phate (NADP) acts as a hydride ion acceptor or donor in many biological redox reactions. NAD has also been shown to be required for important nonredox adenosine diphosphate (ADP) —ribose transfer reactions involved in deoxyribonucleic acid (DNA) repair and calcium mobilization (Kim et al., 1994; Lautier et al., 1993; Lee et al., 1989). The amino acid tryptophan is converted in part to nicotinamide and thus can contribute to meeting the requirement for niacin.

In the form of the coenzymes NAD and NADP, niacin functions in many biological redox reactions. NAD functions in intracellular respiration and as a codehydrogenase with enzymes involved in the oxidation of fuel molecules such as glyceraldehyde 3-phosphate, lactate, alcohol, 3-hydroxybutyrate, pyruvate, and α-ketoglutarate. NADP functions in reductive biosyntheses such as in fatty acid and steroid syntheses and, like NAD, as a codehydrogenase—as in the oxidation of glucose 6-phosphate to ribose 5-phosphate in the pentose phosphate pathway.

Three classes of enzymes cleave the β-N-glycosylic bond of NAD to free nicotinamide and catalyze the transfer of ADP-ribose in nonredox reactions (Lautier et al., 1993). Two of the three classes catalyze ADP-ribose transfer to proteins: mono-ADP-ribosyltransferases and poly-ADP-ribose polymerase (PARP). The third class promotes the formation of cyclic ADP-ribose, which mobilizes calcium from intracellular stores in many types of cells (Kim et al., 1994).

The enzyme PARP is found in the nuclei of eukaryotic cells and catalyzes the transfer of many ADP-ribose units from NAD to an acceptor protein and also to the enzyme itself. These nuclear poly-ADP-ribose proteins seem to function in DNA replication and repair and in cell differentiation. DNA damage greatly enhances the activity of PARP (Stierum et al., 1994); PARP activity is strongly correlated with cellular apoptosis (Stierum et al., 1994).

Absorption of nicotinic acid and nicotinamide from the stomach and the intestine is rapid (Bechgaard and Jespersen, 1977) and at low concentrations is mediated by sodium ion-dependent facilitated diffusion. At higher concentrations, passive diffusion predominates,

Front Matter (R1-R24)

Summary (1-16)

1 Introduction to Dietary Reference Intakes (17-26)

2 The B Vitamins and Choline: Overview and Methods (27-40)

3 A Model for the Development of Tolerable Upper Intake Levels (41-57)

4 Thiamin (58-86)

5 Riboflavin (87-122)

6 Niacin (123-149)

7 Vitamin B6 (150-195)

8 Folate (196-305)

9 Vitamin B12 (306-356)

10 Pantothenic Acid (357-373)

11 Biotin (374-389)

12 Choline (390-422)

13 Uses of Dietary Reference Intakes (423-436)

14 A Research Agenda (437-442)

A Origin and Framework of the Development of Dietary Reference Intakes (443-447)

B Acknowledgments (448-450)

C Système International d'Unités (451-452)

D Search Strategies (453-455)

E Methodological Problems Associated with Laboratory Values and Food Composition Data for B Vitamins (456-459)

F Dietary Intake Data from the Boston Nutritional Status Survey, 1981–1984 (460-465)

G Dietary Intake Data from the Continuing Survey of Food Intakes by Individuals (CSFII), 1994–1995 (466-477)

H Dietary Intake Data from the Third National Health and Nutrition Examination Survey (NHANES III), 1988–1994 (478-501)

I Daily Intakes of B Vitamins by Canadian Men and Women, 1990, 1993 (502-506)

J Options for Dealing with Uncertainties in Developing Tolerable Upper Intake Levels (507-511)

K Blood Concentrations of Folate and Vitamin B12 from the Third National Health and Nutrition Examination Survey (NHANES III), 1988–1994 (512-519)

L Methylenetetrahydrofolate Reductase (520-522)

M Evidence from Animal Studies on the Etiology of Neural Tube Defects (523-526)

N Estimation of the Period Covered by Vitamin B12 Stores (527-530)

O Biographical Sketches (531-536)

P Glossary and Abbreviations (537-540)

Index (541-567)