Cover Image

PAPERBACK
$49.95



View/Hide Left Panel
Click for next page ( 281


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 280
TABLE 1 Dietary Reference Intakes for Thiamin by Life Stage Group DRI values (mg/day) EARa RDAb AIc ULd males females males females Life stage group 0 through 6 mo 0.2 7 through 12 mo 0.3 1 through 3 y 0.4 0.4 0.5 0.5 4 through 8 y 0.5 0.5 0.6 0.6 9 through 13 y 0.7 0.7 0.9 0.9 14 through 18 y 1.0 0.9 1.2 1.0 19 through 30 y 1.0 0.9 1.2 1.1 31 through 50 y 1.0 0.9 1.2 1.1 51 through 70 y 1.0 0.9 1.2 1.1 > 70 y 1.0 0.9 1.2 1.1 Pregnancy £ 18 y 1.2 1.4 19 through 50 y 1.2 1.4 Lactation £ 18 y 1.2 1.4 19 through 50 y 1.2 1.4 a EAR = Estimated Average Requirement. b RDA = Recommended Dietary Allowance. c AI = Adequate Intake. d UL = Tolerable Upper Intake Level. Data were insufficient to set a UL. In the absence of a UL, extra caution may be warranted in consuming levels above the recommended intake.

OCR for page 280
PART III: THIAMIN 281 THIAMIN T hiamin, also known as vitamin B1 and aneurin, functions as a coenzyme in the metabolism of carbohydrates and branched-chain amino acids. Only a small percentage of a high dose of thiamin is absorbed, and el- evated serum values result in active urinary excretion of the vitamin. The adult requirements for thiamin are based on the amount of the vitamin needed to achieve and maintain normal erythrocyte transketolase activity, while avoiding excessive thiamin excretion. Data were insufficient to set a Tolerable Upper Intake Level (UL). DRI values are listed by life stage group in Table 1. Food sources of thiamin include grain products, pork, ham, and fortified meat substitutes. The classic disease of thiamin deficiency is beriberi, which is sometimes seen in developing countries. Severe thiamin deficiency in indus- trialized nations is often associated with chronic heavy alcohol consumption, where it presents as Wernicke-Korsakoff syndrome. Evidence of adverse ef- fects from excess thiamin consumption is extremely limited. The apparent lack of toxicity of supplemental thiamin may be explained by the rapid decline in absorption that occurs at intakes above 5 mg and the rapid urinary excretion of the vitamin. THIAMIN AND THE BODY Function Thiamin (also known as vitamin B1 and aneurin) was the first B vitamin to be identified. It functions as a coenzyme in the metabolism of carbohydrates and branched-chain amino acids. Absorption, Metabolism, Storage, and Excretion Absorption of thiamin occurs mainly in the jejunum. At low concentrations of thiamin, absorption occurs by an active transport system that involves phos- phorylation; at higher concentrations, absorption occurs by passive diffusion. Only a small percentage of a high dose of thiamin is absorbed, and elevated serum values result in active urinary excretion of the vitamin. Total thiamin content of the adult human is approximately 30 mg, and the biological half-life of the vitamin is in the range of 9 to 18 days. Thiamin is transported in blood in both erythrocytes and plasma and is excreted in the urine.

OCR for page 280
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 282 DETERMINING DRIS Determining Requirements The adult requirements for thiamin are based on metabolic studies in which urinary thiamin was measured during depletion–repletion, along with the mea- surement of erythrocyte transketolase activity. Special Considerations Individuals with increased needs: People who may have increased needs for thiamin include those being treated with hemodialysis or peritoneal dialysis, individuals with malabsorption syndrome, and women who are carrying more than one fetus or breastfeeding more than one infant. It was concluded that under normal conditions, physical activity does not appear to influence thia- min requirements to a substantial degree. However, those who engage in physi- cally demanding occupations or who spend much time training for active sports may require additional thiamin. Criteria for Determining Thiamin Requirements, by Life Stage Group Life stage group Criterion 0 through 6 mo Human milk content 7 through 12 mo Extrapolation from adults 1 through 18 y Extrapolation from adults 19 through 50 y Maintenance of normal erythrocyte transketolase activity and urinary thiamin excretion 51 through > 70 y Extrapolation from younger adults Pregnancy £ 18 y through 50 y Age-specific requirement + increased energy utilization and growth needs during pregnancy Lactation £ 18 y through 50 y Age-specific requirement + energy expenditure of human milk production The UL The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse effects for almost all people. Due to insufficient data on adverse effects of excess thiamin consumption, a UL for

OCR for page 280
PART III: THIAMIN 283 thiamin could not be determined. Although no adverse effects have been asso- ciated with excess intake of thiamin from food or supplements, this does not mean that there is no potential for adverse effects resulting from high intakes. DIETARY SOURCES Foods According to the Continuing Survey of Food Intakes by Individuals (CSFII, 1994–1996), the greatest contribution to thiamin intake by U.S. adults came from the following enriched, fortified, or whole-grain products: bread and bread products, mixed foods whose main ingredient is grain, and ready-to-eat cere- als. Other dietary sources of thiamin included pork and ham products, as well as fortified cereals and fortified meat substitutes. Dietary Supplements Approximately 27 percent of adults surveyed took a thiamin-containing supple- ment, according to the 1986 National Health Interview Survey (NHIS). For adults over age 60 years who took supplements and participated in the Boston Nutritional Status Survey (1981–1984), median supplemental thiamin intakes were 2.4 mg/day for men and 3.2 mg/day for women. Bioavailability Data on the bioavailability of thiamin in humans were extremely limited. No adjustments for bioavailability were judged necessary for deriving the EAR for thiamin. Dietary interactions This information was not provided at the time the DRI values for this nutrient were set. INADEQUATE INTAKE AND DEFICIENCY Early stages of thiamin deficiency may be accompanied by nonspecific signs and symptoms that may be overlooked or easily misinterpreted. Signs and symp- toms of thiamine deficiency include the following: • Anorexia • Weight loss

OCR for page 280
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 284 • Mental changes such as apathy, decreased short-term memory, confu- sion, and irritability • Muscle weakness • Cardiovascular effects such as enlarged heart In developing nations, thiamin deficiency often manifests as beriberi. In “wet beriberi,” edema occurs. In “dry beriberi,” muscle wasting is obvious. In in- fants, cardiac failure may occur rather suddenly. Severe thiamin deficiency in industrialized countries is likely to be related to heavy alcohol consumption with limited food consumption, where it pre- sents as Wernicke-Korsakoff syndrome. In severe cases of this syndrome, renal and cardiovascular complications can become life threatening. EXCESS INTAKE There are no reports of adverse effects from the consumption of excess thiamin from food or supplements. Supplements that contain up to 50 mg/day of thia- min are widely available without a prescription, but the possible occurrence of adverse effects resulting from this level or more of intake has not been studied systematically. The apparent lack of toxicity of supplemental thiamin may be explained by the rapid decline in absorption that occurs at intakes above 5 mg and the rapid urinary excretion of the vitamin.

OCR for page 280
PART III: THIAMIN 285 KEY POINTS FOR THIAMIN Thiamin (also known as vitamin B1 and aneurin) functions as a 3 coenzyme in the metabolism of carbohydrates and branched- chain amino acids. The adult requirements for thiamin are based on the amount of 3 the vitamin needed to achieve and maintain normal erythrocyte transketolase activity, while avoiding excessive thiamin excretion. Data were insufficient to set a UL. 3 Food sources of thiamin include grain products, pork, ham, 3 and fortified meat substitutes. The classic disease of thiamin deficiency is beriberi, which is 3 sometimes seen in developing countries. Severe thiamin deficiency in industrialized nations is often 3 associated with chronic heavy alcohol consumption and presents as Wernicke-Korsakoff syndrome. There are no reports of adverse effects from excess thiamin 3 consumption from food or supplements. The apparent lack of toxicity of supplemental thiamin may be 3 explained by the rapid decline in absorption that occurs at intakes above 5 mg and the rapid urinary excretion of the vitamin.