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Three separate types of studies were conducted to explore question of adult protein requirements. The first studies followed the standard UNU protocol and were designed to gather data comparable with those previously gathered from a greater variety of environmental situations. The second set of studies examined the nitrogen balance behaviour of individuals over long periods of time (two to three months), looking at how variable the individual was in terms of these data. Here the question was how representative the data from the standard protocol are. The third set of experiments was directed at the variability among individuals. These experiments examined the balances of large numbers of individuals receiving a constant intake over a short period of time, equivalent to a single level of the standard protocol. The intake levels initially were chosen to test whether the mean requirement plus two standard deviations, as estimated in previous experiments with the same diet, did indeed exceed the requirement of almost all of the larger sample. (This is the fundamental question of whether the response has a Gaussian distribution )
3.1. Standard Protocol
Results of five studies that followed the standard protocol were presented at the meeting; these were conducted in India (paper 8, two studies - one involving men and the other women), China (paper 9), Egypt (paper 10 - women), and Turkey (paper 11). An additional report from Brazil (paper 12) is included, although it arrived too late for review by the group. A summary of the status of each project with respect to the important factors listed in the methodology section is shown in table 5. This table and subsequent discussion include not only these new results, but also those reported to the previous workshop (9). Note that for comparative purposes, results, when possible, are calculated as indicated in the methodology section, and these results may differ from those reported by the original authors. While this method of analysis may not be strictly applicable for all these data sets, in some of which protein quality varied, examination of the actual data suggests that the error introduced is well below the error inherent in the experiment.
TABLE 6. N Requirements of Men in Short-Term Balance Studies in Developing Countries as Predicted by Linear Regression
Number of Subjects
N teal. =0
Mean (Mg N/kg)
|(Chen) paper 9|
|(Ozalp) paper 11|
|(Dutra) paper 12|
|Pooled data mean||114.8|
Studies in Males
Predicted intakes to achieve zero nitrogen balance for the studies involving males are shown in table 6 The predicted N requirements on egg and fish protein diets in three studies from Chile, Taiwan, and Japan range between 94 to 101 mg N/kg/day. The estimate for egg protein from Thailand is higher (124 mg N/kg/day), which could be due to unmeasured sweat losses caused by higher temperature, a habitual rice diet, and a low energy intake in some subjects.
On mixed animal and cereal protein diets, the estimated mean values ranged from 98 to 148. Similar estimates were observed in studies from Chile, Mexico, Taiwan, Brazil, and Japan, the values being 133, 121, 127, 107, and 106 mg N/kg, respectively, and averaging 120 mg N/kg/day. The high estimate of 148 for data from northern China is unexplained, but may be due in part to the large amounts of rice consumed, a situation similar to that in Thailand with the egg-and-rice diet.
Indian subjects showed the surprisingly low estimate of 81 mg N/kg for mean requirement. Factors such as high temperature may have resulted in excessive sweat nitrogen loss, and the poorer quality of the diets supplied at lower levels compared with that at higher levels might have contributed to lower zero nitrogen balance intercepts. Initial examination of the laboratory standardization exercise gives no insight into these results.
Studies in Females
Studies of N requirements of women were presented from India (paper 8) and Egypt (paper 10). Additional data were presented by Calloway from the United States.
Table 7 shows how the important factors in the standard protocol affect each of these studies.
In both the Indian and Egyptian studies, protein quality was not constant at the different levels of dietary protein, and the occurrence of excessive integumental losses was possible as the temperatures were high on some days. Integumental losses were not measured in either study. The predicted effect of these two sources of bias would be a reduction in the predicted N requirement (i.e. intercept).
In the Indian and United States experiments, dietary treatments were presented in descending order only. This design may lead to a higher estimate of the requirement, but other factors in these studies could have the opposite effect, as discussed in the methodology section.
A summary of the results is presented in table 8. When egg albumin was fed, the predicted N requirement was 73 mg N/kg. The two values for a mixed diet were 77 and 94 mg N/kg.
Although there are very few data, it appears that the N requirement for women in mg/kg is not markedly different from that for men. For high-quality protein, the estimate is about 70-75 mg N/kg. The Agarwal study predicted a need of about 94 mg/kg from a mixed diet. If the egg albumin data are adjusted for a dietary protein score of 70, the predicted N need is about 105 mg N/kg, a value close to that of Agarwal.
Additional data were presented by Calloway indicating that, when using short-term balances in women for determining nitrogen IN) requirements, the influence of the menstrual cycle on N utilization should be considered. Recent work by Kurzer and Calloway (10) reported a statistically significant biphasic cycle in urinary N excretion during the menstrual cycle in six healthy young women. Urinary N tended to rise during the follicular phase, to be lowest at about the time of ovulation, to increase sharply during the first week of the luteal phase, and to drop again just before or with the onset of menstruation. Urinary urea excretion had a pattern similar to total urinary N. In one subject, the high and low points of the cycle were about +15 per cent from the cycle mean. Of special interest is that an infectious disorder in one woman during the study appeared to suppress the cycle.
TABLE 7 Evaluation of Bias in Studies on Short-term Nitrogen Requirements in Women
|Egypt (Hussein) Paper 10||USA
|Randomization of treatments||No - descending only||Yes||No descending only|
|Variation of protein quality at different intake levels||Yes||Yes||No|
|Excessive integumental losses||Possibly temp 22-24°C||Probably temp. 25-39°C||No|
|Activity level||Moderate-uncontrolled||Moderate- uncontrolled||Light-controlled|
|Degree of confinement||None||None||Yes|
|Vitamin and/or mineral Supplements||Yes||No||Yes|
|Form of experimental diet||Usual food||Usual food||Formula|
|Use of nitrogen-free diet||Yes||Yes||No|
|Energy level adjusted for Weight maintenance||No||No||Yes|
TABLE 8 Requirements of Women in Short-Term Balance Studies as Predicted by Linear Regression
|Country and||Number of||Dietary Protein||Mean Intercept||CV|
|ref. 1 0|
TABLE 9 Summary of Data for Long-Term Studies in Adults Testing Protein Intakes Derived from Short-Term Studies
Country and Investigator
Number and Sex of Subject
Mean N-balance (0±10 mg N/kg/day)
Energy Intake (kcal/kg/ day)
Body- weight Changes
|Republic of||6 M||50||Metab.||1.08-1.36||30/70||Balancea||37-44||Stable||No adverse|
|Thailand||12 M||90||Field||1.07-1.30||20/80||Balanceb||51-66||Stable||No adverse|
|Philippines||7 F||84||Free-||1.16||40/60||Balancea||40||Stable||No adverse|
|China (Chen)||6 M||90||Free-||0.93||30/70||Balance||45||Stable||No adverse|
|Chile (Yanez)||8 M||90||Metab.||1.0||25/75||Balanceb||40 58||Variable||No adverse|
|USA||8 M||84||Free-||0.8||100 %||Balanceb||39-57||Variable||Stable 40 K|
|USA||6 M||77-87||Free-||0.57||100 %||Balanceb||42-56||Variable||Creatinine|
|USA||6 M||77||Strictly||0.36||100 %||Balanceb||42||Weight||None|
a. Some subjects reached negative N balance in response
to documented stress
b. Some subjects had periods of negative N balance
Failure to take account of menstrual cycles in determining N requirements could lead to larger coefficients of variation. For example, if requirements are predicted at the lowest and highest points of urinary excretion in each cycle, the predicted requirements would be 42 + 15 mg N/kg and 109 + 36 mg N/kg, respectively. Selection of four points at random in the cycle predicted a requirement of 70 + 54 mg N/kg. The CV is much greater, however: 77 per cent versus 27 per cent. Thus, the effect of the menstrual cycle on N requirements should be minimal if there is a random distribution of points in the cycle among the population studied, and if the distribution of dietary treatments is random with respect to the cycle.
3.2. Long-Term Studies
As discussed in the introduction and methodology sections, short-term (5 to 15 days) nitrogen balance studies are not, by themselves, sufficient to estimate protein needs. The group considered it essential that any method used to arrive at estimates of protein requirement should be validated by long-term studies. The variable time required for metabolic adaptation and present limitations in the sensitivity of indicators require that the effects of a given protein intake be studied over a period of at least 60 to 90 days before concluding that it is adequate or insufficient to meet protein energy needs.
There are multiple problems in the design and interpretation of long-term metabolic studies; most of these also confound short-term studies and have been discussed in the methodology section. The key issue in the evaluation of long-term data is the selection of indicators that adequately reflect protein nutritional status at intakes close to requirement level. Currently available methods to determine loss of lean body mass. and functional consequences of insufficient protein intake are not sensitive enough to define requirement precisely. The high variability of nitrogen balance over time within and among individuals further complicates data analysis.
Five studies (papers 9, 13, 14, 15, and 16) were presented in which subjects were fed a predominantly mixed vegetable protein. Intake was kept nearly constant for 50 to 120 days at levels derived from short-term N-balance responses but above the safe FAO/WHO recommended levels. Other studies in which the safe level or a lower level of protein of high quality were tested are included for comparative purpose These results are summarized in table 9.
It appears from these data that most young adults on a predominantly vegetable diet fed about 1.0 g/kg/day of protein are close to nitrogen balance and do not show signs of adverse effects in health or nutritional indicators (e.g., serum albumin, creatinine/height, serum transaminase levels, total body potassium, weight) over a 60-to 90-day period. Several subjects in the various studies showed periods of negative nitrogen balance associated with infections, diarrhoeal episodes, and psychological stresses. In a study by Garza (11 ) where 0.57 g/kg/day of egg protein was given, some subjects showed a decline in TBK and urinary creatinine output and elevation in serum transaminase levels after two months of diet, while their nitrogen balance remained close to zero. The significance of these changes is not fully understood. but suggests that this level is insufficient.
The transient negative N balances in several studies. and especially the stress-related negative Nbalance periods observed in the Korean study, suggest that under real-life conditions protein requirements may change episodically. These added sources of variability interact with the intrinsic variations in N utilization at the individual level.
Based on the variability of measurements of N intake, faecal nitrogen, urinary nitrogen, and the error in using a figure of 5 mg N/kg/day for skin and miscellaneous N losses, a balance of ± 10 mg/kg/day was considered equivalent to equilibrium (N balance of zero). Thus, the data presented here indicate that N equilibrium can be reached at a range of levels of protein intake. On the basis of available indices of protein nutritional status and health indices, it appears that 1.0 kg/day of a predominantly mixed vegetable protein diet, or 0.8 g of a high-quality protein are sufficient to meet longterm protein needs of most normal adults if adequate amounts of energy are provided.
3.3. Large-Sample Studies
Three investigators (Bourges in Mexico, Ozalp in Turkey, and Uauy in Chile) (papers 17, 18, 19) conducted studies using large numbers of subjects to examine the distribution of individuals' short-term balances on constant intakes. All were 10-day balance studies at a single level of nitrogen intake derived from local diets. For two of these studies (Mexico, Chile) the level fed was two standard deviations above the mean requirement, as estimated by previous standard protocol experiments. In the Turkish study the estimated mean requirement was used for the intake level.
The three studies show mean N balances close to zero, and similar intragroup variability (SD = 11.5, 12.0 and 18.7 mg N/kg/day, respectively) (see table 10).
In the Chilean and Mexican studies, 97.5 per cent of the individuals were expected to be in positive nitrogen balance, but 15 of 53 and 9 of 20 were negative, respectively - significantly different from the expected 2.5 per cent. Since in these studies the protein level fed was below the customary intake, the subjects may well have been adapting, with the results reflecting the variability of adaptation rates. Alternatively, these data suggest that the distribution of individual balances for a constant intake may not be Gaussian, but may have heavier tails than would be expected. The implication of the latter would be that more than two standard deviations would be needed to estimate the safe level of intake for a population.
TABLE 10. Large-Sample Experiments (To Estimate Variability of N Balance with Local Diets Fed at the Level)
|Subjects||49 males 19-30 yrs old Healthy Middle class Confined 20°C Sedentary||53 males 18-19 yrs old Healthy Army recruits Lower class Semi-confined 10-20°C Moderate activity||20 males 19-24 yrs old Healthy (but infested by parasites) Field workers Lower class Confined 22-24.5°C Sedentary|
|Diet||Customary energy 98 mg N/kg (estimated mean req.) Usual foods Composition fixed Prot.: 12.3±34% animal 4 meals: 8, 12, 18, 23 furs. Vitamin supplement||3,200 kcal 183 mg N/kg (estimated req. + 36 %) Chemically defined Composition fixed 25% animal, 75% vegetable Wheat, beans, rice, potatoes, 3 meals Vit. and min. suppl||Customary energy using Atwater factors 164.9 mg N/kg (estimated req. + 36 %) Usual foods Composition fixed 100% vegetable 3 meals: 8, 12,19 furs. Vit. and min. suppl|
|Study||1 day protein-free Collection last 5 days of 10-day balance Daily UN measured Faecal pool 5 days||Collection last 5 days of 10-day balance Daily UN measured Faecal pool 8 days Anthropometry days 1, 11 Biochemical initially||1 day protein-free Collection last 5 days of 10-day balance daily UN measured Faecal pool 5 days Anthropometry days 1, 11|
|Main Results (N balance)||3.7±11.5 mg N/kg 16/49 > + 10 mg N/kg 28/49 between ±10 mg N/kg 5149 < - 10 mg N/kg UN: 66.4±9.1 mg N/kg FN: 23 + 5.6 mg N/kg||3 + 12 mg N/kg 17/53 > + 10 mg N/kg 29/53 between ±10 mg N 7/53 < - 10 mg N/kg UN: 143.3±14 mg/kg FN: 31.5 + 6.3 mg/kg||5.1±18.7 mg N/kg 8/20 > + 10 mg N/kg 8/20 between ±10 mg N 4/20 < - 10 mg N/kg UN: 101.4±15.5 mg N/kg FN: 53.3±13.9 mg N/kg|
Sources: Turkey (paper 18). Chile (paper 19). Mexico (paper 17)
Two papers were presented on energy expenditure of women living under their usual conditions (papers 20, 21). McGuire and Torún prospectively studied, for one year, a group of rural Guatemalan women who were engaged in various activities. Observations were conducted on eight who became pregnant and on 15 lactating women during the study period. De Guzman's female subjects were Filipino housewives and peasant women; pregnant and lactating women were not included in her studies. The mean age, body weight, and height were comparable for both studies. The studies included measurements of food intake, specific energy expenditure, and time spent in each activity. Indirect calorimetry was used in both studies to determine the energy cost per unit of time. For basic activities like standing, sitting, and walking, comparable values were obtained in both studies, and they were 0.024, 0.023, 0.046 per kcal, and 0.021, 0.022, 0.05 per kg per minute for Filipino and Guatemalan women, respectively. The values for other tasks performed varied between the two studies, depending on the type of work and manner in which the activities were performed.
The overall daily energy intake for the Guatemalan female subjects was 1,792 + 492 kcal. However, energy expenditure was 1,985 + 483 kcal per day. The observed daily energy intake for the women studied by De Guzman was 1,785 + 553 kcal, while the estimated daily expenditure of these subjects was 2,040 + 328 kcal.
The mean energy intakes for pregnant and lactating Guatemalan women were stated to be 1,678 + 558 kcal, and 1,845 + 483 kcal per day, respectively. The mean energy expenditures of the pregnant and lactating women were estimated to be 2,044 + 167 kcal and 1,981 + 160 kcal per day.
Both studies clearly show that, using available energy balance methodology, there is a significant energy gap for these working women. Torún's study indicates that the pregnant women are in an even more marked negative energy balance It is evident that further research is necessary to assess the current energy requirement for the working woman, especially during lactation and pregnancy, and more knowledge is needed on the possible changes of energy efficiency in populations that appear to be in negative balance.
An additional paper by Ho (paper 22) provided comparable information on the energy expenditure of peasant women in southern China. as well as that of miners and shipbuilders doing various types of work in their respective occupations.
The limitations of indirect calorimetry to assess energy expenditure properly were discussed, and the need to obtain data using newly developed direct calorimeters was expressed.
An interrelationship exists between utilization of dietary protein and dietary energy. In particular, though some studies suggest otherwise, higher energy intakes usually increase nitrogen retention when nitrogen intake levels are marginal or deficient However, protein-energy interactions are not necessarily reflected simply in terms of protein or energy per se. Alterations in the relationship between protein and energy may be expressed indirectly as changes in body weight or composition and level of activity. Changes in the interrelationship may also occur as a result of differences in the composition of the energy source, the level of dietary fibre, and the nitrogen or energy digestibility. Moreover, for an accurate assessment of studies on the adequacy of protein or energy intakes, other factors must be considered, such as the activity level, the degree of sweating (temperature), the presence of disease, infection or parasites, the absence of other nutrients (minerals, vitamins), and the physiological status of the individuals from whom the data were obtained (e.g., with children, normal versus "catch-up" growth).
The research organized by FAD/WHO and sponsored by DANIDA concentrated on the interactions between protein and energy with respect to the following questions:
First, is the present "safe level'' of protein intake adequate: (a) at habitual levels of energy intake; (b) at levels of energy intake that correspond with present estimates of energy requirements?
Second, would habitual protein intakes be adequate if energy intakes corresponded to present estimates of energy requirements?
Other research reviewed at this workshop implicitly addressed these questions and it is appropriate to view their interactions.
5.1. Adequacy of Present "Safe Level" of Protein Intake
This question was specifically addressed by studies in Thailand (paper 23) and (Guatemala (paper 24) with both children and adults. The Thai studies with children 1 to 3 years of age consisted of both a shortterm (11 days) and a long-term (40 days) study period. The Guatemala study was long-term (40 days) and included children 15 to 40 months of age. These studies showed that, if the present FAD/WHO recommended energy level for children (101 kcal/kg/day) is met using the habitual diet, the "safe level" of protein is also met, N balance is positive, and growth (including catch-up growth) occurs. However, at the habitual levels of intake, only 75 to 80 per cent of the current FAO/WHO requirement for energy is satisfied, while more than the current safe level for dietary protein is supplied.
With respect to adults, it was reported that subjects in Thailand, given their habitual diets with ad libitum intake of rice, maintained N balance when protein and energy intakes ranged from 0.82 to 1.08 9 protein/kg/day and 50 to 66 kcal/kg/day, respectively. Both levels are significantly higher than present recommended levels. When protein was fed at the current safe level, nitrogen balance was achieved at about 50 kcal/kg/day, but body weight was lost.
This suggests that habitual levels of protein intake may not be adequate for some populations when fed energy intakes below 50 kcal/kg. It must be stressed that in neither of these studies was the current safe level of protein fed together with energy sufficient to maintain body weight. Therefore, the question of the adequacy of the current safe protein level at habitual energy intakes for adults has not been answered, since the test diets provided more protein than this. No studies were reported in which the current safe level has actually been tested with the energy intake at habitual levels. In the previously published study of Garza et al. (11), the safe level was tested at habitual energy intakes and proved inadequate.
5.2. Adequacy of Habitual Protein Intakes at Estimated Energy Requirements
Several long-term studies were conducted to test the adequacy of estimates of protein-energy needs derived from short-term N balance studies. In the Republic of Korea (paper 13) and Chile (paper 16), groups of healthy men were fed predominantly mixed vegetable proteins (1.0 to 1.36 g/kp/day), primarily from rice and soy, or wheat, beans, rice, and skim milk. In general. subjects maintained nitrogen balance for long periods (52 to 90 days) when their energy intakes were characterized by ad libitum stable intakes of 37 to 58 kcal/kg/day.
Women in the Philippines (paper 15) maintained approximately zero nitrogen balance on 1.16 9 protein/kg/day from rice and fish for 84 days. with a mean energy intake of 40 kcal/kg/day.
A group of 12 village men from Thailand (paper 14), who were allowed to eat rice ad libitum with a fixed intake of fish, required between 51 and 63 kcal/kg to maintain nitrogen balance; this energy value is probably above that generally considered to meet FAOIWHO requirements for men with similar activity levels. However, as the requirement for energy is highly dependent upon the level of activity, energy needs for a group cannot be evaluated unless direct values for energy expenditure are known.
In the case of children, in two long-term studies in Guatemala, children consumed their habitual diets of corn and beans in amounts to supply 1.85 to 2.1 9 protein/kpiday. The diets were supplemented with oil and sugar to provide energy intakes of 92 to 98 kcal/kg/day. These studies were confounded because the children were gaining weight at greater than normal rates, probably indicative of "catch-up" growth and energy intakes in excess of their habitual ones.
It would appear that intake of protein at requirement levels estimated from short-term N balance studies is adequate to maintain nitrogen equilibrium over the long-term in adults, and to promote lean tissue accumulation in children at the recommended level of energy intake. However, if adequate energy intake is defined as that required to maintain constant body weight, the energy intakes needed to support the protein intakes tested may be greater than the currently recommended energy intake level.
5.3. Questions of Adaptation
As noted by Waterlow (Appendix B), adequate intakes of energy and protein for children should allow for: (a) adequate nitrogen retention; (b) normal weight gain; and (c) normal levels of activity. Similarly, adequate intakes of energy and protein for adults should allow for: (a) nitrogen balance; (b) stable body weights; and (c) normal levels of activity.
Adult studies in Guatemala, Chile, Japan, Republic of Korea, the Philippines, Thailand, and the United States demonstrate similar phenomena, but are more difficult to evaluate since the energy intakes required for energy balance, as indicated by steady body weights, were more variable. All these studies have shown that at energy intakes sufficient to maintain body weight, nitrogen balance is maintained at the protein intakes studied (all of them higher than the current recommendation). However, as protein intake falls to a lower level, increased energy is needed to maintain both constant body weight and nitrogen balance (11).
Both Thai and INCAP studies in children show that at the current safe level of protein intake, and at 85 to 90 per cent of the recommended level of energy intake, the diet is adequate as measured by rate of weight gain, nitrogen retention (24-35 mg N/kg/day), and level of activity. Nitrogen balance was maintained even when the energy intake was as low as 75 per cent of the recommended level, but at the expense of a reduced rate of weight gain and/or levels of activity
Preliminary data from Jamaica (paper 26) show comparable results At the lower levels of energy intake, nitrogen turnover increased, with a rise in the rate of both protein synthesis and breakdown. In two children fed a level of protein intake of 0.7 g/kg/day, nitrogen balance was maintained when the energy intake was greater than 90 kcal/kg/day. However, nitrogen turnover and protein synthesis and breakdown were reduced to 50 per cent of that observed at 1.7 9 protein/kgiday. Nitrogen balance could not be maintained at 80 kcal/kg/day.
6.1. Short-Term Studies
Short-term studies (papers 27, 28) following the standard protocol were presented, and their data are summarized in table 11. Other related results were also discussed.
In Chile, seven pre-school children (mean age 45 + 10.5 monthsl were studied to estimate protein requirements, comparing milk and soy-isolate proteins, using four-day balance periods preceded by a four-day period of adaptation to the test diets. Milk and soy-isolate protein were given following a descending order (1.5, 1.25, 1.0, 0.25 g/kg/day), while the energy intake was kept constant at 100 kcal/kg/day. Mean protein intake to retain 24 mg N/kg/day was 0.99 9 for soy proteinikg/day and 0.71 g/kg/day of milk protein. Incorporating the variability of individual intercepts by adding 2 SD, the corresponding requirements for soy and milk are 1.23 and 1.07 g/kg/day, respectively, quite similar to current FAO:WHO figures for this age group.
These results differ from a previous study by Torún et al. using similar protein sources in younger children. These authors used a cross-over experimental design (ascending and descending) with protein intakes of 2, 1.25, 1, 0.25, and 0.5 Their mean nitrogen requirement to retain 24 mg N/kg/day (allowing for integumental losses and nitrogen retention for growth) were 0.61 9 (98 mg N) of milk protein/kpiday and 0.75 9 (120 mg N) of soy isolate kgiday. The safe level of intake was established at 0.94 9 milk proteinikg/day, or 1.01 9 soy isolate protein kg/day. These values are lower than the 1.19 9 milk protein kg/day currently suggested by FAD/WHO.
Data from the Philippines (paper 28) were presented on short-term balance studies in five children 2329 months old, some of them malnourished at the time of the study. Diet was based on rice and mung bean providing a fixed energy intake of 100 kcal/kg/day. The experimental design was of the ascending type, with protein intake starting at 0.75 g/kg/day reaching a maximum of 2 9 at 0.25 9 intervals At intakes under 1 g/kg/day, all subjects were in negative balance. The safe level of intake for 97.5 per cent of this population was estimated to be 229 mg N (or 1.43 9 protein) kp/day. Although this seems high, the authors suggest that this could be due to the higher fibre content and lower digestibility of diets with the highest protein content (mung bean). The apparent digestibilities were only 57 to 62 per cent, and faecal nitrogen was always higher than urinary nitrogen.
Tontisirin presented data from short- and long-term studies in children in Thailand (paper 29). The shortterm study was performed in infants 9 to 36 months in a metabolic-unit setting. Children were fed the typical Thai weaning diet consisting of rice, fish, and banana. In these studies, a fixed amount of protein (1.7 g/kg/day) was fed, while energy intakes were varied from 87 to 118 kcal/kg/day.
At the lowest level of energy intake (87 kcal/kg/day), N retention and weight gain were low. At energy intakes of 100 and 118 kcal/day, N retention was greater than 60 mg/kg/day, and daily weight gain was over 20 9. These data suggest that present safe levels of protein intake (FAD/WHO, 1973) are adequate for this age group provided that energy intake is 100 kcal/kg/day or higher.
6.2. Long-Term Studies
Two long-term studies (papers 24, 29) conducted in children were presented. Their data are summarized in table 12 and discussed below.
In Thailand six otherwise normal infants 8 to 12 months old were studied as they were recovering from malnutrition. Children were followed for up to 120 days in a metabolic unit and fed, throughout the study, a diet providing 1 8 9 protein/kg/day and 100 kcal/kg/day initially. Fat provided 10 per cent of the total energy in their diet; protein digestibility was about 66 to 74 per cent, and energy absorption was 93 to 95 per cent of intake. Metabolic balance studies were performed during the last three days of each 30-day period into which the study was divided.
Energy intakes were close to 100 kcal/kg/day and protein was fed at a level of 1.8 g/kg/day during the first two months of follow-up. For the last two months the energy intakes dropped to 94 kcal/kg/day, and daily protein intake was 1.7 g/kg/day. N retentions of 85 to 100 mg/kg/day were observed initially. During the last two months N retained ranged from 46 to 71 mg/kg/day. Weight gains were close to 10 g/day for the first two months and about 8 g/day for the last two months.
The authors conclude that the recommended safe level of protein intake (2) is adequate for infants provided that energy intakes are 100 kcal/kg/day or more. These conclusions are, however, confounded by the fact that some of the children studied were not fully recovered from malnutrition, although their weight/height ratios were close to normal. The higher weight gains and nitrogen retentions in the first two months could be part of catch-up growth and not exclusively related to higher energy and protein intakes. Weight for height alone is not a sufficient index to assure normal nutritional status in children below 12 months, ideally weight gain should be compared with the weight gain of normal infants of the same age.
TABLE 11. Short-Term Metabolic Balances in Children (All Studies Performed in Metabolic Unit)
|Country/ investigator||Exper. Design||Number of Subjects||Age (months)||Energy Intake (kcal/) kg/day)||Protein Intake (g/kg/day)||Protein Source||Digesti- bility (A-T)a||Nitrogen Balance (mg/kg/day)||Change in Weight||Mean N Requirement (mg N/kg/day)|
|Soy isolate||Descend||7||45±10||100||0.75-1.5||Soy||69-83 (A)||63.0-0.0||Stable||162±26c|
|(b) Milk||7||0.75-1.5||Milk||74-83 (A)||90-28||Stable||116±28|
|(Torun et al )d|
|(a) Soy isolate||Ascend||10||23±4||100||0.5-1.25||Soy||94 (T)||Gained||120c|
|(b) Milk||Descend||10||2.0±0.5||Milk||94 (T)||Gained||98|
|Ascend||4||23-29||100||0.75-2.0||Rice- bean||57-62 (T)||152||Variable||164f|
a. A = Apparent digestibility, T = True digestibility
b. Descending design: 1.5, 1.25, 1.0, 0.75 g protein/kg/day.
c. Calculated at 24 mg/kg N retention kg/day intercept
d. Descending design 2.0, 1.25, 1.0, 0.75, 0.5 g/protein/kg/day; ascending design: 0.5, 0.75, 1.0, 1.25, 1.5 g/protein/kg/day e. Ascending design: 0.75, 1.0, 1.25, 1.5, 1.75, 2.0 g/protein/kg/day rice and mung-bean protein
f. Calculated at the 0 mg N/kg/day intercept
TABLE 12. Summary of Long-Term Study in Children
|Country and Investigator||Number of Subjects||Duration (days)||Envir. Cond.||Protein intake||Energy Intake (kcal/kg/ day)||Body-weight Changes||Clinical Status|
|Guatamala (Torun et al.)||9M, 4F (15-40 months)||63-98||Metab.||1.99 (mixed)||13/87||91-103b||10-15c||One of six did not gain weight satisfactorilly|
|Thailand (Tontisirin et al.)||6M||120||Metab.||1.7 (mixed)||30/70||100||8-11c||No adverse effects|
a. AN = Animal to vegetable protein ratio
b. Net energy intake
c. Some subjects had periods of higher rate of weight gain than normal due to catch-up growth
In Guatamala nine boys and four girls 15 to 40 months old, most of them not fully recovered from malnutrition (defined as weight/height under 92 per cent), were studied for 9 to 14 weeks. These children were fed a diet typical of low-income rural populations in Central America. Children could eat as much as they wanted of the corn/bean diet, but animal protein was fixed not to exceed 16 per cent of the total. Net energy intake was computed individually by subtracting faecal from food energy measured by bomb calorimetry. The aim of the study was to determine if typical diets are capable of sustaining normal growth if enough is available. Black beans and corn provided 70 per cent of the dietary protein, other vegetables, 17 per cent, and animal protein 13 per cent.
Nine children with weights-for-height below 92 per cent of standard gained weight more rapidly than those with normal W/H. The values in the absence of illness were 2.5 and 1.3 g/kg/day, respectively. Children of comparable height gained 0.7 g/kg/day. Gain in length was, on the average, below the 0.33 mm/day expected for normal children of the same size. During episodes of infection, weekly consumption of solid foods dropped by about 10 per cent and weight gain was slower. Energy expenditure derived from the heart-rate/oxygen consumption relationship was 65.8 + 7.0 kcal/kg, with a resulting balance of +31.5 kcal/day.
The Thai and Guatemalan studies show that local diets are capable of sustaining normal growth of children when given in sufficient amounts. However, all the studies were done in the setting of a metabolic ward under closely supervised conditions and utmost care. The conclusions of these studies should be extrapolated to village conditions with great caution, since a multitude of factors may affect, in a positive or negative way, food intake or nutrient utilization. Most important are the frequency of infections and the low energy and protein density of many local diets.
6.3. Turnover Studies
Jackson (paper 26) presented a preliminary report of a nitrogen flux study, on normal or recuperated PEM infants, measured at two fixed levels of protein intake (0.7 or 1.7 g/kg/day) and at three levels of energy intakes (80, 90, and 100 kcal/kg/day) and provided interesting insights into the possible processes of adaptation to changes in dietary protein and energy. There was remarkable variability in kinetic measurements on a given diet. This was consistently greater than variation within a single individual on different diets. Total nitrogen flux appears to be dependent on protein intake, being half on an intake of 0.7 9 of that on 1.7 9 protein. In three children given an adequate protein intake, flux was not modified by decreasing energy from 100 to 90 and appeared to increase as energy intakes fell to 80 kcal/kg/day. The changes in whole body protein breakdown appear to be more sensitive to the level of dietary protein and energy. Although very limited by the few subjects and great variability, this study from Jamaica suggests that dynamic measurements of protein metabolism may be useful to assess adequacy of protein-energy intakes.
One paper, presented by Molla (paper 30), was specifically designed to deal with the effect of infections on protein energy requirements of children in Bangladesh. This study examined, in a hospital setting, the food intake and nitrogen balance in children with diarrhoeal disease of different aetiologies (cholera, rotavirus, enterotoxic Escherichia cold (ETEC), and Shigella). Energy absorption during the acute stage of the disease was 81, 55, 88, and 68 per cent of intake, respectively. Nitrogen absorption dropped to 42, 45, 58, and 41 per cent, depending on the aetiology of the diarrhoea. Children were allowed to eat as much as they wanted of their customary diet and were accompanied by their mothers or other family members. Food intake dropped to 70 per cent of recommended levels during the first week of diarrhoeal disease. Absorption of nitrogen and fat ranged from 40 to 80 per cent; carbohydrate absorption was least affected.
Follow-up studies performed two weeks later showed that most patients had increased food intake to meet or exceed current recommendations, and intestinal absorption had become normal. However, children affected with rotavirus diarrhoea continued to show impaired nitrogen absorption for up to eight weeks after the acute episode.
Nitrogen balance studies showed that. with continued unrestricted food intake, 52 per cent of the patients with cholera, 25 per cent with diarrhoea due to rotavirus, 54 per cent with ETEC, and 33 per cent with Shigella were in positive balance.
In the long-term study by Torún (paper 24), several children suffered 12 episodes of mild infections, during which a tendency to decrease food intake was observed together with increase in urinary nitrogen excretion, weight loss, or a decrease in rate of weight gain.
A study by Espinoza et al. (paper 31 ) explored the effect of fibre on nutrient digestibility. Subjects were fed 0.8 9 of egg protein with either 6 or 20 9 of structured fibre Significant increases occurred in stool weight, nitrogen, and fat content with a concomitant decline in urinary N that compensated so that N balance was maintained.
It would be important to differentiate whether faecal nitrogen increased because of an alteration in absorption of ingested nitrogen, or an alteration in colonic flora producing an increased loss of endogenous nitrogen, or a combination of the two. The study also included measurements of fibre constituents. Digestibility of fibre was 37 per cent and 56 per cent at low and high fibre intake, respectively.
Bressani (paper 32) reported data on the effect of the polyphenol content of beans on protein digestibility. There was a significant increase in faecal N loss when polyphenols were added to the diet.
Scrimshaw described the work of Vysotsky (paper 33) of the USSR that yielded valuable comparative information on obligatory N losses. Urinary N loss in subjects consuming a protein-free diet was 2.5 g/day, which is similar to other reported data.
Rand and Zacharias (paper 34) presented prospective data on the adolescent growth of 338 healthy girls of Newton, Mass., United States. Height measurements were taken monthly, and growth velocities were estimated from a fitting of the data expressed as gain per year starting at 6 years of age and ending at 1 8 years. These results show the variability of prepubertal linear growth as well as of that observed during the growth spurt. Adolescent linear growth also varies significantly among individuals in relation to a specific age in which peak velocity is reached. For example, at age 15 the 25th percentile of the linear growth velocity is 0.3 cm/year, the median is 0.8 cm/year, the 75th percentile is 1.6, and the 95th percentile is 4.2 cm/year. Thus, variability in height gain in normal adolescent populations may be up to five times the median value.
A paper by Rio et al. on protein energy needs during the acute stage of recovery from malnutrition in infants, presented and discussed in the MIT 1980 meeting, is included in this publication (paper 35). This research found that recovery from serious proteincalorie malnutrition, as judged by weight and arm circumference, was more rapid in infants receiving 1 7 per cent protein calories than in those receiving 11 per cent protein calories.
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