HEAPHY 1 & 2 DIAGNOSTIC James HAYES

1. ABSTRACT To estimate population attributable risks for modifiable lifestyle factors and breast cancer in New Zealand. Methods Estimates of the magnitude of modifiable lifestyle risk factors for breast cancer (relative risks and odds ratios obtained from published epidemiological studies) and the prevalence of exposure in New Zealand were used to calculate the population attributable risk percent (PAR%) for each risk factor. The PAR%s calculated for modifiable lifestyle risk factors in New Zealand were used to assess the relative importance of these risk factors, and to give an indication of the potential impact of reducing the prevalence of these lifestyle risk factors on the incidence of breast cancer in New Zealand. Results Six modifiable lifestyle factors were identified for breast cancer. These were obesity, lack of physical activity, high alcohol intake, oral contraceptive use, hormone replacement therapy (HRT), and delayed first birth. The PAR%s for these risk factors ranged from 1% for delayed first birth to 10% for obesity (16% for Maori women and 17% for Pacific women). Conclusions The most important modifiable lifestyle risk factors for breast cancer in New Zealand are obesity, HRT use, hazardous alcohol consumption, and physical inactivity. Other modifiable risk factors of smaller magnitude are delayed age at first birth and oral contraceptive use. In New Zealand obesity, sedentary behavior, and delayed age at first birth have been increasing over the last decade. If these trends continue, they may contribute to an increasing incidence of breast cancer in New Zealand women. If the prevalence of obesity could be reduced, physical activity could be increased, and hazardous drinking could be reduced among New Zealand women, this may reduce the risk of breast cancer. HRT use in New Zealand has declined, as in other Western countries, since the publication of the findings of the Women’s Health Initiative trial, and this is likely to have reduced breast cancer incidence. The most important primary preventive strategies to reduce the risk of breast cancer in New Zealand are promoting lifestyle changes to reduce obesity, promoting regular physical activity (which may in turn reduce the prevalence of obesity), reducing HRT use, and avoiding high alcohol intake. A strategy to promote regular physical activity could also have other benefits, such as reduced risks of cardiovascular disease and diabetes. HEAPHY 1 & 2 DIAGNOSTIC James HAYES Fri 30th Aug 2013 Session 2 / Talk 4 11:33 – 12:00

2. Modifiable lifestyle factors and breast cancer incidence James Hayes

3. Background • Breast cancer is the most commonly registered cancer in women • Second most common cause of cancer death • In 2009 2759 women were diagnosed with, and 658 women died from, breast cancer

4. Aim • To identify modifiable lifestyle factors that could affect the future incidence of breast • It is hoped that the results of this research would provide women with information that would allow them to lower their risk of breast cancer

5. Methods • Relative risks and odds ratios were obtained from published epidemiological studies • The prevalence of exposure in NZ were used to calculate PAR% • PAR% show the relative importance of these risk factors

6. Relative risk and absolute risk • If the absolute risk of developing a disease is 4 in 100 in non-smokers. • And if the relative risk (RR) of the disease is increased by 50% in smokers. • The 50% relates to the 4 - so the absolute increase in the risk is 50% of 4, which is 2. • So, the absolute risk of smokers developing this disease is 6 in 100.

7. Lifestyle risk factors • Include tobacco, diet, alcohol, reproductive factors, lack of physical activity, HRT and oral contraceptive use

8. Results: • Six modifiable lifestyle factors were identified for breast cancer • Obesity, lack of physical activity, high alcohol intake, oral contraceptive use, HRT, and delayed first birth • PAR%s for these risk factors ranged from 1% for delayed first birth to 10% for obesity (16% for Maori women and 17% for Pacific women)

9. Age at first birth • Trend towards later motherhood since 1950s • Maori women tend to give birth at a younger age than other women • First birth at an age ≥ 35 represents PAR% of 1.3%

10. Parity • Parous women have a reduced risk of breast cancer compared with nulliparous women • This protection follows a transient increase in risk in the 10-20 years following delivery • Because the decision to have children or not is determined by many considerations, parity was not considered further as a modifiable lifestyle risk factor

11. Alcohol • 18% of adults have a potentially hazardous drinking pattern • AUDIT score of ≥ 8 (3 + drinks a day) • 24% for Maori women and 14% for Pacific women • PAR% between 3% and 7%

12. Oral contraception • 37% of women aged 25-54 use oral contraception • 81% had used oral contraception at some stage • PAR% of 2.8% • This PAR% will affect the risk of breast cancer in younger women – only 27% of breast cancers occur in women aged less than 50 years

13. HRT • HRT received substantial negative publicity following the publication of a WHI trial • Estimates of cessation ranged between 36% to 58% • In 2002 use in NZ was 11% for women aged 45-64 this represents a PAR% of 7%

14. Tobacco • There is little or no persuasive evidence that tobacco smoking is associated with breast cancer risk • The lack of convincing evidence for an association between smoking and breast cancer meant that smoking was not considered further as a risk factor for breast cancer • Tobacco does not provide protective effect

15. Obesity • Obesity is defined as a BMI of ≥ 30 • For women between 51-70 the prevalence of obesity is 35% • For Maori women this rises to 63% and for Pacific women 68% • PAR% between 9.5% and 17%

16. eleven

17. The increasing prevalence of obesity has been attributed to dietary and physical activity patterns • Reflects an environment that facilitates over-consumption of energy-dense foods and reduces opportunities for physical activity • Mean BMI has been increasing since 1997

18. Lack of physical activity • Sedentary behaviour is defined as less than 30 minutes of exercise in a week • Estimates for the prevalence of this are 10% rising to 15% for Pacific women • PAR% between 3% and 4%

19. The causal evidence is moderately strong that some exercise reduces the risk of breast cancer • The type and amount of exercise needed remains unknown however • Physical activity also reduces the risk of developing diabetes, hypertension, heart disease, and colon cancer

20. Breast feeding • Extended breast feeding is protective against breast cancer • RR of breast cancer adjusted for parity, age, age at first birth, and menopausal status decreases by 4.3% for every 12 months of breast feeding • Women returning to the patterns of breast feeding that were common a century ago is unrealistic

21. Diet • Reports of an association between diet and breast cancer are inconsistent • Prospective studies, including some intervention studies, have not found consistent evidence of any association between dietary factors and breast cancer

22. In summary • Similar PAR%s have been reported for breast cancer in the US and Germany • Reductions in the prevalence of physical inactivity, obesity, alcohol use, and HRT use have potential to reduce the incidence of breast cancer • PAR% estimates differ between countries if the prevalence of risk factors differs, (the PAR% increases with the prevalence of the risk factor).

23. Limitations • PAR% calculations require good evidence of causal associations, and robust estimates of the magnitude of effect and prevalence of risk factors • It is not always possible to find prevalence data that exactly reflect the risk factor measurements used in epidemiological studies

24. A second limitation is that the individual contributions of causal factors must be considered when interpreting PAR%s. • A cancer may have more than one cause (for instance HRT use, hazardous alcohol intake, physical inactivity and obesity may all contribute to breast cancer in the same individual). • The sum of the PAR%s for these risk factors therefore, may add to more than 100%, reflecting the multi-causal nature of disease.

25. Conclusion • PAR% is useful as an indication of the relative importance of modifiable risk factors, and the potential impact of primary preventive strategies • It provides a way to apply the results from international studies, where relative risk estimates are consistent across different populations and associations are biologically plausible,to the New Zealand population.

26. The most important primary preventive strategies to reduce the risk of breast cancer in New Zealand are promoting lifestyle changes to reduce obesity, promoting regular physical activity, reducing HRT use, and avoiding high alcohol intake. • A strategy to promote regular physical activity and reduce obesity could also have other benefits, such as reduced risks of cardiovascular disease and diabetes.