Utilization of thiamin
Intestinal absorption of thiamin takes place throughout the upper intestine but appears to be most effective in the jejunum. Recent research has demonstrated that the mechanism of thiamin absorption depends on the level of intake.45.46 From physiologic doses thiamin is absorbed by an active, probably carrier-mediated mechanism. Presence of sodium and unimpaired Na-K ATPase function are necessary for the release of thiamin from the mucosal cell to the serosa. Both the entry into and the exit from the mucosal cell take place as thiamin- TPP is hydrolyzed in the intestinal lumen prior to its uptake into the mucosa. Although phosphorylation to mono- and pyrophosphate forms takes place within the mucosal cell, followed by dephosphorylation before exit from the cell, these reactions are not considered essential for absorption of thiamin. Passive diffusion is insignificant as a mechanism of thiamin absorption from normal dietary intakes but becomes the major means of thiamin absorption at high levels ofintake.46
The active intestinal transport of thiamin is not shared by other vitamins but is subject to inhibition by other factors. Thiamin absorption is decreased in folate deficiency 47 and in chronic alcoholism, probably due to secondary folate deficiency. The mechanism by which folate deficiency affects thiamin absorption is not known, but it is evident before significant structural alterations in the mucosa are detected. Alcohol also has a direct effect
on thiamin absorption by interfering \vith its release from the mucosal cell, presumably through inhibition of the Na-K ATPase activity.46 This effect is observed only in the presence of alcohol and is not detected with large doses of thiamin. Thus it appears that thiamin absorption in alcoholics may be reduced by two mechanisms-the direct effect by alcohol and a secondary effect due to folate deficiency. Because severe thiamin deficiency is very costly in terms of human suffering and the institutional care that is required for affected individuals, fortification of alcoholic beverages with thiamin has been proposed recen tl y. 48
Thiamin is found in the tissues mostly as TPP, but some free thiamin and its mono- (TMP) and triphosphate (TTP) forms are also present. Thiamin cannot be stored to any extent in the animal body, although certain tissuesheart, brain, liver, and kidney-tend to have higher concentrations than others. Because these amounts decrease quickly when thiamin is not supplied, an adequate daily intake is important. On low intake, urinary excretion of thiamin decreases. When intake is increased, a proportional increase in urinary excretion is observed, thereby providing another measure of the adequacy of thiamin intake. Appreciable amounts of various thiamin metabolites are also found in the urine.
Although some thiamin may be synthesized by bacterial action in the large intestine of humans, it is believed that very little is absorbed.
Human requirement
Because thiamin functions primarily in terms of energy metabolism, the recommended allowances suggested by both the joint FAO/WHO Expert Committee49 and the National Research Council's Food and Nutrition Board30 are based on calorie levels. However, the FAO/WHO Expert Committee set the recommendation at 0.4 mg per 1000 kcal, whereas the Food and Nutrition Board recommends 0.5 mg per 1000 kcal. This latter figure indica tes 1.4 mg for the average man and 1.0 mg for the woman. An increase of 0.4 mg over the allowance recommended for the nonpregnant woman is suggested for the pregnant woman and 0.5 mg for the nursing mother.
