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Assignment 12 Bmr Formula

Energy expenditure of the population under study: the PAL

In the original paper that examined the reported energy intake from 37 different studies,18 EIrep:BMR was compared with a PAL value of 1.55, the WHO value for ‘light’ activity.16 The reasons for choosing this value were several. First, the energy expenditure or physical activity in the studies examined was unknown; second, choosing too high a value might have exaggerated the extent of under-reporting; third, the WHO recommendations for energy requirements16 were well known and widely used; and fourth, calorimetry and early DLW work confirmed the value of 1.55×BMR for light activity as a probable minimum energy requirement for a normally active but sedentary population (not sick, disabled or frail elderly).11

A subsequent review of 74 DLW studies comprising 574 free-living subjects19 indicated that 1.55 was a conservative value and that many groups have higher energy expenditures. That review suggested the following broad categories:

‘The relationship between lifestyle, activity and PAL suggested by the data can be summarised as follows:

  • Chair-bound or bed-bound  1.2

  • Seated work with no option of moving around

  • and little or no strenuous leisure activity 1.4–1.5

  • Seated work with discretion and requirement to

  • move around but little or no strenuous leisure

  • activity  1.6–1.7

  • Standing work (eg housewife, shop assistant)  1.8–1.9

  • Significant amounts of sport or strenuous leisure

  • activity (30–60 min 4–5 times per week)  +0.3

  • Strenuous work or highly active leisure  2.0–2.4n

The data provide little evidence to quantify the energy cost of manual occupations, but the range 2.0–2.4 is suggested as the maximum for a sustainable lifestyle.’

Other PAL values for broad categories are given by the FAO/WHO/UNU Expert Consultation on Energy and Protein Requirement16 and UK report on Dietary Reference Values.20 The WHO report also includes tables showing the factorial derivation of the PAL values. The DLW data from 574 free-living subjects19 were also used to derive mean energy expenditures by age and sex. These are shown in Table 1. However, it should be noted that these subjects were predominantly white collar in origin and very few were identified as having manual occupations. The higher PAL for the age group 18–29 probably reflected more active leisure pursuits, and the lower values in the older individuals, less active leisure. Values for specific groups in the population could be obtained from individual DLW studies. An appendix to the above paper19 listed all 74 studies with details of the subjects and the measured energy expenditures expressed both as mega joules and as PAL. However, such figures should be used with caution since most DLW studies have been conducted on small numbers of selected subjects.

There exists therefore a body of data to suggest suitable PAL values for various population groups. However, knowledge of the activities of the population is required, suggesting that a questionnaire on home, leisure and occupational activity should be included routinely in all dietary surveys.

Between-subject variation in physical activity (CVtP)

CVtP is the coefficient of variation of total variation in energy requirement expenditure calculated from the mean and standard deviation of a study. This figure, although referred to as ‘between-subject’ variation and notated as CVb in the original paper,11 includes within- (CVw) and between- (CVb) subject variation and methodological errors. This is the value to be used when the group mean EIrep:BMR is being evaluated. It is also the value to use when individual EIrep:BMR are being evaluated, since the EIrep:BMR is being evaluated against a population mean that includes both between and within-subject variation. The value of 12.5% used in the original paper was taken from the FAO/WHO/UNU report on energy and protein requirements16 (Section 4.5, p 44), but only one study21 was cited to support this value. The text stated ‘…the measurement of total energy expenditure over a week indicates that the inter-individual variability of expenditure, in a specified group, has a coefficient of variation (CV) of about±12.5% on a body weight basis.21

Accumulated data from doubly labelled water studies (Figure 1) showed a wide range of PAL, from about 1.2 to over 2.0, at all ages.19 The CVtP from these data are shown in Table 1. They ranged from 9.5 to 23.8% in different age–sex groups with a pooled mean of 15.4%. Examination of studies with repeat DLW measurements found the values to be little reduced when CVtP was calculated using subject means derived from more than one DLW measurement.17 A round figure of 15% is suggested as a suitable average value to substitute into the Goldberg equation, but other values may be used if deemed more appropriate for any given study.

Within-subject daily variation in energy intake (CVwEl)

The within-subject day to day variation in food intake (CVwEI) of individuals is large. Figure 2 shows the energy intake of one individual who maintained a weighed diet record every sixth day for 1 y.22 The zero line indicates the mean intake averaged over the year, ie the ‘habitual' intake. Average intakes from 1, 3 or 7 successive days of recording are plotted as the differences from zero. The fine dotted line shows the daily differences and demonstrates the enormous variability from day to day. The 3 day and 7 day average differences show the values obtained from alternative short-term measurements. In the year-long study of dietitians from which these data were taken,23 the within-subject variation in daily energy intake ranged from 10 to 50% in individuals, with a pooled mean of 26%.24

Table 2 show the CVw from other studies reviewed by Bingham25 and Nelson and colleagues.26 These ranged from 14 to 45% with a pooled mean of 23%. Thus 23% is suggested as a suitable average value to substitute into the Goldberg equation, but a specific CVwEI may be calculated for any given study according to the formula

where CVi is the CV calculated for each subject from the number of days of dietary assessment available for that subject, and n is the number of subjects.

Variation in basal metabolic rate (CVwB)

Since energy requirement and intake are expressed as multiples of the BMR, between-subject variation is taken into account and only within-subject variation need be considered in calculating the confidence limits. This includes both measurement error and variation with time on repeated BMR measurements. The classical BMR measurement is made at least 12 h after the last meal, after an overnight sleep in the place of measurement, completely at rest and at thermoneutral temperature (about 23°C). However, practicalities often dictate that these conditions are not fully met, in which case the measurement is called resting metabolic rate (RMR). Either the subject has to walk from the place of sleep to the place of measurement (in-patient RMR) or sleeps at home and travels fasted to the place of measurement in the morning (out-patient RMR), in both of which situations the measurement is done after a rest period of usually 30–60 min. For the out-patient RMR, researchers do not have control over the size of the last meal, length of fast or activity in the previous 24 h all of which influence the RMR. Other factors which may influence RMR and which differ between studies are the nature of the equipment used (face mask, nose clip, ventilated hood, ventilated tent, or whole body calorimeter), subject familiarity with the equipment and procedures, amount of activity on the morning of the measurement, or length of rest period before the measurement, all of which may increase the within-subject variation over that obtained under the well-standardized conditions of the classical BMR.

Under well-standardized conditions CVwB for measured BMR averages about 2.5%. This value was derived from 279 subjects measured in a whole body calorimeter on successive nights on which the antecedent diet and activity had been the same.27 However, in community studies subjects are sampled from the population and variation includes that due to antecedent diet and activity, natural weight fluctuations, menstrual cycle and methodological error. Table 3 shows results from studies that have specifically investigated within-subject variation in BMR; the majority were conducted in young men. The mean CVwB from these 11 studies was 3.9%. Table 4 shows the within-subject variation in BMR obtained from DLW studies with repeat measurements. The mean value from all 12 studies was 5.5%, but the mean from free-living individuals without imposed study conditions was 4.7% (first six studies in Table 4). The mean of these studies and those in Table 3 was 4.2%. A round figure of 4% is suggested as a suitable average value to substitute into the Goldberg equation, but other values may be used if deemed more appropriate for any given study.

Where no measured BMR or RMR is available, it may be calculated from weight and height or weight alone. Many equations are available and each gives a slightly different result.28 The equations based on the largest body of data and most widely used in dietary assessment are those of Schofield,29 modified to provide equations for age range 60–74 and >74 y.20 The value for CVwB used in the original paper11 was a single value of 8%. However, specific values for the different age–sex groups of the Schofield equations are available29 (Tables Al.l and A1.2, pp 31–32 of Schofield's paper). The standard errors for predictions of BMR expressed as a percentage of the mean BMR (ie coefficients of variation for predictions) from these tables are summarized in Table 5. A figure of 8.5% is suggested as a suitable average value to substitute into the Goldberg equation, but other values may be used for studies of specific age–sex groups.

The validity of the Schofield equations for obese subjects

The body of data on which the Schofield equations were based contained few subjects of very high body weight (>83 kg for men or >76 kg for women), and thus probably few of very high body mass index (BMI). Since adipose tissue has a lower metabolic rate than lean tissue, the BMR of obese persons may be over-estimated by the equations and thus the extent of under-reporting as measured by EI:BMRestimated associated with obesity also over-estimated. This question has been examined using the database compiled for the review of DLW energy expenditure.19Figure 3 shows the plot of (BMRestimated−BMRmeasured) against ln(BMI) for males and females aged 18 y and over. The data have been fitted with a quadratic equation. For males there was no significant difference between BMRest and BMRmeas associated with BMI (P=0.4). For females the difference was highly significant (P=0.0001), but only for women with BMI>35 kg/m2 was it of practical importance. Table 6 shows the mean difference by BMI category for both sexes. Since the proportion of the population with BMI>35 kg/m2 is approximately 5–7% only (Health Survey for England 1997, Department of Health press release), it is unlikely that using estimated BMR has caused the association between under-reporting and high BMI to have been exaggerated. The association appears extremely robust; it has been found in many studies including, for example, the USA CSFII 85–86 survey3 and the UK NDNS survey.31 Limited data on the large under-reporting in the massively obese suggest that even adjusting the BMR would not transfer these persons into the category of acceptable reporters. In 324 women with measurements of EI and DLW EE, 18 had BM≥35 kg/m2. In these women mean EI:EE was 0.74 (s.d. 0.33), compared with 0.78 (s.d. 0.25) for those with BMI 25–34 kg/m2 and 0.87 (s.d. 0.22) in those with BMI<25 kg/m2.

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BMR Calculator

The Basal Metabolic Rate (BMR) Calculator estimates your basal metabolic rate—the amount of energy expended while at rest in a neutrally temperate environment, and in a post-absorptive state (meaning that the digestive system is inactive, which requires about 12 hours of fasting). This calculator is based on the Mifflin - St Jeor equation.

Result


BMR = 1,717 Calories/day



The basal metabolic rate (BMR) is the amount of energy needed while resting in a temperate environment when the digestive system is inactive. It is the equivalent of figuring out how much gas an idle car consumes while parked. In such a state, energy will be used only to maintain vital organs, which include the heart, lungs, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin. For most people, upwards of ~70% of total energy (calories) burned each day is due to upkeep. Physical activity makes up ~20% of expenditure and ~10% is used for the digestion of food, also known as thermogenesis.

The BMR is measured under very restrictive circumstances while awake. An accurate BMR measurement requires that a person's sympathetic nervous system is inactive, which means the person must be completely rested. Basal metabolism is usually the largest component of a person's total caloric needs. The daily caloric need is the BMR value multiplied by a factor with a value between 1.2 and 1.9, depending on activity level.

In most situations, the BMR is estimated with equations summarized from statistical data. The most commonly used equation is the Mifflin - St Jeor equation, which is what our BMR Calculator uses:

    BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) + 5         (man)
    BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) - 161     (woman)

BMR Variables

Muscle Mass – Aerobic exercise such as running or cycling has no effect on BMR. However, anaerobic exercise, such as weight-lifting, indirectly leads to a higher BMR because it builds muscle mass, increasing resting energy consumption. The more muscle mass in the physical composition of an individual, the higher the BMR required to sustain their body at a certain level.

Age – The more elderly and limber an individual, the lower their BMR, or the lower the minimum caloric intake required to sustain the functioning of their organs at a certain level.

Genetics – Hereditary traits passed down from ancestors influence BMR.

Weather – Cold environments raise BMR because of the energy required to create a homeostatic body temperature. Likewise, too much external heat can raise BMR as the body expends energy to cool off internal organs. BMR increases approximately 7% for every increase of 1.36 degrees Fahrenheit in the body's internal temperature.

Diet – Small, routinely dispersed meals increase BMR. On the other hand, starvation can reduce BMR by as much as 30%. Similar to a phone that goes into power-saving mode during the last 5% of its battery, a human body will make sacrifices such as energy levels, moods, upkeep of bodily physique, and brain functions in order to more efficiently utilize what little caloric energy is being used to sustain it.

Pregnancy – Ensuring the livelihood of a separate fetus internally increases BMR. This is why pregnant women tend to eat more than usual. Also, menopause can increase or decrease BMR depending on hormonal changes.

Supplements – Certain supplements or drugs raise BMR, mostly to fuel weight loss. Caffeine is a common one.

BMR Tests

Online BMR tests with rigid formulas are not the most accurate method of determining an individual's BMR. It is better to consult a certified specialist or measure BMR through a calorimetry device. These handheld devices are available in many health and fitness clubs, doctor offices, and weight-loss clinics.

Resting Metabolic Rate

While the two are used interchangeably, there is a key difference in their definitions. Resting metabolic rate, or RMR for short, is the rate at which the body burns energy in a relaxed, but not fully inactive state. It is also sometimes defined as resting energy expenditure, or REE. BMR measurements must meet total physiological equilibrium while RMR conditions of measurement can be altered and defined by contextual limitations.

Modern Wisdom

A 2005 meta-analysis study on BMR* showed that when controlling all factors of metabolic rate, there is still a 26% unknown variance between people. Essentially, an average person eating an average diet will likely have expected BMR values, but there are factors that are still not understood that determines BMR precisely.

Therefore, all BMR calculations, even using the most precise methods through specialists, will not be perfectly accurate in their measurements. Not all human bodily functions are well understood just yet, so calculating total daily energy expenditure (TDEE) derived from BMR estimates are just that, estimates. When working towards any sort of health or fitness goals, BMR can aid in laying down the foundations, but from there on it has little else to offer. A calculated BMR and thus TDEE may result in unsatisfactory results because of their rough estimates, but maintaining a daily journal of exercise, food consumption, etc., can help track the factors that lead to any given results and help determine what works, as well as what needs to be improved upon. Tracking progress in said journal and making adjustments over time as needed is generally the best indication of progress towards reaching personal goals.

Reference

* Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR, Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine1. Am J Clin Nutr 2005; 82: 941-948.