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Metabolic Rate And Exercise: Researchers have been trying to puzzle out the effects of exercise on energy expenditure (calorie burning) for decades.

Despite an ever-growing mountain of research documentation, the exercise/calorie-burning story is still far from clear. There's some evidence that exercise can boost your metabolic rate - but, confusingly, regular activity appears to LOWER metabolic rates for some people. Here is a summary of the findings so far, and a round-up of the latest most-popular explanations of just what is going on.

To decode the research, there are some basic definitions to grasp first. In any 24-hour period,we burn a given number of calories - that's referred to as the total daily energy expenditure (TDEE). This total is made up from a number of different types of energy expenditure. For a start, there's energy used up by the Resting Metabolic Rate (RMR), the amount of calories burned just to keep you alive, with all your bodily systems ticking over. This accounts for a big chunk - 60-75% - of total daily energy expenditure.

Secondly, have you noticed that eating tends to warm you up, even when you're eating cold food? Cue the Thermic Effect of Feeding (TEF), the extra energy burned during and after eating anything. This effect is due to the work involved in absorbing food, digestive enzymes getting busy, increased blood flow, and so on. This takes care of about 10% of daily energy expenditure. Finally, physical activity uses up calories. Activity includes formal exercise, as well as 'casual' movement, such as fidgeting or shivering! This component generally constitutes between 15 and 30% of daily energy expenditure.

So - exercise increases calories used due to the need to fuel extra activity at the time. But the question that has kept scientists fascinated (and, as yet, stumped) is: does exercise have an impact on the other calorie-burning mechanisms? Will a jog or an aerobic dance session zap up the amount of energy you burn as you sleep? The answer so far? Maybe.


Aerobic exercise: acute effects
A number of studies have found that aerobic exercise increases energy expenditure in the period of time immediately after activity. But there may be a threshold to clamber over before the effect kicks in. Light aerobic exercise (around 50% VO2max) seems less likely to make a difference, while moderate or intense are more likely to. For example, one review concluded that light exercise could be expected to lead to burning an extra 5-10 calories afterwards; moderate to an extra 12-35 calories. In contrast, strenuous exercise was shown to increase post-exercise energy burning by a huge 180 calories ('Excess post-exercise oxygen consumption - magnitude, mechanisms and practical implications', Bahr et al, Acta Physiol Scand, suppl 605, pp 1-70).

Overall, there's pretty convincing evidence for an acute effect of exercise on energy expenditure. Most studies that have found an elevated resting metabolic rate have made their measurements within 24 hours of the exercise session. One investigation found that any post-exercise increase in RMR faded between 24 and 39 hours. Looking at the information available at present, it seems that to SIGNIFICANTLY boost energy expenditure after exercise, the activity needs to be intense (over 70% VO2max) and prolonged (over 90 minutes); the effect will be most pronounced in the first 12 hours post-exercise.


Long-term training effects
Some investigators have focused on long-term training, trying to find out if athletes who take part regularly in endurance exercise have their metabolic rates affected. This is different from looking at the immediate effects of a one-off exercise session. One expert reviewed the evidence and concluded that metabolic rate is between 5 and 19% higher in highly active compared to sedentary individuals ('Impact of energy intake and exercise on resting metabolic rate', Mole et al, Sports Medicine, vol 10, pp 72-87).

But there may be a gender difference in the metabolic response to exercise. The majority of studies have focused on men. What research there is on women does not convincingly support a difference in RMR between trained and untrained women. At least four studies which looked at both men and women found that total daily energy expenditure did not significantly increase following training in women, but did in men. It appears that women conserve energy more efficiently (ie, they burn fewer calories) at rest and in response to exercise.


Resistance exercise
Resistance exercise is any activity that increases your muscle mass, such as weight training; sports such as cycling and rowing also have resistance elements. There's not so much research data available on this type of activity compared to aerobic exercise. But emerging evidence suggests that resistance exercise is capable of boosting your overall calorie burning.

Muscle tissue burns more energy just ticking over than does fatty tissue. Therefore, increasing your muscle:fat ratio would be expected to give you a higher resting metabolic rate.

Dr Catherine Geissler is a nutrition expert at King's College, London, with a special interest in how the body uses energy. She estimates that the increased lean body mass associated with exercise can increase total daily energy expenditure by between 8% (143 cals per day) for a moderately active person to 14% (286 cals per day) for a highly active person.

Does resistance exercise have an effect beyond the simple one of altering the muscle:fat ratio? So far, the story isn't clear. Eric Poehlman, an expert in metabolism from Maryland University, reviewed the research results on the effects of resistance exercise ('Effects of exercise on daily energy expenditure', Poehlman & Toth, Nutrition Reviews, vol 54, pp S140-149).
He concluded that resistance training has an effect on resting metabolic rate (independent of any change in muscle mass) for the over-50s but not in younger individuals. This type of exercise may therefore be an effective way to offset the decline in total daily energy expenditure which typically occurs with age.


The role of energy efficiency
Some investigators have found LOWER metabolic rates in highly active athletes. One way of making sense of this apparent contradiction is to take into account two related factors: energy balance and energy efficiency.

Here, 'energy' means the energy available from food, measured in calories. When the amount of energy you take in (calories consumed) is equal to the amount of energy you use up (calories burned), you're said to be in 'energy balance'. If you're burning more calories than you eat, you're in 'negative balance'; conversely, eating more calories than you burn puts you in 'positive balance'.

Some experts believe that being in a state of negative energy balance puts your body into a special energy-conservation mode. In other words, the body believes that there's a scarcity of calories, and it has to conserve what energy is available. This is made possible because there is some leeway in terms of how energy-efficient our metabolism is. In a less efficient mode, the body can be quite wasteful with calories, performing metabolic tasks in a way that uses up a lot of energy. In an energy-conserving mode, essential bodily functions are carried out at minimal energy cost.

It's probable that there are genetically-determined differences between individuals in terms of how energy efficient their bodies are in general. But the degree of efficiency also seems to be influenced by whether the body thinks there's a scarcity or abundance of calories available. Thus, some athletes report having to eat LESS to maintain their ideal weight as their training duration increases. It seems that their metabolic rate slows to conserve calories as a defence mechanism, similar to the way metabolism slows in slimmers as a defence against starvation. One way to avoid this effect is to eat more if you're exercising more - that way you're less likely to get into a downward spiral of having to eat fewer and fewer calories to maintain the same body weight.


How does exercise have an effect?
When exercise does boost total daily energy expenditure, there are a number of potential mechanisms:
1. Increased hormonal activity Some invest-igations have found that increased resting metabolic rate following endurance exercise is associated with higher blood levels of adrenaline and noradrenaline. These hormones are controlled by the nerves embedded within muscles, known as sympathetic nerves. Some scientists have looked directly at sympathetic nerves in muscles and found that exercise stimulates the nerve activity. These hormones tend in turn to stimulate various metabolic processes which have the net effect of raising RMR. Other hormones may also play a role. There's some evidence that exercise leads to an increased production of thyroid hormone, which in turn steps up general metabolic activity.

2. Protein resynthesis Several lines of evidence suggest that increased metabolic rate following exercise is associated with altered protein metabolism. Some data show that exercise increases protein breakdown. To keep protein status constant, protein synthesis would need to be stepped up following exercise. There's some indication that exercise causes an increase in the levels of enzymes involved with making proteins. However, so far there's no direct experimental evidence confirming a direct link between increased protein turnover and metabolic rate.

3. High energy flux Two individuals can both be in a state of energy balance, but the number of calories being consumed and burned can be vastly different. A number of researchers believe that the concept of 'energy flux' is all-critical. Basically, a high energy-flux state exists when the increase in calories burned due to an exercise programme is coupled to an increased intake in calories.

For example, one study looked at young male volunteers whose activity levels and calorie intakes were strictly controlled over a 10-day period ('Effects of increased energy intake and/or physical activity on energy expenditure in young healthy men', Goran et al, J Appl Physiol, vol 77, pp 366-372). A significant increase in RMR was seen in the volunteers who were in energy balance at a level of high energy flux. Therefore, increases in RMR may occur following endurance exercise training if dietary intake is stepped up to match the increased caloric expenditure of the exercise.

4. Activity generates even more activity Apart from any direct effects on metabolic rate, there's some evidence that exercising regularly boosts general physical activity throughout the day. Put simply, the fitter you are, the more energetic you're likely to be in your daily life, attacking your daily tasks with extra gusto! For instance, one study found that three young men showed a significant increase in total daily energy expenditure after nine weeks of endurance training. These men had unaltered RMR, and the increased energy expenditure was put down to increased general physical activity.


Conclusions
Despite a huge amount of literature on the subject, differences in experimental procedures mean it's hard to make meaningful comparisons and draw overall conclusions. So far we can only make educated guesses:
1 Exercise may significantly increase metabolic rate for between six and 36 hours after exercise
2 There may be a threshold effect for intensity and duration - possibly 70% VO2max, for 90 minutes or longer
3 Regular endurance exercise may boost levels of activity during the rest of the day
4 Gender seems to be a factor; women's metabolic rates seem less affected by exercise
5 Resistance exercise may be a good way to offset the declining metabolic rate typically found as age increases
6 Going on a strict calorie-restricting diet as well as stepping up exercise may not work as a way of losing weight - you could be better off eating more.

Janet Stansfeld