ANIMAL NUTRITION DISCUSSION AT DUNDEE 421 



determination carried out with pure proteins, carbohydrates 

 and fats. 



These data, which refer to the diet as raw material, must (we 

 have recognised) be qualified by determination of such variants 

 as digestibility, absorbability and the like ; but the amounts of 

 "available" protein and of "available" energy have remained 

 our sole essential criteria of efficiency in diets. That all the 

 assumptions implied in this limitation are become dogma is 

 seen when we read the latest writings of the highest 

 authorities. 



Yet observations made during quite recent years (and I feel 

 that those just detailed for us come into the category) show that 

 our criteria and definitions have been incomplete. The food 

 supply of an animal may, as a matter of fact, contain protein in 

 sufficient amount, also abundant energy and yet may support 

 the animal inefficiently or fail altogether to support it : this, 

 too, when, to the best of our knowledge, the inorganic supply is 

 correctly adjusted. 



We have learnt that the efficiency of the protein supply is 

 not to be defined by its amount alone. Ten or twelve years' 

 work upon the chemistry of "protein" carried out by Emil 

 Fischer and his school, as well as by others, has made it 

 abundantly clear that the term covers a multitude of substances 

 which, however closely related, differ so considerably that they 

 must have different nutritive values for the animal body. We 

 must for the future define an efficient protein supply in terms of 

 quality as well as quantity. 



The nitrogen-free constituents of food we have been prone 

 to consider as sources of energy alone, as so much fuel. Since 

 Rubner has shown that fat and carbohydrate burn isodynami- 

 cally in the body, so that the place of a certain amount of 

 carbohydrate in a dietary can be supplied by a quantity of fat 

 containing its equivalent in energy, without affecting the 

 metabolic balance of the animal, we have troubled ourselves 

 but little about the relative amounts of carbohydrate and fat 

 present in a food mixture. Questions of convenience, digesti- 

 bility and the taste of the animal have, of course, intervened 

 to determine this ratio in practical cases ; but we have looked 

 upon the total energy as the one really essential factor. Yet 

 recent observations have proved abundantly that once an 

 animal is totally deprived of carbohydrate, no matter how much 



