Common maize

Contents - Previous - Next

Protein quality for children

The protein quality of maize evaluated for children recovering from protein energy malnutrition has been reported by various researchers. Table 26 shows the results when lime-cooked maize was supplemented with maize gluten to obtain a product with higher protein content and to facilitate higher protein intakes with lower intakes of solids. The amino acid deficiencies in maize protein were thus magnified and this facilitated their detection using the nitrogen balance technique (Scrimshaw et al., 1958; Bressani et al., 1958, 1963). The results showed decreasing nitrogen retention as nitrogen intake decreased, which was to be expected; however, even at a high nitrogen intake of 469 mg per kg body weight per day, retention was significantly lower than nitrogen retention from milk given in amounts providing the same level of protein. Apparent protein digestibility indicated as nitrogen availability was fairly similar for different nitrogen intakes, varying from 72 to 78 percent. Table 27 refers to nitrogen balance studies in children fed water-cooked maize. Nitrogen retention from maize was significantly lower than from milk at the same level of protein intake. Protein digestibility was 80 percent for milk and 75 percent for maize (Viteri, Martínez and Bressani, 1972). Similar data were obtained with cooked maize endosperm and whole normal maize (Graham, Placko and MacLean, 1980), as shown in Table 28. In this case nitrogen balance was lower for common maize endosperm than for the whole kernel and lower than the results from the reference protein, casein. Graham et al. (1980) calculated that in order to match nitrogen retention from casein, the children would have to obtain 203.9 percent of their energy requirements from maize, which is obviously impossible.

TABLE 26 - Nitrogen balance in children ted lime-treated maize as the sole protein source

Note: Diet consisted of 95% lime-treated maize and 5% maize gluten
Source: Viteri, Martínez and Bressani, 1972

TABLE 27 - Nitrogen balance In children fed common maize and milk

Note: Average values. with dispersion in parentheses
Source: Viteri, Martinez and Bressani, 1972

TABLE 28 - Nitrogen balance in children fed whole common maize kernels and maize endosperm flour

Source: Graham. Placko and MacLean, 1980

As was discussed earlier, germ proteins do contribute significantly to essential amino acids (EAA), so maize food products without the germ, including QPM endosperm, are always lower in protein quality than the whole kernel. Similarly, maize with a high zein content is of a lower quality than maize with lower prolamine content, because of a higher relative lysine deficiency and a higher imbalance of essential amino acids such as leucine relative to isoleucine.

TABLE 29 - Effects on nitrogen retention of additions of lysine tryptophan and methionine to lime-treated maize (nitrogen values in mg/kg/day)

Note: Amino acids used: DL-tryptophan: 0.34%, L-lysine/HCl: 0.56%, DL-methionine: 0.34%
Source: Scrimshaw et al., 1958

Amino acid supplementation

It is widely accepted that maize proteins are deficient in both lysine and tryptophan, as documented from studies with animals. In tests with children, however, the EAA contents of lime-treated maize supplemented with 5 percent maize gluten to raise the protein content (Scrimshaw et al., 1958; Bressani et al., 1958, 1963) were compared with the amino acid contents of the 1957 FAO reference protein. This comparison suggested the following order of amino acid deficiency: tryptophan, lysine methionine, valine, isoleucine and threonine. It also suggested the amounts of amino acids needed to reach the reference level. Representative results from two children fed 3 g of protein per kg body weight per day are shown in Table 29. There was an apparent response to the addition of 148 mg DL-tryptophan per g N which was much improved by the simultaneous addition of tryptophan and lysine the latter in the amount of 243 mg per g N. Addition of methionine decreased nitrogen retention.

TABLE 30 - Response to lysine and tryptophan added alone (nitrogen values in mg/kg/day)

Note: Levels added to give 75-90 mg tryptophan/g N and 180-270 mg L-lysine HCl/g N
Source: Bressani et al., 1958

In other studies, nitrogen balance tests were carried out to learn about the response previously obtained by tryptophan addition alone. The results from two subjects (Table 30) clearly show that tryptophan had no effect on improving protein quality. The addition of lysine on the contrary, appeared to give a response, suggesting lysine to be more limiting than tryptophan.

Similar studies were carried out by feeding children 2 g of protein per kg body weight per day. The results in two children are summarized in Table 31. Tryptophan addition did not induce a positive nitrogen retention, but the addition of tryptophan and lysine with and without isoleucine improved nitrogen balance. Methionine addition decreased retention of nitrogen, as previously demonstrated.

TABLE 31 - Effects on nitrogen retention of additions of lysine tryptophan, isoleucine and methionine to lime-treated maize (nitrogen values in mg/kg/day)

Note: Amino acid levels added: DL-isoleucine 0.45%; other amino acids added in amounts shown in Table 29
Source: Bressani et al., 1958

Nitrogen balance tests were performed with protein intake of 1.5 g per kg body weight per day. The results for one child are shown in Table 32. Although lysine addition did not induce a positive balance, it did tend to cause a decrease in nitrogen losses. The improvement from lysine and tryptophan, with and without isoleucine, is evident. The addition of methionine, even at this level of protein intake, decreased the nitrogen balance as previously indicated for higher intakes of protein.

Because of the consistency of the results, the data obtained for different protein levels under the various dietary treatments were pooled. The results are shown in Table 33. There was a response to tryptophan addition only at the highest level of dietary protein, but the response to lysine was consistent at all protein intake levels, suggesting that this amino acid is more deficient than tryptophan. The response to addition of lysine alone, however, was small and without much nutritional significance, which implies the need to add both amino acids at the same time, as can be done with supplementary foods.

TABLE 32 - Effects of amino acid supplementation of maize at intakes of 1.5 9 protein per kg body weight per day (nitrogen values in mg/kg/day)

^a0.56% L-lysine HCl
^b0.30% L-lysine HCl
Other amino acids added in the amounts shown in Table 29
Source: Bressani et al., 1958

A nitrogen intake level of 239 mg per kg body weight per day is equivalent to 20 g of maize per kg per day, or about the 200 g of maize normally ingested by children. Supplementation with lysine alone would have little effect. When tryptophan is also added, however, the nitrogen retention is significantly higher and even surpasses that of milk at the highest level of dietary protein. The overall conclusion that can be reached from the results obtained by amino acid supplementation of maize is that both lysine and tryptophan must be added to obtain a significant response in protein quality as measured by nitrogen retention. It also appears that the two amino acids are equally limiting in spite of the fact that the addition of lysine alone tended to improve protein quality slightly, while the results from the addition of tryptophan were inconsistent.

The effect of methionine deserves further comment. It was attributed to an amino acid imbalance, because maize already contains enough of this amino acid to meet nutritional requirements.

TABLE 33 - Nitrogen balance in children fed lime-treated maize at various levels of protein intake with and without amino acid supplementation

The results shown in Table 34 indicate that valine also decreases nitrogen retention and that its effect can be reversed by the addition of isoleucine and threonine. A more detailed study in dogs led to the conclusion that there is also a close interrelationship among all four of these amino acids methionine, valine, isoleucine and threonine - as supplements to maize proteins (Bressani, 1962,1963).

It is a point of major importance that children are so sensitive to small changes in amino acid proportions that they are readily detectable in a short period by testing the nitrogen balance. The data presented here emphasize the importance of establishing a proper balance among the essential amino acids if a maximum retention of nitrogen is to be obtained. This is the principle of amino acid supplementation. The results obtained on the amino acid supplementation of maize confirm data derived from studies with rats, pigs and other animals. Results of studies on adult human subjects are shown in the next section.

TABLE 34 - Effect of multiple amino acid supplementation of maize (nitrogen values in mg/kg/day)

^a0.14% DL-methionine in this diet only, all others 0.34% DL-methionine. DL-valine: 0.90%, DL-threonine: 0.22%.
Other amino acids added in amounts shown in Table 29
Source: Scrimshaw e, al., 1958

Contents - Previous - Next