Exercise as it relates to Disease/Walking and vigorous exercise to prevent the risk of coronary heart disease in women

This is a critique of the journal article titled "A prospective study of walking as compared with vigorous exercise in the prevention of coronary heart disease in women" by JoAnn E. Manson, Frank B. Hu, Janet W. Rich-Edwards, Graham A. Colditz, Meir J. Stampfer, Walter C. Willett, Frank E. Speizer, and Charles H. Hennekens published in 1999 in the New England Journal of Medicine.

What is the background to this research?
Coronary heart disease remains a leading underlying cause of death for both men and women around the world. An association between physical activity and a reduced risk of coronary heart disease has been found by a number of studies. However, the amount (intensity, duration and frequency) of physical activity required to reduce the risk of coronary heart disease remains unclear, and the data on women is particularly limited. In addition, one of the most common and accessible forms of exercise among women is walking. But little is known about how walking compares to vigorous exercise for the prevention of coronary heart disease.

Coronary heart disease is a chronic disease of the artery’s which can lead to death. It is characterised by the build-up of plaque inside the coronary arteries and reduces the flow of oxygen rich blood to the heart. When a complete blockage of a coronary artery occurs the heart is starved of oxygen. This is a life threatening event often called myocardial infarction or heart attack. To put this into context, it kills one Australian every 12 minutes and is therefore an important condition to prevent.

Where is the research from?
The research was undertaken in the United States and was supported by research grants from the National Institutes of Health. The research was published in the New England Journal of Medicine which is a world leading peer-reviewed medical journal.

The lead author, Dr JoAnn Manson is a reputable physician from the Harvard Medical School. She is respected for her research on women’s health, cardiovascular disease prevention and population health, with more than 300 published pieces. Along with co-authors Dr Speizer and Dr Colditz, she was a lead investigator on the Nurses’ Health Study of 1976, a landmark women’s health study and the source of data for this research.

What kind of research was this?
The research was a prospective cohort study taken over 8 years. Prospective cohort studies are considered the gold-standard in epidemiological study design. It involves following a cohort of similar individuals into the future whilst recording the development of any outcomes of interest. This was an appropriate choice of methodology, the main advantage being fewer sources of bias than retrospective studies which look backwards. A larger sample size was required but this was not an issue because 72,488 participants were accessible through the Nurses’ Health Study. A cornerstone of evidence based medicine is the levels of evidence classification system, originally described by the Canadian Taskforce on the Period Health Examination 1979. Using this methodology this research is classified as level II evidence, behind randomised control trials which are the highest because of a lower probability of bias.

What did the research involve?
Over an 8-year period between 1986 and 1994 the research studied a cohort of 72,488 female nurses who were between 40 and 65 years old and free of diagnosed cardiovascular disease and cancer. Participants completed questionnaires about their participation in eight types of physical activity in 1986, 1988 and 1992. From this information the researchers calculated a weekly metabolic equivalent (MET) score for total physical activity, vigorous activity, non-vigorous activity and walking. To control for confounding variables, the study also collected information on personal characteristics.The primary end points for the study were coronary events, proven by the review of medical records against the World Health Organisations criteria for coronary heart disease.

Study Limitations

 * The authors tend to downplay the self-reported nature of the physical activity participation by citing a validation study. But the validation study itself may also limited by self-reporting bias and does not offer the precision of an electronic device that measures movement.
 * The relative homogeneity of the study cohort is limited because all participants are nurses, with a similar educational attainment and socioeconomic status. The authors suggest this may serve to enhance the study’s internal validity. Although perhaps true, it may also reduce the external validity of the study and mean the results may not be generalisable to the general population.
 * The authors claims regarding the role of vigorous physical activity may be limited by the fact that only a small number of women studied (26 per cent of the cohort) engaged in regular vigorous physical activity.

Study Strengths
Although the above limitations should be considered, there are also a number of strengths of the study design worth noting including:
 * A large study cohort.
 * Gold standard prospective design.
 * The long term study period.
 * The uniform criteria for diagnosing coronary heart disease.

What were the basic results?
The study found the following key results:
 * A strong graded inverse relationship between energy expenditure and coronary heart disease.
 * Both walking and vigorous exercise were each associated with reductions in the risk of coronary events by approximately 30 – 40 per cent.
 * Walking pace was found to be an independent predictor of the risk of coronary events. But when total energy expenditure was similar the magnitudes of risk reduction associated with walking and vigorous exercise were much the same.

What conclusions can we take from this research?
Overall this study provides women with evidence that any exercise is better than none. This is good news for women who prefer walking and suggests a 30-minute brisk walk each day is likely to provide protection against coronary heart disease. It appears that the amount (intensity, duration and frequency) of physical activity required to reduce the risk of coronary heart disease is reliant on total energy expenditure rather than what type of exercise is undertaken or for how long. This is an important finding, and may be used to help counter misconceptions regarding the need to exercise vigorously to see any benefit.

These findings are supported by recent studies using participants from the Women’s Health Study which also suggests walking is protective against coronary heart disease. A 2018 study found similar evidence but noted, low levels of physical activity must be sustained to be associated with substantial risk reduction and this is worthy of further investigation.

Using the study results, the authors claim one third of coronary events among middle-aged women in the United States are attributable to physical inactivity. This may be an over-statement of the results because the study lacks external validity. Generalisation to the broader population is not recommended without support from other studies.

Practical advice
If you are currently one of the 35% of Australian females aged 15+ living a sedentary lifestyle, there are a number of ways to incorporate walking into your daily routine including:
 * Taking the stairs.
 * Walking meetings.
 * Walk part of your commute.

For those unaccustomed to physical activity, start slowly and build up to the Physical Activity Guidelines. Always consult a health professional if unsure.

Further Information
If you are interested in further information please see the links below:
 * Free heart foundation walking networks https://walking.heartfoundation.org.au/
 * Coronary heart disease resources. https://www.heartfoundation.org.au/for-professionals/clinical-information/coronary-heart-disease