A comparison of the several carbohydrates in their effect upon the metabolism can best be made by a tabular presentation of the averages for the total increments obtained with the different carbohydrates in this series of experiments. Such a grouping has been made in table 178. Comparing particularly the increments for the 100-gram amounts, we see that the differences in the average total increments in the carbon-dioxide production are not so very large. The effect is most pronounced with levulose and least with lactose, that for sucrose lying between the levulose and dextrose increments. According to the standards used in the earlier studies of carbohydrates, in which special emphasis was laid upon the carbon-dioxide excretion, it would be considered that the effect with levulose was much more pronounced than that with sucrose and that the sugars affected the metabolism in these experiments in the decreasing order of levulose, sucrose, dextrose, and lactose. At first sight it is difficult to explain why the carbon dioxide produced should vary for the several sugars, and it is clear that the most careful analysis of the effect of sugar ingestion on the metabolism should not be based upon carbon-dioxide production. An examination of the increments in oxygen consumption shows that in this case the maximum increment was obtained with sucrose, the order of effect being sucrose, levulose, lactose, and dextrose, the lactose being but slightly greater than the dextrose. With the heat production the lowest total increment was found with both dextrose and lactose; the increases with sucrose and levulose were considerably larger, that for the sucrose being one calorie greater than the levulose increment.

In the experiments with the 75-gram amounts, the general picture for the carbon-dioxide production is essentially the same as for the larger amount, the order being levulose, sucrose, and dextrose. For oxygen consumption and heat production the greatest increments were also obtained with levulose, but there were only two experiments with 75 grams of levulose, so that the averages are not perfectly comparable.

Table 178. - Comparison Of Average Increments Of Carbon Dioxide, Oxygen, And Heat After 100 Grams And 75 Grams Of Carbohydrate In Respiration Experiments

From the general picture obtained from all of the experiments, one is justified in saying that if the carbon-dioxide production is used as a basis of comparison, the increment of the ingestion of sugars upon the metabolism decreased in the order of levulose, sucrose, dextrose, and lactose. If the effect is measured by oxygen consumption and heat production, this statement should be revised, for in general the levulose and the sucrose had essentially the same effect, but dextrose had a much less influence than the other sugars. An average of the increments for the individual sugars shows for the 100 grams a general increase for carbon-dioxide production of 14.7 grams, for oxygen consumption of 4.8 grams, and for heat production of 21 calories; the averages for the 75-gram amounts are somewhat smaller.

The statement made that the increment in the metabolism with sugars decreases in the order of levulose, sucrose, dextrose, and lactose, though based on the erroneous assumption that the carbon-dioxide increment with sugars would be proportional to the increment in the total metabolism, has been confirmed in other laboratories, although Lusk properly states that the differences are not very great. The data determined by Lusk1 by indirect calorimetry after the ingestion of 50 grams of carbohydrates show that the increase over the basal metabolism during the second, third, and fourth hours was 30 per cent with glucose, 37 per cent with fructose, 34 percent with sucrose, and but 3 percent with lactose. By direct calorimetry he found a 15 per cent increase with glucose, 24 per cent with fructose, 28 per cent with sucrose, and 4 per cent with lactose. This latter series of values compares more nearly with those observed in our respiration experiments. Perhaps one of the most striking points in a consideration of the data in table 178 for these four sugars is the fact that the carbon-dioxide production, even with pure carbohydrates, is not a suitable measure of the energy transformations.

The clear superiority of levulose and sucrose over dextrose in influencing metabolism is difficult to explain. One may assume that levulose has a special action upon cellular metabolism and that it is the levulose moiety of the sucrose molecule that produces the effect with sucrose, and yet one would expect the effect to be quantitatively considerable less with sucrose than with levulose if this be true. Unfor-tunately the experiments with the smaller amount of levulose, namely, 75 grams, are unsatisfactory and few in number, one of the two being obviously erratic with a larger heat production than in any of the levulose experiments. We are hardly justified, therefore, in drawing definite conclusions regarding the amount of levulose which will produce a maximum stimulating effect. It is conceivable, however, that the effect of the sucrose due to the levulose portion may represent the maximum stimulating effect of levulose.

On the other hand, we have also to consider the energy due to the hydrolysis of the sucrose molecule, which is assumed to be not far from 3.1 per cent. If in the experiments with sucrose we consider that 100 grams of sucrose have an energy content of 400 calories, we should expect somewhat over 12 calories to be produced as the result of hydrolysis. Deducting the 12 calories from the average total increment of 25 calories obtained in our sucrose experiments, we find that there are 13 calories left which can be attributed to the influence of the separate components, levulose and dextrose, upon the metabolism. Assuming that the 100 grams of sucrose result in the formation of 50 grams each of levulose and dextrose, and using the average increments for 100 grams of these substances of 24 and 18 calories, respectively, which were found in our experiments, we would expect to obtain an effect of 12 plus 9 calories, or 21 calories, if the effect is a summation effect. It is clear, therefore, that the explanation of the 25 calories due to the ingestion of 100 grams of sucrose does not rest upon the summation effect of the resultant dextrose and levulose and the heat production due to hydrolysis, but that there must obviously be a compensation. Furthermore, the cells may refuse to react to the indirect stimulus of the result of hydrolysis and the direct stimulus of the two sugars on the basis of a summation effect. With practically all of the sugars except levulose, a somewhat decreased effect was found with the smaller amount of sugar. With levulose, therefore, we have a specific property entirely different from that found with the other sugars and in all probability we have here an intermediary metabolism which may perhaps best be considered in connection with the study of the respiratory quotient.

1Lusk, Journ. Biol. Chem., 1915, 20, p. 590.