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close this bookProtein-Energy Requirements of Developing Countries: Evaluation of New Data (UNU, 1981, 268 p.)
close this folderResearch papers: Protein requirements-adults, standard protocols
close this folderProtein requirements of young male adults with a rural Mexican diet
View the document(introduction...)
View the documentObjective
View the documentExperimental details
View the documentSummary of the main results
View the documentConclusions

Experimental details

1. Subjects
Eight healthy young males participated in the study in which the multiple-level nitrogenbalance technique was used while they consumed a "rural" diet. In order to have reference data, three of these subjects (A.D.F., S.R., U.R.) participated twice, first on the rural diet and later on a milk diet.

All the subjects were born in and are still living in the Malinalco State of Mexico. Their town is 120 km from Mexico City and 1,700 m above sea level, and its main economic activity is agriculture. Table 1 shows the characteristics of the subjects.

2. Physical Activity
During the study period the subjects were sedentary throughout the day with a halfhour period of exercise on a stationary bicycle, except for A.D.F. who, during both the rural and milk diet studies, exercised for 45 minutes per day.

TABLE 1. Characteristics of the Eight Subjects

Age (x S.D.) 21.5 2.92
Sex male
Racial origin mixed Indian-Spanish
Physiological status young adult
Nutritional status  
(ideal weight S.D.) 95.15 6.26
Health status normal

3. Duration of the Study
For each subject there was a stabilization period of approximately 15 days, in which the energy intake was adapted to individual requirements as judged by body-weight changes.

After the stabilization period, four different nitrogen balance studies were conducted on each subject. Each test took the following sequence:

1. One day on a nitrogen-free diet.
2. A 6-day period of adaptation to the test level of protein intake (no collections).
3. A 4 day period for nitrogen balance (collection).
4. A 3 day rest period (same diet but protein intake at 1 g/kg body weight). For the whole study each subject remained in the metabolic unit for approximately 71 days.

4. Diet
Rural Diet: A standard diet was given, consisting of corn, beans, and wheat pasta supplying, respectively, 52, 31.5, and 63 per cent of the total protein at each level.

The typical menu consisted of boiled beans, corn tortillas, pasta soup, fruit, vegetables, and lemonade (fruits and vegetables provided 10.2 per cent of the total protein). The basis for the design of this diet was the information obtained from dietetic surveys in the rural areas of the Mexican plateau. Levels of 0.4, 0.5, 0.6, and 0.7 9 of protein/kg were administered to the subjects.

Milk diet: The milk diet consisted of whole milk, cheddar cheese, and cream supplying 25, 70, and 5 per cent of milk protein, respectively. Fruit, vegetables, cornstarch, corn oil, sugar, candies, and protein free desserts (jelly, preserved peaches, and jam) were added. Fruits and vegetables provided a maximum of 16 per cent of the total protein in the diet.

TABLE 2. Amount and Source of Dietary Energy

A. Rural Mexican Diet: 41.36 2.79 kcal/kg*

Protein intake level (g/kg body weight)

% of calories 0.4 0.5 0.6 0.7
Protein 3.87 4.86 5.71 6.73
0.24 0.28 0.37 0.41
Fat 27.20 29.38 31.34 26.03
8.82 6.52 7 07 7.21
Cabohydrates 68.92 66.13 62.78 66.97
8.94 6.84 7.30 7.12

B. Milk Diet: 42.03 2.91 kcal/kg*

Protein intake level (g/kg body weight)

% of calories 0.3 0.4 0.5 0.6
Protein 2.93 2.83 4.78 5.92
0.17 0.28 0.28 0.17
Fat 25.69 31.66 32.35 31.49
2.92 0.62 3.07 6.10
Carbohydrates 71.40 64.50 62.86 62.57
2.88 10.54 3.19 6.27

* Mean S.D.

The menu essentially consisted of milk with coffee, sweet cornstarch, vegetable soup, cooked vegetables with cheese and cream, vegetable salad, fruit, and jelly. Each subject ate this diet at a level of 0.3, 0.4, 0.5, and 0.6 9 protein/kg.

Fruits, vegetables, and the non-protein ingredients were used to dilute both diets in order to obtain the different protein levels. Each daily ration per subject was prepared individually, carefully weighing the ingredients and using individual containers. The beans and tortillas were prepared in the typical way and the soup was made with pasta, onion, tomato, and chicken broth. Energy values of the diet are given in table 2.

Liquid intake was maintained constant throughout the study, and two capsules of Unicap T were given daily in order to meet vitamin and mineral requirements. A dietitian supervised all the procedures.

5. Measurements
The composition of the diets was determined by analysing the nitrogen content of the main components individually (corn, beans, and wheat pasta) plus a pool of the fruits and vegetables, using the macro-Kjeldahl method. The energy content of the diet was calculated with the aid of tables.

Nitrogen balance: The urinary nitrogen excretion for each intake level corresponds to the average urinary nitrogen excretion for the last four days of each balance period. The faecal nitrogen was determined, by the macro-Kjeldahl method, in each of the corresponding pools for the last four days of the faecal collection in each balance period.

Integumental nitrogen losses were taken as 5 mg N/kg (Calloway et al., J. Nutr., 101: 775 [1971] ). Since environmental temperature in the metabolic unit was around 20 C, no appreciable sweating occurred.