Beyond any other risk factors, a lack of physical activity had the greatest impact on a woman's lifetime risk for heart disease after age 30, Australian researchers found.
In a study of 32,154 Australian women, low physical activity was responsible for greater levels of population risk from age 30 to 80 than any other risk factors, including smoking, high body mass index (BMI), and high blood pressure (BP), according to Wendy J. Brown, PhD, of the University of Queensland in St. Lucia, and colleagues.
For women under 30, smoking was found to be the most important contributor to heart disease when prevalence and relative risk data were combined to estimate population attributable risk (PAR) -- estimated at 59% compared with 15.2% to 29.2% for women ages 47 to 64 and 5.3% for women 73 and older, they reported in the British Journal of Sports Medicine.
The results highlight the fact that population attributable risks for heart disease appear to change throughout women's lives, the researchers concluded.
"They show that continuing efforts to reduce smoking rates in young adult women are warranted, especially during the 20s," Brown's group wrote. "However, from about age 30, the PAR for inactivity outweighs that of the other leading risk factors, including high BMI, which is currently receiving much more attention in Australia than the 'Cinderella' risk factor -- physical inactivity."
While it's known that BMI, smoking, high blood pressure, and lack of physical activity are all leading risk factors for heart disease, a better understanding of the varying contributions of each of these risk factors throughout adult life would help identify the population subgroups most in need of policy action, the researchers wrote.
They analyzed data from the Australian Longitudinal Study on Women's Health, which is a prospective study of women recruited from the national Medicare health insurance database.
For all four risk factors, RRs declined with age. The highest RRs were for smoking in the younger women and the lowest were for high BMI and high BP in the older women.
The PAR analysis revealed that the highest attributable fraction for risk of heart disease was for smoking in the younger cohort (ages 22 to 27 and 25 to 30). The PAR for smoking decreased markedly after this from 59% (ages 22 to 27) to 5% (ages 70 to 75).
In the younger cohort, the PAR for no physical inactivity remained constant throughout life, while the estimate for low activity was lower in the older than in the young and mid-age groups. When these estimates were combined, overall PAR was around 48% in the young cohort, 33% in the mid-age cohort, and 24% in the older cohort.
At baseline, PAR for high BMI was similar in the younger and older groups, and a downward trend in the mid-age cohort was seen. In the older cohort the PAR for high BMI declined with age from 15% (ages 73 to 78) to 11% (ages 85 to 90).
The PARs for hypertension were very low in the younger cohort (from 2% to 3% at all surveys) increasing to 7% to 11% in the mid-age and older cohorts, respectively.
Limitations and Implications
The study had some limitations including participant attrition over time that may have resulted in biases towards inclusion of healthy women. There also was a lack of data needed to adjust estimates for synergistic interactions, and the use of minimal risk criterion for physical inactivity and BMI.
Another limitation was the reliance on self-reports of hypertension and no objective measurements of blood pressure. As a result, the PARs for high blood pressure were likely to be "significantly underestimated," especially in older women.
The study findings highlight the importance of emphasizing regular exercise for reducing cardiovascular disease risk, especially in young adulthood and middle age, the researchers said.
"Our data suggest that national programs for the promotion and maintenance of physical activity, across the adult lifespan, but especially in young adulthood, deserve to be a much higher public health priority for women than they are now," they wrote.
They estimated that "if every woman between the ages of 30 and 90 were able to reach the recommended weekly exercise quota -- 150 minutes of at least moderate intensity physical activity -- then the lives of more than 2,000 middle-age and older women could be saved each year in Australia alone."
Primary source: British Journal of Sports Medicine
Source reference: Brown WJ, et al "Comparing population attributable risks for heart disease across the adult lifespan in women" Br J Sports Med 2014; DOI: 10.1136/bjsports-2013-093090.