6.1.1 Energy requirements
The factors determining energy needs were considered in section 4. As discussed in that section, all energy costs, including total expenditure, are derived as multiples of the basal metabolic rate (BMR). This section is concerned with evaluating these costs.
188.8.131.52 Basal metabolic rate (BMR). Since the BMR depends upon both age and body weight, adults of both sexes have been divided into three age ranges—18–30, 30–60, 60+ (sections 3.5.2 and 3.6.1). Within each age range, values for BMR have been obtained from the body weight by the equations shown in Table 5, as discussed in section 4.2.1 (1).1 Table 6 shows that in adults the BMR per kg varies with actual weight. In the report of the 1971 Committee (1a) it was assumed that the BMR per kg is constant within each age range. The effect of this change on the prediction of BMR is to increase the estimated requirement of smaller and lighter people and to decrease the requirement of those who are larger and heavier.
|0–3||60.9 W- 54||0.97||53|
|> 60||13.5 W+487||0.79||148|
|0–3||61.0 W- 51||0.97||61|
|> 60||10.5 W+596||0.74||108|
a Standard deviation of differences between actual BMRs and predicted estimates.
1 Since the present report was compiled the data base for the equations contained in reference 1 has been slightly expanded. They, therefore, differ from the equations shown in Table 5 but the differences are negligible.
|18–30 years||30–60 years||> 60 years|
|Ht||Wtb||Per kg||Per day||Per kg||Per day||Per kg||Per day|
|(m)||(kg)||per day||per day||per day|
a BMR from equations in Table 5, rounded to 10 kcalth.
b Weight taken as median acceptable weight for height; body mass index (Wt/Ht2 = 22 in men, 21 in women(see Annex 2).
As discussed in section 3, the BMR may be determined either from the actual weight, if this is known, or from the median weight, according to age, sex, and height. Some examples of the differences produced by the two methods are shown in Table 7. The choice of method will depend upon the circumstances and objectives of the user. It will be seen that at a given height the BMRs of subjects at the extremes of the acceptable range of weight differ from those at the median weight by less than 10%.
|Man, age 40, height 1.8 m||Woman, age 25, height 1.5 m|
|Position in rangea||Position in rangea|
|(A) BMRc from actual Wt|
|(B) BMR from median Wt|
a Acceptable range of BMI (see Annex 2A).
b Body mass index = Wt(kg)/Ht2(m).
c Predicted from equations in Table 5.
The converse situation is when subjects of the same weight vary in height and hence in body mass index (BMI). Except in the elderly, such variations, within the acceptable range of weight for height, have no importance in men and relatively little importance in women. Examples of the effect of including height in the prediction of BMR are shown in Annex 1.
184.108.40.206 Baseline energy need. Since the BMR is measured in the postabsorptive state and at complete rest, for an individual to survive an addition has to be made to cover the metabolic response to food (section 4.2.4) and the energy cost of increased muscle tone and minor movement. A value of 1.4 times the BMR during waking hours, for the energy cost of activities such as washing, dressing, and short periods of standing, can be derived from published figures (2). If 8 hours a day are spent in bed at the basal rate of energy expenditure, then the requirement over a period of 24 hours amounts to 1.27 times the BMR. It should be emphasized that this requirement allows for minimal movement; it is not compatible with long-term health and makes no allowance for the energy needed to earn a living or prepare food. It could be called the survival requirement and is of practical value in conditions of crisis only, for estimating the short-term needs of totally inactive dependent people.1
1 See the footnote on page 78.
220.127.116.11 Energy needs for occupational activities. The energy need will vary with the type of occupation, the time spent in doing the task, and the size of the individuals concerned. Annex 5 provides estimates of the requirements per minute for various occupations. These are expressed as multiples of the basal metabolic rate, and thus include the cost of minor movement, muscle tone, and the specific metabolic response to food.
Classification of occupational activities. Traditionally the occupations of men and women have been classified into those which involve light, moderate, and heavy physical activity. This has facilitated the broad assessment of the energy requirements of populations and has been helpful when the energy needs of a particular occupational group have not been specifically studied. Annex 5 lists the activities and occupations that can be classified in this way.
Table 8 (page 76) shows how approximate values for the energy costs of occupations involving the three degrees of activity have been obtained. On this basis one can estimate the gross energy expenditure on occupational work at light, moderate, and heavy levels of activity as 1.7, 2.7, and 3.8 times the basal metabolic rate in young men, and 1.7, 2.2, and 2.8 times the BMR in young women.
Clearly, care is needed to ensure an accurate description of the activity and the time spent on it. Thus the energy demand on workers undertaking specific jobs, such as farming, mining, shipbuilding, or tree felling, may vary enormously, depending on the degree of mechanization.
For estimating the requirement per day, weekly working hours have to be averaged over 7 days. Thus for those who work for 8 hours per day for 5 days a week, the average would be 5 hours 43 minutes daily over the entire week. For other groups with different work patterns the calculation of time will have to be adjusted.
18.104.22.168 Discretionary activities. The principles underlying the inclusion of discretionary activities when estimating energy requirements have been given in sections 3 and 4. Some activities will be short-lived but require considerable rates of energy expenditure, whereas others have only modest costs but are undertaken for longer periods. Socially desirable physical activities have been calculated to be equivalent to walking, but may involve a variety of activities, of which some examples were given in section 4.2.3. It is likely that many of these discretionary activities, particularly the more vigorous ones, will not be performed every day, and it is only possible to make a nominal allowance for them.
In sedentary people the allowance for discretionary activity includes provision for short periods of vigorous exercise to maintain physical fitness and promote cardiovascular health. Five times the BMR represents a steady state of exercise at about 60% of maximal work-load (5). In this report 20 minutes a day is suggested as a reasonable period of time for such exercise. This is unlikely to be excessive, since in one study it was shown that adolescent boys need at least one hour per day at this rate to achieve a significant increase in aerobic capacity (6).
The energy requirements of the elderly differ from those of the young not only because they often reduce their occupational activities, but also because their basal energy requirements decline, as discussed in section 4.3. An additional allowance has been made for an extra hour of socially desirable activity at this age.
22.214.171.124 Estimating total energy requirements. Once the separate components of energy expenditure have been identified and evaluated, the total requirement can be calculated. Some examples are shown in Tables 9–14. As far as possible, these have been calculated from observed patterns of activity described in the literature. On the basis of such patterns, approximate estimates of the total daily energy expenditure corresponding to light, moderate, and heavy work can be derived as multiples of the BMR. These estimates are shown in Table 15.
It must be emphasized that these figures are intended to be general guidelines. As far as possible, users should make their own calculations, according to the characteristics of the population concerned. This could be done in two stages. The first step is to develop the appropriate BMR factor. Adjustments can readily be made to accommodate differences in the time spent at work and in discretionary activity. Most occupations involve static activity which develops muscle strength but not cardiorespiratory efficiency. In general, therefore, discretionary activity of a dynamic nature is beneficial. However, this type of exercise may not be possible for those whose work involves heavy labour and physical fatigue. All additional times when individuals are not engaged in occupational or discretionary activities are considered to require a minimum energy expenditure at 1.4 times the BMR, except for the 8 hours assigned to sleeping at a rate equal to the BMR.1
The second step is to use the equations in Table 5 to obtain the value for the BMR appropriate for the body weight. For convenience section 8 gives tables of the total daily energy requirement for sex and age at different body weights and different values of the BMR factor.
1 The evidence available was insufficient to enable the Consultation to recommend an operational “maintenance” requirement. Any figure chosen would reflect a value judgement on what levels of activity above the minimum for survival could be appropriately included in the term “maintenance” (see page 73). The cost of an additional 1.5 hours a day of walking or about 2 hours of standing would increase energy expenditure to 1.4 times the BMR over 24 hours (3, 4). This figure should provide a guide for assessing the maintenance requirements until further published information becomes available.
|Cost/min||Average cost ×BMR||Cost/min||Average cost ×BMR|
|75% of time sitting or standing||1.51||6.3||1.79||7.5|
|25% of time standing and moving||1.70||7.1||2.51||10.5|
|25% of time sitting or standing||1.51||6.3||1.79||7.5|
|75% of time spent on specific occupational activity||2.20||9.2||3.61||15.1|
|40% of time sitting or standing||1.51||6.3||1.79||7.5|
|60% of time spent on specific occupational activity||3.21||13.4||6.22||26.0|
a Times and energy costs of sitting, standing, moving around, and work tasks are composite values derivedfrom published and unpublished data (Annex 5).
b Based on young adult females (18–30 years), Wt 55 kg, BMR 0.90 kcal th (3.8 kJ)/min (Table 5).
c Based on young adult males (18–30 years), Wt 65 kg, BMR 1.16 kcalth (4.9 kJ)/min (Table 5).
|Age 25 years, weight 65 kg, height 1.72 m, BMI 22|
Estimated basal metabolic rate: 70 kcalth (290 kJ) per hour
|In bed at 1.0 × BMR||8||560||2340|
|Occupational activities at 1.7 × BMR||6||710||2970|
|• Socially desirable and household tasks at 3.0 × BMR||2||420||1760|
|• Cardiovascular and muscular maintenance at 6 × BMR||1/3||140||580|
|For residual time, energy needs at 1.4 × BMR||7 2/3||750||3140|
|= 1.54 × BMR|
Note: These data may be compared with those from Garry et al. (7) on office clerks in the mining industry, who on average measured 1.72 m and weighed 64.6 kg with an energy expenditure of 11 715 kJ per day. The energy spent during sleep was 17.9%, in occupational activities 31.8%, and in non-occupational activities 50.3%. This compares with the present suggested proportions for the population of 19.3%, 32.8%, and 47.9% for each group of activities.
|Age 25 years, weight 58 kg, height 1.61 m, BMI 22.4|
Estimated basal metabolic rate: 65 kcalth (273 kJ) per hour
|In bed at 1.0 × BMR||8||520||2170|
|Occupational activities at 2.7 × BMR||7||1230||5150|
|• Socially desirable and household tasks at 3.0 × BMR||2||390||1630|
|• Cardiovascular and muscular maintenance—not needed if moderately active||-||-||-|
|For residual time, energy needs at 1.4 × BMR||7||640||2680|
|= 1.78 × BMR|
Note: These data compare with 24-year-old Kaul males in Papua New Guinea, 1.62 m tall and weighing 57.4 kg. Their actual energy expenditure was 10 960 kJ (8).
|Age 35 years, weight 65 kg, height 1.72 m, BMI 22|
Estimated basal metabolic rate: 68 kcalth (284 kJ) per hour
|In bed at 1.0 × BMR||8||545||2280|
|Occupational activities at 3.8 × BMR||8||2070||8660|
|Discretionary activities at 3.0 × BMR||1||205||860|
|For residual time, maintenance energy needs at 1.4 × BMR||7||670||2800|
|= 2.14 × BMR|
|Age 75 years, weight 60 kg, height 1.6 m, BMI 23.5|
Estimated basal metabolic rate: 54 kcalth (225 kJ) per hour
|In bed at 1.0 × BMR||8||430||1810|
|• Socially desirable at 3.3 × BMRa||2||355||1490|
|• Household tasks at 2.7 × BMR||1||145||610|
|• Cardiovascular and muscular maintenance at 4 × BMR||1/3||70||300|
|For residual time, energy needs at 1.4 × BMR||12 2/3||960||4020|
|= 1.51 × BMR|
a Because the elderly man has no occupational demands on his time, an extra hour has been allocatedfor walking and other similar activities.
|Age 25 years, weight 55 kg, height 1.5 m, BMI 24|
Estimated basal metabolic rate: 54.5 kcalth (230 kJ) per hour
|In bed at 1.0 × BMR||8||435||1 820|
|• Extra housework, a at 2.7 × BMR||1||150||630|
|• Socially desirable and household tasks at 3.0 × BMR||2||330||1 380|
|• Cardiovascular and muscular maintenance at 6 × BMR||1/3||110||460|
|For residual time, energy needs at 1.4 × BMR||12 2/3||965||4 040|
|Total||1 990||8 330|
|= 1.52 × BMR|
a Housewives are envisaged as needing to spend an extra hour, over and above the hour per dayapplicable to all adults, in household tasks requiring moderately high physical activity (i.e., at a gross costof 2.7 × BMR). The remaining household activities—such as sewing or knitting, ironing, some parts of foodpreparation, etc.—are included in maintenance.
|Age 35 years, weight 50 kg, height 1.6 m, BMI 19.5|
Estimated basal metabolic rate: 53 kcalth (220 kJ) per hour
|In bed at 1.0 × BMR||8||425||1 780|
|• Housework, preparing food, etc. at 2.7 × BMR||3||430||1 800|
|• Working in fields, at 2.8 × BMR||4||595||2 490|
|Discretionary activities at 2.5 × BMR||2||265||1 110|
|For residual time, energy needs at 1.4 × BMR||7||520||2 180|
|Total||2 235||9 360|
|= 1.76 × BMR|
6.1.2 Adult protein requirements
Young men. To derive the protein requirement of young male adults, the Consultation reviewed evidence from both short- and longer-term nitrogen balance studies. The short-term studies accepted for this purpose are summarized in Table 16. In all these studies protein was fed at several levels below and above an amount expected to promote N equilibrium (zero balance). The aggregated data provide an estimated mean requirement of 0.63 g/kg of highly digestible, good-quality protein. This mean value is slightly higher than the safe level recommended by the 1971 Committee (la), which was intended to be 2 SD above the mean requirement. As discussed in section 5.5, three factors contribute to the difference in estimates: studies before 1971 involved relatively high energy intakes, promoting more positive N balances; many of the earlier balance studies did not include enough levels of intake in the region of zero balance, so that the efficiency of utilization was overestimated; finally, the 1971 Committee's figures allowed 5 mg of N/kg for miscellaneous losses, in contrast to the 8 mg/kg assumed by the present Consultation.
|Protein source||No. of subjects||Mean requirementa|
(g protein/kg per day)
|Coefficient of variation (CV)||Reference|
|Single, high-quality proteins|
Usual, mixed diets
|China, Province of Taiwan||15||0.80||20.3||(15)|
a Recalculation with 8 mg of N/kg per day for miscellaneous losses.Pooled coefficient of variation = 16.2%.
|Source of protein|
and intake level
|No. of subjects||Total length|
of study (days)
|Summary evaluation of major findings||Reference|
|Egg: 0.59 g/kg per day||6||81 – 89||4 subjects in negative N balance. Body composition changes.||(23)|
|Abnormal blood biochemical changes|
|Egg: 0.57 g/kg per day||6||77 – 87||Balance improved with excess energy intake but N balance negative in 5 subjects at estimated required energy intakes.||(24)|
|Weight gain at high energy intake. Abnormal biochemical changes reversed with increase in protein intake|
|Egg: 0.57 g/kg per day||4||59 – 77||3 subjects in negative balance; improved by increased energy intake||(25)|
|Egg: 0.57 g/kg per day + a nonessential amino acid mixture|
( = 0.23 g protein/kg per day)
|6||58 – 79||Addition of non essential amino acid improved N balance. Body weight stable. Lower energy intakes required to maintain N balance than at lower N intake||(26)|
|Egg: 0.36 g/kg per day||6||77||2 subjects in negative balance; 3 in marginal balance.|
|5 subjects showed weight loss. No adverse biochemical changes, except for small decrease in Hbb|
|Milk: 0.61 g/kg per day||4||36||N balance in marginal range. No significant changes in body weight or Hb||(27)|
a Interpretation of the published data takes into consideration the present estimate of 8 mg of N/kg per day for miscellaneous N losses. This may be an overestimatewhen N intake, and hence blood urea N and sweat N concentrations are low.
b Durkin, N., et al. unpublished data, 1981.
A few longer-term balances (1–3 months) have been measured, at single levels of intake, since the report of the 1971 Committee was published. Taken as a group, these studies provide information about the range of requirements for protein. A total of 28 men were fed egg or milk protein at about the 1973 safe level (0.57–0.61 g/kg) in five separate studies (Table 17). N balance was negative in 12 men and marginal (within laboratory error) in 6 others. In another study (Table 17) five of six men fed a lower level (0.36 g of egg protein/kg) for 77 days lost weight; one man was in positive N balance, two were in negative balance, and values for the other three were in the marginal range. Unpublished studies of men fed 0.73–0.80 g/kg show that N balance was adequately maintained in all subjects at that level (28, 29). These results suggest that 0.36 g/kg may be regarded as approximating to 2 SD below the mean requirement or less and 0.73–0.80 g/kg as 2 SD above the mean or more. The average amount fed, 0.58 g/kg, is a reasonable estimate of the mean requirement of healthy young men whose habitual intake is well above this level.
In a long-term study carried out for another purpose (30), 21 men received a good mixed diet supplying 6.5 g of N/day or about 0.64 g of protein/kg. Only urinary N was measured, but at 109 days the average excretion was 5.2 g, indicating that on the average the men were in N equilibrium. Four men continued on the diet for much longer; average urinary N was 5.19, 5.08, and 5.07 g at 319, 410, and 525 days, respectively. The original records of this study cannot be located so that it is not possible to determine whether some men were consistently in negative balance, or whether most or all subjects were in balance averaged across several days. Body weight was 63 kg initially and 62 kg at day 250. These findings suggest that the average figure proposed at least meets and may exceed the average requirement of adapted individuals.
In the absence of better estimates of the average requirement, the Consultation decided to accept the mean of the values derived from the two sets of balance data. The mean values of 0.63 g/kg derived from the short-term balance studies and 0.58 g/kg from the longterm balance studies give a figure of 0.605 which could for most purposes be rounded to 0.6 g/kg per day, representing the average requirement for proteins of high quality, such as those from meat, milk, egg, and fish. The Consultation recognized that this value may be higher than the requirement of fully adapted persons but there was not enough information to improve this estimate.
In order to translate this estimate of average requirement into a level sufficient to cover individual variations within a population group (safe level of intake as defined in section 2), the coefficient of variation of the requirements must first be estimated. To obtain the coefficient of variation in the absence of data on variability from long-term studies at various levels of protein intake, the Consultation used the available information from short-term N-balance studies performed at different levels of protein intake around zero N balance (Table 16). These data show that the coefficient of variation in estimates of requirements averaged 16.2%. Assuming that this variation is approximately equally partitioned between and within subjects (where the within-subject variability includes measurement error as well as biological variability), the Consultation estimated that the true coefficient of variation of the protein requirements of adults was 12.5%. Consequently, a value of 25% (2 SD) above the average physiological requirement would be expected to meet the needs of all but 2.5% of individuals within the population. This level of good-quality protein (0.75 g/kg per day) is, therefore, thought to correspond to the lower end of the safe range of protein intakes.
Nitrogen balance data are also available from short-term studies in which men were fed several levels of protein from ordinary mixed diets (Table 16). These studies predict the mean daily dietary requirement to be 0.54–0.99 g of protein per kg of body weight. The diets required in larger amounts are mainly those that are poorly digested and the requirement for net absorbed protein does not appear to differ between the high-quality proteins and practical adult diets. The method of correcting for digestibility is discussed in section 7.3.
Young women. There are less extensive data available for adult women. The 1971 Committee (1a) concluded that obligatory urinary nitrogen losses per basal kcalth do not differ between young men and women and more recent studies support this conclusion (31). Furthermore, on the basis of short-term N-balance studies (32) performed on young women receiving proteins from different sources, there is no evidence to suggest that the efficiency of utilization of dietary protein for meeting their physiological requirements is substantially different from that of young adult men when expressed per unit of body weight.
In industrialized countries young women generally have a higher proportion of body fat than young men and therefore a lower metabolic mass per kg. This has not been found in some less privileged communities (33). However, it would be unwise to suppose that it is always the case. Therefore, the Consultation concluded that there is no justification for making a distinction between adult males and females when setting the safe intake of protein. Accordingly, the safe intake of good-quality, highly digestible protein was set at 0.75 g/kg per day for both sexes.
Older adults and the elderly. Since many age-related body changes appear to occur continuously throughout adult life, protein allowances for adults should ideally be those that best preserve bodily functions from early adulthood to old age. Protein needs might be expected to change progressively during aging, since body composition, physiological functional capacity, physical activity, total food intake, and frequency of disease alter with age (see sections 3 and 4). However, there is not, at present, sufficient information to establish firm recommendations based on such a continuum. Nevertheless, the elderly make up an important section of the population for whom estimates of protein requirements must be developed as a public health measure.
Some recent observations on age-related changes in body composition and protein metabolism, especially relating to muscle, suggest that utilization of dietary protein and essential amino acids may differ between the young and old adult (34). Direct studies of the amount of dietary protein needed to bring older adults and the elderly into N equilibrium and maintain protein nutritional status are limited (34). Unfortunately, four recent studies (35–38) do not provide a consistent picture of the protein needs of the elderly. In one case, 0.8 g/kg per day of egg protein was not enough to maintain N balance in the majority of elderly men and women over a 30-day period (38). However, another study, on a group of slightly less elderly subjects, found this level of protein to be adequate (36). In both these studies body weight was maintained but energy intake was less in the former than in the latter, suggesting that activity patterns may have been different in the two groups.
It is improbable that high intakes of protein can prevent the aging process in adults, since measurements showing loss of lean body mass and tissue function with age have been made in Western countries in which the daily consumption of protein by adults is customarily about twice the estimated lower limit of the safe protein intake of 0.75 g/kg. It is not known whether populations living at the level of 0.75 g/kg of dietary protein or less show different losses of lean body mass and tissue function.
In view of these considerations, the Consultation concluded that the safe intake of protein should not be lower than 0.75 g/kg per day for older adults and the elderly. This figure is higher than that for younger adults in relation to lean body mass, because it is an accepted fact that protein utilization is less efficient in the elderly.
6.2.1 Requirements during pregnancy
126.96.36.199 Energy. During pregnancy extra energy is needed for the growth of the fetus, placenta, and associated maternal tissues. Basal metabolism rises (37–41), partly due to the increased mass of active tissue (fetal, placental, and maternal), the cost of increased maternal effort (e.g., cardiovascular and respiratory work), and the cost of tissue synthesis.
In well nourished populations in the developed countries, the weight gain during pregnancy is about 12.5 kg and the median infant birth weight is 3.3 kg, with a coefficient of variation of 15%. The average extra energy cost of this typical pregnancy has been calculated to be about 335 MJ (80 000 kcalth) over the 9-month period (42), distributed, according to the report of the 1971 Committee (1a), as an extra 630 kJ (150 kcalth) day during the first trimester and 1465 kJ (350 kcalth)/day during the second and third trimesters.
It is difficult to calculate accurately the energy needs during pregnancy. Women of small stature tend to have small babies and would logically fall in the lower range of normal weight gains and hence need less additional energy than the average. Obese women need to gain less fat than slimmer women, and women who are underweight for their height should need to gain more than the average. The extra dietary energy requirement in pregnancy also depends on the extent to which mothers can and do reduce their physical activity. It is clearly desirable to increase dietary intake to spare maternal tissue, allow for satisfactory growth of the fetus, adnexa, and breast tissue, and to sustain a desirable pattern of physical activity. The need for generous fat reserves is arguable, but deposition of some fat is associated with a more satisfactory infant birth weight.
Many recent studies of food intakes of well nourished pregnant women (43) indicate that these extra energy requirements for tissue deposition are not always accompanied by commensurate increases in intake. Nevertheless, women receiving less than these extra energy intakes seem to deposit enough extra body fat to provide the reserve needed for subsequent lactation, and the fetal and maternal tissues grow satisfactorily (44). Although the evidence is only tentative, it appears that the physical activity of such women is reduced. It is also possible that metabolic changes occur in pregnancy that result in a greater economy of energy utilization. Some (41, 45) but not all (46) studies of well nourished pregnant women indicate that the slowing of self-paced work, e.g., climbing stairs, is usual, so that the energy expended per unit time is maintained at approximately the same level as in the non-pregnant state. However, the energy cost of fixed-pace work is increased, as would be expected from the increases in BMR and body mass, and shows no evidence of improved efficiency under conditions of unrestricted food intake.
If women begin pregnancy with marginal nutritional reserves (e.g., some teenagers in developed countries and many women in developing countries), and if they cannot reduce their previous level of activity, it was the Consultation's view that every effort should be made to provide the full energy allowance.
Because some fat should be deposited early in pregnancy, and because appetite and periodic work requirements vary greatly, there is little evidence to suggest that the extra energy requirement differs between the three trimesters. The Consultation advised an average addition of 1200 kJ (285 kcalth) daily throughout pregnancy. Where healthy women reduce their activity, it is considered reasonable to reduce the average additional allowance to 840 kJ (200 kcalth) daily.
188.8.131.52 Protein. The total protein requirement of a woman gaining 12.5 kg during pregnancy and delivering a 3.3-kg infant has been estimated to be 925 g (42), or 3.3 g per day throughout pregnancy. The rate of storage is not constant; estimates provided for the first, second, third, and fourth quarters are, respectively, 0.64, 1.84, 4.76, and 6.10 g of protein per day. Previous committees have used these figures to derive estimates of the extra protein needs of pregnant women.
A second approach has been to study protein needs during pregnancy by N balance. These studies indicate that in the second half of pregnancy, there are retentions about 50% greater than can be accounted for by the tissues included in the sum given above (fetus, placenta, maternal tissues, and blood). The difference between the two estimates is about 330 g of protein, or approximately 1.6 kg of lean body mass. If the higher figure predicted from N balance is correct, then the amount of fat storage during the second half of pregnancy must be far lower than is currently supposed. Analysis of the regression of N balance on weight gain measured in a recent study (47) shows the composition of the gain in the second half of pregnancy to be 12% protein, a figure which is compatible with the body composition of the newborn (see section 3.3). There are not sufficient data for the first half of pregnancy to enable a judgement to be made on the quality of the available information.
Clearly, these discrepancies merit further investigation, but for the time being the Consultation felt that protein needs should continue to be assessed on the calculated increment of 925 g protein, the average gain, plus 30% (2 SD of birth weight), which should cover the protein gains during pregnancy of nearly all normal women. These figures for gain must be adjusted for the efficiency with which dietary protein is converted to fetal, placental, and maternal tissues. There is no direct evidence on this subject. Two studies (48, 49) in which graded N levels, all presumed to be above requirement, were fed to pregnant women indicate the efficiency of utilization to be low (25–35%) and quite variable. At intakes nearer the requirement, the efficiency is unlikely to be so low, and therefore an efficiency factor of 0.70, derived from growth data in children (see Table 6) is accepted as applying also to pregnant women. The safe levels of additional protein computed in this manner are 1.2 g, 6.1 g, and 10.7 g per day in the 1st, 2nd, and 3rd trimesters, respectively (Table 18). However, there is evidence (50) from animals that more protein may be deposited early and somewhat less very late in pregnancy so that the distribution of deposition by trimesters may be arbitrary. Thus it is estimated that the protein requirement should be increased by an average of 6 g/day throughout pregnancy. These amounts should be added to the non-pregnant allowance and the sum corrected for digestibility (see section 7.3).
|Trimester||N gain (g/day)a||Efficiencyb||Additional proteinc|
a Estimated tissue N gained in a pregnancy producing a 3.3-kg infant; CV of birth weight 15%.
b Assuming 70% efficiency of conversion of dietary to tissue protein (see text).
c In terms of absorbed protein.
More work is required in this area of crucial importance to maternal and child health, and the Consultation recommended that it should be made a research priority.
6.2.2 Requirements during lactation
184.108.40.206 Energy. The energy cost of lactation is the energy content of the milk secreted plus the energy required to produce it. The 1971 Committee (1a) based its allowance for lactation on the assumption that about 850 ml of milk with an energy content of 3kJ (0.72 kcalth)/ml is secreted daily, with an 80% efficiency of conversion of dietary energy to milk energy. Thus, average milk production was assumed to demand 3.1 MJ (750 kcalth)/day. Since then, WHO has sponsored a collaborative study of breast-milk volume and composition (51) that suggests a slight revision of the figures. In the five populations examined (Guatemala, Hungary, Philippines, Sweden, Zaire), the volume of milk ingested by infants increased between the second and third months and then remained relatively stable until 6 months. Very few data are available from Western countries after 6 months of age. In the studies in the developing countries, milk consumption fell between the sixth and twelfth months and the level was even further reduced in the second year (Table 19). Among these five populations, breast-milk consumption in the first few months was lower for infants whose birth weights had been lower.
The present analysis of the energy requirements for lactation is based on the median milk consumption of breast-fed Swedish infants for the first 6 months and on more limited data from all populations for the later periods. Median milk volume is close to the mean value and the coefficient of variation is about 12.5%. Consumption data have been adjusted upwards by 6% to compensate for an observed underestimation of milk secreted versus milk consumed (25). The WHO study (51) found the average energy content of breast milk to be 2.9 kJ (0.70 kcalth)/ml, which agrees with earlier estimates (52, 53). Since there is no new information, the efficiency of conversion of food energy to milk is taken to be 80%. The resulting values are given in Table 19.
|Month||Median volumea||Energy content of milkb||Energy cost of lactationc|
a Data derived from the results of the WHO Collaborative Study on Breast-feeding (51).
b Taken as 0.7 kcalth (2.9 kJ) per ml.
c Assumed efficiency of conversion 80%.
If maternal reserves have not been depleted during pregnancy, the amount of dietary energy needed by the average woman for lactation should not be higher than that computed in Table 19, plus the energy required for basal metabolism and daily activity. The pattern of activity of lactating women may be changed, depending upon the circumstances of life. A woman may spend 2–2 ½ hours a day breast-feeding; this is described as “active seated work”(54).
There is no evidence yet available to support the suggestion that the BMR may also be altered during lactation. It has been postulated that an enhanced efficiency of energy metabolism might continue from pregnancy into lactation; studies on energy metabolism during lactation, as well as in pregnancy, are clearly a priority for future research.
If the recommendations for pregnancy have been met, the average woman will start lactation with some 150 MJ (36 000 kcalth) of fat reserves. A normal body composition should be re-established within 6 months by utilizing this reserve, which would thus provide about 835 kJ (200 kcalth) per day. In this case, the additional average energy requirement during the first 6 months of lactation would be about 2090 kJ (500 kcalth)/day, rather than the 2930 kJ (700 kcalth)/ day indicated in Table 19. Allowances during this and subsequent periods will need to be adjusted according to maternal fat stores and patterns of activity. During the later stages of lactation the full allowance of about 2090 kJ (500 kcalth)/day should be provided. This requirement must be increased if more than one child is being breast-fed.
220.127.116.11 Protein requirements during lactation. The average protein content of breast milk (N × 6.25) has been taken as 1.15 g per 100 ml, except during the first month, when the value is approximately 1.3 g per 100 ml (51) (see section 6.3.2). It is accepted that, as for growth in children, an efficiency factor of 70% is necessary to adjust for the conversion of dietary protein to milk protein. While there may be a small amount of tissue protein available from accretion during pregnancy, e.g., from involution of the uterus, this is not a significant factor in providing the extra requirement for lactation.
The coefficient of variation of breast-milk volume has been taken in the previous section as 12.5%. The safe level of provision for the mother should allow for those who are producing, or are capable of producing, more than average amounts of milk. Upward adjustment of the median milk volume, and hence the amount of protein secreted, by 2 SD allows for this.
The figures in Table 20 suggest a safe level of extra protein intake of about 16 g per day during the first 6 months of lactation, 12 g per day during the second 6 months, and 11 g per day thereafter. These amounts should be added to the normal estimate of the woman's protein requirement and corrected for the digestibility of the dietary protein (section 7.3).
|Month||Breast milk secreted||Maternal extra protein|
|Volumea||Proteinb||Averagec||+ 2 SDd|
a Data derived from the results of the WHO Collaborative Study on Breast-feeding (51).
b Average protein content taken as 1.3 g per 100 ml in first month; thereafter 1.15 g (see section 18.104.22.168).
c Allowing for 70% efficiency of utilization.
d CV of infant's birth weight taken as 12.5%.