ENERGY RELATIONSHIPS. 341 



values ranging from 2 to 23 per cent, the average value is approximately 

 10 per cent. 



The three experiments with milk on March 21, 22, and 23 were 

 planned to be comparable as the same amount of milk was given the 

 subjects; the fuel value ranged from 382 to 444 calories. The incre- 

 ments obtained are somewhat irregular. A minimum of 11 calories 

 was found with H. R. D. on the first day and fairly comparable values 

 of 59 and 51 calories, respective^, were obtained on the two succeeding 

 days. The fuel value of the milk taken in the last two experiments 

 was practically the same. We see no reason for omitting the experi- 

 ment on the first day, and hence the three experiments represent an 

 average cost of digestion of milk of approximately 10 per cent. 



In the previous comparisons, the fuel value did not exceed approxi- 

 mately 1,900 calories, except in one experiment with butter and potato 

 chips, in which the intake of energy was 3,202 calories. In a group of 

 experiments with an excess amount of food, characterized as "heavy- 

 breakfast" experiments, the fuel value ranged from 2,142 to 4,378 

 calories. The cost of digestion in these experiments was fairly uniform, 

 ranging from 4 to 6 per cent, with an average of 5 per cent. There 

 were also two experiments of much longer duration than any of the 

 other experiments included in this table, viz, those with H. R. D., 

 April 10-11, and A. L. L., April 6-7, in which "heavy suppers" were 

 taken with high fuel values. The cost of digestion was 10 and 12 per 

 cent respectively. These higher figures may be due to faulty basal 

 values or unusual activity in the food experiment, or the experiments 

 may have been long enough to obtain all the increment which actually 

 took place. They do not lend themselves, however, to very critical 

 analysis. 



Under ordinary conditions the normal individual rarely eats a meal 

 containing a pure nutrient or a meal in which there is an excessive pro- 

 portion of any single nutrient, but usually a fairly balanced combina- 

 tion of nutrients. It is accordingly of considerable practical signifi- 

 cance that the six experiments with a heavy breakfast show such 

 uniform percentages. While the use of average figures for the several 

 groups of carbohydrates, fats, and proteins may be somewhat question- 

 able, with mixed diets we may fairly state that the excess heat produc- 

 tion as a result of ingesting such a diet is 5 per cent of the fuel value of 

 the intake. In all of the heavy-breakfast experiments, the basal 

 metabolism was not reached during the experimental period; the value 

 of 5 per cent is therefore probably somewhat low and a value of 6 per 

 cent would be more nearly in accord with the actual facts. We sug- 

 gest, therefore, that as a general factor a heat production equivalent to 

 6 per cent of the fuel value may be expected as the result of the inges- 

 tion of a mixed diet. 



