The metabolic impact of exercise – or how your body works out while you put your feet up

You finished your workout a couple of hours ago and now you are relaxing as you get your regular fix of Peak Performance. But in fact your body may not be quite as relaxed as you think. There are a number of ways in which exercise can exert lasting physiological effects that persist for long after you have showered and headed home. And knowledge of these processes will help you to optimise your performance and recovery, as well as managing your body weight, writes John Shepherd.

Weight is an important issue for sportsmen and women. Rugby or American football players, for example, need powerful lean muscle and body mass to hit their opponents hard and absorb the impacts of the game. A top player can burn 3,000 calories or more on a typical training day – enough to cause a worrying loss in body weight and lean muscle if calories are not replaced consistently and appropriately.

Endurance athletes are less concerned than rugby players with putting weight on, but must also be careful to ingest enough food calories to maintain their body weight, maximise recovery and optimally fuel their activities.

Both types of athlete may be assiduous in calculating the number of calories they need for their respective activities; but the reality is that they may underestimate their true calorific requirements by as much as 20% by failing to take account of the following factors:

  • A consistently elevated metabolic rate, resulting from regular endurance training, that can increase calorific expenditure by as much as 17%;

Both of these factors are affected by sex and age, of which more later.

Let’s begin by understanding metabolic rate. Total daily energy expenditure (TDEE) is just that – the sum total of energy expended over a day. A very significant proportion (60-75%) of TDEE is used to maintain the resting metabolic rate (RMR), which fuels a broad range of invisible essential bodily functions, including heart, lung and mental function. (Calculations of RMR are made over a 24-hour period but do not include the calories burned during sleeping.) You may be surprised to learn that physical activity accounts for no more than 15% of TDEE.

However, numerous scientific studies have demonstrated a training-induced rise in RMR of up to 20%. This response is known as excess postexercise oxygen consumption (EPOC). EPOC appears to have two phases: a first lasting less than two hours and a second with a more prolonged effect, lasting up to 48 hours. The former is thought to be more significant in terms of calorie burning than the latter.

The mechanisms underlying short-term EPOC created by endurance training are well known, involving the following bodily processes:

  • Replenishment of oxygen stores;
  • Re-stocking of prime muscle fuels adenosine triphosphate (ATP) and creatine phosphate;
  • Removal of excess lactate from the bloodstream;
  • Increased body temperature, circulation and ventilation rate.

The mechanisms involved in the longer lasting EPOC are less well understood, although they may include a sustained enhancement of circulation, ventilation rate and body temperature. Interestingly, little is known about the mechanisms underlying EPOC after resistance exercise, of which more later.

If endurance training can affect EPOC significantly, how is this effect mediated by training intensity and frequency?

It appears that a high intensity of training is needed to generate a significant metabolic EPOC. As Pohleman of the University of Vermont in the United States writes: ‘An exercise prescription for the general population that consists of exercise of low (less than 50% VO2max) or moderate intensity (50-75% VO2max) does not appear to produce a prolonged elevation of post-exercise metabolic rate that would influence body-weight.’ (1)

Higher exercise intensities induce greater metabolic responses that take more time to dissipate. Paradoxically, though, athletes (particularly endurance athletes) can actually slow their RMR when training intensely and for prolonged periods. This tends to happen when calories are consumed in insufficient quantities to fuel energy expenditure plus the additional increase in RMR.

In such situations the body can ‘hang on’ to this inadequate energy supply, thus slowing RMR. This ‘starvation mode’ is a legacy from our prehistoric ancestors who often had to go for long periods without food and whose bodies consequently developed the ability to use food sparingly in order to sustain life.

To avoid inducing this paradoxical response, sportsmen and women should ensure they eat enough and, crucially, that they eat regularly, with as many as five meals spread across the day and snacks consumed as needed before, during and after workouts.

It is important to understand that eating itself is a significant booster of metabolism in that the thermic effect of feeding (TEF) can account for up to 10% of TDEE. TEF refers to the energy cost of all the processes involved in the consumption and digestion of food.

If high-intensity workouts boost the body’s metabolic rate, what is the impact of high- frequency training, eg twice daily workouts? Ronsen et al from Norway addressed this question in a study of nine elite male athletes, described in the box above.

Athletes seeking to boost their metabolism through frequent exercise should be careful, in the light of Ronsen’s findings, not to allow too long a gap between sessions. Essentially, the briefer the interval between sessions, the greater the combined energy expenditure.

Most athletes train with weights to increase their power and injury resistance, but are often unaware of the fact that their increased lean muscle mass needs more feeding. It is said that every 0.45kg increase in muscle needs an extra 50 calories a day just to maintain it, which can obviously have a significant effect on calorific intake.

Within the fitness industry weight training is widely advocated as a way to lose weight on the grounds that the leaner you are, the more efficient you will be at burning fat. In general this is true; however, research by Lemmer et al from the US suggests that weight training has a lesser metabolic impact on women than on men (3).

The research team compared the age and gender effects of a 24-week strength training programme on RMR, energy expenditure of physical activity (EEPA) and body composition.

The following groups were involved in the study:

  • 10 men and 9 women aged 20-30;
  • 11 men and 10 women aged 65-75.

When results from all the subjects were pooled, absolute RMR increased by a significant margin of 7%. However, when the groups were considered separately some clear gender differences emerged, with only the men showing a significant rise in RMR.

There are two possible explanations for this apparent difference:

  1. The relatively brief duration of the trial. Had the women continued with strength training for a longer period they might have been able to increase their lean mass to a more significant level, thus giving a greater kick to RMR;
  2. Women are biologically programmed for less significant lean muscle adaptation than men because of their lack of the male growth hormone testosterone.

However, subsequent research by Dionne et al of Canada indicated that younger women might derive a greater boost to RMR from strength training than their older counterparts (4). The researchers found that younger women who weight trained for six weeks managed to increase their RMR, specifically from 1,379 to 1,451 calories a day, while older women did not experience similar benefits.

A round-up of research on EPOC carried out in Norway concluded: ‘The relationships between the intensity and duration of resistance exercise and the magnitude and duration of EPOC have not been determined, but a more prolonged and substantial EPOC has been found after hard- versus moderate-resistance exercise. Thus, the intensity of resistance exercise seems to be of importance for EPOC.’(5)

A final factor to note in terms of the effects of training on metabolism is the likelihood that men and women engaged in sport and fitness burn more calories than sedentary people by virtue of increased energy levels that make them more active in general. Again, this additional energy expenditure needs to factored into calorific calculations if adequate body fuelling is to be maintained.

Ronsen’s study of high-frequency training

This study was set up to consider:

  1. The impact of prior exercise on metabolic responses to a subsequent exercise session;
  2. The effect of different recovery periods between two daily exercise sessions on metabolic responses to the second bout.

The athletes each completed four 25-hour trials, as follows (2):

  • One bout of exercise only;
  • Two bouts of exercise separated by three hours of rest and one meal;
  • Two bouts of exercise separated by six hours of rest and two meals;
  • No exercise.

All the exercise bouts consisted of 10 minutes of cycling at 50% of VO2max, followed by 65 minutes at 75% of VO2max.

Increased metabolic stress – including a higher mean oxygen uptake, heart rate, rectal (core) temperature and EPOC and a lower respiratory exchange ratio – was observed when strenuous exercise was repeated after only three hours of recovery. But metabolic stress was reduced when a longer recovery period, including an additional meal, was given.

  • Summary points

  • Training can boost the resting metabolic rate (RMR) by up to 20%;
  • High-intensity training has a greater effect on RMR than low-intensity training;
  • The briefer the interval between two exercise sessions performed on the same day, the greater the combined energy expenditure;
  • Weight training has a lesser metabolic impact on women than on men;
  • However, younger women are more likely to benefit than older ones.

 

John Shepherd MA is a specialist health, sport and fitness writer and a former international long jumper.

References

  1. Sports Med 1991 Feb; 11(2): 78-101
  2. Eur J Appl Physiol 2004 Aug; 92(4-5): 498-507
  3. Med Sci Sports Exerc 2001 Apr; 33(4): 532-41
  4. Exp Gerontol 2004 Jan; 39(1): 133-8
  5. Sports Med 2003; 33(14): 1037-60