%, which indicates moderate-high heterogeneity. However, authors report that the sensitivity analysis demonstrated that the stability of the overall treatment effects was good. Forest plot analysis showed absence of publication bias when the BMI was analyzed continuous variable, but when analyzed as a categorical variable, the analyses were likely to be underpowered authors note.
Authors searched the following databases Cochrane Library, PubMed and Embase up to December 2016 for articles evaluating the effects of BMI on DR in patients with Type 1 and Type 2 diabetes. The key words related to body mass (“body mass index” or “body mass” or “BMI” or “body weight” or “obesity” or “overweight’” or “adiposity”) and “diabetic retinopathy” were used to search for the relevant articles published in English. Moreover, reference lists and conference abstracts were also examined to retrieve potential relevant studies.
Studies were included if they fulfilled the following criteria: (a) observational study type; (b) participants with type 1 or 2 diabetes or both; (c) outcomes included DR; and (d) BMI was analyzed as a categorical or continuous variable. The categorical levels of BMI were assessed according to the WHO-recommended BMI classifications. BMI levels within 2kg/m2 of standard categories were also considered to be acceptable in case losing studies were available. The exclusion criteria were as follows: (a) the adjusted risk estimates were unavailable; (b) certain publication types (e.g., reviews, letters, case reports, comments, conference abstracts, and editorials); and (c) studies with duplicate or insufficient data.
Two authors independently screened studies for inclusion. For the included studies, the basic characteristics including the first author, publication year, study location, age range, design of study, sample size, diagnostic basis, grading standard, and the outcome were extracted. The Newcastle–Ottawa scale (NOS) score was used to evaluate the quality of the observational studies.
Authors report that neither being overweight nor obesity conferred an increased risk of DR, which is consistent with previous studies, where BMI was analyzed as continuous variable.
Authors do not discuss the study results to low- and middle-income settings.
Summary of quality assessment:
Overall, there is medium confidence in the conclusions about the effects of this study. Authors did not conduct a thorough search of the literature to ensure that all relevant studies were included in the review. In addition, it is not clear if methods to extract data of included studies were rigorous avoiding biases. However, authors appropriately address limitations of the review and included studies.