372 PHYSIOLOGY: HARRIS AND BENEDICT 
production decreases about 7.15 while in women it decreases about 2.29 
calories per year. Women are smaller than men and have a lower heat pro- 
duction. When the decrease in metabolism with age is expressed in calories 
per kilogram of body weight or in calories per square meter of body surface, 
the results for the two sexes are much more nearly identical. 
The problem of the difference in the metabolism of men and women, dealt 
with in the past by a number of writers, has been reconsidered on the basis 
of the larger series of data now available. The average daily (24 hours) 
basal heat production of men is 1632 calories whereas that of women is 1349 
calories. Thus women have an average daily heat production about 300 ca- 
lories less than that of men. But women are smaller than men. If correc- 
tion for body size be made by expressing heat production in calories per kilo- 
gram of body weight, it is 25.7 calories in the 136 men as compared with 24.5 
calories, or 1.2 calories per kilogram less, in the 103 women. On the basis 
of heat production per square meter of body surface as estimated by the 
Du Bois height-weight chart the men show an average daily heat production 
of 925 calories as compared with 850 calories, or 75 calories less, in the 103 
women. The most critical test of the difference of men and women in the 
level of metabolism is that furnished by a modification and extension of the 
selected group method of Benedict and Emmes. In the new method the con- 
trol values for the several groups of women are not the empirical constants for 
men of as nearly as possible like stature and body weight but are determined 
from equations taking into account stature, weight and age in all the available 
data for men. Analysis shows that, however expressed, the metabolism of 
American women is lower than that of the men. Our results show that the 
differentiation of the sexes is not evident in infancy. They do not confirm 
the conclusion of Sonden and Tigerstedt that the difference between men and 
women tends to disappear with age. Instead we find the difference in the 
metabolism of men and women well-marked throughout the period of adult 
life. 
The validity of the so-called body surface law, according to which metab- 
olism is proportional to the superficial area of the individual, i.e., 
h = ah a 
where a = superficial area and h a = mean heat production per unit of time 
per square meter of body surface in a standard series, has been critically 
tested. It has been shown that the supposed proofs of its validity hitherto 
adduced are erroneous. Heat production is not 'proportional to body sur- 
face but not to body weight' as has been asserted to be the case, but is highly 
and about equally correlated with both body weight and body surface. It 
has been shown that as a basis for predicting the heat production of a sub- 
ject the above relationship is less satisfactory than multiple regression equa- 
tions involving stature, weight and age. Thus the 'body surface law' is 
deprived of its unique significance as a basis for the prediction of the me- 
