Food and nutrition.pdf

98 Function of Foodutilization (NPU) is a measure of the amount or percentage of proteinretained in relation to that consumed. As an example, Table 16 gives thechemical score and NPU of the protein in five foods.It is not usual or easy to obtain NPU values in people, and in moststudies rats are used. Table 16 suggests that there is a good correlationbetween the values in rats and in children, and that chemical scoreprovides a reasonable estimate of protein quality.For the professional involved in nutritional activities to help people -be it a dietitian in a health facility, an agricultural extension worker or anutrition educator what is important is that the protein value differsamong foods and that mixing foods improves the protein quality of themeal or the diet. Table 17 gives the protein content and the limiting aminoacid score of some commonly eaten plant-based foods. Because lysine ismost commonly the limiting amino acid in many foods of plant origin,the lysine score is also given.Protein Digestion and AbsorptionProteins consumed in the diet undergo a series of chemical changesin the gastrointestinal tract. The physiology of protein digestion iscomplicated; pepsin and rennin from the stomach, trypsin from thepancreas and erepsin from the intestines hydrolyse proteins into theircomponent amino acids. Most of the amino acids are absorbed into thebloodstream from the small intestine and thus travel to the liver and fromthere all over the body. Any surplus amino acids are stripped of the amino(NH2) group, which goes to form urea in the urine, leaving the rest of themolecule to be transformed into glucose. There is now some evidence thata little intact protein is taken up into certain cells lining the intestines.Some of this protein in the infant may have a role in the passive immunityconveyed from the mother to her newborn child.A little of the protein and amino acids released in the intestines isnot absorbed. The unabsorbed amino acids, plus cells shed from theintestinal villi and acted upon by bacteria, together with gut organisms,contribute to the nitrogen found in faeces.Table. Chemical Score and Net Protein Utilization in Selected FoodsFood Chemical score NPU determined in NPU determinedchildrenin ratsEggs (whole) 100 87 94Milk (human) 100 94 87Rice 67 63 59Maize 49 36 52Wheat 53 49 48