G. Inoue, Y. Fujita and Y. Nüyama
Young male subjects were first fed for about one week a standard diet providing 1.25g of mixed protein, and 45 kcals per kilo bodyweight. N-balances were then carried out over a three week period during which each individual was given a fixed daily amount of protein derived either from whole egg or from rice, and which ranged from 0.3 to 0.7 g/kg as between individuals. Two series of studies were made, one with calorie intakes of 45 kcals/kg (an adequate level) and another series at 57 kcals/kg (a surplus level). In a further series of experiments, three different amino acid mixtures were fed, in combination with calorie intakes ranging from 35 to 60 kcals/kg. These provided either 6.8 g total N with 0.6 or 4.0 g of essential amino acids patterned after rice, or 10.0 g total N providing 4.0 g essential amino acids.
The total N lost during the first week after transfer from the standard diet was taken as a measure of labile protein loss. The amount of N loss depended upon both N and calorie intake, falling from about 17 g at an intake of 45 mg N/kg, to 12 g at an intake of 85 mg N/kg when calories were supplied at 45 kcals/kg and from 12 g to 5 g for the same N intakes, when the calories were given at a rate of 57 kcals/kg.
As compared with measurements during the week on the standard diet, body weights declined slightly in all subjects fed the adequate level of calories, regardless of the protein intake, the average loss being 1.1 &+-; 0.5 kg. At the high level of calories, no change in body weight was found.
Regression lines relating N balance to either intake or absorbed N, were calculated for the individual balances during the third week. The slopes of these lines can be regarded as measures of the relative NPU and BV of the two protein sources measured at both adequate and surplus calorie intakes. Table 1 shows the values obtained. For both proteins, the effect of the higher calorie intake was to increase the efficiency of utilization by about 30%. At the higher level of calories, the BV of rice and egg were not significantly different.
|45 kcals/kg||57 kcals/kg|
* The digestibility of whole egg was assumed to be 100%
The low values for NPU and BV particularly of egg may be due to the fact that the relation between balance and intake is not in fact linear. In fact a non-linear relationship at low intake levels is suggested by the work of Nasset and Anderson, and more recently by Said and Hegsted.
The fact that efficiency of utilization is increased by addition of extra calories, even when the level of N intake is a constant, is an illustration of the N sparing effect of added calories. This sparing effect becomes increasingly marked at N intakes greater than 0.25 g/kg. This effect is evidence that some absorbed N is used as a calorie source even when protein intake is limited.
The amounts of protein needed to maintain equilibrium at a calorie intake of 45 kcals/kg were 0.65 g/kg of egg, and 0.87 g/kg of rice. However at 57 kcals/kg requirements were reduced to 0.46 g/kg of egg and 0.58 g/kg of rice.
The figure of 0.46 g/kg for egg, represents a lower limit of adaptation, and compares well with Hegsted's estimates. However the authors consider that the value of 0.65 g/kg obtained at the adequate level of calories is a more appropriate estimate of requirements. To this an amount should be added to allow for dermal losses, and on 22 subjects this has been estimated at 10.5 mg N/kg. This would bring the total requirement to 0.72 g/kg/day, a figure which is (fortuitously) very close to the FAO/WHO (1965) estimate of the safe practical allowance.
This series of balance studies shows that for a fixed level of N intake, N balance increases with increasing calories, although there was a limit at about 55 kcals/kg above which no further retention was seen. For each of the mixtures, the regression line of N balance against calorie intake differed, so that equilibrium was attained at different levels of calories according to the amino acid mixture fed. The greatest calorie intake being necessary with the mixture containing the lowest proportion of essential amino acids. It is concluded that maintenance of N can be achieved with various different combinations of amino acid proportions and calorie levels.