Blood Concentration of Thiamin

In experimental animals and in depletion studies, measurement of the concentration of thiamin in plasma or whole blood provides an indication of the progression of deficiency. The normal method is by the formation of thiochrome, which is fluorescent ordy free thiamin, and not the phosphates, undergoes 

Urinary excretion over 4 h after a 19 nmol (5 mg) parenteral dose  

Sources From data reported by Brin, 1964 Sauberlich et at, 1974 Finglass, 1993.  

Erythrocytes and leukocytes contain mainly thiamin diphosphate, whereas plasma contains free thiamin and thiamin monophosphate. The concentration of thiamin diphosphate in erythrocytes is normally between 110 and 330 nmol per L of packed cells. The total thiamin concentration in erythrocytes is about 4- to 5-fold higher than in plasma and that in leukocytes is 10-fold higher again. 

Whole blood total thiamin below 150 nmol per L is considered to indicate deficiency. However, the changes observed in depletion studies are small. Even in patients with frank beriberi, the total thiamin concentration in erythrocytes is only 20% lower than normal whole blood thiamin is not a sensitive index of status. 

4- to 5-fold higher than in pleisma and that in leukocytes is 10-fold higher ageun. 

---

The transport system is saturated at relatively low concentrations of thiamin (about 2 /xmol per L), thus limiting the amount of thiamin that can be absorbed. As a result, increasing test doses of thiamin from 2.5 to 20 mg have only a negligible effect on the plasma concentration of thiamin or on urinary excretion. By contrast, the absorption of lipid-soluble aUithiamin derivatives is not apparendy saturable, and they can be used to achieve high blood concentrations of thiamin. 

PDH deficiency results in raised blood concentrations of pyruvate, lactate and alanine. Some patients respond to supplementation with lipoic acid or thiamin (coenzymes for PDH). Treatment with a low carbohydrate, ketogenic diet has been advocated but with limited success. (The ketone bodies readily cross the blood-brain barrier and their catabolism produces acetyl CoA independently of PDH.)... 

Thiamine is excreted in the urine, the amount being dependent on dietary intake and the relative saturation of the tissue stores. Determination of thiamine excretion in the urine, especially after a test dose of thiamine has been administered, is one of the methods used in evaluating nutritive status relative to this vitamin. After intramuscular injection of 1 mg. of thiamine, persons who are adequately nourished excrete at least 100 fig. in the subsequent 4 hr., whereas patients with signs of thiamine deficiency usually excrete less than 50 /xg. during this period. Estimation of the concentration of thiamine in blood has also been used in nutritional appraisal. Mean... 

The decarboxylation and oxidation of pyruvate to form acetyl CoA requires the coenzyme thiamin diphosphate, which is formed from vitamin (section 11.6.2). In thiamin deficiency, this reaction is impaired, and deficient subjects are unable to metabolize glucose normally. Especially after a test dose of glucose or moderate exercise they develop high blood concentrations of pyruvate and lactate. In some cases this may be severe enough to result in life-threatening acidosis. 

In the tissues of animals, most thiamine is found as its phosphorylated esteis (4—6) and is piedominandy bound to enzymes as the pyrophosphate (5), the active coen2yme form. As expected for a factor involved in carbohydrate metaboHsm, the highest concentrations ate generally found in organs with high activity, such as the heart, kidney, Hver, and brain. In humans this typically amounts to 1—8 p.g/g of wet tissue, with lesser amounts in the skeletal muscles (35). A typical healthy human body may contain about 30 mg of thiamine in all forms, about 40—50% of this being in the muscles owing to their bulk. Almost no excess is stored. Normal human blood contains about 90 ng/mL, mostly in the ted cells and leukocytes. A value below 40 ng/mL is considered indicative of a possible deficiency. Amounts and proportions in the tissues of other animal species vary widely (31,35). 

At this point, every patient with altered mental status should receive a challenge with concentrated dextrose, unless a rapid bedside blood glucose test demonstrates that the patient is not hypoglycemic. Adults are given 25 g (50 mL of 50% dextrose solution) intravenously, children 0.5 g/kg (2 mL/kg of 25% dextrose). Hypoglycemic patients may appear to be intoxicated, and there is no rapid and reliable way to distinguish them from poisoned patients. Alcoholic or malnourished patients should also receive 100 mg of thiamine intramuscularly or in the intravenous infusion solution at this time to prevent Wernicke s syndrome. 

In a rare autosomal recessive condition (discovered in 1954) the urine and perspiration has a maple syrup odor/ High concentrations of the branched-chain 2-oxoacids formed by transamination of valine, leucine, and isoleucine are present, and the odor arises from decomposition products of these acids. The branched-chain amino acids as well as the related alcohols also accumulate in the blood and are found in the urine. The biochemical defect lies in the enzyme catalyzing oxidative decarboxylation of the oxoacids, as is indicated in Fig. 24-18. Insertions, deletions, and substitutions may be present in any of the subunits (Figs. 15-14,15-15). The disease which may affect one person in 200,000, is usually fatal in early childhood if untreated. Children suffer seizures, mental retardation, and coma. They may survive on a low-protein (gelatin) diet supplemented with essential amino acids, but treatment is difficult and a sudden relapse is apt to prove fatal. Some patients respond to administration of thiamin at 20 times the normal daily requirement. The branched-chain oxoacid dehydrogenase from some of these children shows a reduced affinity for the essential coenzyme thiamin diphosphate.d... 

Thiamine is absorbed by a pathway that is saturable at concentrations of 0.5-1.0 jumol/L. Oral doses in excess of 10 mg do not significantly increase blood or urine concentrations of vitamin Bi. In the human, absorption occurs predominantly in the jejunum and ileum. Some ferns, shellfish, fish, and species of bacteria contain thiami-nase, which cleaves the pyrimidine ring from the thiazole ring. This enzyme causes thiamine deficiency in cattle. In plasma, thiamine is transported bound to albumin and, to a small extent, other proteins. TPP is synthesized in the liver by thiamine pyrophosphokinase. 

Thiamin (vitamin Bi) Thiamin in the body is chiefly found in the phosphorylated form thiamin pyrophosphate (TPP) which is a coenzyme. The majority (80%) of thiamin in the blood is found in the erythrocytes and assay of blood thiamin is a more reliable indicator of deficiency than assay of erythrocyte transketolase. The phosphorylated vitamers are enzymically converted to thiamin in samples using diastase following deproteinization. To reach the low picomolar concentrations the thiamin compounds are oxidized by ferricyanide to form thiochromes, which are highly fluorescent. The thiochromes are then separated by reversed-phase HPLC and detected by their emission at 425-450 nm. 

Standard methods for assessment of thiamine status used to be determination of erythrocyte transketolase (a-ETK) activity (EC 2.2.1.1) with and without stimulation of this enzyme by addition of TDP cofactor (TOP TK effect). A TDP TK effect >15% is considered to show some degree of deficiency, whereas values >22% are considered to indicate severe deficiency. Technical difficulties, including standardization of the assay, instability of the enzyme during storage, and various conditions possibly influencing apoenzyme concentrations led to an increasing use of direct determination of TDP in whole blood, e.g., by HPLC in order to assess thiamine status. The HPLC assay is more robust and easier to perform. Thiamine... 

Nutritional status assessment for thiamine is generally carried out by assaying the total thiamine in whole blood or erythrocytes, or by measuring the activity of erythrocyte transketolase before and after incubation with exogenous thiamine pyrophosphate. The latter serves as the sensitive index of thiamine nutritional status (Brin 1980). In addition to the enzymatic test, a measure of urinary thiamine in relation to dietary intake has been the basis for balance studies to assess the adequacy of intake. When thiamine excretion is low, a larger portion of the test dose is retained, indicating a tissue s need for thiamine. A high excretion indicates tissue saturation. In the deficient state, excretion drops to zero. Plasma pyruvate and lactate concentrations have also been used to assess thiamine status. 

Also called thiamine, occurs widely in food, bnt mostly in small amounts. The best source of thiamine is dried brewers yeast. Other good sources include meat (pork, lamb and beef), ponltry, whole-grain cereals, nuts, pulse and dried legumes. Because thiamine has a high tnmover rate and is not appreciably stored in the body, a continuous supply is required. The heart, kidney, liver and brain have the highest concentrations, followed by the leukocytes and red blood cells. 

Beriberi is caused by a deficiency of thiamin (also called thiamine, aneurin(e), and vitamin Bj). Classic overt thiamin deficiency causes cardiovascular, cerebral, and peripheral neurological impairment and lactic acidosis. The disease emerged in epidemic proportions at the end of the nineteenth century in Asian and Southeast Asian countries. Its appearance coincided with the introduction of the roller mills that enabled white rice to be produced at a price that poor people could afford. Unfortunately, milled rice is particularly poor in thiamin thus, for people for whom food was almost entirely rice, there was a high risk of deficiency and mortality from beriberi. Outbreaks of acute cardiac beriberi still occur, but usually among people who live under restricted conditions. The major concern today is subclinical deficiencies in patients with trauma or among the elderly. There is also a particular form of clinical beriberi that occurs in patients who abuse alcohol, known as the Wer-nicke-Korsakoff syndrome. Subclinical deficiency may be revealed by reduced blood and urinary thiamin levels, elevated blood pyruvate/lactate concentrations and a-ketoglutarate activity, and decreased erythrocyte transketolase (ETKL) activity. Currently, the in vitro stimulation of ETKL activity by thiamin diphosphate (TDP) is the most useful functional test of thiamin status where an acute deficiency state may have occurred. The stimulation is measured as the TDP effect. 

In food, thiamin occurs mainly as phosphate coenzymes and the predominant form is TDP (also called thiamin pyrophosphate and cocarboxylase). The phosphate coenzymes are broken down in the gut by phosphatases to give free thiamin for absorption. Thiamin is absorbed mainly from the upper intestine, and less thiamin is absorbed on an empty stomach than when taken with a meal. The latter could be due to the alkaline conditions in the duodenum, which are prevented by the presence of food. Absorption of up to 2 mg per meal occurs by an active saturable process involving a sodium-dependent adenosine triphosphatase and against a concentration gradient. During absorption, thiamin is phosphorylated to the monophosphate ester (TMP). Thiamin is absorbed via the portal venous system. Further phosphorylation to TDP occurs on entry into all tissues. TDP can cross the blood-brain barrier, where a portion is converted to TTP, although even in the brain, TDP is the predominant form of thiamin. A second passive absorption process operates when intakes of thiamin are >5 mg but the maximum that can be absorbed from an oral dose is 2-5 mg. 

In yet another variant [91], the clinical course was relatively mild, a developmental quotient of 37 and a diffusely abnormal E.E.G. being the only features, and the concentrations of valine, leucine and isoleucine in the blood were five times the normal. On giving extra thiamine, 10 mg per day, the body chemistry quickly became normal, the features of leucinosis reappearing when thiamine was withdrawn. On the high thiamine intake the leucocytes had 40% of the normal ability to oxidize leucine. 

At autopsy, multiple areas of necrosis are found in the grey matter of the central part of the brain and in the spinal cord [141]. Leigh compared these changes to Wernicke s encephalopathy, though with a different distribution. This suggested either thiamine deficiency or inability to utilize thiamine normally, but treatment with thiamine or thiamine pyrophosphate has no effect on the course of the disease [142]. Lipoic acid, like thiamine pyrophosphate involved in the oxidative decarboxylation of pyruvic acid, has been given to some patients [137] the concentration of pyruvic acid in the blood fell and clinical improvement was claimed. Others have tried lipoic acid treatment with less success—the pyruvic acid content of the blood fell, but there was no effect on the clinical course of the disease [136]. It is now known that the enzymes dec2irboxylating pyruvic acid are normal and that the metabolic error results from a lack of pyruvate carboxylase. 

---