Study question: What is the contribution of diet, physical activity and sedentary behaviour to body mass index (BMI) in women with and without polycystic ovary syndrome (PCOS)?

Summary answer: PCOS status, higher energy intake and glycaemic index and lower physical activity were independently associated with BMI.

What is known already: Obesity worsens the clinical features of PCOS and women with PCOS have an elevated prevalence of overweight and obesity. It is not known whether there is a contribution of lifestyle factors such as dietary intake, physical activity or sedentary behaviour to the elevated prevalence of obesity in PCOS.

Study design, size, duration: This study is a population-based observational study with data currently collected at 13 year follow-up. The study commenced in 1996. For this analysis, data are analysed at one time point corresponding to the Survey 5 of the cohort in 2009. At this time 8200 participants remained (58% retention of baseline participants) of which 7466 replied to the questionnaire; 409 self-reported a diagnosis of PCOS and 7057 no diagnosis of PCOS.

Participants/materials, setting, methods: Australian women born in 1973-1978 from the Australian Longitudinal Study on Women's Health.

Main results and the role of chance: Mean BMI was higher in women with PCOS compared with non-PCOS (29.3 ± 7.5 versus 25.6 ± 5.8 kg/m(2), P < 0.001). Women with PCOS reported a better dietary intake (elevated diet quality and micronutrient intake and lower saturated fat and glycaemic index intake) but increased energy intake, increased sitting time and no differences in total physical activity compared with non-PCOS. PCOS status, higher energy intake and glycaemic index and lower physical activity, as well as age, smoking, alcohol intake, occupation, education and country of birth, were independently associated with BMI.

Limitations, reasons for caution: The weaknesses of this study include the self-reported diagnosis of PCOS, and the women not reporting PCOS not having their control status clinically verified which is likely to underrepresent the PCOS population. We are also unable to determine if lifestyle behaviours contributed to the PCOS diagnosis or were altered in response to diagnosis.

Wider implications of the findings: The strengths of this study include the community-based nature of the sample which minimizes selection bias to include women with a variety of clinical presentations. These results are therefore generalizable to a broader population than the majority of research in PCOS examining this research question which are performed in clinic-based populations. This study is in agreement with the literature that PCOS is independently associated with elevated BMI. We provide new insights that diet quality is subtly improved but that sedentary behaviour is elevated in PCOS and that PCOS status, higher energy intake and glycaemic index and lower physical activity are independently associated with BMI.

Study funding/competing interest(s): L.J.M. was supported by a South Australian Cardiovascular Research Development Program (SACVRDP) Fellowship (AC11S374); a program collaboratively funded by the National Heart Foundation of Australia, the South Australian Department of Health and the South Australian Health and Medical Research Institute, S.A.M. was funded by an Australian Research Council Future Fellowship (FT100100581), S.Z. was funded by a Heart Foundation Career Development Fellowship (ID CR10S5330) and H.J.T. was funded by an NHMRC fellowship (ID 545888). None of the authors has any conflict of interest to declare.

Trial registration number: Not applicable.