"Heart disease warning: Lack of exercise is worse risk for over-30s women than smoking or obesity," The Independent reports. It is important to stress that this headline is based on a result that is applicable to a population, not to an individual.
The Australian study the headline is reporting on looked at population attributable risk, or PARs. PARs can be used to estimate the proportion of cases of a disease, such as heart disease, that would not occur in a population if the risk factor, such as inactivity, was eliminated.
The researchers wanted to determine the proportion of heart disease that was attributable to four specific risk factors: smoking, physical inactivity, high body mass index, and high blood pressure. They looked at groups of women of different ages.
Two key findings of the study were that:
- smoking had the greatest PAR in women under 30 – if women aged under 30 gave up smoking, approximately 55-60% of cases of heart disease would be eliminated
- physical activity had the greatest PAR in women aged 31 and older – if women aged 31 to 36 increased their physical activity, about 51% of cases of heart disease could be eliminated
It pays to emphasise that factors could have the greatest PAR simply because they are the most common, rather than because they are associated with the greatest increase in risk for the individual. In fact, smoking was associated with the greatest increase in risk of heart disease at all ages.
It would be unwise to think you could offset one risk against another. Just because you take regular exercise does not mean that it is safe for you to smoke.
Where did the story come from?
The study was carried out by researchers from the University of Sydney and the University of Queensland in Australia.
Some of the data came from the Australian Longitudinal Study on Women's Health, funded by the Australian Department of Health. One of the researchers was supported by the Australian National Health and Medical Research Council.
The study was published in the peer-reviewed British Journal of Sports Medicine.
The results of the study were reported well by BBC News and The Daily Telegraph. However, the Daily Mail has misinterpreted the meaning of the figures reported in the study – specifically, how the population attributable risk tool "works".
It reports that individual women in their thirties who are inactive are almost 50% more likely to develop heart disease. However, the study refers to outcomes at the population level.
The 50% figure actually refers to the proportion of heart disease cases that could be eliminated from the population as a whole if this inactivity was not present.
It could be the case that eliminating a risk factor reduces the number of cases the most because it is the most common risk factor in a population, rather than because it is associated with the greatest increase in risk for the individual.
What kind of research was this?
This was an analysis of data from cohort studies. It aimed to determine the proportion of heart disease attributable to four specific risk factors in Australian women of different ages.
The figure the researchers were calculating is called population attributable risk, or PAR. It indicates the proportion of cases of a disease that would not occur in a population if a risk factor was eliminated. PAR depends on how common a risk factor is (its prevalence) and the strength of its association with the disease.
Researchers and policy makers can use these figures to help them decide which risk factors they should be targeting to get the greatest reduction in disease in the population as a whole.
What did the research involve?
The researchers calculated population attributable risk for heart disease that was attributable to four risk factors:
- high body mass index (BMI)
- high blood pressure
- physical inactivity
To do this, they used relative risks of heart disease associated with high BMI, smoking, high blood pressure and physical activity from the Global Burden of Disease reports.
The relative risks give a measure of the strength of the association between each risk factor and heart disease. The Global Burden of Disease reported relative risks based on pooling of results (meta-analyses) of epidemiological studies.
As the risk associated with risk factors varies by age and with sex, the researchers used relative risks specifically for women and the age groups they were looking at.
Relative risks in the Global Burden of Disease reports compared the risk of heart disease for:
- high BMI (>23kg/m2) versus low BMI (23kg/m2)
- current smokers versus non-smokers
- high blood pressure (>115mmHg average) versus low blood pressure (<115mmHg average)
- no, low and moderate physical activity versus high physical activity
The latter was assessed using what is known as MET (metabolic equivalents), a calculation of how much energy is burned off over a minute during certain activities. For example, for most people, running at 10 mph is equal to 16 METs.
The researchers used estimates of how common each risk factor was (prevalence) in Australian women from the Australian Longitudinal Study on Women's Health between 1999 and 2012. This study surveyed women born from 1973-78 (the younger cohort), 1946-51 (the mid-aged cohort), and 1921-26 (the older cohort) every three years.
The risk factors were defined as:
- high BMI (>23kg/m2)
- current smoking
- high blood pressure (defined as being diagnosed or treated for hypertension)
- no or low physical activity (defined by the Global Burden of Disease study) – MET minutes per week were calculated from reported time spent walking briskly and in moderate and vigorous leisure time activities
The researchers used the relative risks and prevalence estimates to calculate population attributable risks using standard methods.
What were the basic results?
The risk of heart disease associated with each risk factor varied across age groups, as did the prevalence of each risk factor.
Smoking was associated with the greatest increase in risk of heart disease at all ages. Of the four factors assessed, smoking had the highest population attributable risk in women aged 22 to 27 (59%) and 25 to 30 (56.6%).
The population attributable risk associated with smoking was lower in women aged 47 to 64 and in the older cohort, and was 5% in women aged 73 to 78 (the oldest group of women with smoking data available).
In women aged 31 to 90, physical inactivity (no or low physical activity) had the highest population attributable risk of the four factors assessed. The population attributable risk of physical inactivity in women aged 31 to 36 was 50.9%.
On average, the population attributable risk was:
- 48% in the younger cohort (aged 22 to 39)
- 33% in the mid-aged cohort (aged 47 to 64)
- 24% in the older cohort (aged 73 to 90)
How did the researchers interpret the results?
The researchers concluded that, "From about age 30, the population risk of heart disease attributable to inactivity outweighs that for other risk factors, including high BMI.
"Programmes for the promotion and maintenance of physical activity deserve to be a much higher public health priority for women than they are now, across the adult lifespan."
This study has found that the proportion of heart disease attributable to four risk factors (smoking, high BMI, high blood pressure and physical inactivity) in Australian women changes with age.
The figures calculated in this study – called population attributable risk – indicate the proportion of cases that would not occur in a population if risk factors were eliminated. Population attributable risk depends on the increase in risk of heart disease associated with the factor, and the number of women with the risk factor.
A risk factor could have the highest population attributable risk if it is the most common risk factor in a population. But this is not necessarily because it is associated with the greatest increase in risk for the individual.
This study has found that smoking had the highest population attributable risk in women under 30. If women in this age group gave up smoking, approximately 55-60% of cases of heart disease would be estimated to be eliminated.
In women aged 30 or over, physical inactivity (low or no physical activity) had the highest population attributable risk of the four factors assessed. If inactive women aged 31 to 36 increased their physical activity, about 51% of cases of heart disease could be eliminated.
The population attributable risk for inactivity was lower in older age groups, but if women aged 47 to 64 increased their physical activity, 33% of heart disease cases could be eliminated. If women aged 73 to 90 did the same, 24% of heart disease cases could be eliminated.
One important factor to note is that these population attributable risk figures are estimates designed to give an indication of the maximum effect that might be achieved by removing these risk factors. Achieving this change may be difficult.
The estimates also do not take into account interactions between these and other risk factors. They therefore may overestimate the impact of each factor individually.
As population attributable factors take into account the prevalence of risk factors, they will also change depending on how common a risk factor is, and will therefore differ across populations with different behaviours and characteristics.
Overall, the findings of this study do not change the message for individuals about the importance of reducing unhealthy behaviours such as smoking, and making sure we stay active.
If you are concerned about your fitness level, why not try the NHS fitness plan, which is designed to get fitness phobes up to speed in 12 weeks.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Independent, 9 May 2014
Daily Mail, 9 May 2014
The Daily Telegraph, 9 May 2014
Links to the science
British Journal of Sports Medicine. Published online May 8 2014