Everyone knows that you need to exercise to stay healthy. The list of benefits related to regular physical exertion ranges across all areas of health and provides a convincing argument for going out of your way to make time to be physically active (see box below).
But as the diet and fitness industry has become more of a moneyspinner, many questions remain as to just what constitutes optimal exercise and whether, as far as exercise is concerned, there can be too much of a good thing.
Thankfully, belief in the 'no pain, no gain' mantra of the 1970s has finally waned. Nevertheless, other equally worrisome myths persist: that there is a single 'best' form of exercise; that exercise equals weight loss; that regular intense workouts are the only path to fitness.
Among the biggest myths of exercise is the idea that it can reduce your appetite, boost your metabolism and so help you lose weight. But, while the promise of an improved metabolism and weight loss, of shapelier thighs or a flatter midsection is what gets most people off the couch and into the gym, fat loss is one of the lesser benefits of exercise. Its effect on metabolism also seems to be short-term at best.
Exercise can be an important part of a multifaceted weight reduction programme, but exercise alone is not the most effective way to boost your metabolism or lose weight.
Our resting metabolic rate (RMR) accounts for 60-75 per cent of our total daily energy expenditure. The energy required to digest and absorb food (the thermic effect of feeding) uses an additional 5-10 per cent and the energy expended in physical activity (the thermic effect of activity) constitutes 20-30 per cent of our daily energy output (Life Sci, 1981; 28: 1821-7).
Many things influence the RMR, including age, gender, hormones, genetics, body temperature, body surface area and, in women, the phase of the menstrual cycle (Prog Food Nutr Sci, 1977; 2: 395-403; Am J Clin Nutr, 1983; 38: 1006-17; Metabolism, 1989; 38: 364-70; Med Sci Sports Exerc, 1989; 21: 515-25; Am J Clin Nutr, 1986; 44: 614-9).
However, except for daily activity, these factors are beyond our control, which is why health enthusiasts encourage us to take charge of that 20-30 per cent that is under our control in an attempt to cause our RMR to increase.
One hour of vigorous exercise burns far fewer calories than most of us believe and, according to some experts, aerobic exercise that produces sweating and heavy breathing is essentially a sugar-burning exercise.
Obese individuals need exercise that burns fat - usually those that involve slow sustained activity. This means that 45-60 minutes of vigorous walking, every day if possible, is probably better for you than a few bursts of intense aerobics each week.
Indeed, there is evidence to show that just brisk walking can lower the body's set point (the point at which the body releases fat to be burned for fuel) and increase metabolic rate (Am J Clin Nutr, 1989; 49: 409-16).
To enhance the effects of exercise, many people engage in low-calorie diets. But, in reality, restricted eating negates the benefits of exercise. Dieting can cause the RMR to drop - at least temporarily - and whether this decrease is due to fat or protein loss is still a matter of debate.
Nevertheless, a meta-analysis of 22 studies of dieting and metabolism concluded that the RMR decreases significantly when the daily calorie intake is restricted to 941 calories or less (Int J Sport Nutr, 1996; 6: 41-61).
In a study in which women consumed a liquid diet supplying 522 calories and engaged in daily exercise (either weight training or endurance exercises, or both), there was no significant improvement in metabolism. Instead, RMR fell by 7-12 per cent in all of the study participants (Am J Clin Nutr, 1991; 54: 56-61).
Other studies (in rats and so not necessarily applicable to humans) have found that the more severe the diet, the less exercise benefit on the RMR (J Appl Physiol, 1991; 70: 2303-10). Determination of the ideal balance between diet and exercise remains elusive (Sports Med, 1991; 11: 78-101).
While adding exercise to a low-calorie regime appears to prevent some of the metabolic loss usually seen with such a diet, this positive effect is too small while the adverse effects of starvation are too vast.
The bottom line is that food - not food restriction - is what fuels the furnace of metabolism, and exercise can stoke the fire.
Too much of a good thing
Sprains, strains, contusions, bruises, fractures and more serious injuries are all becoming more and more common as usually sedentary adults and children are encouraged to become more active.
While it may be true that there is still less risk with being active than with continued inactivity, it is worthwhile being aware of just how prevalent sports injuries are becoming. A recent German study, spanning 25 years and including 30,603 sportsmen, noted a 25 per cent increase in injuries from tennis, skiing and physical exercise, and a fourfold increase in cycling injuries (Sportverletz Sportschaden, 1999; 13: 38-52).
When researchers in Sweden surveyed 41,000 adults who participated in leisure activities such as soccer, basketball, volleyball, handball and hockey over a 12-month period, the injury rate was 22.5 per 1000 individuals (Scand J Soc Med, 1996; 24: 282-92).
The medical lexicon of common injuries - from 'tennis elbow' and 'runner's knee' to 'swimmer's shoulder' and 'dog-walker's syndrome' - is testimony to the way activities that require repetitive movements using a single group of muscles can increase the risk of injury.
One study found that the rates of back injuries among squash players increased with how often they played the game. Among those who played seven matches per week, 100 per cent of them also reported a back injury. Of those who played five or six matches per week, 73.7 per cent reported a back injury and, with three or four matches per week, 53.5 per cent reported injury. Even with one or two matches per week, 50 per cent admitted to having back injuries (J Sports Med Phys Fitness, 1998; 38: 337-43).
Some areas of our bodies, usually in the lower body, are more vulnerable to injury than others. Among US Marine Corps recruits, the injuries reported over the standard 12-week training period followed a distinct pattern. For the 1296 young men involved in one study, the overall injury rate was 39.6 per cent, with 78 per cent of these resulting from overuse and 82 per cent occurring in the lower extremities. The most frequent site of injury was the ankle/foot (34.3 per cent) followed by the knee (28.1 per cent) (Med Sci Sports Exerc, 1999; 31: 1176-82).
These findings are consistent with those of previous studies in civilians. A survey in Alberta, Canada, of 3790 individuals who engaged in regular sports or recreational activities reported knee and ankle injuries in 21 and 14 per cent, respectively (Can J Public Health, 1998; 89: 53-6).
As women are more encouraged to engage in sports, their injury rates have also risen. While some studies suggest that there is no difference between men and women in overall injuries (Int J Sports Med, 2001; 22: 420-3), others show that rates for women may sometimes be higher than that for men.
When the University of Medicine and Dentistry of New Jersey studied 11,780 collegiate and scholastic basketball players of both sexes, the female players had a 25 per cent greater risk of sustaining a grade I ankle sprain compared with their male counterparts (Clin Orthop, 2000; 372: 45-9). Other studies have also found a greater level of injury among women (Am J Sports Med, 1980; 8: 265-9). The reason, according to some, may be the differences in the structure of the knee and thigh muscles of men compared with women, but there are also changes in oestrogen levels when women menstruate - causing a softening of the ligaments at certain times of the month (Sports Med, 1998; 25: 369-406).
Children, too, can suffer the pain of muscle overuse (Int J Sports Med, 1999; 20: 103-8). New activities such as in-line skating and the use of scooters can lead to a whole range of both major and minor injuries (Am J Phys Med Rehabil, 1999; 78: 7-10; Pediatrics, 2001; 107: E64). A recent US study involving 11,840 children, aged 5 to 17 years, concluded that sport injuries accounted for 36 per cent of all reported injuries in that age group (Arch Pediatr Adolesc Med, 1995; 149: 1009-16). Injuries that are not properly rehabilitated in childhood can go on to plague an individual as an adult.
Overtraining can also lead to other problems, such as oxidative stress. The more energetic and long-lasting your exercise, the more oxygen you take into your body. Thus, during intense exercise, the body can end up with up to 20 times the normal volume of oxygen. This is generally considered a good thing, but more oxygen also means an increased demand on the body as it struggles to metabolise all this extra oxygen. The result is a well-known, but little publicised, adverse effect of intensive training - oxidative stress, and the production of muscle- and organ-damaging free radicals.
While supplementing with antioxidants is thought to be one way to protect your body from exercise-induced free-radical damage, evidence of efficacy is mixed. In one study, supplementation with the antioxidant vitamins A, C and E were found to protect cyclists from the harmful effects of environmental ozone (Occup Environ Med, 1998; 55: 13-7). In another study, vitamin E (1200 IU daily for two weeks) appeared to reduce the amount of muscle damage caused by intense exercise (Med Sci Sports Exerc, 1998; 30: 67-72).
However, other studies have found that antioxidants, such as a combination of vitamin E and coenzyme Q10, did not reduce oxidative stress induced by exhaustive exercise (Free Radic Res, 1998; 29: 85-92). Similarly, 1000 IU of vitamin E was shown to be inadequate in preventing the abdominal pain and bleeding - another sign of stress and damage to the muscles - commonly seen in marathon runners (Nutrition, 1999; 15: 278-83).
Overtraining can lead to both too-high and too-low levels of the stress hormone cortisol. At normal levels, cortisol is protective, but too much or too little has important effects on health. When levels are high, muscle-tissue breakdown is increased (which may explain why strength may decrease with high levels of training) (Townsend Lett Docs, 1998; 184: 38-40) whereas too little cortisol leads to an inability to deal with other stressors (Med Sci Sports Exerc, 1998; 30: 407-14).
What's more, it's not just aerobic exercise that can lead to this altered hormonal pattern. Although aerobic exercise is the most studied type of exercise, overtraining with weights, for example, can also lead to unfavourable neuroendocrine responses (Sports Med, 1997; 823: 106-29).
Exercise versus activity
The best way to protect yourself from injury and overdoing it is to include a balanced amount of aerobic, stretching and weight-bearing exercises suited to your level of fitness (and your schedule) in your exercise routine.
While each of this triad of exercise types has its proponents, the American College of Sports Medicine claims that, to be really fit, you must partake of all three types. Those who favour aerobic exercise may increase their risk of injury if they are not flexible. Likewise, improved muscle strength may help to support flexibility exercises like yoga. But no single type of exercise can help you achieve optimal health.
Many of us believe that to be fit, we must take part in a structured physical activity several times a week, preferably one that makes us puff and sweat. But recent research suggests that this may not be the case. It appears you can get just as much benefit from walking around the block a couple of times or climbing up and down the stairs more often than usual.
Researchers from the Cooper Institute for Aerobics Research in Dallas, Texas, put a group of 235 sedentary men and women through two years of either a traditional exercise programme (including up to one hour of aerobic activity three to five times per week) or a lifestyle programme that included activities such as walking around the local shopping mall for at least half an hour, seven days a week. Both groups showed significant - and similar - improvements in cardiorespiratory fitness and blood pressure. Neither group lost a significant amount of weight, but both showed similar reductions in the percentage of body fat (JAMA, 1999; 281: 327-34).
Brisk walking, like structured exercise, can also prevent non-insulin-dependent (type II) diabetes. The ongoing Nurses' Health Study has shown that the brisker the walking pace, the lower the risk of developing diabetes (JAMA, 1999; 282: 1433-9). Others have found that brisk walking can improve heart health. Again, data from the Nurses' study, based on 72,488 female nurses aged 40 to 65 years, found that brisk walking for three or more hours a week lowered the risk of coronary heart disease by 30-40 per cent, the same reduction as seen with 1.5 hours of vigorous exercise per week (N Engl J Med, 1999; 341: 650-8).
This type of activity can also improve the mind. Much research has focused on the mental health benefits of regular exercise (Res Q Exerc Sport, 1995; 66: 292-7; J Roy Soc Health, 1996; 116: 7-13; J Psychiatr Ment Health Nurs, 1996; 34: 38-43). When a team from the University of Illinois randomly assigned 124 sedentary adults, aged 60 to 75 years, to a programme of either brisk aerobic walking or non-aerobic stretching and toning exercises, they found that the walking group not only improved physically, but also experienced a clear improvement in cognitive function (Nature, 1999; 400: 418-9).
Less well-studied but just as important, pursuing a well-chosen activity or exercise can open up a range of insights into who you are and what makes you tick. As Dr Alan Gaby noted (Townsend Lett Docs, 1996; 153: 112-3), most of us interested in holistic health have embraced the psychological/spiritual path to health, but . . . many of us have not traversed the physical path - the one on which you must use your legs and lungs and blood and sweat and guts to reach the pinnacle.
Using your body as something other than a vehicle for carrying briefcases, shopping and children can be a real eye-opener. Writes Gaby, 'We have the capability of discovering ourselves in new ways through our bodies, just as many of us have done by exploring our minds and spirit.'
The golden rules for safely integrating exercise into your life are simple:
* Doing something is better than doing nothing
* No single activity - no 'magic bullet' - can provide all the benefits
* In some cases, one type of activity can be substituted for others without losing benefit
* Variety in the types of activities you choose is the key to all-round health.