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The most prominent worker in the field of the "A" vitamin measurement in America is Steenbock. His basal diets are a combination of those already described.

Steenbock's basal diets

This was his original basal diet but later he modified it by adopting the McCollum method of carrying his "B" vitamin on the dextrin. This was usually the alcohol extract of 20 grams of wheat embryo. In the following diets the presence of this extract is indicated by the letter (x) following the dextrin.

Steenbock's salt mixtures

The very nature of these basal diets suggests their use. In general however their utilization for testing purposes is based on the following principles: Since the basal diet supplies all the requirements of a food except the vitamin for which one is testing, it is simply necessary to add the unknown substance as a given percent of the diet and observe the results. If the amount added is small it is assumed that its addition will not appreciably effect the optimum concentrations of nutrients, etc., and for such experiments no allowances are made for the constituents in the unknown. For example let us assume that we wish to test the value of a yeast cake as a source of "B" vitamin. We first select a sufficient member of rats of about thirty days age to insure protection from individual variations in the animals.

The age given is taken as an age when the rats have been weaned and are capable of development away from the mother and as furnishing the period of most active growth. These rats are now placed on one of the basal diets which in this case supplies all the requirements except the "B" vitamin. In this experiment any of the diets of Osborne and Mendel or of McCollum will do that have been labelled "A" only. After a week or so on this diet they will have cleared the system of the influence of previous diets and their weight curves will be either horizontal or declining. If now we make the diet consist of this basal diet plus say 5 per cent of yeast cake, the weight curve for the next few weeks will show whether that amount supplies enough for normal growth, comparison being made with the normal weight curve for a rat of that age.

In this method it is assumed that the amount of yeast cake added will not derange the proportions of protein fat, etc., in the basal diet enough to affect optimum conditions in these respects. This is a curative type of experiment. If we wish to develop a preventive experiment the yeast cake may be incorporated in the diet from the first and the amount necessary to prevent deviation from the normal curve determined. Both methods are utilized, the one checking the other. If however the amount of the substance necessary to supply the vitamin required for normal development is large such addition would of course disturb the proportions of nutrients in the normal diet and in that case analysis must be made of the substance tested to determine its protein, fat, carbohydrate and salt content and the basal diet corrected from this viewpoint so as to retain the optimum proportions of these factors. McCollum's cereal testing combinations are illustrative of such methods applied to cereals.

Still another method is to add a small per cent. of the unknown and then add just enough of the vitamin tested to make sure that normal growth results. Such a method gives the results in terms of a known vitamin carrier. For example, if we add to a basal diet, sufficient in all but the "A" vitamin (Steenbock's mixture for example), a small per cent of a substance whose content in "A" is unknown and note that growth fails to result we can then add butter fat until the amount just produces normal growth. If now we know just what amount of butter fat suffices for this purpose when used alone we can calculate the part of the butter which is replaced by the per cent of unknown used. To put this in terms of figures will perhaps make the idea clearer. Let us assume that 5 per cent of butter fat in a given diet is sufficient to supply the "A" necessary for normal growth. Assume that the addition of 5 grams of the unknown in 100 grams of the butter-free diet fails to produce normal growth but that by adding 2 per cent of butter fat normal growth is reached. It is obvious under these conditions that 5 grams of the unknown is equivalent in "A" vitamin content to 5 minus 2 grams of butter fat, i.e., is equivalent to 3 grams of butter fat or expressed in per cents the substance contains 0.6 or 60 per cent of the "A" found in pure butter fat.

Experience has shown that it is dangerous to draw conclusions from experiments of too short duration or to base them on too few animals. For complete data the experiments should be carried through the complete life cycle of the rat, including the reproductive period. Otherwise it may turn out that the amount in the unknown while apparently sufficient for normal growths is incapable of sustaining the drain made in reproduction. It is this consideration that makes the accumulation of authoritative data on vitamin contents of foodstuffs so slow and tedious and one of the reasons why we lack satisfactory tables in this particular at present.

Osborne and Mendel raise another point of methodology and believe that more accurate results will be obtained if the source of the vitamin is fed separately than if mixed with the basal diet. It is easily possible that since one of the effects of lack of vitamin, especially of the "B" type, is poor appetite, the amount necessary to produce normal growth may be smaller than would appear from results obtained by mixing it in the basal diet. When so mixed the animals do not get enough to maintain appetite and really decline because they do not eat enough rather than because the amount of vitamin given is inadequate to growth. Details of this kind are matters however that particularly concern the experimentalist and as our purpose here is to merely describe the methodology we may perhaps turn now to other types of testing.

Tags: Vitamins