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Amino acid composition of food groups
The amino acid compositions of selected food groups are shown in table 24. These values, in milligrams per gram of protein, are from the Massachusetts Nutrient Data Bank. They were calculated from food table data, in milligrams per 100 g of food, mainly originating from the US Department of Agriculture , which in turn originated from a number of sources, including FAO . All composite dishes (i.e., those containing mixtures of foods) were excluded, so that the mean values and variations shown are only for individual foods. Other than this no further selection was made.
TABLE 24. Amino acid composition of major food groups from the Massachusetts Nutrient Data sank (mg/g protein)
|Amino acid||Food group|
|Animal mean (± SD)||Cereals mean (± SD)||Food group Legumes mean (± SD)||Nuts/seeds mean (± SD)||Fruits/vegetables mean (± SD)|
|Iso||46.7 (4.7)||39.8 (4.6)||45.3 (4.2)||42.8 (6.1)||38.5 (10.8)|
|Leu||79.6 (6.0)||86.3 (26.3)||78.9 (4.2)||73.5 (9.0)||59.1 (19.6)|
|Lys||84.3 (7.1)||30.5 (9.8)||67.1 (3.8)||43.5 (12.7)||49.2 (13.3)|
|Saa||37.7 (3.3)||41.1 (4.3)||25.3 (2.8)||37.7 (11.7)||23.6 (7.2)|
|Aaa||74.9 (8.2)||83.0 (9.2)||84.9 (6.3)||88.0 (16.9)||64.0 (18.4)|
|Thr||43.4 (2 6)||33.6 (5.4)||40.0 (3.3)||37.9 (5.4)||35.1 (8.7)|
|Trp||11.4 (1.5)||12.1 (3.3)||12.3 (2.4)||15.4 (4.6)||10.8 (3.9)|
|Val||51.2 (5.6)||51.1 (6.9)||50.5 (4.0)||55.6 (10.3)||45.9 (12.6)|
It should be noted that many of the values, especially in the animal food group, are not independent variables, since they may never have been individually analysed for amino acid composition. For example, amino acid data (milligrams per 100 g food) tabulated for a large number of cuts and preparations of beef could well originate from a single mean reported amino acid composition analysis for beef (milligrams per gram of protein) multiplied by the protein content (grams per 100 g) of the cut. Data in food tables, as such, do not always allow for exact determination of the origin of the data.
The following discussion of amino acid composition is in quality terms throughout, i.e., milligrams per gram of protein. Since the values are not true independent variables, full statistical analysis was not possible and the differences cited can only be suggestive. Nevertheless, several observations can be made from these data. For the majority of the amino acids, namely isoleucine, leucine, aromatic amino acids, threonine, tryptophan, and valine, the differences between the means for the various food groups are generally small. However, low levels of leucine and aromatic amino acids in fruits and vegetables and high levels of tryptophan in nuts and seeds may be noted.
Most importantly, however, major differences between the mean values for the food groups exist, especially for lysine but also for sulphur amino acids. Both fruits and vegetables and legumes are lower in sulphur amino acids than are the other food groups. For lysine the largest differences exist between the animal, cereal, and legume groups. The values for grains (cereals), with a mean level of 30.5 mg/g protein, were far lower than those for animal foods (84.3 mg/g protein) and legumes (67.0 mg/g protein), with intermediate values for nuts and seeds and for fruits and vegetables.
The special nature of lysine distribution between the food groups is demonstrated in table 25, where the amino acid data are expressed as ratios in relation to the grain group. Ratios for animal foods and for legumes compared to cereals are 2.8 and 2.2, respectively, with the maximum ratio for any other amino acid being 1.3. The relatively high levels of sulphur amino acids in cereals can also be noted; compared with cereals, the ratios both for legumes and for fruits and vegetables are 0.6. These observations, drawn from amino acid composition data, reinforce the conclusions drawn from the CV% data for lysine and the other essential amino acids shown in tables 9, 19, 22, and 23, which originate from food balance sheet calculations.
TABLE 25. Ratios for the amino acid compositions (mg/g protein) of the major food groups compared to cereals = 1.0
|Amino acid||Food group|
Source: calculated from data in the Massachusetts Nutrient Data Bank.
Summary and conclusions
According to ICN reports , hunger and malnutrition remain devastating problems facing the majority of the world's poor. One of five persons in the developing world is chronically undernourished; nearly 200 million children suffer from protein-energy malnutrition; and over 2,000 million people have micronutrient deficiencies. The most prevalent recognized micronutrient deficiencies are lack of iron, iodine, and vitamin A. However, the relative lack of the essential amino acid lysine may also be widespread in vast areas of the world where diets are heavily based on cereals. This includes much of South Asia. Strategies to alleviate micronutrient deficiencies must include improving dietary diversity as well as nutrient fortification.
Comparisons of food availability data among various regions and countries have demonstrated that as wealth (gross national product) decreases, less food energy and protein are available for consumption. In addition, however, there are major changes in the pattern of foods selected, including much greater dependence on cereals as a source of protein. The essential amino acid lysine showed a much greater CV% in several groups of data than did the other essential amino acids. This confirms the greater variability for lysine as a consequence of the very different dietary patterns of the rich and the poor, especially in relation to their dependence on cereals as a source of protein.
When dietary and health data are stratified by gross national product, it can be demonstrated that the poorest countries, with a population of nearly 3,000 million, have the lowest mean daily availability of food energy, total protein, animal protein percentage, and lysine They also have diets with the highest proportion of their protein from plant sources. In addition, these are the countries with the lowest life expectancies and the highest under-five mortality rates. Furthermore, when food balance sheet data are examined and comparisons are made between 1961 and 1992, changes in lysine availability can be related to changes in wealth and consequent changes in dietary patterns.
Dietary survey data from both India and Pakistan have confirmed the conclusions from country-wide data originating from food balance sheets. There were, additionally, wide intra-country variations in diet and lysine availability. Average lysine values were low in both countries and would indicate considerable risk of inadequate levels of intake in many subsections of the population.
When amino acid composition data were examined, it was observed that, for the majority of the amino acids, the differences between the mean values for the food groups were generally small. For lysine however, these differences could be large, especially between cereals and animal foods. In practice it was observed that the amino acid compositions of animal, pulse, and cereal proteins were sufficiently distinctive from each other to allow the use of food group data for predicting the lysine value of diets.
I am grateful to the Global Cereal Fortification Initiative, Tokyo, for their financial support of this study. I am also grateful to Drs. N. S. Scrimshaw and V. R. Young of the Massachusetts Institute of Technology for their encouragement and for many useful discussions concerning this and other studies.
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