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No. 24. 



THE COMPARATIVE VALUES OF PROTEIN, FAT, AND CARBOHY- 

 DRATE FOR THE PRODUCTION OF MILK FAT. 



By E. J. SHEBHY, F.R.C.Sc.L, B.Sc. (Hons.), M.R.I.A., 

 Biochemical Laboratory, D.A.T.I. 



(Read Apeil 24. Printed Jdne 21, 1923.) 



While the composition of milk yielded by an animal depends primarily on the 

 nature of the inherited milk producing factors, it must be conceded that certain 

 constituents of milk are subject to alteration by feeding. Jordan, Hart, and 

 Patten (1906) found that on a low phosphorus ration the percentage of fat in 

 cows' milk was noticeably reduced, and vice versa. The author (1922) showed 

 that the percentage of fat in the milk of goats fed on an insufficient ration was 

 increased by adding protein, fat, or carbohydrate to the ration, and that the 

 reverse change took place in the milk fat when the ration was considerably 

 reduced. Eekles and Palmer (1916) were unable to affect the composition of 

 the milk of cows by feeding superabundant rations, but, by feeding rations 

 insufficient to supply the cow's requirem.ents for maintenance and milk yield, 

 they did effect a change in the percentage of fat and protein. Evidently the 

 percentage of fat in milk is influenced to an extent by feeding, and there is 

 some evidence that the percentage of nitrogen is subject to variation by feeding 

 also. Our knowledge of the physiology of milk fat production is not yet 

 complete, but there is fairly conclusive evidence from the experiments of Meigs, 

 Blatherwick, aiid Gary (1919), and the author (1921), that the precursor of 

 milk fat in the organism is some soluble phospholipoid, in which form fat is 

 absorbed from the blood by the mammary gland cells, to be synthesised into the 

 glyceride esters of fatty acids found in milk. Bodj^ fat may be produced from 

 fat in the food — Lebedev (1882), and from carbohydrates — Lawes and Gilbert 

 ( 1852) ; and there is strong evidence in favour of its formation from protein — 

 Lusk (1909) : the direct formation of fat from protein with the omission of the 

 carbohydrate stage is suggested by the recent work of Atkinson, Rapport, and 

 Lusk (1922). Milk fat, likewise, is derived from more than one source. That 

 ingested fat is converted, at least partly, into milk fat in a lactating animal is 

 shown by the fact that the nature of the milk fat produced, after the consumption 

 of a particular fat, is modified to resemble the latter — Rosenfeld (1902-03). The 

 experiments of Jordan (1897 and 1901) and his collaborators showed that the 

 carbohydrate of the food may be converted into milk fat. In a recent paper 

 (1922) and in the present one the author produces evidence to show that the 

 protein of the food may be utilized for the same purpose. 



In this paper data are presented which enable the comparative values of 

 protein, fat, and carbohydrate, as sources of milk fat, to be determined. It is 

 obvious that sufficient protein must be supplied in the diet of a lactating animal 

 to provide for the nitrogen requirements of the body, and also to supply material 

 for the milk protein, if nitrogenous equilibrium is to be maintained. Surplus 

 proteins are de-aminised; the nitrogen is excreted in the form of urea, and 

 the remainder of the molecule converted into carbohydrate. The comparative 

 feeding value of protein, fat, and carbohydrate, for milk fat production, was 



SCIENT. PEOC. E.D.S., VOL. XVII, NO. 24. 2 Q 



