Summary of main findings

Results from a single large study reporting mortality rates24 showed that there was no difference in cardiovascular or respiratory tract disease mortality rates in same-sex married compared with opposite-sex married women, but higher mortality rates in same-sex cohabiting women compared with opposite-sex cohabiting women.

Meta-analyses of adjusted ORs of disease prevalence showed no differences in CVD, hypertension or diabetes mellitus prevalence, but a higher prevalence of asthma in SMW compared with heterosexual women.

Discussion of main findings

A key finding was the higher prevalence, from the adjusted OR meta-analysis, of asthma in lesbians and bisexual women. Asthma is caused by a mixture of genetic and environmental factors. Higher rates are associated with anxiety, but it is not known if asthma causes psychological problems or if psychological problems lead to asthma.33 Nevertheless, studies have shown higher rates of mental health problems including anxiety in SMW.34 35 Asthma is also more common among those who are economically disadvantaged, and a consistent finding in studies included in the systematic review was that SMW had below-average incomes.15–17 21 29 Asthma is also more common among current or former smokers. Several included studies showed higher rates of smoking or tobacco use among SMW.15 16 19–21 23 25 26 28 However, only one of the studies reporting asthma prevalence clearly controlled for smoking behaviour.15

The finding of lower hypertension prevalence and no difference in the adjusted OR meta-analysis in lesbians and bisexual women was unexpected. Higher rates of hypertension are associated with lack of exercise and obesity. Several of the included studies demonstrated higher rates of obesity,15–18 20–22 25 26 and a recent systematic review on obesity in SMW36 also found consistently higher rates of obesity among SMW compared with heterosexual women. However, the rates of physical exercise in SMW are less clear. Two of the included studies showed higher rates of physical activity or exercise in lesbians and bisexual women compared with heterosexual women,16 28 while four showed no differences.20 22 25 26 Hypertension is also associated with mental health difficulties, particularly depression,37 and there are higher rates of depression in SMW.34 35

No difference in rates of diabetes mellitus was found in the meta-analysis of adjusted ORs, but in the meta-analysis of unadjusted prevalence higher rates were found in bisexual women but not lesbians. It is unclear as to why this would occur. Risk factors for type 2 diabetes mellitus include hypertension, overweight/obesity, physical inactivity and unhealthy diet. Evidence on the first three are discussed above; however, there is much less information available about diet. Dilley et al 19 reported that the proportion eating insufficient fruits and vegetables was higher in bisexual women than lesbians and heterosexual women, but Garland-Forshee et al 25 showed no differences between lesbians, bisexual and heterosexual women in the proportion who met the US Centers for Disease Control and Prevention (CDC) recommendations on fruit and vegetable intake.

Three of the included studies calculated that lesbians and bisexual women were at higher risk of CVD.18 21 31 Farmer et al 21 and Clark et al 31 calculated the risk scores using the Framingham General CVD Risk Score and both calculated that SMW had higher CVD risk scores. Farmer et al 21 calculated that SMW was 13.9% (95% CI 8.55% to 19.3%) older in vascular terms than their chronological age, and that this was 5.7% (95% CI 1.5% to 9.8%) greater than heterosexual women. Clark et al 31 found that an average of 30-year CVD risk was raised in all sexual minority groups of women, significantly so in mostly heterosexual and mostly homosexual women. Conron et al 18 also calculated CVD risk using the presence of obesity and smoking plus one other risk factor, including lack of moderate physical activity, lifetime diabetes mellitus, hypertension and high cholesterol. They estimated that lesbians and bisexual women were at higher risk of CVD than heterosexual women.

It is known that there are higher rates of several CVD risk factors in SMW, including overweight/obesity, diabetes mellitus, tobacco use (all discussed above), high cholesterol and harmful use of alcohol (discussed below). Hence the finding of no difference in CVD rates was surprising. Also, since the systematic review found higher rates of asthma, if this was due to higher rates of smoking, it would be expected that there would be correspondingly higher rates of CVD.

Several of the included studies reported higher rates of harmful alcohol use in lesbians and bisexual women compared with heterosexuals.16 19–21 23 25 26 Several also reported cholesterol levels—one found lower cholesterol levels in lesbians and bisexual women,20 but most found no significant differences.19 23 25 Matthews and Lee28 found that twice as many lesbians and bisexual women than heterosexual women were not having their cholesterol checked (32.5% vs 13.8%), but the implications of this are unclear.

Strengths and weaknesses of the study

The strengths of the current systematic review include extensive searches from a number of different sources; a minor weakness is that the searches were conducted to December 2016 and more studies may have been published since then. We used a wide definition of SMW to include identity, behaviour and partnership. It is acknowledged that these are different concepts and women can identify as lesbian or bisexual without being sexually active or being in a partnership. Also some women identify as lesbian while having sex with men, and some women identify as heterosexual while having sex with women. Most of the studies also used self-report for the physical conditions, and this may result in responder bias, but it is unclear why responder bias might be stronger in SMW than heterosexual respondents. Also, a major limitation is that almost all of the included studies were conducted in the USA, so results may not be generalisable to other countries. Also, it is known that SMW have less insurance coverage and poorer access to healthcare in the USA.38 The precise questions on health used in the BRFSS questionnaires asked whether the respondent had been ‘told by a health care professional’ that they had had the named condition. If SMW have less access to healthcare, it could be assumed that fewer would have been told they had one of the conditions investigated here. So it is possible that all of the rates may have been underestimated, and the increased rates of asthma may be even higher than found here. In the reported results, the prevalence of physical conditions was weighted to better reflect the underlying population in some of the included studies but not in others. Where the sexual minority samples were younger than the heterosexual population with which they were compared, it might be expected that the lack of weighting by age would result in underestimation of the difference in prevalence of physical health conditions, particularly CVD, hypertension and diabetes mellitus, where prevalence rises by age. There were insufficient studies to be able to conduct meaningful subgroup analyses by whether or not the study had controlled for age. Furthermore, two of the studies16 23 were unclear as to whether they weighted the reported prevalence or whether the reported weighting factors referred to the adjusted ORs that they also report. Some of the studies weighted by factors such as education and income, which may also impact on the estimated prevalence of physical conditions. Some important factors were often not controlled for; for example, for asthma, it would be usual to include smoking rates, which differ between SMW and heterosexual female populations.

In the meta-analyses, considerable efforts were made to avoid double-counting of participants from different studies when entering data, and hence some studies were excluded for one or more reported outcomes.19 23 31 32 Random-effects models were used because of clinical heterogeneity of the study samples. The heterogeneity between studies in the weightings that were used for the prevalence estimates in the unadjusted meta-analyses may have introduced some bias from this loss of information about differences between the two groups. Hence there may be some inconsistencies between the AORs reported in the results tables and the ORs used in the meta-analysis. The meta-analyses of AORs mitigate some of these effects, and all AORs used were adjusted for age. However, in both types of meta-analyses, there was heterogeneity in outcome measures (eg, one study measured hypertension, six using self-report hypertension and one study using hypertensive medication use), although we do not expect that this impacted on the observed differences between groups, our main outcome of interest.

Strengths and weaknesses in relation to previous research

The previous systematic reviews12 13 found fewer studies and did not conduct meta-analyses so did not quantify the physical health disparities they had found. For CVD prevalence Eliason12 included 7 studies, of which 4 were published before 2010, and for hypertension it included 12 studies of which 4 were published before 2010. For asthma it included 13 studies, 4 of which were published before 2010. Some relevant results from included studies were not described, and the study by Garland-Forshee et al 25 was omitted. Eliason12 concluded that asthma was more common in SMW, but no differences were consistently found in the other chronic physical conditions she investigated, including diabetes, hypertension and CVD. Simoni et al 13 had a very brief summary of results. For CVD it found one study, for hypertension one study and for asthma four studies. All of these were included in the systematic review by Eliason.12 Simoni et al 13 found evidence of disparities in the one included study reporting CVD22 and in asthma, but that evidence was lacking in diabetes and hypertension. There is also little information on the prevalence of these conditions in men according to sexual orientation and no relevant systematic reviews.10

Implications for clinicians and policy-makers

If there are higher rates of asthma in lesbians and bisexual women, this might have implications for health service delivery, particularly in primary care. Urwin and Whittaker39 published an evaluation of the English General Practice Patient Survey (n=2 807 320 in total, 1 556 909 women) looking at inequalities of general practice (GP) use by sexual orientation for various conditions. They found that lesbians but not bisexual women were less likely to visit the GP than heterosexual women in the previous 3 months for asthma or long-term chest problem (adjusted OR=0.84 (95% CI 0.71 to 0.98) and OR=0.85 (95% CI 0.69 to 1.04)). So it is likely that SMW, particularly in the UK and possibly elsewhere, are not accessing services despite ill-health. A recent systematic review found that sexual minority populations generally have difficulties with access to health services for a variety of reasons, including communication difficulties, internalised homophobia, prejudicial conduct adopted by health professionals, breach of confidentiality during consultations and institutional homophobia.40 Combined with the evidence shown in this systematic review, this suggests potentially considerable latent demand for primary care services among SMW and that there may be particular issues for lesbians accessing primary healthcare services for asthma. This evidence contributes to a bigger picture about inequality for SMW in a wide range of aspects.5 8

This systematic review highlights the need for better routine data collection on SMW, as much of the current research has small sample sizes and is based on one country with different healthcare access and social norms around sexual identity to other countries. The introduction of a UK National Health Service (NHS) information standard on sexual orientation in April 201741 will start to introduce routine data capture across hospital episode statistics and disease registries, alongside training across the NHS to support staff having positive conversations about sexual orientation, which will build over time a much clearer picture of the health inequalities in this group and potentially help to reduce them.

Implications for research

This rigorously conducted systematic review has reported some important new findings on health inequalities in SMW that are hard to explain. Further research would be useful on these health inequalities, including their causes. This would be supported by routine collection of sexual identity measures in population-level epidemiological studies, and the results published. Robust multilevel modelling (including sexual identity) should be conducted with large databases and cohort studies. For asthma, results from large cohort studies, controlled for risk factors such as smoking and overweight/obesity, would be useful to further examine these findings. Regarding hypertension and CVD, the findings are also unexpected, so investigation into potential causes would be very useful, such as possible differences in hormone levels, or other environmental, social, physiological, psychological or genetic factors that might be contributing to these results.