

DIGESTIBILITY OF CEREAL BREAKFAST FOODS. 189 
outside the body. In computing the quantity of energy available 
to the body, account must therefore be taken of the energy of 
these incompletely oxidized residual products of protein, which 
are excreted in the urine. 
Two methods of estimating the amounts of energy, for which 
such allowance must be made, are common. In one case, the 
total amount of urine for the experimental period is collected, 
and the heat of combustion of the organic matter in it is deter- 
mined. ‘This period is not chosen with the idea that the nitro- 
gen of the urine, thus collected, represents the actual protein 
katabolism of the food under investigation; but in the absence 
of any means of marking the urine for a given period, similar 
to that followed in the case of the feces, and in lack of definite 
knowledge concerning the nitrogen lag, that is, the time between 
the ingestion of nitrogen in the food and its excretion in the 
urine, the period of the experiment is most convenient. The 
chief uncertainty of this method is in the impossibility of tracing 
any exact relation between the protein of the food eaten on a 
given day and the nitrogen of the urine. 
In the other case the energy lost in the urine is computed. 
Since urea is the most abundant of the organic materials in 
(normal) urine, it has sometimes been ‘assumed that all the 
nitrogen from the katabolism of protein is excreted in this sub- 
stance, and allowance made for the heat of combustion of an 
amount of urea corresponding to the amount of nitrogen in the 
urine. According to such an assumption, 0.87 calorie* of 
the energy latent in each gram of available protein would be 
lost to the body in the urea formed from the nitrogen of the 
protein. In a considerable number of experiments made at this 
Station, however, it has been found that the average heat of 
combustion of the organic matter of the urine corresponding to 
I gram of available protein amounts to 1.25 calories.y It is 
believed that the energy of urine as calculated by use of this 
factor, derived from a relatively large number of determina- 
tions in experiments extending over considerable periods, so 
that the error due to nitrogen lag would be very appreciably 
diminished if not altogether eliminated, is reasonably accurate. 

*U.S. Dept. Agr., Office of Experiment Stations, Bul. 44, p. 40. 
¢ Storrs Expt. Sta. Rpt. 1899, p. Ioo. 
