There has been increasing recognition of the inextricable link between nutrition and physical activity over the last decade. This evolution in thinking is evident if one reviews the Dietary Guidelines for Americans, a publication that is produced jointly by the U.S. Department of Health and Human Services and the U.S. Department of Agriculture and updated every 5 years to provide science-based advice “to promote health and to reduce the risk for chronic diseases” in the population. Recommendations about physical activity have become increasingly prominent and detailed over successive revisions of this document from 1995 through 2005 because physical activity is a major determinant of energy expenditure, a key component of energy balance. Weight gain and the increase in obesity in the United States is a major public health concern and can be attributed to excessive caloric intake, inadequate energy expenditure in the form of physical activity, or a combination of the two. The Dietary Guidelines recommend that individuals consume a wide variety of foods to meet their nutrient needs. However, adequate nutrient intake must be achieved within calorie needs if neutral energy balance is to be maintained and weight gain avoided, and this can be difficult when energy expenditure is low. For these reasons and because of the recognition of the overall health benefits related to physical activity, its prominence has steadily increased to the point that the third of nine key messages conveyed by the Dietary Guidelines for Americans, 2005 Edition reads, “be physically active every day,” and the dietary pyramid was modified to include physical activity.
EXERCISE AND ITS BENEFITS IN THE GENERAL POPULATION
It is necessary to review some general aspects of the terminology and physiology related to exercise and exercise training in the general population before applying these principles to chronic kidney disease (CKD) populations. Physical activity refers to any movement of the body that results in an increase in energy expenditure and includes activities of daily living as well as home maintenance and occupational activities. In contrast, exercise generally refers to physical activity that is planned, structured, repetitive, and purposive in the sense that improvement or maintenance of one or more components of physical fitness is the objective (Table 17-1
). Physical activity is often classified by its intensity, which itself can be characterized on an absolute or a relative scale. The most common way of classifying activity is as a function of energy expenditure, in units of metabolic equivalents (METs), where one MET is the energy expended (or oxygen consumed) at rest. In this classification, light activity constitutes activity that
requires less than 4 METS (or less than 3 for older individuals), moderate activity requires 4 to 6 METs (or 3 to 6 for older individuals), and vigorous activity requires ≥6 METs (see Table 17-2
for specific examples of moderate and vigorous activities). Another way to categorize the intensity of physical activity is according to an individual’s rating of perceived exertion, or (RPE). Gunnar Borg, a Swedish exercise physiologist, developed the Borg RPE Scale®, which starts at 6, corresponding to no exertion and goes to 20, or maximal exertion. Moderate activities are those for which the perceived exertion is in the range of 11 to 13 or “somewhat hard,” and vigorous activities are in the range of 14 and above or “hard” to “very hard.” It has been noted that in healthy individuals, heart rate during exertion can be approximated as 10 times the RPE.
Table 17-1. Glossary
Activities of daily living (ADLs)—Personal care activities necessary for everyday living, such as eating, bathing, grooming, dressing, and toileting.
Aerobic training—training that improves the efficiency of the aerobic energy-producing systems and that can improve cardiorespiratory endurance or fitness.
Body composition—A health-related component of physical fitness that relates to the relative amounts of muscle, fat, bone, and other vital parts of the body.
Cardiorespiratory endurance or fitness—A health-related component of physical fitness that relates to the ability of the circulatory and respiratory systems to supply oxygen during sustained physical activity.
Disability—The inability to perform or a limitation in the performance of actions, tasks, and activities usually expected in specific social roles that are customary for the individual or expected for the person’s status or role in a specific physical environment.
Endurance training—repetitive, aerobic use of large muscles (e.g., walking, bicycling, swimming).
Exercise—Planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness.
Flexibility—A health-related component of physical fitness that relates to the range of motion available at a joint.
Functional limitation—A deficit in the ability to perform a discrete task, such as stair climbing.
Instrumental activities of daily living (IADLs)—The more complex skills, such as maintaining a home, performing household chores, shopping, and managing finances, that are important components of independent living.
Maximal heart rate—The highest heart rate value attainable during an all-out effort to the point of exhaustion (often estimated as 220-age).
Maximal oxygen consumption (VO2max)—The maximal capacity for oxygen consumption by the body during maximal exertion. Also known as aerobic power, maximal oxygen uptake, and cardiorespiratory endurance capacity.
Metabolic equivalent (MET)—A unit used to estimate the metabolic cost (oxygenconsumption) of physical activity. One MET equals the resting metabolic rate of approximately 3.5 mL O2/kg/minute.
Moderate physical activity—Activity that causes some increase in breathing or heart rate and is perceived as light to somewhat hard (e.g., brisk walking).
Muscle fatigue—The decline in ability of a muscle to generate force as a result of prolonged or repetitive contraction.
Muscular endurance—The ability of the muscle to continue to perform without fatigue.
Physical activity—Bodily movement that is produced by the contraction of skeletal muscle that increases energy expenditure above basal levels.
Physical fitness—A set of attributes that people have or achieve that relates to the ability to perform physical activity.
Physical function or functioning—A fundamental component of health status describing the state of those sensory and motor skills necessary for usual daily activities.
Physical performance—The execution or accomplishment of specific physical tasks (e.g., walking, stair climbing).
Rating of perceived exertion (RPE)—A person’s subjective assessment of how hard he or she is working. The Borg RPE scale® is a numerical scale for rating perceived exertion.
Resistance training—Training designed to increase strength, power, and muscular endurance.
Strength—The ability of the muscle to exert force.
Vigorous physical activity—Activity that causes a large increase in breathing or heart rate and is perceived as hard or very hard.
Exercise is additionally classified as endurance (or aerobic) exercise or resistance (or strengthening) exercise. Endurance exercise involves repetitive, dynamic, and rhythmic use of large muscles (e.g., walking, running, bicycling) and is the major form of exercise that can improve cardiorespiratory fitness or maximal oxygen consumption (VO2max
). Resistance exercise generally involves lifting weights or all or part of one’s body weight, or moving the body against an externally imposed resistance (e.g., using a strength training machine or stretching elastic bands). There is overwhelming evidence that both types of exercise have large health benefits in the general population, including prevention of disease and disability as well as improvement in symptoms or management of chronic disease or disability. Higher levels of physical activity have been linked to lower risk of overall and cardiovascular mortality and to reduced risk of outcomes such as cardiovascular events and development of diabetes mellitus, hypertension, colon cancer, and depression. In addition, regular physical activity can improve the control of hypertension and diabetes among those with established disease, increase bone density
and improve symptoms of arthritis, and improve physical functioning and psychological well-being among those with limitations. In 1996, the U.S. Surgeon General developed a report on physical activity and health, which concluded that “sedentary living habits clearly constitute a major public health problem.” The report summarized the expected benefits of physical activity and included a series of conclusions and recommendations, among which were:
People of all ages, both male and female, benefit from regular physical activity.
Significant health benefits can be obtained by including a moderate amount of physical activity (e.g., 30 minutes of brisk walking or raking leaves, 15 minutes of running, or 45 minutes of playing volleyball) on most, if not all, days of the week. Through a modest increase in daily activity, most Americans can improve their health and quality of life.
Additional health benefits can be gained through greater amounts of physical activity. Physical activity reduces the risk of premature mortality in general and of coronary heart disease, hypertension, colon cancer, and diabetes mellitus in particular. Physical activity also improves mental health and is important for the health of muscles, bones, and joints.
More than 60% of American adults are not regularly physically active. In fact, 25% of all adults are not active at all.
Table 17-2. Examples of moderate and vigorous physical activities
Exercise and leisure
Walking briskly, dancing, leisurely bicycling, or roller skating, horseback riding, canoeing, yoga
Jogging or running, fast bicycling, aerobic dance, martial arts, jump rope, swimming, step climbing
Volleyball, golfing, softball, baseball, badminton, doubles tennis, downhill skiing
Soccer, field hockey or ice hockey, lacrosse, singles tennis, racquetball, basketball, cross-country skiing
Mowing the lawn with a push power mower, general lawn and garden maintenance, scrubbing floors or washing windows, vacuuming, sweeping
Mowing the lawn with a hand mower, shoveling, carrying and hauling, masonry, carpentry, moving or pushing furniture
Walking and lifting as part of work, general
Heavy manual labor (e.g., digging ditches, carrycarpentry ing heavy loads)
The report also included some discussion of the dose-response relationship between physical activity and health benefits, but it was noted that the wide variation among studies summarized made it difficult to define an optimal dose that includes duration, intensity, and frequency of activity. Nevertheless, it was noted that there “appears not to be a lower threshold [to the dose-response relationship], thereby indicating that any activity is better than none.” However, this finding was not highlighted, nor were specific recommendations given for implementation of increased physical activity among elderly individuals or those with chronic diseases.
In 1998, the American College of Sports Medicine issued a position stand on exercise and physical activity for older adults, which was updated to include individuals with chronic conditions and was endorsed by the American Heart Association as well in 2007. They noted that the goals of exercise appropriate to younger adults, such as prevention of cardiovascular disease, cancer, and diabetes, and increases in life expectancy, should perhaps be replaced in the oldest adults with a new set of goals, which include minimizing biological changes of aging, reversing disuse syndromes, the control of chronic diseases, maximizing psychological health, increasing mobility and function, and assisting with rehabilitation from acute and chronic illnesses. In addition, these guidelines highlighted the importance of defining the intensity of physical activity on a relative rather than an absolute scale. In other words, what would be light activity to a younger, healthier individual might well qualify as moderate intensity activity for an elderly individual or an individual with “clinically significant chronic conditions and/or functional limitations” (such as CKD).