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aData adapted from: Paul AA (1969) The calculation of nicotinic acid equivalents and retinol equivalents in the British diet. Nutrition (London) 23: 131-136,a and supplements to McCance and Widdowson's The Composition of Foods (Holland B, Welch AA, Unwin ID, Buss DH, Paul AA, and Southgate DAT (1991), The Royal Society of Chemistry and MAFF),a and from Bressani R et al. (1961) Effect of processing method and variety on niacin and ether extract content of green and roasted coffee. Food Technology 15: 306-308.

bAmount available for absorption. In the case of bread, rice, and maize, the total amounts present are 1.7, 1.5, and

1.2 mg per 100g, but apart from the niacin added in the fortification of white flour, 90% of this is unavailable for utilization by humans.

cAssuming that 60 mg tryptophan yields 1 mg niacin equivalent.

dNiacin is released from trigonelline in coffee beans by the roasting process.

aData adapted from: Paul AA (1969) The calculation of nicotinic acid equivalents and retinol equivalents in the British diet. Nutrition (London) 23: 131-136,a and supplements to McCance and Widdowson's The Composition of Foods (Holland B, Welch AA, Unwin ID, Buss DH, Paul AA, and Southgate DAT (1991), The Royal Society of Chemistry and MAFF),a and from Bressani R et al. (1961) Effect of processing method and variety on niacin and ether extract content of green and roasted coffee. Food Technology 15: 306-308.

bAmount available for absorption. In the case of bread, rice, and maize, the total amounts present are 1.7, 1.5, and

1.2 mg per 100g, but apart from the niacin added in the fortification of white flour, 90% of this is unavailable for utilization by humans.

cAssuming that 60 mg tryptophan yields 1 mg niacin equivalent.

dNiacin is released from trigonelline in coffee beans by the roasting process.

metabolic components. Both facilitated diffusion (which is sodium- and energy-dependent and saturable), and passive diffusion (which is nonsaturable) contribute to tissue uptake from the bloodstream. With the exception of muscle, brain and testis, within the body nicotinic acid is a better precursor of the coenzyme form than is nicotinamide. The liver appears to be the most important site of conversion of tryptophan to the nicotinamide coenzymes.

Of the two pyridine nucleotide coenzymes, NAD is present mainly as the oxidized form in the tissues, whereas NADP is principally present in the reduced form, NADPH2. There are important homeostatic regulation mechanisms which ensure and maintain an appropriate ratio of these coenzymes in their respective oxidized or reduced forms in healthy tissues. Once converted to coenzymes within the cells, the niacin therein is effectively trapped, and can only diffuse out again after degradation to smaller molecules. This implies, of course, that the synthesis of the essential coenzyme nucleotides must occur within each tissue and cell type, each of which must possess the enzymatic apparatus for their synthesis from the precursor niacin. Loss of nicoti-namide and nicotinic acid into the urine is minimized (except when the intake exceeds requirements) by means of an efficient reabsorption from the glomerular filtrate.

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