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In nutritive value maize is quite similar to other cereal grains. In fact, it is somewhat superior to wheat flour and only to a small extent below rice. These are the three cereal grains most consumed by people throughout the world. The problem with maize lies in the diet of which it is a component, a diet mostly deficient in the kind of supplementary foods necessary to upgrade the nutrients ingested in relatively large amounts of maize. Maize-consuming populations would be nutritionally better off if the maize consumed had the lysine and tryptophan genes of QPM or if it were consumed with a sufficient amount of protein foods such as legumes, milk, soybeans and amaranth seeds and leaves. This, however, will not occur in the near future, and therefore other measures should be taken. In this section, a number of possibilities, the results of studies to improve the nutritional quality of maize-based diets, are presented.
Throughout the world, particularly in developing countries, the diet of populations is based on the consumption of a cereal grain, usually maize, sorghum or rice, and a food legume, either common beans or any of the others known. Results of many studies have shown that these two types of basic food nutritionally complement each other. A complementary effect was observed, for example, when animals were fed diets in which the protein was derived from maize and common beans in various proportions from 100:0 to 0:100. When each component provided close to 50 percent of the protein in the diet a high quality was obtained, higher than the individual qualities of the components alone. The reason for this is the essential amino acid make-up of each component. Maize protein is deficient in lysine and tryptophan but has fair amounts of sulphur-containing amino acids (methionine and cystine). On the other hand, the protein of food legumes is a relatively rich source of lysine and tryptophan but is low in sulphur amino acids (Bressani and Elías, 1974). From these studies it was concluded that the protein of beans or food legumes complements the protein of maize best in the proportion of 30 parts beans to 70 parts maize.
This complementarily is also found in maize with cowpeas, with mung beans, with soybeans and with other food legumes. The response is the same even if the protein level in the diet is not fixed as in the example above but varies depending on the protein content of each component. Beneficial results have been obtained with oil added to the diet in amounts from zero to 10 percent. It is also of significance that food intake was highest at the maximum level of complementation. That is, a higher energy intake was also observed.
The great importance of protein quality has been missed by those who claim that energy is more limiting in diets than protein. The complementary effect described above has also been shown to occur in humans. Nitrogen balance was tested in studies of children fed lime-treated maize and beans in two fixed ratios and ad libitum, as selected by the children (Figure 6). The nitrogen balance at the fixed ratio in Phase 1 (76:24) was lower than when they were fed a ratio of 60:40 maize to beans (Phase 2). Nitrogen balance improved when the children were allowed to select, and the selection was close to seven parts maize and three parts beans by weight. Equally important is the fact that total food intake also increased. The usual intake of maize and beans established by dietary surveys in the 1960s varied between 11:1 and 18:1; therefore the supplementation of beans was relatively small. More recent data (Garcia and Urrutia, 1978) for three-year-old children gave an 8:4 maize-to-bean ratio, and the ratio was even poorer for children 6 to 11 months of age.
Combinations of maize and bean protein, although of relatively high protein value in tests in animals, are not adequate for the treatment of children with protein malnutrition. Furthermore, the increase found by Arroyave et al. (1961) in plasma amino acid levels following a test meal of milk was much higher after a period of treatment with a 1: I maize-bean combination than the response observed when milk protein was given after treatment with either milk or a vegetable mixture made of maize, cottonseed flour, torula yeast and minerals (Bressani and Scrimshaw, 1961). These results confirmed the inadequacy of the maize and bean diet. Likewise, nitrogen balance results in children fed maize and bean mixtures as compared with milk and other vegetable proteins have been relatively low. Gómez et al. (1957) reported on nitrogen balance experiments made on eight children, one to five years old, with chronic severe malnutrition, who were kept on a diet of maize meal and beans during the experiments. Both nitrogen absorption and retention were extremely variable from child to child; four children had a positive nitrogen balance, and four negative. The addition of tryptophan and lysine to the maize-bean diet greatly improved nitrogen absorption and nitrogen retention in four cases. In these studies, no indication was given on the amounts of maize and beans mixed, and protein intake varied from 1.53 to 8.50 g per day. Frenk (1961) also found poor performance in children fed maize and beans. A significant improvement was obtained upon supplementation with fish meal.
FIGURE 5 - Nitrogen retention of children receiving maize/bean diets
In common with other investigators, Hansen (1961) found that milk initiated the cure of kwashiorkor without difficulty; a two-component mixture of 66 percent maize meal and 3 3 percent cowpea meal, however, did not initiate a cure in the three cases treated with it. A three-component mixture made up of equal parts of maize meal, maize germ and cowpea (Vigna sinensis) brought about satisfactory recovery in the one case in which it was employed.
It would require 238 g of the dry three-component mixture and 267 g of the two-component mixture to supply the essential amino acids contained in 100 g of skim milk. Since the vegetable formulas also require greater dilution, it is difficult to supply enough of them to meet protein needs.
Scrimshaw et al. (1961) considered that excessive bulk relative to protein content was a major reason for the lack of success in initiating cure of kwashiorkor with mixtures of maize and beans. Hansen et al. (1960) stated that the differences in biological value of the proteins tested were clearly reflected in nitrogen retention, which averaged 13 to 14 percent for milk, 8.8 percent for the two-component mixture and only 5.7 percent for the three component mixture. It was concluded that the two- and three-component mixtures were each adequate to prevent kwashiorkor after initial recovery from the disease but that only the three-component mixture had proteins sufficient in concentration and quality to be satisfactory for use in treatment.
It should be noted that the two-component mixture of 66 percent maize meal and 33 percent cowpea meal is not the best combination of these two sources of protein. Bressani and Scrimshaw (1961) reported that in the best mixtures of these two foods, cowpea provided 50 to 75 percent of the protein, and maize 50 to 25 percent.
In other studies by Hansen et al. (1960) and Brock (1961), the nutritive value of maize alone and of maize supplemented with lysine and tryptophan, with pea flour and fish flour and with pea flour and milk was measured by means of nitrogen balance. Nitrogen retention was increased significantly by each form of supplementation, but at protein intakes of less than 2.5 g per kg body weight per day N retention was significantly less with the lysine and tryptophan supplement or the pea-flour supplement than with a milk diet. These differences disappeared at higher intakes of protein. The maize-pea mixture supplemented with 12 percent milk or with 10 percent fish flour resulted in nitrogen retentions comparable to those of a milk diet at all levels of protein intake. These variable results for bean and other legume seed proteins may be due to the type of legume seed used, to amino acid deficiencies or to some unknown factor. They deserve further investigation, because legume seeds have good potential for helping to solve the nutritional problems of the world.
Baptist and de Mel (1955) obtained a highly satisfactory response in 23 Ceylonese children one to six years old fed a mixed diet of three cereals and four legumes supplemented with skim milk. On the other hand, Navarrete and Bressani (1981) reported from nitrogen balance studies in adults that a bean diet produced nitrogen equilibrium at an intake of 114 mg N per kg per day; however, an 87:13 maize/bean mixture induced nitrogen equilibrium with an intake of 98 mg N per kg per day.
All these studies suggest that even though maize protein is improved in nutritive value upon addition of beans, its quality is still not fully adequate to feed infants and preschool children. This was evident when high-quality protein supplements were also tested with the maize/bean diet. Bulkiness limiting intake and nutritional quality are two factors of importance in maize/bean mixtures or diets.
It has been shown that adding 0.3 percent L-lysine HCl and 0.10 percent DLtryptophan to a diet of 90 percent maize and 10 percent beans resulted in significant increases in weight gain and protein quality. These did not increase further when methionine was also added (see Table 42). The significance of protein quality in a system based on maize and beans was observed when diets of mixtures of maize and beans plus methionine were offered. The results confirmed the limitation of this amino acid in beans, since a response was observed when more beans were included in the diet. Likewise, those diets with maize and beans and methionine also induced the subjects to consume greater amounts of food or of energy, demonstrating thus the value of protein quality in stimulating food intake (Contreras, Elías and Bressani, 1980, 1981). The results also served to demonstrate that even with the best combination-that is, a 7:3 maize-to-beans ratio-the diet is still short of an adequate quality for small children, and it is even more so when the proportion of beans is lower.
TABLE 42 - Effect on the nutritive value of a 90/10 maize/bean diet of the addition of lysine and tryptophan to maize or methionine to beans
|Dietary treatment||Ave. weight gain (g/28 days)||PER|
|Maize + lysine + tryptophan
Beans + methionine
|Maize + lysine + tryptophan
Beans + methionine
Note: Lysine 0.3% (L-lysine MCI); tryptophan 0.1%; methionine
Source: Gómez-Brenes, Elías and Bressani, 1972
Vitamins and minerals
A diet of maize and beans in the ratio of 7:3 responds to the single addition of a complete B-vitamin and fat-soluble mixture and more so to a complete mineral supplement, but not to calories or to lysine and tryptophan. The best results from double combinations have been obtained from minerals plus amino acids, minerals plus vitamins, minerals plus calories, vitamins plus amino acids and vitamins plus calories. The addition of calories plus amino acids did not significantly improve either the weight gain of the subjects or the PER of the diet. For triple combinations an adequate intake of vitamins and minerals is needed before an effect from the amino acids can be obtained, since animals fed with amino acid enriched diets probably develop a vitamin and mineral deficiency. Although this may be obvious, it is usually not acted on in practice.
It was observed that animals on a diet enriched with amino acids developed vitamin and mineral deficiencies, and many died. This was attributed to a depletion of these nutrients caused by the catalytic effect of the improved protein quality on the potential of the animal to respond to this stimulus.
Provision of additional calories in the diet resulted in a slight decrease in the quality of the diet. This suggests that the addition of calories lowered the protein intake of the diet, which in turn reduced its quality by enhancing essential amino acid deficiencies in the mixture of maize and beans. Similar results were found by Contreras, Elías and Bressani (1980, 1981) using young growing rats and pigs fed maize/bean mixtures in either an 87:13 or a 70:30 weight ratio. These authors confirmed the results previously reported and indicated that one of the main constraints in maize/bean diets is their bulkiness, which does not permit greater intakes. Results of some of these supplementations in rats are summarized in Table 43.
A number of studies have been carried out to learn whether an increase in the protein content of the diet from an increase in maize and bean proteins would improve animal performance. These showed that the use in the maize/ bean diet of a maize with 13 percent protein to replace one with 8.3 percent protein resulted in some increase in weight gain and in utilizable protein in spite of the fact that protein quality decreased as shown by PER and relative nitrogen value (RNV) figures. This was expected, since utilizable protein is the result of protein quantity and quality. When the two maize samples (low and high protein content) were supplemented in this maize/bean diet with lysine and tryptophan, a greater improvement in weight gain and utilizable protein was obtained than from the diet with the high-protein maize.
TABLE 43 - Nutritive value of a 90/10 maize/bean diet supplemented with vitamins, minerals, calories and amino acids
|Supplement||Ave. weighs gain (g/28 days)||PER|
|None (basal diet)||26 ± 2.3||1.11 ± 0.07|
|+ Vitamin mixture||49 ± 4.0||1.55 ± 0.06|
|+ Mineral mixture||65 ± 4.3||1.94 ± 0.06|
|+ Calories (5% oil)||23 ± 1.2||0.95 ± 0.05|
|+ Amino acidsa||26 ± 2.5||1.13 ± 0.08|
aLysine (0.3%): DL-tryptophan (0.10%)
Source: Bressani, 1990
Increases in weight gain and utilizable protein compared with the basal diet also resulted when the proportion of beans in the diet was increased from 10 to 20 percent, but these were lower when compared with the respective amino acid supplemented diets. These data were interpreted to mean that diets of maize and beans in a 90:10 ratio are limiting first in protein quality and to a lower extent in protein quantity (Gómez-Brenes, Elías and Bressani, 1972; Elías and Bressani, 1971; Bressani, Elías and de España, 1981). This is in agreement with the conclusions of Arroyave (1974), who indicated that for one- to two-yearold children to obtain an adequate nitrogen retention from maize and beans, similar to that from 1.27 g milk protein per kg body weight per day, 1.7 g protein per kg per day were required. These results show that the protein of common maize in the diet is improved by the addition of lysine and tryptophan.
Various studies conducted with animals demonstrated that methionine is the limiting amino acid in diets containing more than 30 parts of beans, while those diets containing more than 70 parts of maize are limiting in lysine The diet giving the highest quality is deficient in both amino acids (Bressani, Valiente and Tejada, 1962). At the same time such diets are low in total protein content. Therefore, in order to improve the quality of maize/bean mixtures, it is necessary to add protein sources rich in both amino acids. Studies with animals fed diets based on maize, beans and various animal protein sources such as chicken or beef indicated that a 20 to 30 percent addition of animal protein would result in significant increases in nutritive value (Bressani, 1987). In experiments by other researchers, animals were fed ad libitum with 1, 2,3 and 4 g of milk as a daily supplement to a diet of maize and beans. The results demonstrated that approximately I to 2 g milk per day added to a basal diet intake of 15 g per day was enough to increase the nutritional quality of the diet, evaluated from the protein quality point of view. In these studies 12 percent milk was found to be the minimum necessary to induce a relatively high improvement in the quality of the maize/bean diet. Furthermore, the effect of the supplement was more consistent when it was given on a daily basis. With growing dogs as experimental animals, Murillo, Cabezas and Bressani (1974) found 20 percent milk as the minimum complement to give the highest nitrogen balance to a maize/bean diet. This was not obtained when the basal maize/ bean diet was supplemented with lysine methionine and tryptophan as found in milk proteins. Torún and Viteri (1981) and Torún et al. (1984) showed in metabolic studies with children that a diet of maize and beans in a weight ratio of 85: 15 with 18 percent animal protein (milk) would induce good and consistent biological responses. These authors concluded from the diet used in the study that protein intakes were adequate when energy intakes corresponded to the estimates of energy requirements.
Replacement of common maize by QPM is another alternative that could improve the quality of maize/bean diets. The results obtained by feeding animals with mixtures of QPM and beans showed that, as with common maize, optimum complementation takes place at approximately a 50:50 diet protein ratio, equivalent to 70:30 maize/beans by weight (Bressani and Elías, 1969). However, there are two differences that should be noted. One is that both weight gain of the animals and protein quality were higher with the QPM/bean blends than with the common maize/bean mixtures. The second point, possibly even more important, is that the weight gain and protein quality of mixtures with more than 70 parts of maize were no different from the values found for the best mixture, a 70:30 diet. Likewise, diet intake in a 28-day experimental period increased from 224 g per animal to 388 g at the maximum point and remained constant in all other diets with higher levels of QPM in the mixture.
In other series of studies the protein quality of QPM as a component of a maize/bean diet of 82.8 percent maize and 10.5 percent cooked beans was evaluated in young and adult dogs fed at two levels of protein (Bressani & Elías, 1972; Murillo, Cabezas and Bressani, 1974). The QPM/bean diet was compared with similar diets of common maize and beans and common maize supplemented with lysine and tryptophan and beans. Nitrogen balance data showed that nitrogen retention levels for young or adult dogs fed QPM/bean diets were as high as or higher than those in which common maize in the diet was supplemented with lysine and tryptophan, and the levels were significantly higher with both diets than with maize and beans alone.
These studies, as well as studies conducted with growing pigs, indicated also that maize/bean diets are bulky, which limits the amount that can be ingested to meet nutritional needs fully (Contreras, Elías, and Bressani, 1980, 1981).
High-quality food mixtures
In many developing countries and for quite a long time, many efforts have been made to develop high-quality food mixtures that would supply the nutrients, particularly protein, provided by animal food products. Most of these foods have a relatively high protein content with a good essential amino acid pattern which can to some extent correct deficiencies of amino acids and of other nutrients in maize/bean diets, if consumed in the appropriate amounts. Studies have shown this supplementary effect to be present. Young growing animals were fed a basal diet of about 85 percent lime-treated maize and 15 percent cooked black beans. This diet was properly supplemented with minerals, vitamins and energy. Groups of animals were fed daily I, 2, 3 and 4 g of a high protein food based on maize, soybeans and skim milk. The results demonstrated that these levels, particularly the highest, effectively supplemented the basal diet, as judged by weight gain, protein utilization and biochemical parameters (de Souza, Elías and Bressani, 1970).
These diets with animal foods and with high-quality foods are effective because they are able to provide nutrients still deficient in diets based on maize and beans. Therefore, any food of animal origin and some foods of vegetable origin, such as soybeans and green leafy vegetables, would improve the quality of such diets.
An examination of a maize/bean diet shows that besides protein quality, other nutrients are deficient. The effect of adding vitamins and/or minerals to such a diet has already been described. Other studies were conducted in which the basal maize/bean diet was supplemented with small amounts of leafy vegetables such as amaranth, spinach and chipilín (crotalaria). These leafy vegetables provide not only essential amino acids and protein, but vitamins and carotenes which supply to some extent the vitamin A needs of the animal.
Various vegetables as supplements to maize/bean diets have been reported on and results are shown in Table 44. Two sets of diets were tested, one with vitamins added and the other without. The level of addition was 5 percent dry weight. All vegetables in both sets of diets improved weight gain and increased diet intake. Utilizable protein was also higher in the maize/bean diets with vegetables than in the control, and it was highest with the leafy vegetables. Nutritional values were higher with added vitamins than without. These studies clearly indicate that nutritional improvement of 87:13 maize/bean diets is possible by providing vitamins, some additional protein and essential amino acids.
TABLE 44 - Effect of various vegetables added to improve the nutritive value of a common (87/13) maize/bean diet
|Ave. wt. gain (8/28 days)||Food intake (g)||PER||RNV||Utilizable protein (%)||Ave wt. gain (g/28 days)||Food intake (g)||PER||RNV||Utilizable protein (%)|
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