Several lines of procedure offer prospects of successful comparison of the relative biological values of mixtures of proteins. One is to feed a series of groups of young and growing animals a diet similarly constituted and deriving its protein from the same source, but with the per cent of protein adjusted in the series from very low to very high levels. From the rate of growth of the animals an idea can be gained as to the least amount of protein from the sources studied, which just suffices to maintain animals without growth and to induce growth at subnormal rates, or at the maximum rate.

Another method of investigation is to utilize female animals nursing litters of young. By adopting a standard size for the litters, and by observing the success with which the mothers induce growth in their young when the diet contains different percentages of protein but is otherwise of uniform composition, an approximately quantitative comparison of the values of the proteins from different sources can be made.

Osborne and Mendel (2) made attempts, in their studies with isolated proteins, to determine their relative values by observing the growth of animals which ate the same amount of food in the same number of days and gained the same amount in weight. The protein factor was the only variable. It is very difficult to conduct experiments in this manner, and Osborne and Mendel (3) attempted to develop another method by restricting the protein so that it became the factor which determined the rate of growth, and then attempted to discover the concentration of protein in the diet promoting the greatest gain of body weight relative to the protein ingested. To accomplish this result they supplied the experimental animals with foods containing different percentages of the same protein. Using this method they secured data which is illustrated by the following table:

Protein.

Initial

Body

Weight.

Protein in Food.

Intake Per Gram of Gain.

Gain Per

Gram of

Food.

Gain Per

Gram of

Protein.

Grams.

Food.

Grams.

Protein. Grams.

Grams.

Per Cent.

Lactalbumin .

62

16.2

3.3

0.52

0.31

1.93

61

"

2.6

0.42

0.39

2.41

73

"

3.4

0.54

0.30

1.86

67

"

3.8

0.60

0.27

1.67

Average...

3.3

0.52

0.32

1.97

Lactalbumin .

71

10.3

3.6

0.37

0.28

2.74

63

"

4.0

0.42

0.25

2.41

Average...

3.8

0.40

0.27

2.58

Lactalbumin .

64

7.9

4.0

0.31

0.25

3.18

70

"

4.5

0.35

0.22

2.83

Average...

4.3

0.33

0.24

3.01

Lactalbumin .

66

6.2

6.3

0.39

0.16

2.58

67

"

6.0

0.37

0.17

2.69

Average...

6.2

0.38

0.17

2.64

Lactalbumin .

63

4.9

8.6

0.42

0.12

2.40

63

"

8.9

0.43

0.11

2.32

Average...

8.8

0.43

0.12

2.36

Lactalbumin .

68

3.3

18.7

0.62

0.05

1.63

67

"

24.6

0.82

0.04

1.23

Average...

21.7

0.72

0.05

1.43

In a similar study in which casein was the protein used, the protein intake was in most cases higher than in the lactalbumin

Protein...................................................................

Casein

Per cent in food...........................................................

17.4

Weights of experimental animals.............................................

58-76 grams

Intake per gram of gain-

Food..3.1; 4.0; 3.0; 3.5; 3.9; 3.1; 3.5; 3.0; 3.4. Average, 3.4.

Protein. .0.54; 0.70; 0.53; 0.61; 0.68; 0.53; 0.61; 0.52; 0.59. Average, 0.59.

Protein...................................................................

Casein

Per cent in food...........................................................

14.7

Weights of experimental animals.............................................

64-67 grams

Intake per gram of gain-

Food....4.1; 3.5; 3.0. Average, 3.5

Protein. 0.60; 0.51; 0.44. Average, 0.52

.

Protein...................................................................

Casein

Per cent in food...........................................................

12.0

Weights of experimental animals.............................................

63-69 grams

Intake per gram of gain-

Food..4.2; 3.5; 3.5. Average, 3.7

Protein.0.50; 0.42; 0.42. Average, 0.45

Protein...................................................................

Casein

Per cent in food...........................................................

9.3

Weights of experimental animals.............................................

64-65 grams

Intake per gram of gain-

Food..8.9; 6.4; 7.4. Average, 7.6.

Protein.0.83; 0.59; 0.68. Average, 0.70