Distribution of Thiamine
The vitamin is widely distributed in both plant and animal tissues. In plants the seeds generally contain the highest concentration. Dry peas, beas, and soyabeans are excellent sources. The vitamin is concentrated in the outer layers of the grain kernels. Bran and rice polishings are thus rich sources. Whole wheat bread and white bread made from enriched flour contain a good supply of thiamine. Yeast is an outstanding source of thiamine and of certain other members of the B complex.
Many nuts have a high concentration of the vitamin. Peanuts contain a good supply, and pecans and brazil nuts have still more. Milk contains only one tenth the amount of thiamine that is found in peanuts but because of the comparative quantities consumed, milk is by far a more important dietary source. Many cheeses contain thiamine in amounts comparable to that of milk. Avocados, gooseberries, and dried prunes contain the highest thiamine content among the fruits. Oranges have around hill e quantity found in figs or prunes. Many animal tissues are excellent sources of the vita m. Pork chops and fat ham have exceptionally high contents. Many beef cuts are also excellent source.
It is of interest that though live yeast contains a high concentration of thiamine not only
the vitamin unavailable to humans, but the yeast also appears to complete with the body in the gastrointestinal tract for other dietary thiamine. Parsons and co-workers fed live yeast :0 individuals and noted a marked drop in urinary thiamine excretion to less than the basal level (without yeast). On removal of the yeast from the diet, urinary excretion rose. The thiamine of killed yeast, on the other hand, is almost completely available.
Various workers have demonstrated that the ingestion of viable yeast leads to increased loss of free thiamine in the feces, primarily in the cells of yeast and other organisms, and lessened urinary excretion, which is a good indication of decreased absorption from the intestine.
Some Bacteria synthesize thiamine. There has been considerable controversy over the question of availability to the body of teh vitamin from this source. Alexander and Landwehr presented evidence which indicates that neither free nor bound (diphosphate) thiamine of the feces is of nutritional value to the individual. They found that most of the vitamin exists in the bacterial cells and that the greatest part was present as bound thiamine, which is not absorbed until it is dephosphorylated. They point out that probably no enzyme capable of bringing about this hydrolysis exists in the large intestine. They administered both forms of thiamine by retention enema to an individual and found that the urinary thiamine excretion was not increased and that all the thiamine and cocarboxylase administered were recovered in the next 24-hour stool. By the use of C14-labeled thiamine it was shown that rats did not absorb thiamine from sources other than that supplied in the diet. This would indicate that under the conditions employed in these experiments intestinal microorganisms did not add to the total vitamin supply available to the host. Mickelsen has reviewed the subject of intestinal synthesis of vitamins.
Determination of Thiamine
Biological methods for thiamine determination have ~argely been replaced by physicochemical techniques. Of the former, the rat curative procedure has been widely used. The test material is administered to acutely polyneuritic rats. The duration of cure of rats given various levels of the unknown is compared to the duration after giving known amounts of thiamine.
From such data the approximate vitamin content can be calculated. Yeast fermentation methods and microbiological methods have been developed and widely used. The chemical procedures are increasing in popularity because of ease and rapidity compared to animal assays.
The most widely employed method involves the isolation of thiamine from the test material, the oxidation of the vitamin to thiochrome, and the comparison of the fluorescence of this compound and the fluorescence produced by oxidizing a known amount of pure thiamine. Alkaline ferricyanide is used to oxidize thiamine. The blue fluorescing thiochrome can be separated from many interfering substances by isobutyl alcohol extraction. Special equipment is required to make a quantitative comparison of the fluorescence in the sample and in a standard solution. Fluorescence attachments are available for various types of spectrophotometers:
Developments in colorimetric, polarographic, and chromatographic techniques for thiamine, as well as separation procedures for mono-, di-, and tri-phosphoric esters are summarized by Horwitt. Mickelsen and Yamamoto have reviewed analytical methods including animal, physicochemical, enzymatic, and microbiological techniques developed by many investigators over the years.
