Monday November 2 2009
Subjects of the study had existing heart problems
Half an hour of aerobic exercise can cut the risk of a fatal heart attack by 60%, according to The Daily Telegraph.
The news comes from research that was also covered by The Daily Express, but neither newspaper made it clear that the study behind these findings was in people with existing heart problems and not in the general population.
This study assessed the effect of exercise regimes in patients with coronary artery disease in relation to their stress levels. It concluded that stress increases the risk of death in these patients, and that exercise can reduce the proportion of people who experience stress. They say this may partially explain the reduced risk of death seen in the patients who exercised.
This research may have implications for the treatment of people with coronary artery disease but it has little relevance to the general population.
Where did the story come from?
The study was carried out by Dr Richard Milani and Dr Carl Lavie from the Ochsner Clinic Foundation in LA. The authors declare that their research received no additional funding. The study was published in the peer-reviewed American Journal of Medicine.
The news headlines that describe this study may be misleading and could imply that the findings of the study apply to healthy people. Neither The Daily Telegraph nor The Daily Express emphasise clearly enough that the study was in people with coronary artery disease and only investigated the effects of stress and exercise on outcomes for these patients and not the general public.
What kind of research was this?
This was a case series study which investigated the effects of cardiac rehabilitation and exercise training for patients who had experienced a coronary event (heart attack, coronary bypass surgery or a percutaneous coronary intervention). The study assessed the effect of exercise training on psychosocial stress and other factors potentially linked to the risk of death. It also investigated how exercise itself affected the risk of death.
The researchers looked at whether the patients’ stress levels at the start of the study affected their outcomes. To do this, the effects of exercise training were compared between groups of people with high and low levels of psychosocial stress (assessed based on symptoms such as depression, anxiety and hostility).
What did the research involve?
The researchers enrolled 522 patients in their study, all of whom had been assigned a course of cardiac rehabilitation and training between January 2000 and July 2005. This exercise regime began between two and six weeks after the patients’ coronary event and lasted for 12 weeks (36 sessions). This patient group included people with high and low stress levels, which had been assessed using a behavioural questionnaire.
The patients also included a subgroup of 27 people who had high scores on the psychosocial stress test but who dropped out of their exercise programme after two weeks. These patients served as a control group of subjects with high stress who did not follow an exercise programme.
Exercise programmes were tailored to the patients’ individual needs but generally consisted of 10 minutes of warm-up, followed by 30 to 40 minutes of aerobic exercise (walking, rowing, jogging), light activities such as weights and then a cool-down period. The programme also included lectures and learning about lifestyle and diet.
Height, weight, age, gender, blood fats and other measures, including peak oxygen uptake (a measure of how much effect the exercise was having on fitness), were taken at the start of the study and again a week after the exercise programme was complete.
The researchers looked at the benefits of exercise in the groups who had initially high and low stress levels, in other words the change in various measures over time for each of these groups. There were only 53 people in the high stress group, so the findings from this analysis must be interpreted with caution.
The researchers also compared the effects of exercise between these groups. Patients were divided into two groups based on their response to exercise during the programme. Those with a great improvement in oxygen uptake (10% or more) were considered ‘high exercise change’, while those with minimal or no improvement were termed ‘low exercise change’.
Mortality at the end of the study, after an average follow-up of about three-and-a-half years, was compared between these two groups of patients. Participants were not randomly assigned to the comparison groups (for example, those receiving no exercise programme and those receiving the programme). This means that there may have been imbalances between these groups that could have contributed to any differences seen, rather than the exercise itself.
What were the basic results?
The study found that people with high levels of psychosocial stress were four times more likely to die than those with low stress (22% compared with 5%).
The exercise programme improved exercise capacity in people with high stress levels. It also improved stress levels and all behavioural measures, as well as increasing the levels of good cholesterol. In people with low stress levels, exercise led to reduced BMI and other improvements.
Overall, exercise reduced the proportion of people with stress from 10% to 4% across the sample.
How did the researchers interpret the results?
The researchers conclude that psychosocial stress “is an independent risk factor for mortality in patients with coronary artery disease”. They say that exercise training can reduce stress and this may explain, at least in part, the beneficial effect of exercise on mortality.
The findings from this research may have implications for the management of people with coronary artery disease but have little relevance to the general population, who were not the subject of this research. The research also has limitations that affect the interpretation of its results, even within people with coronary heart disease:
- This is an observational study that did not randomise people when assigning them to the different experimental groups. This means that the results could have been caused by imbalances between these groups and not by the exercise programme itself. As such, this research should be considered as useful in generating an initial hypothesis.
- The optimal study design to investigate whether exercise reduces mortality and what role stress plays in this relationship would be to randomise patients to groups performing or not performing exercise, adjusting for their stress levels or analysing subgroups with high and low levels of stress.
The findings from this research do not challenge the current recommendations on exercise for the general population, which is 30 minutes of moderate physical activity five times a week.