FORMATION OF SUGAR FROM PROTEWS AND FATS 523 



CH 3 

 changed into lactic acid, CHOH. Lactic acid, as can be seen from 



COOH 



the formulae, is closely related to glyceric aldehyde, which in turn may 

 readily be condensed into dextrose, as follows : 



CH 2 OH CH 2 OH CH 2 OH 



CHOH + CHOH = (CHOH) 4 



CHO CHO CHO 



(glyceric aldehyde) (dextrose) 



Serin, oxy-amino-propionic acid, also a constituent of the proteid 

 molecule, is even more closely related to dextrose, as shown by its 



CH 2 -OH 

 formula, CH-NH 2 . 



COOH 



Extreme difficulties exist in such experimental work, because of the 

 numerous possible sources of error which are introduced by the following 

 conditions : (1) More or less glycogen is retained in the tissues during 

 starvation ; (2) the proteids of the foods contain preformed carbohydrate 

 radicals ; ( 3) carbohydrates may be formed from fats ; (4) proteids and 

 ainmo-acids may be oxidized in place of carbohydrates, which thus 

 escape destruction and cause increased glycosuria. This has made most 

 of the experimental evidence on this question of uncertain value ; con- 

 sequently, while we find, in a recent review by v. Noorden, the origin 

 of sugar from the proteid molecule treated as an established fact, at the 

 same time Macleod, in his review of the literature, 1 states that "there is 

 no unequivocal evidence, so far" that glycogen formation can result 

 from feeding with proteids that contain no carbohydrate group. How- 

 ever, Macleod also states that by the indirect method "the evidence 

 undoubtedly points to sugar formation from all proteids." 



FORMATION OF SUGAR FROM FATS 



In starving animals with glycosuria, or in diabetics on a restricted 

 diet, the amount of sugar eliminated often seems to be greater than can 

 be accounted for by destruction of the proteid of the food and tissues, 

 as measured by the nitrogen excretion. It would, therefore, seem prob- 

 able that sugar may, in these conditions, be formed from the fats. 2 

 There is no a priori reason why this should not occur, but the proof that 

 it does, occur seems to be scanty. 



Glycerin might readily form sugar, as follows : 



1 " Kecent Advances in Physiology and Biochemistry," 1906. 

 2 Not in phlorhizin diabetes, according to Lusk (loc. cit.). 



