This study examined prevalence of disability and the disabling impact of risk factors and disease in a sample of the German population of age 65 and above. Minimal disability was highly prevalent in all age groups. Aging was associated with increasing prevalence of disability, with a steep increase in participants over 80. Female sex, lower per capita income, physical inactivity and malnutrition were factors significantly associated with disability when adjusting for age and disease. Not surprisingly, when analyzing the specific contribution of single diseases on disability prevalence on the population level we found that disability attributed to causes other than the diseases included in the analysis increased with age. Stroke and neurological diseases were strongly associated with disability, but joint diseases and eye disease contributed most to the burden of disability in this population.
Direct comparisons of prevalence of disability between studies are difficult because of the multitude of measures applied, and because of diverse populations and their differing living conditions. A study based on the LEILA 75+ survey in Leipzig observed a disability prevalence of 63.6% for community-dwelling persons aged 75 or above, but did not analyse prevalence of disability for other age groups . The observed increase of disability prevalence with age, stratified by sex, in our study is consistent to findings of other studies [19, 34]. A report of the German health reporting system based on the socio-economic panel also observed a sudden increased need for personal assistance at home for persons aged 80 or older .
However, sample sizes, specifically in age groups over 80, are usually small, thus making evidence in that age group inconclusive. Our study, with a sample size of 780 participants over 80, showed that disability increased in a curvilinear way with a steeper onset in the 8th life decade. A study in Netherlands included a similar number of persons in this age group, but observed a lower prevalence of disability for both sexes: 20% for men and 37% for women. A part of this difference can be explained by the varying conceptualisation of disability .
Also, in line with the literature [34–36], we found that prevalence of disability was higher in women across all age strata, levels of severity, and domains of disability. Sex differences in disability are often explained by co-morbidity and greater female longevity. It is argued that social and health related issues largely contribute to the higher prevalence of disability in women . However, the association of sex and disability remained relevant in our study even when controlling for major health conditions and even when stratifying for age. Additionally, we found that differences were more pronounced in locomotor functions such as reach, grip, and hygiene, indicating that disability might be associated with general fitness and strength as a consequence of an inactive lifestyle (see Additional file 1). Since more women than men reported living alone in our study (41.5% vs. 14.9%) lack of assistance may have contributed to this finding. Indeed, more women than men reported needing assistance.
We could confirm the association of low socioeconomic status and disability at old age in our study. This association is well known from the literature . Other studies have also shown that lower socioeconomic status increases the risk for physiological impairment, i.e. factors predisposing disability, such as muscle strength, range of joint motion and visual acuity .
The impact of physical activity on health and mortality is conclusive . As was shown before the association of physical activity and disability persisted in our study, even after controlling for diseases .
The attribution method revealed joint diseases as being the strongest contributing factor to disability prevalence. A similar result was obtained in the study by Klijs et al. . We could additionally show that diseases of the eye explained about 10% of the prevalence of disability. This result confirms findings from previous study on the role of eye on functional ability in the aged [42, 43].
Increasing prevalence of disability by age, even in disease-free individuals, can be explained by increasing frailty. This is likely to be associated with decreasing muscle strength, decreased radius of activity and reduced cardiopulmonary fitness [44–47]. Joint disease and fractures contributed most to the burden of disability by being frequent diseases with a strong association to disability. This is in line with recent findings reporting musculoskeletal disease as the main contributors of disability . Additionally, we could show in our study that diseases of the eye explained about 10% of the prevalence of disability. This result confirms findings from previous study on the role of vision on functional ability [42, 43]. The results of our study might be promising because fractures as a consequence of falls are known contributors of disability  and can be prevented. Joint disease can be modified by exercise and weight control. Vision is also amenable to intervention. Nevertheless, more detailed data of the nature of eye and joint disease is needed to inform on the appropriate measures for intervention. .
We acknowledge several limitations in our study. Due to the observational, cross-sectional study design, causal associations could not be examined. Thus, disability may not only be a consequence of a disease, but also be a cause for a disease . In situations like this where variables show complex interactions, reverse causation cannot be ruled out. To give an example, the true nature of the interaction of education, BMI and disability has to be examined more closely.
Additionally, information was collected by personal or proxy interview which is, in theory, prone to information bias. However, previous studies have shown that self report of health conditions in this study setting was reliable .
We also acknowledge that the sample might not be entirely representative of the general population of aged adults due to the exclusion of individuals who either chose not to participate because of disabling conditions or for whom no proxy information was available if they did participate. The issue of non-participation in one of the baseline surveys has been studied in detail before . It was shown that non-participant include a higher fraction of persons with worse health and that severely impaired persons are less likely to participate in our study. As a result, our study may underestimate the true prevalence of disability and consequently also the true impact of disease on disability. Furthermore, the city of Augsburg and surroundings is not representative for Germany in terms of socio-economic status and deprivation .
We used years of education instead of the highest level of education, as this variable combines information on both vocational training and school education. This allowed us to differentiate between those with minimal school education and those with additional vocational training.
The attribution model itself also has several shortcomings and assumptions that need to be considered. A basic assumption of the model is that the distribution of diseases does not change over time. Moreover, disease and background morbidity are assumed to act as independently competing causes for disability that add up to the total risk. Also, it is assumed that all persons belonging to an age group have the same background disability risk.