PROTEIN AND THE REQUIREMENTS
Protein requirements are defined as the lowest level of dietary protein intake that a person will need to balance nitrogen losses from the body in persons maintaining an energy balance at modest levels of PA.In growing children and pregnant or lactating women, this requirement includes the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health.
During growth and development, more protein is necessary to build up the body; that is, about 2 g/kg body weight at the first through third months is recommended.
The period from 6 to 12 months is clearly most critical because of the rapid growth that occurs during this time, and because the child increasingly relies on supplementary foods. The mean rate of nitrogen accretion during growth can be estimated using the expected daily rate of weight gain, which corresponds to the 50th percentile of usual growth standards and to the nitrogen (N) concentration in the body. This is low at birth and increases to the adult value by the age of 5 years or sooner. The extent of the increase is important between 6 and 12 months when growth is rapid. Reported values for body N concentration at different ages were obtained using three different methods. At some ages, the values were not in agreement.
It should not be assumed that growth always proceeds at exactly the same rate from day to day, even in normal healthy children with adequate and regular food intake. The cause, extent, and significance of these fluctuations in growth rate are difficult to assess. The variability of gain is much greater than the variability of intake. These differences may also represent day-to-day differences in the proportions of deposited fat and lean tissue, which depends on both nutritional and PA regimens. This applies even more to periods of recovery from malnutrition and/or from a disease, etc.
In order to maintain a satisfactory overall growth rate, any failure to take in protein on one day must be compensated for on a subsequent day. The human body has a very limited capacity for storing amino acids or for drawing on the free amino acid pool for protein synthesis. Even during short periods such as 12 h without food, the nitrogen balance becomes negative. It follows that since it is impossible to foretell on which days the growth rate will be lower or higher, it is necessary to provide enough energy and protein everyday for the possible extra demand even when some sparing mechanisms under conditions of malnutrition may appear. The estimated safe levels of N and protein calculated in this way are shown in Table 5.5 (WHO 1985). These values are given for a joint group of boys and girls. This RDA refers to high-quality protein (from sources such as egg white, milk, beef, fish, and the like). Proteins should account for 12–13% of the EI. This applies to all age categories. In very young children, this proportion is higher: in a 4-year-old boy with a body weight of 18 kg about 18% of energy is covered by protein, as follows from the calculations of the intake of protein in grams per kilograms of body weight and total energy RDA.
Habitual home diets of some population groups may provide the same foods in different proportions, and their constituent proteins may not supply an adequate combination of essential amino acids. In such a case, when the quality is poor, a higher intake of protein is required. The safety margin is narrower for children than for adults.
Energy and protein are the two main components of foods that cannot be substituted with anything else. Energy is also derived from fat and carbohydrate and, when the EI is not sufficient, from protein. This is especially detrimental during growth, and sufficient amounts of both energy and protein are, therefore, essential. However, as mentioned in Chapters 3 and 4, a higher protein intake at the beginning of life, exceeding 16% of EI, can influence AR, which occurs earlier than usual (normal age 5.5–6 years), and thus increases the risk of easier development of obesity later during growth. Even though this was not proved in all studies, it is necessary to adhere to adequate protein intake, which has to be balanced and adjusted as exactly as possible to the needs of the growing organism. This can guarantee adequate development of the child at present and also later on.
