Background
Adults with intellectual disability have poorer health than their non-disabled peers. However, little is known about the health of the ‘hidden majority’ of adults with primarily mild intellectual disability who do not use intellectual disability services. The aims of the present study were: to estimate the physical health status of a population-based sample of British adults with and without mild intellectual disability while controlling for any potentially confounding effects resulting from between-group differences in gender, age, socio-economic disadvantage and neighborhood social capital.

Methods
Secondary analysis of data from Understanding Society, a new longitudinal study focusing on the life experiences of UK citizens. We identified 299 participants aged 16–49 (1.2 % of the unweighted age-restricted sample) as having intellectual disability, and 22,927 as not having intellectual disability. Multivariate logistic regression was used to investigate between group differences adjusting for potential confounding personal characteristics (e.g., gender).

Results
Unadjusted comparisons indicated that British adults with intellectual disability have markedly poorer health than their non-disabled peers on the majority of indicators investigated including self-rated health, multiple morbidity, arthritis, cancer, diabetes, obesity, measured grip strength, measured lung function and polypharmacy. Adjusting for between-group differences in age and gender had a marginal impact on these estimates. Further adjusting for between-group differences in socio-economic disadvantage and neighborhood quality had a more marked impact on estimates with the number of statistically significant differences reducing from 13 to 8 and statistically significant attenuation of odds on three indicators (self-rated health, SF-12 physical component and multiple morbidity).

Conclusions
The ‘hidden majority’ of adults with primarily mild intellectual disability who do not use intellectual disability services have significantly poorer health than their non-disabled peers. This may, in part, reflect their increased risk of exposure to well established ‘social determinants’ of poorer health.