In the balance method, subjects are fed varying amounts of protein or amino acids and the balance of a particular parameter—usually N balance—is measured. An adequate amount of dietary protein is that level of intake that will maintain a neutral or slightly positive N balance. The balance method can be used to titrate N intake in infants, children, and women during pregnancy, when the end point is a balance positive enough to allow appropriate accretion of new tissue. The balance method is also useful for testing the validity of the factorial method estimates. In general, N balance studies in which dietary protein intake is titrated give higher protein requirements than predicted by the factorial method.
There are several reasons for this result. The N balance method has important errors associated with it that are not minor ( 48, 158). Urine collections tend to underestimate N losses, and intake tends to be overestimated. Miscellaneous cutaneous and hair losses, which are "best guesses," may have small but substantial errors. However, these factors affect both methods. In the balance method, which "titrates" dietary intake to determine zero balance, the response to increasing protein intake is nonlinear. As protein intake is increased from a grossly deficient status toward an adequate status, the improvement in N balance is at first proportional to the amount of protein added to the diet. As the balance point is approached, however, the slope of the N balance-protein intake curve progressively decreases, i.e., more protein is required to achieve a zero N balance than is predicted by linear extrapolation ( 158). This refractoriness in reaching N equilibrium has been estimated to add 30% to the intake needed to equal output. As a result, the factorial method estimate of 0.44 g/kg/day is increased to a recommended intake of 0.57 g/kg/day (45).
Most studies of N balance have been performed at presumably adequate levels of energy intake. However, N balance is affected by energy intake. Energy intake below requirements causes the N balance measurement to become negative when protein intake is near the requirement. Most recommendations are based, nonetheless, on the assumption that energy intake is adequate (159). Another factor affecting N balance indirectly is the quality and digestibility of the protein being consumed. Generally it is assumed that protein of lesser quality and digestibility than egg white will be consumed. A correction factor that increases protein requirements by 25 to 30% is added to compensate for consumption of lesser-quality protein (156).
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