Introduction
Age-related macular degeneration (AMD) is a major cause of vision loss with an estimated 196 million people affected globally in 2020.1 Early or intermediate stages of the disease are usually asymptomatic, so the disease is often detected at a late stage.2 AMD risk factors can be categorised as non-modifiable, for example, genetics, or modifiable, for example, smoking, diet, adiposity and physical activity.3–7 Vision loss which accompanies late-stage AMD is generally irreversible; hence, a clear understanding of modifiable risks to enable primary or secondary prevention via behaviour modification would be a worthwhile goal. While obesity has been shown to induce AMD in animal models,8 evidence from human studies has been less straightforward. There are several means of identifying excessive adiposity. Anthropometric measures (AnthM) of anatomical proportions highlight excessive adiposity across the entire body [eg, body mass index (BMI)], in particular regions [eg, waist circumference (WC), waist/height ratio (WHR)] or measure adiposity deposited just under the skin (ie, skinfold thicknesses). Skinfold thicknesses across four regions (biceps, triceps, subscapular and iliac) are used to calculate body density and total fat.9 AnthM have been used to capture adiposity in large population-based studies.10 However, a recent systematic review by Ng Yin Ling et al found conflicting results in 16 epidemiological studies whereby AnthM (namely, BMI, WC and WHR) and incident AMD were positively associated in several prospective cohort studies, while others presented no or even inverse associations.11 Furthermore, AnthM including BMI have been criticised for their inability to distinguish fat versus lean mass in total mass hence leading to inaccurate measures of adiposity in individuals with high lean mass (eg, athletes), low lean mass (eg, elderly and those affected by sarcopenia, ie, loss of lean muscle mass) and for those with comorbidities such as oedema.12 13
Multislice MRI and CT are considered the reference standard for measuring total and regional adiposity in living participants14 15; however, they are costly and technically laborious with CT requiring radiation exposure thus preventing their use in large-scale studies.16 Another imaging technique uses dual-energy X-ray absorptiometry measure (DEXAM) which uses much lower (and thus safer) levels of radiation, is low-cost and easier to operate. Several cross-sectional studies have found strong agreement between DEXAM and MRI; DEXAM is therefore a valid alternative to measure adiposity.17 Furthermore, DEXAM differentiates between tissue (lean vs fat) types and captures the mass of each tissue type, unlike AnthM which are proxy measures of adiposity and may be inaccurate on some occassions.13 Excessive dual-energy X-ray absorptiometry (DEXA)-measured android adiposity (that is, fat around the waist) is deemed particularly harmful as it has been positively associated with diabetes and cardiovascular disease: greater android adiposity may impair insulin resistance, glucose tolerance and unfavourably affect levels of high-density lipoproteins (HDLs), low-density lipoproteins (LDLs) and triglycerides.18 An aetiological effect of android fat on AMD may exist: one case–control study reported that patients with late AMD compared with matched controls had significantly higher BMI and DEXAM of central-abdominal-to-total body fat ratio.19 However, the number of cases was small (n=54) and all had late-stage AMD; thus, the association may be different in a larger sample including those with asymptomatic, early/intermediate-stage AMD.
To our knowledge, there are limited studies that have compared DEXAM, particularly android fat, among those with and without AMD in a population setting or explored whether there are differences in the association between AnthM and DEXAM with AMD. The National Health and Nutrition Examination Survey (NHANES) collected and graded retinal images for the presence of AMD in a representative sample of older adults in the USA (2005–2006) and undertook DEXAM in a subsample thus enabling exploration of the association between DEXAM and AMD.