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The high consumption of maize by the human population in a number of countries in Latin America and Africa and the well-established lysine and tryptophan deficiencies in maize protein motivated the search for a maize kernel with higher concentrations of these essential amino acids in its protein. The possibility of finding better varieties of maize appeared feasible on the basis of three facts. One was that by selection, oil content in the maize kernel could be increased from about 4 to 15 percent (Dudley and Lambert, 1969). This increase was obtained by increasing the size of the germ, the part of the kernel where the oil is concentrated. The same researchers showed that it was possible to increase total protein content from about 6 to 18 percent by increasing the prolamine (zein) fraction in maize endosperm. The third finding was the wide variability in lysine content reported among varieties and selections of maize.
TABLE 35 - Summary of the nitrogen balances of children fed whole milk and opaque-2 maize (nitrogen values in mg/kg/day)
|Treatment||Nitrogen intake||Faecal nitrogen||Urinary nitrogen||Nitrogen absorbed||Nitrogen retained||% N intake absorbed||% N intake retained|
|1.8 g protein/kg/day|
|1.5 g protein/kg/day|
Source: Bressani, Alvarado and Viteri, 1969
The search for a high quality protein maize succeeded when Mertz, Bates and Nelson (1964) announced their discovery that the opaque-2 gene used as a marker in maize breeding significantly increased lysine and tryptophan in the cereal protein.
Results from initial alkaline processing studies of opaque-2 maize (cultivated in Indiana, United States, in 1965) showed that the process did not induce significant nutritional changes in the dough or in the tortillas, as concluded from chemical data and biological trials with rats.
The protein quality of alkali-processed opaque-2 maize was evaluated in children using the nitrogen balance index (the relationship between nitrogen absorption and retention). Six healthy children were used in two studies. The average nitrogen balances, at protein intake levels of 1.8 and 1.5 g per kg body weight per day, are presented in Table 35 (Bressani, Alvarado and Viteri, 1969). As can be observed, there were no significant differences in nitrogen retention among the children fed the diets based on milk and on alkali-processed opaque-2 maize when the level of protein intake was 1.8 g per kg per day. The data demonstrate differences in nitrogen absorption.
The apparent protein digestibility for processed opaque-2 maize averaged 73.5 percent in these studies. Based on faecal metabolic nitrogen determined in the children, true protein digestibility was 83.8 percent. From these results, it was concluded that the quantities of opaque-2 maize ingested by the children were 16.3 to 16.7 g and 12.9 to 14.5 g per kg body weight to take in 1.8 and 1.5 g protein per kg per day, respectively. These figures are equivalent to a total maize intake of 140 to 227 g per day, amounts similar to those commonly ingested by children in Guatemala.
With the data obtained in this study and data on urinary endogenous nitrogen, the relationship between nitrogen absorption and retention from milk and from opaque-2 maize was calculated. This nitrogen balance index constitutes a good measure for the biological value of proteins. The index was 0.80 for milk and 0.72 for opaque-2 maize, establishing then that the protein value of opaque-2 maize is equivalent to 90 percent of the biological value of milk. When the figure for true digestibility was used, the biological value of opaque-2 maize protein was calculated to be 87.1 percent. The figures also indicated that 90 mg of nitrogen must be absorbed from opaque-2 maize to obtain a nitrogen equilibrium.
For comparative purposes the same type of analysis was carried out for common maize in children (Scrimshaw et al., 1958; Bressani et al., 1958, 1963). Data on the nitrogen balance index were obtained from various studies in which children were fed with maize proteins as the only protein source in their diet. The biological value calculated was 32 percent. These data demonstrated again the low quality of common maize protein.
The difference between the nutritive value of opaque-2 maize protein and that of common maize is clearly shown in Figure 2, obtained from data in the studies described above. This figure shows the nitrogen retention in groups of children fed exclusively with opaque-2 and in others fed with common maize, in both cases at different protein intake levels. The supplementation effect of lysine and tryptophan on common maize is also shown. Even at intakes of 400 or 500 g of common maize, nitrogen retention is quite low, and this decreases to lower levels when the intake is reduced to200 or 300 g per day. With opaque-2, on the contrary, intakes of 140 or 230 g per child per day induced positive retentions that exceeded even those obtained with common maize supplemented with lysine and tryptophan. This suggests that it may be necessary to supplement common maize with other amino acids to make it comparable in protein value to opaque-2 maize.
FIGURE 2 - Nitrogen retention of children fed milk, common maize (alone and supplemented) and opaque-2 maize
The difference between opaque-2, common maize and the latter supplemented with lysine and tryptophan can be attributed to the better essential amino acid pattern found in opaque-2 maize, since the digestibility of the three is essentially the same. QPM maize also has a lower leucine content, implicated in the low nutritional value of maize.
The information presented clearly indicates the superiority of opaque-2 maize protein to that of common maize, a fact that is of great importance for populations consuming large quantities of maize as part of their habitual diet.
In a study by Luna-Jaspe, Parra and Serrano (1971) the nitrogen retention of common maize, Colombian opaque-2 maize (ICA H-208) and milk was compared in three children aged 24 to 29 months and weighing 5.9 to 10.1 kg. The protein and calorie intakes were approximately 1 g and 100 calories per kg body weight daily. Nitrogen retention was negative when the children received the opaque-2 maize. Common maize, however, gave an even lower or more negative figure. When milk was given, one child showed a negative balance and the other two a positive one, with the average balance on the positive side.
The authors indicated that the apparent protein digestibility of common maize was 61.5 percent, opaque-2 maize 57.9 percent and milk 66.4 percent. They also concluded that opaque-2 maize is of a higher nutritional value than common maize. They pointed out, however, that its use for young children with a rapid growth rate should be carefully controlled, and they could not recommend it as the main source of daily protein intake.
The results of these investigators are in agreement with those reported by other workers (Bressani, Alvarado and Viteri, 1969). The latter found that with 90 mg N absorbed per kg body weight per day, nitrogen equilibrium was obtained. The investigators in Colombia found that 90 mg absorbed nitrogen yielded a relatively low negative retention, while 100 mg absorbed nitrogen yielded nitrogen equilibrium. The differences between the results were not significant, and they could be explained by the age of the children, who were younger in the Colombian study and had lower body weight than the subjects used in the 1969 study. A more important factor was the lower protein intake. In any case, the data suggest that a minimum daily intake of approximately 125 g of opaque-2 maize might guarantee nitrogen equilibrium. This could not be obtained by using even twice the amount of common maize.
Similar studies were conducted by Pradilla et al. (1973) using the same variety of maize but with the opaque-2 gene (H-208 opaque). A crystalline endosperm containing the opaque-2 gene was also tested. The results are shown in Table 36, in which similar values may be observed for digestibility, biological value and nitrogen retention for the two maize selections containing the opaque-2 gene. These values were slightly lower than those for casein but significantly higher than values for common maize. In more recent studies Graham et al. (1989) evaluated QPM Nutricta, a maize variety containing the opaque-2 gene. This maize is high yielding, has a hard endosperm and contains high levels of lysine and tryptophan, although not as high as those in the original opaque-2 maize first studied. These authors used six male children aged 7.9 to 18.5 months who were recovering from malnutrition. Common maize and QPM as well as a casein diet were fed to provide 6.4 percent of the calories as protein. Total energy intake was approximately 125 kcal per kg per day, which was calculated to support weight and growth at previously established rates. The nitrogen balance results are shown in Table 37. Nitrogen absorption from QPM and common maize was 70 and 69 percent respectively, and 82 percent from casein. Nitrogen retention as a percentage of intake was 32 percent for QPM as compared with 41 percent for casein and 22 percent for common maize. These results, like others previously reported, confirm the great superiority of opaque2 maize to common maize as food for children.
TABLE 36 - Comparative nitrogen balances in children fed QPM and common maize
|Protein||Protein digestibility (%)||Net protein utilization (%)||Biological value (%)||Nitrogen source retention (g/day)|
aHigh lysine and tryptophan
Source: Pradilla et al., 1973
TABLE 37 - Digestibility and energy and protein use from common maize, quality protein maize and casein, measured in six infants
Graham et al. (1980) and Graham, Placko and MacLean (1980) also reported on studies of eight convalescent malnourished children, 10 to 25 months of age, who were fed opaque-2 and sugary-2 opaque-2 endosperm and the whole kernel. Protein was fed so as to provide 6.4 percent of total calories, and the diets provided 100 to 125 kcal per kg body weight per day. The results showed an apparent N retention from the endosperm meal lower than that from the whole kernel meals, and both were lower than from casein. The difference between whole kernel and endosperm nitrogen retentions can probably be attributed to the amino acids contributed by the germ. The same researchers reported on plasma-free amino acids in the studies described above and concluded that the types of maize tested were possibly limiting in lysine tryptophan and isoleucine.
These authors also reported that for the children to match N retention from casein, presumably equal to the requirement, they would have to consume 203.9, 148 or 122.5 percent of their energy requirements as common, opaque2 or sugary-2 opaque-2 endosperm meals, which is impossible. For whole meals, they would have to consume 108.2, 90.3 or 84.2 percent of their energy as common, opaque-2 or sugary-2 opaque-2 maize.
Growth studies of children fed QPM have been conducted by various workers, among them Amorin (1972) and Valverde et al. (1981). In all reports, QPM was significantly superior to common maize and only slightly below the growth response observed when milk was fed.
Graham et al. (1989) stated: "To anyone familiar with the nutritional problems of weaned infants and small children in the developing countries of the world, and with the fact that millions of them depend on maize for most of their dietary energy, nitrogen and essential amino acids, the potential advantages of quality protein maize are enormous. To assume that these children will always be given a complementary source of nitrogen and amino acids is a cruel delusion."
Two studies evaluating the protein quality of opaque-2 maize for human adults have been published. In the first, Clark et al. (1967) used ten university students as subjects in two experiments. The maize utilized was finely ground and included the whole grain. It contained 11 to 12 percent protein, 4.65 g lysine per 16 g N and 1.38 g tryptophan per 16 g N. values similar to those of the opaque-2 maize used in the study of children by Bressani, Alvarado and Viteri (1969). The maize was given in quantities of 300, 250, 200 and 150 g per day, which provided 5.58, 4.65, 3.72 and 2.79 g nitrogen per individual per day. The results of one experiment are shown in Table 38. All the individuals were in positive balance with an intake of 300 g of the maize and all of them were in equilibrium when they were administered 250 g. The 200 and 150 g levels resulted in a negative balance. With these data the regression equation between nitrogen balance and maize consumed was calculated. On the average, nitrogen equilibrium was obtained with an intake of 230 g.
TABLE 38 - Average daily nitrogen balance in adult human subjects fed at different intake levels of opaque-2 maize
|Maize kernels||Human weight (kg)||
aTotal nitrogen intake: 6.00 g
Source: Clark et al.. 1967
The same authors studied the effect of lysine or tryptophan supplementation alone. Only one subject showed improved nitrogen retention. The addition of methionine did not induce any change. This indicated that the protein of opaque2 maize was not deficient in these three amino acids for adult human subjects. Similar results were reported by Clark et al. (1977) for adult human subjects fed QPM and sugary-2 opaque-2 maize.
Unfortunately no studies have been done on adult human subjects comparing opaque-2 and common maize in the same study. The protein quality of common maize has, however, been evaluated in human adults by Kies, Williams and Fox (1965). In one study ten subjects were fed degermed maize to provide nitrogen intakes of 4, 6 and 8 g per day. The results clearly indicated that when the degermed maize provided 4 and 6 g of nitrogen, the average nitrogen balance was negative. When the intake increased to 8 g of nitrogen per day, the balance became positive. The regression between nitrogen intake and nitrogen retained was calculated. From the equation it was calculated that 6.9 g of degermed maize nitrogen was necessary to give nitrogen equilibrium. The regression coefficient, multiplied by 100 and divided by the protein digestibility, gives the biological value of the protein. In the present case this value was 46.5 percent.
Based on 8.0 g protein per 100 g degermed maize, an intake of 6.9 g nitrogen is equivalent to 539 g maize. This figure is close to levels of maize consumed by adults in Mexico, Guatemala and El Salvador.
In the study described above lysine and tryptophan added alone did not produce changes in average nitrogen retention. When both amino acids were added, however, nitrogen retention increased - not necessarily because of the higher amount of nitrogen being administered with the addition of these two amino acids. This possibility may be discarded in view of the response obtained when non-specific nitrogen was added. These data demonstrate that the common maize protein is deficient in lysine and tryptophan for adult humans, as it is for children (see above in this chapter).
The results of these studies of amino acid intake from QPM and common maize (Clark et al., 1967; Kies, Williams and Fox, 1965) are compared in Table 39. As shown earlier in this chapter, twice as much common maize is necessary to obtain nitrogen equilibrium in adults. This is equivalent to a protein intake of approximately 1.6 times more from common maize than from opaque-2. EAA intake follows the same trend as total nitrogen intake.
Using a biological value of 82 percent for opaque-2, of the 28 g ingested about 23 g are retained, which is the approximate amount (21 g) retained from common maize, which has a biological value of 46.5 percent. These data indicate the great losses of nitrogen occurring with common maize. With the exception of lysine and tryptophan, common maize provides a greater quantify of essential amino acids. They are, however, a load the body has to discard, a load that is greater in the cases of leucine, tyrosine and valine. The physiological cost of metabolizing these unnecessary amino acids is unknown, but it should be estimated.
TABLE 39 - Protein and amino acid Intake of opaque-2 and common maize needed to obtain nitrogen balance (g/day)
|Total amino acids||11.70||18.99|
aProtein digestibility of opaque-2 maize, 76.5%;
biological value of common maize protein, 46.5%
Sources: Clark et al.. 1967: Kies, Williams and Fox, 1965
Furthermore, the amino acid intake pattern is unbalanced, which may be an additional reason for the poor biological value of the common maize protein. Another method of analysing intake of individual amino acids is to express it as a percentage of the total amino acid intake, a calculation which magnifies the deficiency in lysine and tryptophan in common maize and also indicates the excess of other amino acids. This information, in reference to adults as well as children, demonstrates once more the excellent quality of opaque-2 maize protein and the poor quality of common maize protein.
No direct comparative studies are available on the digestibility and biological value of common and opaque-2 maize proteins, so to make a comparison between them the studies of common maize by Truswell and Brock (1961, 1962) and of opaque-2 maize by Young et al. (1971) will be used. In one of the studies conducted by Truswell and Brock (1962), the experimental subjects received 90 percent of their nitrogen intake from maize and 10 percent from other foods. A positive nitrogen balance was obtained when the nitrogen intake was more than 7 g per day, although great variability was found as in other studies. The authors calculated the biological value, which averaged 45 percent at a high intake level and 57 percent at a lower level of nitrogen intake. The difference was to be expected, since the biological value of a protein depends on the level of protein intake. Since all the experimental subjects showed a positive nitrogen balance when the intake was high, the authors concluded that the biological value of maize is close to the 57 percent figure. Similar results were found by Young et al. (1971). Truswell and Brock (1961) also found that in adult human subjects fed maize, the addition of lysine tryptophan and isoleucine increased nitrogen balance from 0.475 to 0.953 g N per day in one study and from 0.538 to 1.035 g N per day in a second study. The flour fed was degermed maize flour, in which deficiencies are more apparent.
The biological value of opaque-2 maize protein was studied by Young et al. (1971). Egg protein was used as reference, fed at intakes of 2.64 to 3.95 g nitrogen per day. The authors calculated true protein digestibility and biological value from the faecal metabolic nitrogen and urinary endogenous nitrogen. The protein digestibility of opaque-2 maize protein varied from 67 to 106 percent, with an average for the eight individuals in the study of 92 percent, while the variability for egg protein was from 78 to 103 percept with an average of 96 percent. The average biological value for opaque-2 maize was 80 percent, and for egg the average was 96 percent.
Practical significance of protein evaluation of opaque-2 maize
The evidence presented from studies in both children and adults clearly indicates the superiority of opaque-2 maize over common maize. In spite of this, of the maize-consuming countries only Colombia and Guatemala have made efforts during the last few years to introduce this superior maize into agricultural production systems. The reasons are not clear, since agronomic studies conducted in a number of locations have shown that there are no differences between QPM and common maize in cultural practices, yield per unit of land and physical quality of grain. Furthermore, the plants look alike; QPM kernels are crystalline and grain yields are comparable to those of common maize. These factors are perhaps more important to growers than the nutritional advantages offered by QPM.
Energy content is alike in both types of maize, but the protein content of QPM is higher and is better utilized because of its better essential amino acid balance. The protein value of opaque-2 maize, however, can be analysed from other points as well. The information in Table 39 could be used to decide whether to introduce the opaque-2 maize varieties in grain-consuming countries.
It has been established that the intake of both types of maize as well as their nitrogen content (protein) are alike, but their digestibility percentages are very different: of 48 g of nitrogen intake from common maize, only 39.4 g are absorbed and 8.6 g are lost in the faeces. In the opaque-2 maize, of the 48 g of nitrogen intake, 44.2 g are absorbed and 3.8 g are lost in faeces.
The factor that should be considered, then, is the biological value, which is defined as the amount of absorbed nitrogen needed to provide the necessary amino acids for the different metabolic functions. The biological value of common maize is 45 percent; from the 39.4 g absorbed, 17.7 g are retained and 21.7 g are excreted. In opaque-2 maize the biological value of the protein is 80 percent; of the 44.2 g of absorbed nitrogen, 35.4 g are retained and 8.8 g are excreted. The total amount of nitrogen lost when common maize is consumed equals 30.3 g, while only 12.6 g are lost when the same amount of opaque-2 maize protein is consumed. In other words, only 37 percent of the common maize intake is utilized, compared to 74 percent from opaque-2 maize. The production and consumption of QPM in maize-eating countries would therefore have a significant beneficial effect on the nutritional state of populations, with important economic implications from the better use of what is produced and consumed.
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