Over the course of the last decade, physical activity has been recognized as a critical component in achieving and maintaining optimal health and weight. Health Canada, Dietitians of Canada, The Heart and Stroke Foundation and the Canadian Diabetes Association along with numerous other national and international nutrition, exercise and health organizations, have taken strong proactive roles in promoting exercise. Health Canada offers physical activity guides for children, youth, adults and older adults, while the recently released “Eating Well with Canada’s Food Guide” includes physical activity benefits and recommendations accompanied by visuals depicting active living.
The benefits of physical activity are well documented. Regular, moderate-intensity physical activity reduces risk for chronic diseases such as high blood pressure, stroke, coronary artery disease, type 2 diabetes and osteoporosis. It is also an important factor in achieving or maintaining a healthy body weight and appears to promote psychological well-being. It is interesting to note that different types of physical activity may provide different benefits.
Dr. Kenneth Cooper coined the term aerobics in 1968 (1). Aerobic activity uses large muscle groups in continuous, rhythmic motions for sustained periods of time. It is characterized by an increased heart rate, respiratory rate, blood pressure and body temperature. Examples of aerobic activity are brisk walking, running, cycling and swimming.
Research showed that engaging in approximately 20 minutes of continuous aerobic activity, such as walking or jogging, on three days per week resulted in significant cardiovascular improvement after 12 weeks (2). Additionally, moderate-intensity exercise boosts natural immune functions and host defenses for several hours post-activity (3). Moderate-intensity aerobic activity has also produced significant positive alterations in mood states, such as depression, and increased mental acuity specific to planning, organizing and juggling intellectual tasks (4). For example, Blumenthal and colleagues showed that moderate-intensity exercise versus high-intensity activity was effective in reducing depression. Adults who were 50 to 77 years of age walked briskly for 30 minutes three times per week for 16 weeks. Findings showed that the exercise group matched a drug therapy group in relieving minor depression (2, 4). Aerobic exercise also appears to offset mental decline with aging through improved blood flow to the brain (5). Further, it assists in maintaining body weight through energy expenditure (6).
Anaerobic activity is characterized by short-burst, high-intensity exercise (e.g., “stop and go” activities). Examples are sprinting, vaulting, basketball and resistance training (weight-lifting). Anaerobic activity has also been shown to have benefits for longevity and health. For example, resistance training has produced increases in bone mineral resorption (1), reduced muscle tissue loss caused by aging or energy restricted diets (1-3, 6), and improved balance and increased fat-free mass (muscle tissue), which enhances overall functional fitness in adults (1, 2).
Aerobic and anaerobic activities are fueled through different physiological energy systems. Aerobic activities such as low to moderate-intensity walking, jogging, swimming, cycling or hiking require a continual supply of oxygen. From a fuel standpoint, protein and fat require oxygen for combustion, while carbohydrate can be burned with or in the absence of oxygen. In the case of physical activity that is aerobic in nature, approximately 40% of the energy expended in a 60-minute walk is derived from carbohydrate, 55% from fat and 3-5% from protein. The low to moderate pace of aerobic activity allows the cardiovascular system to gradually adapt to the increased oxygen demands of working muscles, which allows more fat to be used as a fuel source (1).
In contrast, anaerobic activities burn energy rapidly for short durations without oxygen. Rapid anaerobic energy transfer maintains a high standard of performance for maximal short-term efforts such as sprinting in a pool, stopping and starting over and over again on a basketball or tennis court, or doing repetitive movements in weight lifting. Anaerobic movements rely on three fuel sources that do not require oxygen to be burned: adenosine triphosphate (ATP), creatine phosphate (PCr) and carbohydrate in the form of glycogen (1). Compared with fat and protein catabolism, carbohydrate remains the preferential fuel during high-intensity anaerobic exercise because it rapidly supplies ATP during oxidative processes. High-intensity anaerobic exercise uses approximately 70% glycogen, 15% fat and 5-8% protein (1).
Regardless of the type of physical activity undertaken, energy is burned. The number of kilocalories burned with exercise depends on a number of factors, including: the duration of the activity (e.g., 30 minutes vs. 60 minutes), the frequency of the activity (e.g., once per week vs. five times per week), the intensity of the activity (e.g., walking 4 kilometres per hour vs. 6 kilometres per hour) and an individual’s weight (1, 3, 6).
Recommendations for physical activity vary largely on the specific benefits being sought. For example, to gain and maintain cardiovascular fitness, Health Canada recommends engaging in aerobic activity four to seven times per week for 30-60 minutes per session (7). Benefits (many of which are also linked to decreased body weight) include increased maximal cardiac output, decreased myocardial oxygen demand for the same level of external work performed, decreased blood pressure and increased high-density lipoprotein (HDL) cholesterol. Regular aerobic training also has significant beneficial effects on insulin sensitivity. Further, recent research finds a role in the prevention and/or treatment of osteoporosis and certain cancers, such as colon cancer (1). Strength and flexibility exercises which improve balance, mobility and muscle and joint health should also be performed 2-7 times weekly (7). If participating in physical activity for a minimum of 30 minutes to start is a challenge, activity can be broken up into shorter sessions over the course of a day (7).
Engaging in physical activity every day or nearly every day can play a central role in protecting against weight gain over time. Exercise produces cumulative calorie-expending effects; 0.45-kg (1 lb) body fat loss occurs when the calorie deficit equals 3500 kcal. Consequently, physically active men and women of all ages maintain a more desirable body composition and weight than do their sedentary counterparts (1, 8). According to the National Weight Control Registry that looks at what previously overweight individuals have done to lose and maintain weight loss, sustainable results occur with at least 60 minutes of exercise daily (9).
One weight-loss study compared food restriction to exercise (9). The food-restricted group showed greater total body weight losses; however, the exercise group showed greater body fat losses. Both groups showed decreases in insulin, triglycerides, serum leptin and low-density lipoprotein- (LDL) and very low-density lipoprotein-(VLDL) cholesterol concentrations, but only the exercise group showed an increase in fat-free mass, which led to greater improvements in blood lipid profiles than those in the food restricted group.
Endurance activities such as marathon running, triathlon training and long distance cycling or swimming require that participants make a conscious effort to supply energy to their working muscles throughout the duration of the activity. In such events, glycogen availability becomes a critical factor in delaying fatigue.
The average person stores about 1600 to 1800 calories of muscle and liver glycogen. Within approximately 60 minutes of physical activity, 50% of stored glycogen has been used. Within 90 minutes, 75% of the glycogen supply is depleted and fatigue sets in, impacting speed, mental concentration and the ability to continue the activity at all. Because endurance-trained athletes may pass up opportunities to eat or drink during competition for fear of losing precious time, it is essential to emphasize the importance of eating or drinking a carbohydrate source before fatigue occurs. Eating or drinking carbohydrate-rich sport bars, gels or beverages designed for rapid digestion, absorption and metabolism during endurance activities helps by elevating blood glucose levels and sparing remaining glycogen stores (4).
A primary dietary difference between serious, endurance-trained athletes and those engaged in regular physical activity for health and weight management is recommended daily calorie intake (1, 3, 6). Another important consideration is adequate hydration.
Engaging in physical activity for 30 to 60 minutes on five days per week promotes energy balance, while being physically active for 90 to 120 minutes on five to six days per week promotes energy deficit and weight loss. For example, a 60 minute walk at a pace of three-miles per hour burns approximately 300 calories (300 calories per hour). Compare this energy expenditure to a 90 minute run that burns closer to 1000 calories (670 calories per hour). Even more striking—an endurance athlete engaging in a 7 hour hike would burn about 300 calories per hour, for a total of 2100 calories—a full day’s worth of calories for many people. Such differences in intensity and duration of physical activity are the reason that even regular exercisers need to control portion sizes and caloric intake, unlike endurance athletes, who typically need extra calories (1).
Determine total calorie needs, using the following formula:
|Gender Factor x Daily Rate x Activity Factor = Total Calorie Needs|
|1.0 for males||x||weight) lbs.||x||24 hrs.||x||0.64 (low) |
|0.9 for females||x||weight) lbs.||x||24 hrs.||x||0.64 (low) |
|Example: Determine calorie needs for moderately-active 180 lb.(82 kg) man:|
|Example: Determine calorie needs for an active 120 lb.(54 kg) woman:|
Dispelling misconceptions about nutrition and promoting healthful eating are two of the most important educational efforts that health professionals can offer coaches, athletic trainers and athletes. One area rife with misconceptions is how best to achieve weight loss. Some coaches may encourage athletes to drop weight quickly to reach a specified weight class, run faster or otherwise improve performance. Athletes may skip meals, severely restrict fluids or energy intake, and eliminate foods or entire food groups, thus compromising their energy levels and nutritional status, to reach their goals. Slower weight loss through moderate caloric reduction (500-700 kcal/day) reduces loss of fat-free tissue and helps ensure adequate energy for training or competition. Ideally, weight loss should be postponed until the athlete’s competitive season has concluded.
People exercise for a variety of reasons, in a variety of ways, at varying levels of exertion and duration. Therefore, dietary advice for physically active people should be based on their existing diets and individual goals for losing, maintaining or gaining weight.
Eating Well with Canada’s Food Guide is an excellent tool to use for planning a balanced diet. Frequent or endurance exercisers who find it difficult to eat enough calories to maintain a desirable body weight should be guided toward nutrient and energy dense foods that provide calories in small portions. Examples are peanut butter (190calories/2 tablespoons), nuts (~160 calories/ounce), cheese (~100 calories/ounce), dried fruit (130 calories/1.5 ounces) and 100% fruit juices (~120 calories/cup). Calculating protein, carbohydrate and fat needs based on body weight can benefit active people, who frequently, and often erroneously, consider dietary supplements to be necessary for weight gain or increasing muscle mass.
Health Canada. Eating Well with Canada’s Food Guide, available at www.healthcanada.gc.ca/foodguide.