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While METs describe activity intensities relative to a resting metabolic rate (RMR), the physical activity level (PAL) is defined as the ratio of total energy expenditure (TEE) to basal energy expenditure (BEE). Thus, the actual impact on PAL depends to some extent on body size and age, as these are determinants of the BEE (Figure 12-1). The impact of these factors can be judged by examining the ratio of MET (extrapolated to 24 hours) to BEE. It is noteworthy that the errors that this introduces in the calculation of PAL values, at least over the normal range of body weights, is of minor importance in comparison to the very large uncertainties generally inherent in the assessment of the duration and intensity of physical activities in individuals and populations.

For a typical 30-year-old reference man and woman 1.77 m and 1.63 m in height and weighing 70 kg and 57 kg (Chapter 1, Table 1-1), BEEs are 1,684 and 1,312 kcal/day, respectively (calculated from the predictive BEE equations in Chapter 5. These correspond to 0.95 and 0.91 times the 1,764 and 1,436 kcal/day obtained by extrapolating a rate of 1.0 MET4 to 24 hours for reference men and women (1,764 kcal/day = 1 MET X 1,440 min X 0.0175 kcal/kg/min X 70 kg and 1,436 kcal/day = 1 MET X 1,440 min X 0.0175 kcal/kg/min X 57 kg). The following equations, derived for reference body weights of 70 kg for men and 57 kg for women, were utilized to determine the change in PAL for each of the activities in Table 12-1.

Men: APAL = (# of METs - 1) X 1.34 X (min/1,440 min), where 1.34 = 1.15 percent (EPOC) 0.9 percent (TEF) 0.95 percent.5

Women: APAL = (# of METs - 1) X 1.42 X (min/1,440 min), where 1.42 = 1.15 percent (EPOC) 0.9 percent (TEF) 0.91.5

4Defined as 0.0175 kcal/kg/min.

5Correction to cover EPOC and TEF.

888 DIETARY REFERENCE INTAKES

Body Weight (kg)

FIGURE 12-1 Relationship of basal energy expenditure (BEE), metabolic equivalents rate and body weight in 30-year-old adults. The upper panel shows the impact of body weight on BEE in men (o) and women (□) and on a MET-rate of 1.0 (x) extrapolated to 24 h. Points with body mass indexes (BMIs) from 18.5 up to 25 kg/m2 are filled in. The lower panel shows the ratio of BEE divided by an MET rate of 1.0 for a given body weight for men (o) with reference heights of 1.75 m or reference height ± 1 standard deviation (i.e., 1.64 or 1.86 m), and for women (□) with reference heights of 1.62 m or reference height ± 1 standard deviation (i.e., 1.55 or 1.70 m), and BMI of 18.5, 22.5 (men) or 21.5 (women), 25, 30, and 35 kg/m2.

Body Weight (kg)

FIGURE 12-1 Relationship of basal energy expenditure (BEE), metabolic equivalents rate and body weight in 30-year-old adults. The upper panel shows the impact of body weight on BEE in men (o) and women (□) and on a MET-rate of 1.0 (x) extrapolated to 24 h. Points with body mass indexes (BMIs) from 18.5 up to 25 kg/m2 are filled in. The lower panel shows the ratio of BEE divided by an MET rate of 1.0 for a given body weight for men (o) with reference heights of 1.75 m or reference height ± 1 standard deviation (i.e., 1.64 or 1.86 m), and for women (□) with reference heights of 1.62 m or reference height ± 1 standard deviation (i.e., 1.55 or 1.70 m), and BMI of 18.5, 22.5 (men) or 21.5 (women), 25, 30, and 35 kg/m2.

The coefficients given in Table 12-1 can then be used to arrive at an estimate of an individual's PAL by cumulating the effects of the various activities performed on the basis of their duration and intensities (see below, "Physical Activity for Adults").

Because it is the most significant physical activity in the life of most individuals, walking/jogging is taken as the reference activity, and the impact of other activities can be considered in terms of exertions equivalent to walking/jogging, to the extent that these activities are weight bearing and hence involve costs proportional to body weight. The effect of walking/jogging on energy expenditure at various speeds is given in Table 12-1 in terms of METs and is also shown in the upper panel of Figure 12-2. The middle panel describes the energy expended in kcal/hour for walking or jogging at various speeds by individuals weighing 70 or 57 kg (the reference body weights for men and women, respectively from Table 1-1. The figure's lower panel describes the total cost of walking or jogging one mile at various speeds, including the increments in energy expenditure above the resting rate during and after walking or jogging plus a commensurate increase in TEF. The energy expended per mile walked or jogged is essentially constant at speeds ranging from 2 to 4 miles/hour (1 kcal/mile/kg for a man [70 kcal/mile/70 kg] to 1.1 kcal/mile/kg for a woman [65 kcal/mile/57 kg], or approximately 1.1 kcal/mile/kg body weight; lower panel, Figure 12-2), but increases progressively at higher speeds.

According to the formulas shown above, walking at a speed of 4 mph (4.5 METs, upper panel, Figure 12-2) for 60 minutes causes an increase in the daily APAL of 0.195 ([4.5 METs - 1] x 1.34 x 60 min/1,440 min) in men and 0.204 ([4.5 METs - 1] x 1.42 x 60 min/1,440 min) in women, or a APAL of approximately 0.20 as given in Table 12-1. Walking or jogging at speeds of 4.5 mph raises the metabolic rate to 6 METS (upper panel, Figure 12-2), increasing the impact on changing the daily PAL by half to 0.30 for sixty minutes (APAL in men = [6 METs - 1] x 1.34 x 60 min/ 1,440 min = 0.279, APAL in women = [6 METs - 1] x 1.42 x 60 min/ 1,440 min = 0.296). Indeed, walking or jogging to cover 4.5 miles in 60 minutes, at a cost of 107 kcal/mile (lower panel, Figure 12-2) or 1.53 kcal/mile/kg (107 kcal/mile 70 kg) in men, or performing some equally demanding activity for 60 minutes, will cause an increase in PAL of approximately 0.30.

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