The present study, using a representative sample of the whole national population, shows that the prevalence of diabetes mellitus in Spain is 13.8%, with 6.8% having UKDM discovered during the study fieldwork.
A total of 13 studies from nine European countries (three from Spain) were included in the Decode Study, involving 7,680 men and 9,251 women aged 30–89 years. The conclusion reached was that in the majority of European countries the prevalence of diabetes and IGR is moderate or low (<10% in people younger than 60 and 10–20% in people aged 60–80 years) .
Other studies on diabetes mellitus prevalence have been undertaken in other European countries with data obtained from case records or from structured interviews, but with no OGTT. In other cases, local studies are extrapolated to the total national territory, and this may or may not represent the prevalence over the whole country. There may also exist important differences between countries in the prevalence of obesity, physical activity or eating patterns, which might partially explain the variation in diabetes prevalence .
In a recent Portuguese study using the same methodology as ours, the total prevalence of diabetes mellitus was 11.7%, very similar to that for the Spanish population . In both studies diabetes mellitus prevalence was significantly higher in men than women. In our study, the prevalence of KDM was somewhat greater and UKDM somewhat less than that published in earlier studies in Spain  and Portugal . The prevalence of IFG and IGT was also lower than that found in earlier Spanish studies  or recently in Portugal , where the prevalence of IFG and IGT was 23%. Different health strategies, different methodologies or a different prevalence of obesity or other metabolic risk factors might explain these differences in contemporary studies in which the overall diabetes mellitus prevalence was very similar.
In our study, diabetes mellitus and IGR were significantly associated with a greater frequency of obesity, high blood pressure, hypertriacylglycerolaemia and low HDL-cholesterol as expected . However, people with KDM probably receive statin therapy more frequently, which may explain the lower level of LDL-cholesterol in people with KDM.
Finally, people with a low educational level had a 28% increased risk of having diabetes mellitus after adjustment for other risk factors closely associated with diabetes. A lower socioeconomic level has been associated with a poorer state of health, higher rates of mortality and cardiovascular diseases, and an increase in diabetes prevalence .
The main strengths of this study are first, the sampling was representative of the whole national territory, and second, the diagnosis of diabetes was made by OGTT in the majority of cases. The study, however, has a few limitations: the participation was relatively low (56%) and there was a greater participation of women and older people, meaning all the prevalence and analysis data were corrected for age and sex. In addition, not all the participants underwent the OGTT, but the prevalence of diabetes mellitus and IGR was calculated taking this into account, as indicated in the Methods section. Although, for clinical purposes, an OGTT needs to be reassessed to establish diagnostic status, it is widely accepted that one OGTT is enough in the setting of epidemiological studies. Another limitation is that the information was self-reported, although this, too, is common practice in large epidemiological surveys.
In summary, this study contributes information for the first time on the prevalence of diabetes mellitus and IGR in Spain. The results will provide our public health authorities with data that should encourage the urgent implementation of clinical and preventive intervention programmes to tackle the increasing health and economic burden of diabetes in Spain.