This study examined the association between overweight and obesity and several chronic diseases using nationally representative survey data from the Republic of Ireland. It can be concluded from the results that firstly, there is a high prevalence of overweight and obesity (BMI ≥25 kg/m2) in Irish adults, particularly among men (59% vs. 42%). Secondly, overweight and obesity is a major contributor to a range of chronic diseases and carries a significant disease burden in the Republic of Ireland, particularly among women. A small reduction in BMI at a population level would potentially lead to substantial gains in terms of reduced prevalence of chronic disease.
In this study, hypertension and raised cholesterol in men, and osteoarthritis, diabetes, CVD, and hypertension in women were significantly more prevalent in the overweight category. In the obese category, statistically significant associations were observed for osteoarthritis, hypertension, and raised cholesterol in men, and for lower back pain, osteoarthritis, diabetes, hypertension, raised cholesterol, and chronic bronchitis in women. As the RRs generally increased with increasing BMI, this implies a direct association between increasing BMI and increasing prevalence of related chronic disease. Previous cross-sectional studies assessing the overall burden of chronic disease show similar findings for a number of conditions [22–25]. Although most of the following did not reach statistical significance, graded associations were generally seen for lower back pain, asthma, chronic bronchitis, anxiety, and depression. Lack of a statistically significant association between overweight and obesity and diabetes in men may reflect weight loss following diagnosis. Similar considerations may apply in relation to the apparently negative association with CVD, along with the small number of study participants with this condition.
The PAFs indicated that a large proportion of a number of chronic diseases are attributed to overweight and obesity, suggesting that obesity is an important cause of morbidity, with a significant impact on health care costs. A recent report  highlighted the substantial direct and indirect costs associated with overweight and obesity in the Republic of Ireland, with 2009 estimates at €1.13 billion. Direct healthcare costs accounted for 35% and indirect costs accounted for 65% of the total costs. As there is a high level of indirect costs associated with chronic diseases such as lower back pain and osteoarthritis, and direct costs associated with chronic diseases such as CVD, reductions in these diseases are likely to reduce costs incurred with obesity.
As the importance of population-based strategies has long been recognised , a one unit population reduction in BMI was assessed to estimate the effect such a strategy may have on the prevalence of chronic disease. As the prevalence of overweight and obesity is high, and a considerable proportion of the population are at risk of a number of chronic diseases, the population approach targeting the entire population is likely to be more effective and potentially less costly than targeting high-risk individuals, in reducing the prevalence of overweight and obesity in the population and thus the burden of disease attributable to overweight and obesity. In men, there was a 4% reduction in chronic disease (reduced from 595 to 568 cases per 1,000 men) associated with a one unit population-wide decrease in BMI. In women, there was a 4% reduction in chronic disease (reduced from 676 to 648 cases per 1,000 women) associated with a one unit population-wide decrease in BMI.
A number of limitations need to be taken into account. The cross-sectional study design cannot provide evidence of a temporal relationship or causality. Findings on associations with individual conditions such as CVD and diabetes must be interpreted cautiously given the potential for reverse causation. Compared to longitudinal studies, risk estimates are likely to be reduced . Thus, estimates in this study may under-represent the actual risk of developing a disease. Although the SLÁN 2007 survey had a reasonable response rate (62%), there is the potential for selection bias, for example, less healthy individuals may be more likely to refuse to participate , which may have resulted in an underestimated prevalence of overweight and obesity and of chronic disease.
The use of self-reported height and weight may have resulted in reporting bias. Evidence suggests that a considerable number of Irish adults underestimate their body weight , therefore, prevalence estimates in this study are likely to be underestimated. There may also have been sex differences in the reporting of height and weight. While there is a general trend of under-reporting for weight and over-reporting for height, the degree of this trend varies for men and women . One study found that while females tended to underestimate their weight, males were inclined to slightly overestimate their weight . A recent study examining trends in misclassification patterns of measured and self-reported BMI in the SLÁN surveys did not find a trend related to gender bias , suggesting that the extent of sex differences in the reporting of BMI in this study is not likely to be considerable. The use of self-reported chronic conditions may also have resulted in reporting bias, resulting in an underestimation of the true prevalence of chronic disease and thus an underestimation of the strength of their association with overweight and obesity. Similar to the reporting of height and weight, there may also have been sex differences in the reporting of chronic conditions; women have been shown to more accurately self-report diagnoses compared to men . Nonetheless, it has been found that there is considerable agreement between medical record diagnosis of disease and patients' self-reports for a number of chronic diseases [32, 33]. The known association between excess weight and chronic conditions increases the likelihood of diagnosis in heavier people and may represent an additional source of bias. For example, it is likely that diagnosis of high blood pressure and raised cholesterol is greater in overweight and obese individuals due to more frequent measurements in these individuals compared to those that are of normal weight.
The use of BMI as a measure of excess body weight may lead to some misclassification as it does not distinguish between fat and muscle mass . The use of other methods including waist circumference, waist-to-hip ratio, and skin-fold thickness may provide more accurate estimates, however, these were not available in the SLÁN dataset.
It must be noted that for many specific conditions, such as CVD, the survey produced a relatively small number of cases, thus possible associations may have been missed. The findings on individual conditions are constrained by limited statistical power and random sampling error. Furthermore, the burden of conditions with a short natural history for which there are known associations with overweight and obesity, such as a number of major cancers, cannot be captured in this type of study. Therefore, the overall burden of disease associated with overweight and obesity is underestimated.
Despite these limitations, the current analyses highlight and quantify the burden of prevalent chronic disease associated with overweight and obesity and the findings are similar to other nationally representative cross-sectional studies [34–36]. The results of this study are applicable at a population level as a result of applying sampling weights. A number of important potential confounding factors were adjusted for in the analyses and a number of models were employed to assess potential differences in association depending on the adjustment of certain factors. PAFs, which are useful for informing public health interventions, were calculated, although some authors argue that in order to inform public health interventions, such interventions should be precisely defined in the estimations . A one unit reduction in BMI was assessed to strengthen the evidence of the burden of chronic disease associated with overweight and obesity and the potential decrease in this burden if overweight and obesity was reduced or eliminated in the population.