As early as the 5^th century B.C. commentators began debating the health benefits of exercise, with one of the most famous quotes attributed to Hippocrates:

“All parts of the body, if used in moderation and exercised in labors to which each is accustomed, become thereby healthy and well developed and age slowly; but if they are unused and left idle, they become liable to disease, defective in growth and  age quickly.”
—Hippocrates, 5^th century B.C.

Currently physical inactivity has been identified as the fourth leading cause of mortality globally and the CDC report that an estimated 300,000 deaths in the US each year are related to physical inactivity and poor diet.¹ The economic costs (both to individuals and the country) of physical inactivity are increasingly being viewed as an issue of concern.

HOW DOES EXERCISE IMPROVE HEALTH?

Research demonstrates an overwhelming positive link between exercising throughout life and overall health. A 2018 review of the literature estimated that physical activity had an important role to play in delaying the onset of around 40 chronic conditions and diseases including type-2 diabetes, cardiovascular disease and obesity,² while a second review found exercise to be associated with a reduced risk of several site-specific cancers including breast, colorectal and endometrial.³ A dose-response reduction in dyslipidemia among an overweight cohort was achieved following the introduction of an exercise regimen,⁴ while depression appears to be increased among those who are physically inactive, although the causal pathway is unclear.⁵ Does depression cause inactivity or inactivity cause depression? Exercise is also a key factor within energy expenditure and weight control/overweight and obesity – which further play a role in an individual’s risk of developing upwards of 20 health conditions including type-2 diabetes, cardiovascular disease and cancer.⁶ In essence, levels of physical activity and overweight and obesity could be viewed as interacting to impact upon health outcomes. Given that heart disease, stroke and cancer are the leading lifestyle-related causes of death in the US, it is imperative that we understand the factors which contribute to their occurrence.

The biological mechanisms underpinning the associations between physical activity and health are diverse and relate to improved body composition, adiposity, lipoprotein profiles, HDL, LDL, glucose and insulin, blood pressure and flow, endothelial and immune function and inflammation.^7,8

HOW MUCH EXERCISE IS RECOMMENDED FOR IMPROVED HEALTH?

The U.S. Department of Health and Human Services (HHS) recommends that adults undertake at least 150 minutes (two and a half hours) per week of moderate-intensity aerobic physical activity, while the World Health Organization recommends 75 minutes or more of vigorous exercise or 150 minutes or more of moderate exercise or a combination of the two.^9,10 For those who want to significantly improve their health and achieve substantial weight loss, upwards of 300 minutes of moderate exercise or 150 minutes of vigorous exercise per week is recommended¹⁰. Both organizations also agree that such activity should take place in blocks of more than ten-minute intervals, with anything less than that appearing to have no positive impact upon health.⁹ Finally, at least twice a week, adults should diversify their exercise routine to include activities which work to strengthen the muscles within the body, such as weight-lifting and push-ups/sit-ups.

IS ALL EXERCISE CREATED EQUAL?

Although there appears to be a dose-response relationship between physical activity and health outcomes,⁷ there may also be differences dependent upon the level of intensity performed. One study found that high-intensity training was more effective in improving cardiorespiratory fitness, among men who had been previously sedentary, when compared with moderate intensity training.¹¹ The study also found alterations in coronary heart disease risk that were associated with the intensity of the exercise undertaken. A second study concluded that “if the total energy expenditure of exercise is held constant, exercise performed at a vigorous intensity appears to convey greater cardioprotective benefits than exercise of a moderate intensity”.¹² Of note these studies were undertaken in the early 2000s and only focused upon cardio health. More recent studies have found no difference in improvements in measures of metabolism and adiposity between those who exercised intensely or moderately.^13,14 Studies that have sought to explore the impact exercise frequency versus duration has upon health have produced varied results.^15,16 Finally, walking and participation in sport, but not domestic physical activity, was found to improve health related to cardiovascular risk (e.g. pulse and blood pressure and BMI).¹⁷

Colloquially referred to as ‘weekend warriors’, this group of people undertakes intense workouts over the weekend, while being relative sedentary during the week. Although the evidence related to the health benefits of exercising in this way are somewhat limited, a 2017 study from the UK did find reduced mortality among this group¹⁸. Using self-reported data from ~63,000 individuals, the study compared overall, cardiovascular and cancer mortality among those who were classified into the following groups: 1) inactive (no moderate or vigorous activity); 2) insufficiently active (<150 minutes per week moderate and <75 min/wk in vigorous activity); 3) weekend warrior (≥150 min/wk in moderate or ≥75 min/wk in vigorous activities from 1 or 2 sessions); and 4) regularly active (≥150 min/wk moderate or ≥75 min/wk in vigorous activity from ≥3 sessions). For all-cause, cardiovascular and cancer each group had reduced mortality when compared with those who were inactive. The weekend warriors also had no statistically significant differences in mortality compared to the other active groups. Further research, which analyzes the prevalence of diseases and conditions within populations undertaking different exercise regimens, is required.

A note of caution for ‘weekend warriors’, the American Osteopathic Association previously recommended that individuals avoid exercising in this way. If the weekends are your only opportunity to exercise, try to exercise in ways that do not cause strain to parts of the body and avoid trying to do too much in too little time. That way you reduce the risk of injury and muscle/joint strain.

SEDENTARY BEHAVIOR

While there is a consistent association between physical activity and health, there is also a growing body of evidence which suggests that sedentary behaviour can be detrimental to health. Although the evidence is mixed there may also be a threshold within the association; above this point, the more time an individual remains sedentary, the more likely they are to develop a chronic condition.¹⁹ The impact of sedentary behavior may also be somewhat attenuated by regular exercise, as evidenced by the research focused upon weekend warriors.^18,20 Among those who are the most sedentary, breaking up the time spent sitting or lying down,²¹ and the introduction of light amounts of physical activity, can produce large and significant improvements in health outcomes. One study found that, among this group, replacing one hour of sedentary behavior with either light or moderate-to-heavy exercise reduced mortality by 18 and 42 percent, respectively.²²

CONCLUSION

There appears to be a dose-response relationship between physical activity and health, with those who are the most unfit at the start of a new exercise regimen benefitting the most from it. Whether you take a series of ten-minute walks or hit the gym for an extended session, current research appears to support the adage that “exercise is medicine”.² Therefore individuals should endeavor to make exercise a consistent part of everyday life and reduce sedentary behavior.

Author: Dr. Vanessa Gordon-Dseagu gained her PhD from University College London in 2014 and subsequently moved from the UK to Washington, D.C. to take up a postdoc fellowship within the National Cancer Institute’s (NCI) Metabolic and Epidemiology Branch. Since leaving NCI she has been working as a freelance epidemiologist, research and policy consultant. She has written and published numerous peer-reviewed papers, as well as several white papers and blog posts, and presented her work at internationally-renowned conferences. Currently, her work focuses upon a diverse array of epidemiological, public health and policy issues – including pancreatic cancer survival rates, the provision of end of life care in different countries, hypertension and healthcare costs, and the use of mobile phone applications to support the work of health professionals.