This kinetic, bioavailability, and metabolism study of RRR-α-tocopherol in healthy adults suggests lower intake requirements than previous estimates.

Kinetic models enable nutrient needs and kinetic behaviors to be quantified and provide mechanistic insights into metabolism. Therefore, we modeled and quantified the kinetics, bioavailability, and metabolism of RRR-α-tocopherol in 12 healthy adults. Six men and 6 women, aged 27 ± 6 y, each ingested 1.81 nmol of [5(-14)CH(3)]-(2R, 4'R, 8'R)-α-tocopherol; each dose had 3.70 kBq of (14)C. Complete collections of urine and feces were made over the first 21 d from dosing. Serial blood samples were drawn over the first 70 d from dosing. All specimens were analyzed for RRR-α-tocopherol. Specimens were also analyzed for (14)C using accelerator MS. From these data, we modeled and quantified the kinetics of RRR-α-tocopherol in vivo in humans. The model had 11 compartments, 3 delay compartments, and reservoirs for urine and feces. Bioavailability of RRR-α-tocopherol was 81 ± 1%. The model estimated residence time and half-life of the slowest turning-over compartment of α-tocopherol (adipose tissue) at 499 ± 702 d and 184 ± 48 d, respectively. The total body store of RRR-α-tocopherol was 25,900 ± 6=220 µmol (11 ± 3 g) and we calculated the adipose tissue level to be 1.53 µmol/g (657 µg/g). We found that a daily intake of 9.2 µmol (4 mg) of RRR-α-tocopherol maintained plasma RRR-α-tocopherol concentrations at 23 µmol/L. These findings suggest that the dietary requirement for vitamin E may be less than that currently recommended and these results will be important for future updates of intake recommendations.

Retinol to retinol-binding protein (RBP) is low in obese adults due to elevated apo-RBP.

Elevated serum retinol-binding protein (RBP) concentration has been associated with obesity and insulin resistance, but accompanying retinol values have not been reported. Assessment of retinol is required to discriminate between apo-RBP, which may act as an adipokine, and holo-RBP, which transports vitamin A. The relations between serum RBP, retinol, retinyl esters, BMI, and measures of insulin resistance were determined in obese adults. Fasting blood (> or =8 h) was collected from obese men and women (n = 76) and blood chemistries were obtained. Retinol and retinyl esters were quantified by HPLC and RBP by ELISA. RBP and retinol were determined in age and sex-matched, nonobese individuals (n = 41) for comparison. Serum apo-RBP was two-fold higher in obese (0.90 +/- 0.62 microM) than nonobese subjects (0.44 +/- 0.56 microM) (P < 0.001). The retinol to RBP ratio (retinol:RBP) was significantly lower in obese (0.73 +/- 0.13) than nonobese subjects (0.90 +/- 0.22) (P < 0.001) and RBP was strongly associated with retinol in both groups (r = 0.71 and 0.90, respectively, P < 0.0001). In obese subjects, RBP was associated with insulin (r = 0.26, P < 0.05), homeostatic model assessment of insulin resistance (r = 0.29, P < 0.05), and quantitative insulin sensitivity check index (r = -0.27, P < 0.05). RBP was associated with BMI only when obese and nonobese subjects were combined (r = 0.25, P < 0.01). Elevated serum RBP, derived in part from apo-RBP, was more strongly associated with retinol than with BMI or measures of insulin resistance in obese adults. Investigations into the role of RBP in obesity and insulin resistance should include retinol to facilitate the measurement of apo-RBP and retinol:RBP. When evaluating the therapeutic potential of lowering serum RBP, consideration of the consequences of vitamin A metabolism is paramount.

Current Capabilities and Limitations of Stable Isotope Techniques and Applied Mathematical Equations in Determining Whole-Body Vitamin A Status.

BACKGROUND: Retinol isotope dilution (RID) methodology provides a quantitative estimate of total body vitamin A (VA) stores and is the best method currently available for assessing VA status in adults and children. The methodology has also been used to test the efficacy of VA interventions in a number of low-income countries. Infections, micronutrient deficiencies (eg, iron and zinc), liver disease, physiological age, pregnancy, and lactation are known or hypothesized to influence the accuracy of estimating total body VA stores using the isotope dilution technique. OBJECTIVE: Our objectives were to review the strengths and limitations of RID methods, to discuss what is known about the impact of various factors on results, and to summarize contributions of model-based compartmental analysis to assessing VA status. METHODS: Relevant published literature is reviewed and discussed. RESULTS: Various equations and compartmental modeling have been used to estimate the total body VA stores using stable isotopes, including a newer 3-day equation that provides an estimate of total body VA stores in healthy adults. At present, there is insufficient information on absorption of the isotope tracer, and there is a need to further investigate how various factors impact the application of RID techniques in field studies. CONCLUSIONS: Isotope dilution methodology can provide useful estimates of total body VA stores in apparently healthy populations under controlled study conditions. However, more research is needed to determine whether the method is suitable for use in settings where there is a high prevalence of infection, iron deficiency, and/or liver disease.

Performance of the CRAFTi portable fluorometer comparing with the HPLC method for determining serum retinol.

OBJECTIVE: To determine the performance of a portable fluorometer for measuring serum retinol (SR) concentration. DESIGN AND METHODS: Serum samples were obtained from 75 factory worker women and 143 school children. SR concentration was quantified using a portable fluorometer ('CRAFTi') and HPLC analysis. RESULTS: SR by HPLC (1.23 ± 0.43 µmol/L) and CRAFTi (1.16 ± 0.46 µmol/L) was significantly correlated. Sensitivity and specificity were 85.3% and 78.0% (cutoff of 1.05 µmol/L). Kappa statistics showed moderate agreement. CONCLUSIONS: CRAFTi portable fluorometer is a promising field-friendly tool for screening vitamin A deficiency.

Stable isotope dilution techniques for assessing vitamin A status and bioefficacy of provitamin A carotenoids in humans.

Vitamin A deficiency is a major global public health problem. Among the variety of techniques that are available for assessing human vitamin A status, evaluating the provitamin A nutritional values of foodstuffs and estimating human vitamin A requirements, isotope dilution provides the most accurate estimates. Although the relative expense of isotope dilution restricts its applications, it has an important function as the standard of reference for other techniques. Mathematical modelling plays an indispensable role in the interpretation of isotope dilution data. This review summarises recent applications of stable isotope methodology to determine human vitamin A status, estimate human vitamin A requirements, and calculate the bioconversion and bioefficacy of food carotenoids.

Analysis of retinoids and carotenoids: problems resolved and unsolved.

Progress in nutritional biochemistry has always depended on progress in analysis of nutrients. Animal growth assays were fundamentally important in the discovery and initial isolation of the fat-soluble vitamins. Chromatography, initially introduced by Tswett for separation of plant pigments (including carotenoids), quickly became indispensable for separation of carotenoids and vitamin A compounds; the early open-column methods were eventually superseded by more efficient HPLC techniques, and reversed-phase HPLC has become the current method of choice for analysis of retinoids and carotenoids in biological tissues. Detection and quantitation of retinoids and carotenoids most often has depended on their unparalleled spectral properties; the conjugated polyene structures of these compounds give them unique light absorption spectra and high molar absorptivities, and hence outstanding lower limits of detection. Other techniques, such as gas chromatography (GC) and mass spectroscopy (coupled with GC and HPLC), immunoassays, supercritical fluid chromatography, and capillary electrophoresis, have proven useful in certain applications. Analysis of retinoid-binding proteins has been mostly by conventional protein methods, although the fluorescence of the retinol ligand has been useful in some instances to provide a highly specific assay. Current challenges in retinoid and carotenoid analysis include the resolution of stereoisomers, and quantitation of these compounds at ultratrace levels in biological tissues. Possible new approaches include accelerator mass spectroscopy, and use of gene expression assays to assess vitamin A status.

Vitamin A depletion is associated with low phosphoenolpyruvate carboxykinase mRNA levels during late fetal development and at birth in mice.

Expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene is repressed during fetal liver development and activated at birth. It has been shown that the PEPCK gene is a retinoid-responsive gene, but whether it is regulated by vitamin A in the fetus has not been established. In this study, we found that PEPCK mRNA can be detected in the murine fetal liver as early as gestational d 17. In addition, expression and cAMP induction of the PEPCK gene during late gestation and at birth require vitamin A sufficiency in the fetus and neonate. The PEPCK promoter contains several regulatory elements that bind a diverse array of transcription factors and nuclear coregulators, although it is largely unknown which of these factors are expressed early in liver development. Expression of some of these nuclear factors in livers of fetal mice was investigated by immunohistochemistry (IHC). Fetuses were from dams that were fed from the beginning of gestation diets that were adequate or devoid of vitamin A. Hepatocyte nuclear factor 4alpha (HNF4alpha) was expressed at the earliest stage of liver development on d 11, whereas retinoid X receptor alpha (RXRalpha) and nuclear coactivator CREB-binding protein (CBP) were expressed from d 16 onward. Although expressions of RXRalpha and CBP in livers of vitamin A-sufficient and vitamin A-depleted fetal mice did not differ, the level of HNF4alpha was consistently lower in the latter. Our findings strongly suggest that vitamin A is required during liver development for staged expression of the PEPCK gene and that HNF4alpha may be involved in mediating vitamin A regulation of the PEPCK gene at these critical periods.