Introduction to Human Nutrition

38 Introduction to Human Nutrition

FFM is dependent on the quality of the FFM, in terms
of hydration and relative contribution of the different
organs that make up the FFM. For example, skeletal
muscle constitutes approximately 43% of total mass
in an adult, but contributes only 22–36% of the RMR,
whereas the brain, which constitutes approximately
only 2% of mass, contributes 20–24% of the RMR. In
addition, the metabolic cost of each kilogram of FFM
decreases with developmental progression, probably
owing to developmental increases in the muscle mass
to organ mass ratio within FFM. Thus, the relation-
ship between RMR and FFM is not linear across all
ages and is estimated to be 331.8 kJ/kg between the
ages of 0 and 2.5 years, 151.2 kJ/kg in children aged
4–7 years, 88.2 kJ/kg during adolescence, and 151.2 kJ/
kg in adulthood.
Although fat mass is generally thought to be meta-
bolically inert, it signifi cantly contributes to varia-
tions in RMR. This is likely explained, at least in
part, by neurobiological effects (e.g., changes in
sympathetic nervous system activity) resulting from
variations in fat mass which affect the metabolism of
other tissues. RMR is also infl uenced by fat mass,
even though fat mass is generally thought to be meta-
bolically inert. Fat mass contributes in the order of
42.0–54.6 kJ/kg to RMR. This difference is indepen-
dent of the gender difference in FFM; in other
words, if one studied a group of males and females of
identical FFM and similar age, RMR would be
higher in males than in females by around 210.0 kJ/
day. This gender difference is consistent across the
lifespan, and the source of the difference is not
well understood (Table 3.1). More active people
tend to have a higher RMR than inactive individuals.
This difference may be explained in part by the resid-
ual effects of chronic exercise on metabolic rate. In
other words, RMR appears to be elevated because of
the long-lasting effects of the thermic effect of exer-
cise. However, other factors are also involved, since
the higher RMR in more active individuals persists
long after the last bout of exercise has been com-
pleted. Collectively, FFM, fat mass, age, gender, and
physical activity explain 80–90% of the variance in
RMR. In addition, a portion of the unique variance
in RMR across individuals has been ascribed to
genetic factors, although the specifi c source of this
genetic variation has not yet been identifi ed. Other
factors that have been shown to infl uence metabolic
rate include thyroid hormones (higher levels increase

metabolic rate) and sympathetic nervous system

activity.

Several prediction equations have been developed

to estimate RMR from other simple measures. These

equations are often useful for making estimates in

clinical situations when measurement of RMR cannot

be achieved, or for estimating energy needs for other

individuals. The classic equations of Harris and

Benedict are frequently used for this purpose. These

equations were developed from limited measures per-

formed in the early 1900s, and predict RMR from age,

height, and weight, and may be of limited accuracy.

More recent equations have been developed in larger

groups of subjects and can predict RMR from body

weight (Table 3.2). These new equations have been

shown to be more accurate.

Thermic effect of feeding

The thermic effect of meal ingestion is primarily

infl uenced by the quantity and macronutrient quality

of the ingested calories. The thermic effect of food has

also been termed meal-induced thermogenesis, or the

specifi c dynamic action of food. The increase in meta-

bolic rate that occurs after meal ingestion occurs over

an extended period of at least 5 hours; the cumulative

energy cost is equivalent to around 10% of the energy

ingested. In other words, if one consumed a mixed

meal of 2.1 MJ, the body would require 210.0 kJ to

Table 3.1 Variation in total energy expenditure (TEE) as a function of

resting metabolic rate (RMR) among various populations

Study group Average TEE/RMR (range)

5-year-old children in Arizona, USA 1.37 (1.15–1.70)

Obese women in the UK 1.39 (1.20–1.77)

Elderly women in Vermont, USA 1.42 (1.25–1.82)

5-year-old children in Vermont, USA 1.44 (1.11–1.77)

Elderly men in Vermont, USA 1.50 (1.30–2.11)

Obese Pima Indians 1.56 (1.03–1.99)

Adolescents in the UK 1.56

Dutch adults 1.64

Obese women in New York, USA 1.68

Young men in Boston, USA 1.70 (1.38–2.32)

Obese women in New York, USA 1.73

Elderly men in Boston, USA 1.74

Young men in the UK 1.88 (1.44–2.57)

Young men in Boston, USA 1.98 (1.57–2.60)

Mount Everest climbers 2.0

Tour de France cyclists 5.3

Burns patients 1.3

Range of TEE/RMR is given in parentheses for studies in which the

individual data were reported.