Careful examination of the existing data suggests that the optimal dose of aerobic exercise, more specifically the exercise dose required to produce the most benefit, varies considerably across key outcomes of clinical relevance. In clinical practice, this means that the prescription of an exercise dose may best be accomplished by identifying a specific therapeutic target for each individual.Benefits of daily exercise

Plasma Lipids

The impact of aerobic exercise training on serum lipoproteins has been studied extensively. The response of high-density lipoprotein (HDL) to exercise training appears to be modest but favorable. The estimated minimal weekly exercise energy expenditure and duration required for this HDL response were 900 kcal and 120 minutes, respectively, which approximates the minimum exercise dose recommended by current PA guidelines of at least 150 minutes of moderate-intensity exercise or 75 minutes of vigorous-intensity exercise weekly. The impact of aerobic exercise training on serum low-density lipoprotein (LDL) is not consistent across available studies. Two large meta-analytic studies found that HDL but not LDL was significantly improved by exercise training when confounding variables (diet, body weight, etc) were considered. Routine exercise has clearly been demonstrated to have a beneficial impact on serum lipids. The effect of exercise appears strongest and most consistent for HDL, whereas the impact of exercise appears both less consistent and smaller in magnitude for LDL, subfractionated lipoproteins, and triglycerides. Although the magnitude of the impact of routine exercise on lipids at the individual patient level is small, lowering of total cholesterol by as little as 10% through either dietary or pharmacological intervention has been shown to result in a 27% reduction in incident CVD.

Blood Pressure

The relationship between exercise training and blood pressure (BP) has been the topic of extensive study. In large, observational studies, greater amounts of leisure-time PA have been associated with a reduction in incident hypertension, although this relationship has been less consistent among women and blacks. Controlled exercise intervention studies and meta-analyses that have examined the BP response to exercise training suggest a modest but clinically significant effect. Specifically, exercise training appears to translate into a 3– to 5–mm Hg reduction in systolic BP (Δ2%–4%) and 2– to 4–mm Hg reduction in diastolic BP (Δ2%–3%). BP reduction attributable to exercise is generally greater among hypertensive individuals (reductions of 6–8 mm Hg in systolic BP and 5–6 mm Hg in diastolic BP) and slightly less in normotensive individuals (reductions of 2–3 mm Hg in SBP and 1–2 mm Hg in DBP). The impact of exercise on BP has been clearly demonstrated in studies that have used exercise doses in the range of current PA recommendations. However, a clear dose response has not been demonstrated, and there is neither definitive evidence to support a minimum exercise threshold for BP lowering nor any data to support an upper limit of exercise dose beyond which the BP reduction is diminished.

Insulin and Glucose Metabolism

Exercise increases insulin sensitivity and non–insulin-mediated skeletal muscle glucose metabolism, and epidemiological data suggest that exercise training improves metabolic heath. weight loss rather than exercise was the most important determinant of incident DM risk reduction. In the US Diabetes Prevention study, exercise was not significantly associated with reduced risk of DM when adjusted for weight change. However, participants in the Finnish study, who increased their exercise exposure the most, even after adjustment for weight loss and dietary changes, appeared to reduce the risk of DM significantly more than those whose exercise habits did not change. the potential superiority of moderate-intensity exercise for attenuating incident MS was similarly shown for serum triglycerides. This raises the possibility that moderate-intensity exercise uniquely affects the metabolic pathways that link insulin resistance, impaired postprandial lipolysis, and triglyceride excursion.Exercising on the beach

Body Mass

Weight loss is among the most popular reasons that exercise is prescribed because it is indisputable that changes in body mass are driven by changes in net energy balance. To what degree a given exercise intervention will favorably affects energy balance depends on concomitant changes in caloric intake. Numerous organizations have published guidelines that address the role of exercise for the prevention and treatment of obesity. To achieve substantial weight loss (>5% decrease in body mass), 2 approaches have been proposed: a concomitant dietary intervention (either maintenance of pre-exercise intake or reduction in intake) or exercise of a sufficient dose to expend a high amount of calories, at least 26 MET-h/wk (eg, walking at 3 mph for 60 minutes daily or jogging at 6 mph for 20 minutes daily).

The minimum exercise dose required to maintain optimal weight or to stimulate weight loss appears to be substantially higher than the minimum dose required to favorably affect other CVD risk factors (lipids, BP, insulin sensitivity). There is a clear dose response with exercise

and weight loss. Available data suggest that a dose of at least approximately double that of the current PA guidelines is necessary to reliably establish and maintain a healthy weight and that more activity is required to achieve more substantial weight loss. In clinical practice, we routinely emphasize the importance of coupling exercise with deliberate control of caloric intake for patients who seek to reduce body mass.

Current Exercise Recommendations:

The Data Behind the Dose

As evidence supporting the link between exercise and favorable health outcomes continued to mount, 3 independently generated PA guidelines were published in the 1990s to 2000s.

Writing groups representing the American College of Sports Medicine and the Centers for Disease Control, the National Institutes of Health, and the US Department of Health and Human Services (HHS) each concluded that routine moderate- intensity exercise was an effective means to reduce the overall risk of chronic disease. The consistent recommendation across these initial guidelines was that all people should engage in at least 30 minutes of moderate-intensity exercise on most, preferably all, days of the week. This approximate recommended dose remains the cornerstone of public health PA guidelines.

The most recent guidelines by the HHS in 2008 and the World Health Organization in 2010 suggest a total of at least 150 minutes of weekly moderate-intensity activity (3–6 METs) or ≈450 to 900 MET-min/wk.14,100 More recent guidelines by the American Heart Association and American College of Cardiology recommended a similar total amount of exercise, although divided into sessions of longer duration (40 versus 30 minutes) and performed less frequently (3–4 rather than 5 sessions per week), on the basis of their review of the literature specific to reduction in BP and improvement in lipids. Walking at 3 mph (3.3 METs) for 150 minutes achieves the minimum target of ≈500 MET-minutes that all of these guidelines have in common. However, for those who choose higher-intensity activity (>6 METs), the HHS guidelines endorse a shorter total duration of exercise. For example, jogging at 6 mph (10 METs) for a weekly total of 75 minutes amounts to 750 MET-minutes, thus meeting the dose target. It is noteworthy that the HHS guidelines recognize an exercise dose of 450 to 900 MET-min/wk as the minimum necessary to promote health and acknowledge the potential role of higher total doses and intensities of exercise for individuals with specific goals, including fitness optimization or weight management.

Conclusions

Decades of scientific inquiry have led to the indisputable fact that routine exercise or high levels of PA confer positive cardiovascular health benefits. Exercise and PA, much like medications used in clinical practice, are best measured and prescribed by consideration of dose, which is a function of 3 principal attributes: intensity, duration, and frequency. PA guidelines are based on a solid epidemiological foundation, and numerous RCTs have begun to delineate the mechanisms by which exercise leads to health and longevity. In clinical cardiovascular practice, exercise dose and PA habits can and should be addressed with each patient with an ultimate goal of individualized counseling and exercise prescription. Although progress has been made, there is much to be learned. Refinements in our understanding

of how exercise dose across the spectrum affects cardiovascular health are needed. Future gains will best be accomplished by the use of complementary strategies that include prioritized scientific funding, widespread application of technology designed to measure exercise dose both in clinical trials and in real-world living, and focused translational work geared toward delineating cellular and biochemical responses to exercise.