The study's key finding is that 39% of the male prison population had three or more CVRF compared to only 10% of disadvantaged men of similar age in the community. It is important to highlight that all prisoner participants in this study were smokers, so they all had at least one CVRF, while the community comparison included both smokers and non-smokers. Though this comparison is imperfect, it is not considered to be inappropriate as the proportion of male prisoners who are current smokers is 75%, compared to 16% in the general community, which suggests that most male prisoners do have smoking as a CVRF [8, 20]. Further, we propose that these findings are likely to be an underestimate of the number of CVRF among prisoners as the study explicitly screened out prisoners with diagnosed psychiatric illness and/or cardiac disease. The 2009 NSW Inmate Health Survey recruited 996 male prisoners, of whom 19% reported that they had been told by a doctor that they had a heart problem . This suggests that the CVRF profile of prisoners is significantly poorer than the findings reported here.
Cardiovascular risk factors for male prisoners in this study were similar to that reported in the 2009 NSW Inmate Health Survey (IHS). Both studies identified 23% of male prisoners with insufficient physical activity, similar rates for not completing year 10 (56% vs 52% IHS), similar rates for depression (30% vs 33% IHS) and for risky alcohol consumption (69% vs 63% IHS). As this study only recruited males who smoked tobacco, the proportion for that cardiovascular risk factor was 100%, compared to 75% of males in the IHS who reported they currently smoked tobacco. As this study also excluded men with a history of cardiac disease, the proportion of participants found with high blood pressure (2%) was significantly lower than found in the IHS (15%), which suggests that this study underestimates the extent of cardiovascular risk among male prisoners .
This study found that significantly more Aboriginal prisoners had three or more CVRF than non-Aboriginal prisoners. The burden of disease among Aboriginal Australians has been estimated to be 2.5 times greater than among non-Aboriginal Australians . One reason for this is that Aboriginal people have a greater likelihood to have multiple risk factors such as tobacco smoking, physical inactivity, poor nutrition and risky alcohol consumption. These factors have been found to be higher among Aboriginal prisoners in NSW compared to non-Aboriginal prisoners . Prisons are an important setting in which health interventions can be developed for hard to reach populations, such as Aboriginal people, who may not access health care in the community . Improving the health of Aboriginal people in prison can contribute towards attempts by the Australian government to reduce gaps in health and well-being between Aboriginal and non-Aboriginal Australians .
This study also found that prisoners are at even higher risk for developing cardiovascular disease compared to males of similar age in the general population from the most disadvantaged quintile in our community. This suggests that the highly disadvantaged backgrounds of prisoners are more severe than is found among the most disadvantaged in the community. There is a direct relationship between the number of CVRF and the development of atherosclerosis, which correlates with reduced life expectancy and greater health care costs . The cumulative effects of multiple risk factors may be additive or synergistic. A focus on moderate reductions in several risk factors is likely to have more benefit than focussing on achieving a major reduction in one factor .
Limitations of our study
As our study was not primarily designed to investigate CVRF we did not measure all possible risk factors. We did not collect objective data on diabetes or cholesterol or weight (to determine presence of overweight/obesity, a major CVD risk factor). Another limitation is that a second blood pressure reading was only taken if an abnormal first blood pressure reading was obtained. Therefore our ascertainment of CVRF is subject to the vagaries of self-report. The 2009 NSW Inmate Health Survey reported that 55% of male prisoners were found to be overweight or obese  so we would expect a similar proportion in our study to have been overweight or obese. Thus we have underestimated the prevalence of CVRF by not including overweight prisoners. Further, as this was part of a randomised control trial, our eligibility criteria excluded prisoners with current cardiac disease and those who were on current medication for a mental illness, including depression. Hence, a further limitation of our study is that prisoners with these cardiovascular risk factors were excluded from our study. Therefore, the number of risk factors among prisoners is an underestimation.
A further limitation is that the study participants were all smokers compared to only a portion of the general community sample of men. This may bias the results among prisoners to a higher risk group compared to the general community. However, as 75% of male prisoners are current smokers, this limitation is relatively consistent with the risk profile of prisoners .
The Framingham Risk Equation (FRE) is widely used to estimate cardiovascular risk. However, there are limitations with the FRE as it does not include some significant risk factors including overweight/obesity, physical inactivity, family history of CVD, socioeconomic status, psychological factors [25, 26]. Further, the FRE has been shown to overestimate CV risk in populations with low CV mortality, but greatly underestimate risk among those with high mortality, including those who are socioeconomically deprived groups , such as prisoners and Indigenous people [15, 28].
Clinical and research implications
The CVD mortality rate among the most disadvantaged males in Australia was 112% higher compared to the least disadvantaged males . Higher death rates from CVD are due to socioeconomic inequality between the most and least disadvantaged which has widened over the past decades [10, 29, 30]
Community studies have reported that CVD is 1.3 times more prevalent among Aboriginal Australians compared to non-Aborigines . In our study we found that CVRF were more prevalent among prisoners, particularly Aboriginal prisoners, when compared to the most disadvantaged males in the general community of similar age. Excess mortality from CVD is likely to be even greater among prisoners.
Re-entry into the community can be a stressful experience involving adjustment to relationships and family, seeking employment and housing. Several studies have assessed ex-prisoners' post-release death rates and have shown that they are at increased risk of death [32–34]. Mortality in Australian ex-prisoners released from custody was significantly higher compared to the NSW population . Deaths were particularly high from mental and behavioural disorders, suicide, drug overdose and homicide. Ex-prisoners have been found to be 3.5 times more likely to die just after release compared to the general public from CVD, drug overdose, homicide and suicide . The first 6-12 months following release from prison is a high-risk time. Released Aboriginal men in Western Australia (WA) have an almost 10 times greater risk of death than the general WA population and an almost three times greater risk of death compared with their Aboriginal peers residing in the community . Suicide, high risk drug and alcohol consumption and motor vehicles accidents are the main causes of death.