Recent studies suggest that increasing the frequency or intensity of exercise beyond a certain limit is not associated with incremental health benefits

In the last few decades, more and more people have taken to a regular regime of exercise . While exercise intuitively appears to be better that physical inactivity, less is known about what is the optimal intensity or duration of exercise. You have finished your regular daily routine of vigorous physical exercise program, shed some fat around your tummy and toned your muscles.  Hence, you would have expected that your risk of stroke and heart attack should be lower .While this would seem rational  and logical, the Million Women Study, a large prospective study of  healthy middle aged women in the United Kingdom published recently in 2015  in the Circulation journal seems to show that more may not be better.

Exercise versus physical inactivity

Exercise appears to be associated with health benefits when compared to inactivity.  In the Million Women Study which followed up about 1.1 million women over an average period of 9 years, when comparing women who were physically inactive with those who were physically active , the incidence of heart attacks, stroke and clot formation in the leg veins were significantly reduced in the group that was physically active.

In a 2014 study published by Lee in the Journal of the American College of Cardiology on more than 55 000 adults who were followed for a mean of 15 years, compared with non-runners , runners had  a 3-year life expectancy benefit.

The Copenhagen City Heart Study which compared non-joggers and joggers who were followed up for 35 years reported that there was an increase in survival of about 6 years for those who jogged.Hence , the data is very consistent that a physically active lifestyle can provide real health benefits over a sedentary lifestyle.

Maximal benefit from exercise

While one would expect that increasing the frequency of physical activity should be associated with an increase in beneficial health effects, the Million Women study did not show this to be so.  In this study, “strenuous activity” was defined as activity that was “enough to cause sweating or a fast heart rate” and “any physical activity” included mainly activities such as walking, gardening, and housework. The study examined both the intensity of the physical exercise and the frequency of the physical exercise.

The lowest risks were seen in those with moderate physical activity. For women who were physically active, there was no progressive reduction in the risks with increasing frequencyof “strenuous exercise” or “any physical exercise”.

For stroke risk reduction, the largest benefit was seen in those who exercised 2 to 3 times weekly, and the risk increased significantly with those who engaged in daily physical exercise.  A similar trend was observed when considering the risk of developing blood clots in the leg veins, where those who exercised daily had a significantly higher risk of developing clots in the legs when compared to those who exercised 2 to 3 times weekly.

For heart disease, this trend of increasing risk with increasing exercise was significant only in those who engaged in “strenuous activity” but for those with “any physical activity” , although it showed a similar trend, it was not statistically significant.

The  study by Lee showed that the physical activity of running, even 5 to 10 minutes a day and at slow speeds of less than 6 miles/hour, was associated with significantly reduced risks of death from all causes, heart attack and stroke. These benefits were similar across the different intensities and frequency of physical exertion, and increasing the degree of exercise was not associated with increasing health benefits.

The Copenhagen City Heart Study also found that the intensity of physical exercise and not the duration of exercise that was most important in relation to death from heart attacks and all causes. While most would have expected that increasing the intensity of exercise would result in greater health benefits, the study showed that those with the least intensity of jogging had the greatest health benefit.

Jogging 1 to 2.5 hours per week at a slow or average pace and a frequency of ≤3 times per week was associated with the lowest death rates. Those who jogged more than 4 hours per week, at a fast pace, and more than 3 times per week appeared to lose many of the longevity benefits that was seen with less strenuous doses of jogging.

Hence, the death rate for the strenuous joggers was similar to those who were sedentary.  This U shaped curve where more exercise seems to be associated with worse outcomes appears to be consistent these recently published studies.

The data from all these studies suggests that exercising 2 to 3 times a week at a slow to average pace for short periods of time derived the maximal amount of health benefits. Exercises of increasing intensity, frequency and duration did not show any incremental health benefits and in some of these studies , the strenuous exercise was associated with poorer outcomes as compared  to those with mild to moderate exercise.

Is there a safe upper limit for exercise?

Current recommendations from the American Heart Association suggest that, for reduction of heart attacks and stroke, adults should be doing the equivalent of at least 30 minutes of moderate-intensity aerobic activity at least 5 days per week in bouts of ≥10 minutes, or 25 minutes of vigorous aerobic activity at least 3 days per week.  This level of activity would equate to about 8 to 12 MET-hours per week.

A metabolic equivalent or MET is the ratio of the metabolic rate of an activity to the rate of energy expended while resting, and  1 MET is the rate of  energy expenditure while at rest. An activity such as housework is deemed as requiring the expenditure of 3 times the energy compared to physical inactivity and is accorded an energy expenditure of 3 MET.  Hence doing housework for one hour a day, for 4 days in a week will be equivalent to 12 MET-hours per week.

The first landmark study by Paffenbarger , an internationally known exercise authority and Stanford Professor ,published in the New England Journal of medicine in 1986 , found that exercise such as walking, stair climbing, and sports were inversely related to total death, which was primarily attributable to death due to heart, stroke  or lung causes. The study found that death rates declined gradually as energy expenditure increased from less than 500 to 3500 kcal per week.  Energy expenditure beyond 3500 kcal per week was associated with an increase in death rates.

The equivalent of a weekly energy expenditure of 3500 kcal would be running  about 50 km per week or walking about 70 km per week . Large epidemiological studies have found that this is about the safe upper limit for deriving optimal benefits for long term risk reduction for heart disease and for improving life expectancy.  The study estimated that  by the age of 80 years, the amount of additional life attributable to adequate exercise, as compared with physical inactivity, was one to more than two years.  Hence, the current data suggests that there is an upper limit of physical tolerance by the human body beyond in which exercise becomes detrimental to health.

Endurance exercises

Strenuous exercises, such as half-marathons or marathons,  have become more popular in the last 3 decades.  Although the absolute number of sudden cardiac death in such races is very low , the rate of sudden cardiac death in a marathon ( 10.1 per million ) is about 4 times that of a half-marathon ( 2.7 per million ).

Studies on those who train and participate in endurance events such as marathons, iron man triathlons and extreme distance cycling have demonstrated acute swelling of the upper heart chambers and the right lower heart chamber.  This is associated with transient decrease in the right heart chamber pumping function, elevation of proteins released from damaged heart muscle cells and abnormal heart rhythms.

Prolonged participation in such extremely strenuous exercises can lead to scarring of the heart muscles in both the upper and lower heart chambers , potentially increasing the likelihood of abnormal heart rhythms arising from the upper or lower heart chambers. Other detrimental changes associated with these extreme sports include calcification of the heart arteries, stiffening of the heart muscles and stiffening of the wall of the large arteries.

Exercise sensibly

Exercise provides health benefits when compared to a sedentary lifestyle.  These benefits can be realised with mild to moderate levels of exercise. The current evidence shows that increasing the frequency or intensity of exercise beyond a certain limit  is not associated with incremental health benefits and may even be detrimental to health. There is a difference between exercising to reduce the risk of  heart attacks and increased life expectancy , and strenuous endurance training designed to enhance physical performance.  While repetitive endurance training may improve physical performance, there is accumulating evidence it can cause both transient and permanent damage to the heart .Hence, where exercise is concerned, more is not better.