While exercise has a beneficial effect in the fight against heart disease, its effect on risk factors is only modest. By Dr Michael Lim
PHYSICAL inactivity is considered to be a risk factor for heart disease. An analysis of data from many epidemiological and observational studies reveal that the physically inactive have about twice the incidence of death from heart disease when compared to the physically active. But how is exercise beneficial for health?
Many believe that they can burn off their low density lipoprotein cholesterol (LDL-C) or “bad” cholesterol by engaging in greater physical exercise. However, they will be disappointed to know that studies correlating changes in physical fitness with LDL levels and studies assessing the impact of exercise on LDL cholesterol have shown mixed results. There is no consistent data to show that regular physical exercise can result in significant reduction in LDL-C.
Some have argued that perhaps there may be a difference if the level of exercise was of a higher intensity. So, does intensity of exercises make a difference? A few studies, including the randomised Training Level Comparison Study trial in men with heart disease published in the American Journal of Cardiology in 2001, assessed the impact of different exercise intensities on LDL cholesterol and they were unable to find any significant impact on LDL-C in the different exercise intensity groups.
It is commonly believed that exercise will help to increase the “good” cholesterol. An analysis by Kodama in the Archives of Internal Medicine in 2007 of the combined data of 25 studies which assessed the impact of exercise intensity on high density lipoprotein cholesterol ( HDL-C ) or “good” cholesterol found an overall small increase of 2.5 mg/dl. Hence, while exercise seems to have a beneficial impact of HDL-C, the beneficial effect appears to be rather modest.
Does long-term physical activity in the young reduce the likelihood of developing high blood pressure in the future? Physicians attempted to answer this question in the CARDIA study published in the American Journal of Public Health, in which a group of about 4,000 young men and women between the ages of 18 and 30 years were followed up for up to 15 years to examine the relationship between physical activity and the development of high blood pressure. The study showed that for an increment of weekly exercise quantity that is sufficient to support weight loss, there was a marginal 17 per cent decrease in the incidence of high blood pressure. These findings were also supported by other observational studies. A limitation of these studies, is that they rely on self-reporting and regular follow up over a period of time, resulting in a high default rate. In the CARDIA study, more than one third of the initial group were eventually excluded from the final analysis of the data as a result of default in follow up.
Observational studies have also reported that regular exercises result in lower resting blood pressure in those with or without high blood pressure. An analysis of 54 randomised controlled trials by Whelton published in the Annals of Internal Medicine in 2002 reported that there was a statistically significant reduction of systolic (upper) and diastolic (lower) blood pressure of almost 4mm/Hg and almost 3mm/Hg, respectively. In another analysis of 48 studies of exercise rehabilitation in heart patients by Taylor in the American Journal of Medicine in 2004, there was a decrease of systolic blood pressure of slightly more than 3mm/Hg with exercise. While the large body of evidence suggests that exercise has beneficial impact on blood pressure, both in reducing the incidence of high blood pressure and in reducing the blood pressure values, it is to be noted that these effects are modest.
Not unexpectedly, lifestyle intervention studies which resulted in a regular exercise regime and weight loss in diabetic patients have reported better blood sugar control. The benefit from exercise seems to extend beyond better sugar control and appears to have a preventive effect as well. Data from the Diabetes Prevention Program Research Group published in the New England Journal of Medicine in 2002 showed that over a period of almost three years, exercise of more than 150 minutes per week when combined with weight loss, resulted in a 58 per cent reduction in the incidence of non-insulin requiring diabetes mellitus when compared with a control group.
Can I start exercising?
If you are middle aged and you are about to start on a vigorous exercise programme, do you need to see your doctor before starting on an exercise programme? According to the 2013 American Heart Association Scientific Statement, for men older than 45 years and women older than 55 years of age who are undergoing vigorous exercise, a visit to the doctor is advised if you have diabetes mellitus or two other risk factors for heart disease such as high cholesterol levels, high blood pressure, smoking or a family history of heart disease. For those who are younger, a visit to the doctor is also advised if there is a family history of sudden death at a young age, or if the risk factors for heart disease are poorly controlled. For those with known heart disease or lung disease, medical advice should be sought before commencing vigorous physical exercise.
While exercise has a beneficial effect on risk factors of heart disease by elevation of HDL-C, lowering of blood pressure values and better diabetic control, one must remember that its effect on these risk factors is modest and the exercise intensity has to be sufficient enough to result in a weight loss for a benefit to be realised. Moreover, excessively vigorous exercise can result in injuries which may compromise physical mobility and in those with underlying heart disease, it can increase the risk of sudden death. Hence, one must weigh the benefits and risks of exercise and tailor the exercise regime appropriately when embarking on an exercise programme.