Biochemical Status Assays Recommended Intakes

The detection of subclinical niacin deficiency and the confirmation of a clinical deficiency require the objective measurement of biochemical status (and, if possible, of dietary intake, which is usually a more time-consuming task) in order to provide confirmatory evidence, especially because the typical clinical signs and symptoms of pellagra are not entirely specific and pathognomic. Biochemical status estimates can be used to characterize a population, particularly any high-risk subgroups, and to monitor the efficacy of any anti-pellagra interventions. For most micronu-trients, robust, specific, and sensitive blood component status assays have been developed. However, for niacin the only promising blood-based assay, namely of intracellular pyridine nucleotide (NAD(P)) concentrations, has not been developed into a definitive and generally accepted biochemical status assay with well-defined normal ranges and a demonstrated association between low concentrations and clinical deficiency signs and symptoms. It has been suggested that a ratio of NAD to NADP below 1.0 in erythro-cytes may provide evidence of niacin deficiency, but this requires confirmation.

The practical measurement of niacin status has mainly depended on urinary assays of the excretory products of niacin metabolism, namely N1-methyl nicotinamide (NMN), N1-methyl-2-pyridone-5-carboxamide (2-pyridone), and N1-methyl-4-pyridone-3-carboxamide (4-pyridone), which can be quantitatively estimated by high-performance liquid chromatography separation followed by UV absorption-detection. The Interdepartmental Committee on Nutrition for National Defense has selected as the preferred principal index of niacin status an NMN excretion rate of 5.8 mmol (0.8 mg) per day in 24 h urine samples as defining the junction between biochemical deficiency and sufficiency. If only casual (spot) urine samples are available, then the ratio of NMN to 2-pyridone may provide a useful alternative index, and one study suggested that <8.8 mmol of the combined excretion of NMN plus 2-pyridone can be considered as defining borderline adequacy, corresponding to a niacin intake in the region of 6mg NE/day. The average adult NE requirement has been estimated from depletion-repletion studies to be approximately 5.5 mg NE/1000kcal food energy/day, and thus with a 20% allowance for individual variation to cover the needs of the majority of healthy individuals, an RNI (UK) of approx. 6.6 mg NE/1000kcal (4200 kjoule) food energy/day translates into the broad ranges of UK RNI values that are shown in Table 3. In the USA, the basis for the calculation is

Table 3 Reference and recommended intakes of niacin equivalents3

Age group

United Kingdomb

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