174 Vitamins in Human Nutrition
stimulated if minute amounts of thiamin are added to the
soil or water. This is the reason why cuttings grow better if
they are split at the bottom and an oat seed inserted.
Most animals, including man, depend upon outside sources
for their thiamin, but some, such as cattle, harbor bacteria
in the rumen of the stomach which make it for them in their
own alimentary canals. It is for this reason that cow’s milk
always contains a fair amount of thiamin regardless of the
diet, which is not true of human milk. In the Philippines,
for instance, mortality from beri-beri is particularly high in
breast-fed infants.
The potency of various foods in vitamin B1 has been meas-
ured by various methods of “biological assay.” A method
which was formerly extensively used was to determine the
amounts of a particular food which, if fed daily in addition
to a diet entirely lacking in vitamin B1, would enable rats
which had been depleted of the vitamin to make a certain
very slight gain in weight each week for two or three weeks.
This amount of the food was said to have one Sherman unit
of vitamin B1. In the last few years better methods have
been developed. Rats are depleted of the vitamin until they
shows signs of polyneuritis. They are then given a test dose
of a known quantity of pure thiamin. They recover in a few
hours and are then watched to see how many days elapse
before the symptoms return. They are then given a certain
amount of the food to be tested and the curative effect com-
pared with that produced by thiamin. On the basis of the
results obtained smaller or larger doses are given after the
rats have relapsed again, until finally the quantity equalling
the dose of thiamin is reached. Some assayers prefer to use
the rate of the heart beat as a criterion. After two weeks of
vitamin B1 depletion a rat’s heart slows from 500 beats per
minute to 250, and a dose of thiamin brings it back to nor-