FACTORS THOUGHT TO MODIFY THE METABOLIC RATE

Fat-free mass. The amount of FFM (which includes all non-fat components such as muscle, organs, bones, blood) is by far the biggest determinant of MR. The higher the FFM, the higher the RMR and total MR. Obese people have higher FFM (mainly muscle) in order to carry around their extra weight. Thus, they also have a higher MR compared to lean people. Skeletal muscle metabolism accounts for approximately 50 per cent of RMR and about 75 per cent of MR when active during exercise. At a given body weight, an individual with a greater proportion of FFM to fat mass (FM) would have a higher metabolic rate.

Fat mass. As body fat levels increase so does RMR. Most of this is due to the increased FFM, but even at the same amounts of FFM, the person with the higher fat mass (FM) has a higher RMR. Similarly, as FM decreases, RMR will decrease and tend to ‘brake’ the reduction in body fat which might otherwise occur.

Age. Recent research estimates that RMR decreases at a rate of approximately 2 per cent per decade between the ages of 20 and 70 years. This reduction is accounted for primarily by decreases in FFM which is mostly due to reduced overall physical activity. However, at the same levels of FFM and FM, older people have a slightly lower RMR.

Gender. For a given body weight, females have a lower RMR than males due to a lower FFM to FM ratio. Females also have a slightly lower RMR than males independently of FFM, FM and age, most probably due to the effects of sex hormones on RMR. Several studies have shown that energy expenditure varies within the female menstrual cycle. During the preovulatory (progesterone) stage, RMR appears to be 6-15 per cent lower than during the pre-menstrual (oestrogen) stage. RMR may also be reduced in women taking progesterone-only oral contraceptives.

Genetics. FFM, FM, sex and age account for about 70-80 per cent of the differences in RMR between people. Other genetic factors account for some of the remaining differences. This suggests that RMR is at least partially genetically determined. These findings are supported by studies involving identical (monozygotic) and non-identical (dizygotic) twins which showed that identical twins are more alike with respect to RMR than non-identical twins.

Sympathetic nervous system (SNS) activity. The SNS is that part of the nervous system involved in generally speeding up the body’s responses, for example through increased heart rate, constriction of blood vessels and release of hormones such as adrenalin. The SNS also stimulates lipolysis resulting in the release of free fatty acids (FFA). The degree of SNS activity may have a small influence on RMR.

Ambient environmental temperature. When the environmental temperature is low, maintenance of normal body temperature at 37°C is an active process requiring extra energy. For this reason more energy needs to be expended in cold conditions than in warm. The cooling process (for example, sweating) is more energy efficient than the heating process (for example, shivering). The use of a sauna bath may result in fluid loss (and therefore weight loss), but not significant fat, or energy loss. Similarly, exercising while rugged up to increase sweating is only likely to have a short term effect on fluid loss and no extra effect on fat loss over and above the exercise. On the other hand exercising in the cool or cold, while only lightly dressed, would require that the body increase its energy use to maintain body temperature. Of course, this needs to be balanced by the injury protection benefits of warming up muscles before exercising.

Physical activity. One question which is often asked is whether physical activity has a carry-over effect on RMR? In the short term immediately after exercise, there is an excess post-exercise oxygen consumption (EPOC) which reflects an ongoing excess metabolic rate. The duration of EPOC is dependent on the duration and intensity of the exercise, but it may last for a few hours. In the longer term, the question of whether fit people have a higher RMR after the EPOC has disappeared (say 24 hours after an exercise bout), is less clear. A recent review of all the major work earned out in this area’ suggests that regular physical activity does have a positive effect on increasing RMR but this is mainly because of the relative increase in FFM in the body. The longer term benefit of regular exercise on metabolism is probably more related to changes in the respiratory exchange ratio (RER) than RMR.

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