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Original Research Communications |
1 From the Program in International Nutrition, Department of Nutrition, and the Facility for Advanced Instrumentation, University of California, Davis; and the International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh.
2 Supported by US Department of Agriculture grant 93-37200-9382.
3 Reprints not available. Address correspondence to MJ Haskell, Program in International Nutrition, 3150 Meyer Hall, One Shields Avenue, University of California, Davis, CA 95615. E-mail: mjhaskell{at}ucdavis.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Objective: Our objective was to determine the responsiveness of the DRD technique to 3 different daily supplemental vitamin A intakes during a period of 2.5–4 mo.
Design: Two oral doses of [2H4]retinyl acetate [52.4 µmol retinol equivalent (RE)] were administered on study days 1 and 91 to 26 men (18–32 y of age) who were consuming controlled, low–vitamin A diets, and receiving daily either 0, 5.2, or 10.5 µmol RE of unlabeled supplemental retinyl palmitate during a 75- or 129-d period. Plasma isotopic ratios of [2H4]retinol to retinol on day 115 were used to estimate final vitamin A body stores per Furr et al (Am J Clin Nutr 1989;49:713–6).
Results: Final ( ± SD) estimated vitamin A pool sizes were 0.048 ± 0.031, 0.252 ± 0.045, and 0.489 ± 0.066 mmol in the treatment groups receiving 0, 5.2, and 10.5 µmol RE/d, respectively (P < 0.001). Estimated mean changes in vitamin A pool sizes were similar to those expected for the vitamin A–supplemented groups [estimated:expected (95% CI of change in pool size): 1.08 (0.8, 1.2) and 1.17 (1.0, 1.3)].
Conclusions: The DRD technique can detect changes in total body stores of vitamin A in response to different daily vitamin A supplements. However, abrupt changes in dietary vitamin A intake can affect estimates of total-body vitamin A stores.
Key Words: Vitamin A • deuterated-retinol-dilution technique • DRD technique • pool size • isotope dilution • stable-isotope study • supplementation • Bangladesh
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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20 d in adults; 3). Total body stores of vitamin A are estimated according to the principles of isotope dilution, a set of assumptions regarding retention of the dose of labeled vitamin A, the ratio of specific activities of vitamin A in plasma to that in liver, and the irreversible loss of vitamin A over time (2). As described previously, total-body vitamin A stores = F x dose x (S x a x [(1/D:H) - 1]); where F is a factor for the efficiency of storage of an orally administered dose of vitamin A, which is estimated to be 0.5 on the basis of the previous work of Bausch and Rietz (1); the dose is the amount of isotope administered, expressed as µmol retinol equivalents (RE); S is the assumed ratio of [2H4]retinol to retinol in plasma to that in liver; a is the half-life of vitamin A turnover in the body; and D:H is the isotopic ratio of [2H4]retinol to retinol measured in plasma. The dose of labeled vitamin A does not truly equilibrate with vitamin A body stores because 1) unlabeled vitamin A is continually consumed in the diet, and 2) newly absorbed dietary vitamin A contributes preferentially to the plasma pool (2). For these reasons, the isotopic ratios in plasma and liver do not reach equality. On the basis of the observed mean ratio of specific activities of radiolabeled vitamin A in serum to those in liver in a group of rats with a wide range of dietary vitamin A intakes and hepatic vitamin A concentrations (0.02–0.12 µmol/g liver; 4), a factor of 0.65 is used to estimate S to correct for the inequality of the [2H4]retinol-retinol ratios in plasma and liver. The factor a is used to correct for the irreversible loss of vitamin A and is based on the half-life of vitamin A turnover in the body, which is estimated as 140 d (5). This factor is assumed to be independent of the size of total-body vitamin A stores and time invariant (a = e-kt, where k is 0.5%/d and t is time in days since the dose). Finally, the value -1 corrects for the contribution of the dose of labeled vitamin A to total-body vitamin A stores.
The DRD technique has been validated in well-nourished surgical patients in the United States (2) and in surgical patients with low-to-adequate vitamin A status in Bangladesh (6). The results of these studies have shown that the technique provides an accurate quantitative estimate of total-body vitamin A stores for groups of subjects. The DRD technique may also be useful for assessing the effect of vitamin A supplementation programs by measuring the change in vitamin A pool size in response to an intervention. However, the sensitivity of the technique for detecting changes in total-body vitamin A stores is not known. Therefore, the present study was carried out to evaluate the usefulness of the DRD technique in assessing changes in total body stores of vitamin A in response to supplementation with 3 different amounts of vitamin A. We expected that final estimated total-body vitamin A pool sizes would vary in relation to the total amount of unlabeled vitamin A consumed over a 90-d supplementation period.
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SUBJECTS AND METHODS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Study site
The study was carried out at the International Centre for Diarrhoeal Disease Research in Dhaka, Bangladesh. The subjects reported to a research study ward daily from 0730 to 1930 for a period of 129 d. During that time, they consumed all of their meals (breakfast, lunch, and dinner) and a midafternoon snack in the study cafeteria under supervision. Because of space limitations in the research ward, the study was conducted in 2 separate rounds, each lasting a period of 129 d, with 15 subjects participating in the first round and 11 subjects in the second round.
Treatment groups
Subjects in round 1 (n = 15) were ranked in groups of 3 according to their initial plasma retinol ncentration and those within each group were randomly assigned to 1 of 3 treatment groups to receive a basal low–vitamin A diet and a daily capsule containing one of the following: corn oil only (0 mg RE), 5.2 µmol RE retinyl palmitate/d (1.5 mg RE/d) in corn oil, or 10.5 µmol RE retinyl palmitate/d (3.0 mg RE/d) in corn oil. Subjects in round 2 (n = 11) were ranked similarly according to their initial plasma retinol concentration and randomly assigned to treatment groups. The subject who fell in the middle of the retinol concentration ranking (no. 6) was selected for the placebo group (n = 1) and the remaining subjects were assigned to the vitamin A–treatment groups (n = 10). Thus, considering both rounds, there was a total of 6 subjects in the placebo group and 10 subjects in the each of the vitamin A–supplemented groups (total n = 26).
Basal diet and administration of capsules
The basal diet consisted primarily of rice and lentils with small amounts of curried meats (mutton, chicken, and fish), vegetables (cabbage, cauliflower, white squash, and white potato), and fruit (banana), all with low vitamin A content. We estimated that the basal diet provided 0.94 µmol RE/d (270 µg RE/d) (7). Subjects were allowed to consume rice and lentils ad libitum at mealtimes so that they could self-adjust their food intake to satisfy their energy needs; however, portions of vegetables, meat, and fruit were controlled. Each subject received a single portion of these foods, which was weighed out onto a tared plate. All meals and snacks were prepared in the hospital kitchen under the supervision of a dietitian. Subjects were free to return home after the evening meal, but were instructed not to consume any food after leaving the study ward. If they did consume any food (which was reportedly rare), they were asked to inform the supervisor the next morning.
In both rounds 1 and 2, the placebo and unlabeled vitamin A capsules were administered in midafternoon with a high-fat snack (shingara, deep-fried white potato pastry). Subjects were weighed and measured at baseline and examined by a physician weekly for clinical evidence of vitamin A deficiency throughout the study periods. At the end of the full set of studies, subjects in the placebo group received a single high-dose vitamin A capsule (210 µmol RE as retinyl palmitate in corn oil).
Isotope-dilution studies
Subjects began consuming the basal diet and their assigned dose of unlabeled vitamin A 1 d before initiating the isotope-dilution studies (study day 0), then continued with their assigned treatment for a period of either 75 d (round 1; n = 15) or 129 d (round 2; n = 11). On study day 1, the subjects received a single oral dose of 52.4 µmol RE (15 mg RE) [2H4]retinyl acetate. Blood samples (7 mL) were drawn 12 h after the dose and on days 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 20, 30, 60, 75, and 90 for measurement of the plasma isotopic ratio of [2H4]retinol to retinol. The purpose of collecting multiple samples during the first 90 d of the study was to evaluate the effect of consumption of different amounts of vitamin A on the plasma isotopic ratio of [2H4]retinol to retinol. Plasma isotopic ratios on day 20 were also used to estimate vitamin A pool sizes at baseline (2).
On study day 91, a second dose of 52.4 µmol (15 mg RE) [2H4]retinyl acetate was given orally and blood samples were drawn 9, 15, 24, and 36 d after the dose for measurement of the plasma isotopic ratio of [2H4]retinol to retinol. We intended to use the plasma isotopic ratio on study day 115 (24 d after the second dose) to estimate total-body vitamin A stores on day 91 (2) for subjects in both rounds 1 and 2. This would also help us to evaluate the ability of the DRD technique to detect differences in vitamin A pool size in response to the different amounts of vitamin A provided during the 90-d supplementation period. However, because of a problem with the original study design, which is described below, it was not possible to estimate reliably the final body stores of vitamin A for the subjects in the vitamin A–supplemented groups in round 1.
Before estimation of total-body vitamin A stores on day 91 for subjects in round 2, the plasma isotopic ratios on day 115 were corrected for the amount of isotope remaining in plasma from the first dose of [2H4]retinyl acetate by using the terminal slope of each individual's initial plasma kinetic curve to estimate, by extrapolation, the contribution of the first dose to the observed isotopic ratio on day 115. Although supplementation with unlabeled vitamin A continued throughout the 129-d study period for those in round 2, the estimates of vitamin A pool sizes based on the plasma isotopic ratios on study day 115 theoretically reflect the pool sizes on the day that the second dose of labeled vitamin A was administered (study day 91). (The "e-kt" term in the prediction equation for estimating total-body vitamin A stores corrects the isotopic ratios for irreversible loss of vitamin A during the period between administration of isotope and measurement of the plasma isotopic ratio.) Thus, the estimated pool sizes reflect the effect of 90 d of supplementation with 3 different amounts of vitamin A on final vitamin A stores.
Laboratory procedures
Plasma retinol concentrations were determined by HPLC (8). The within- and between-day CV of the mean plasma concentration of replicate, pooled plasma samples was <5%. The accuracy of the measurements was assessed by analyzing the retinol concentration of control serum (Fat Soluble Vitamins, 968A; National Institute of Standards and Technology, Gaithersburg, MD). The measured concentration of the control serum was within 2% of the certified retinol concentration. The isotopic ratio of [2H4]retinol to retinol in plasma was determined by gas chromatography–mass spectrometry (9) as described previously (6). Briefly, retinol was isolated from plasma by HPLC and the tert-butyldimethylsilyl (tBDMS) derivative of retinol was formed. Isotopic ratios were estimated by gas chromatography–mass spectrometry on a Shimadzu QP-5000 quadrupole mass spectrometer (Shimadzu Scientific Inc, Columbia, MD) using 1.12 x 10-17 J (70 eV) electron ionization. Selected ion monitoring was carried out for fragment ions of the tBDMS derivatives at a mass-to-charge ratio of 255 (retinol) and 259 ([2H4]retinol). The within-run precision of the isotopic ratio measurements was determined by analyzing standards with each set of plasma samples. The CV for the mean isotopic ratio measurements for the standards was <5%.
Statistical analysis
Mean plasma isotopic ratios of [2H4]retinol to retinol were calculated for each of the 20 time points and compared by study group using repeated-measures analysis of variance. Total-body vitamin A pool sizes were estimated by using the method of Furr et al (2) and were compared by treatment group by using analysis of variance and Tukey's test for multiple comparisons. Mean plasma retinol concentrations were calculated on study days 1, 30, and 90 and compared by treatment group using two-factor repeated-measures analysis of covariance, with initial values controlled for, and by Tukey's test for multiple comparisons.
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RESULTS |
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. Their body weights ranged from 39 to 58 kg and their body mass indexes (in kg/m2) ranged from 15.5 to 21.1. The mean values for each of these measurements did not differ by treatment group. None of the subjects developed clinical symptoms or signs of vitamin A deficiency during the study period.
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). The mean plasma retinol concentrations of the 2 vitamin A–supplemented groups increased significantly by day 30 (P < 0.05) when compared with values on day 1, whereas there was no significant change in the placebo group. The mean plasma retinol concentration of the group that received 5.2 µmol RE/d decreased slightly between days 30 and 90; however, the day 90 value was still significantly higher than the day 1 value (P < 0.01). The mean serum retinol concentration of the group that received 10.4 µmol RE/d did not change significantly from day 30 to day 90.
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). Plasma isotopic ratios were similar for all groups on day 1 but were significantly different by day 4 and thereafter (P < 0.001). In round 1, the mean plasma isotopic ratios of both vitamin A–supplemented groups increased significantly from day 75 to day 90 (P 
0.011), after daily consumption of supplemental unlabeled vitamin A had been discontinued (Figure 1). By contrast, the mean plasma isotopic ratio of the placebo group decreased significantly from day 75 to day 90 (P < 0.01), although the rate of decrease was not significantly different from that measured from day 30 to day 75. For subjects in round 2, supplementation with corn oil or unlabeled vitamin A was continued throughout the 129-d period. Thus, there was no perturbation of the plasma kinetic curves after day 75 for any of the 3 treatment groups, and the differences in patterns of isotopic ratios between days 75 and 90 that were observed in round 1 did not occur (Figure 2).
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Because of the perturbation of the plasma isotopic ratio from day 75 to day 90 described above in the 2 vitamin A–supplemented groups in round 1, the protocol was modified in round 2. The estimated final total-body vitamin A stores are presented for the second group of supplemented subjects (n = 10) and compared with the subjects who received placebo in both rounds (n = 6).
Estimated final total-body vitamin A stores in round 2
As shown in Table 4, the estimated final total-body vitamin A stores were significantly different across the three treatment groups following continuous supplementation with different levels of unlabeled vitamin A (P < 0.001). The estimated and expected theoretical differences in total-body vitamin A pool sizes for the vitamin A–supplemented groups are shown in Table 4. The expected theoretical differences in pool size were estimated as 50% of the total amount of supplemental vitamin A provided from day 1 to day 90, adjusted for the irreversible loss of vitamin A over time (0.5%/d) (5, 10). The difference in pool size was estimated as the observed mean final pool sizes of the vitamin A–supplemented groups in round 2 minus the observed mean final pool size of the placebo group (rounds 1 and 2 combined), assuming that the groups had similar pool sizes initially. A mean (±SE) difference in pool size of 0.204 ± 0.024 mmol was estimated for the group that received 5.2 µmol RE/d (1.5 mg RE/d) and a difference of 0.441 ± 0.033 mmol was estimated for the group that received 10.5 µmol RE/d (3.0 mg RE/d). The ratio of estimated to theoretical pool sizes was 1.08 (95% CI: 0.82, 1.33) for the group that received 5.2 µmol/d (1.5 mg RE/d) and 1.17 (95% CI: 1.00, 1.34) for the group that received 10.5 µmol/d (3.0 mg RE/d). The estimated vitamin A pool size of the combined placebo group at baseline and after 90 d of supplementation was 0.040 ± 0.023 mmol and 0.048 ± 0.031 mmol, respectively, and did not change significantly over time.
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DISCUSSION |
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Theoretically, the DRD technique estimates total body stores of vitamin A on the day that the test-dose is administered because the term e-kt in the prediction equation corrects the observed plasma isotopic ratio for the irreversible loss of vitamin A over time. Thus, if our assumption is correct that the total vitamin A reserves of each of the supplementation groups were similar initially, the DRD test, with isotope being administered on day 1, should have provided similar estimates of total vitamin A pool size in each group. By contrast with this theoretical situation, the plasma isotopic ratios of [2H4]retinol to retinol were significantly lower in the vitamin A–supplemented groups than in the placebo group by day 4 and thereafter, and the estimates of total body stores of vitamin A based on plasma isotopic ratios on day 20 were correspondingly 4–7-fold greater in the vitamin A–supplemented groups than in the placebo group. Plasma isotopic ratios may have been lower in the supplemented groups on day 20 for 3 possible reasons: 1) a true difference in pool size on day 1, 2) a true increase in the vitamin A pool size between days 1 and 20 and incomplete correction for this with the assumptions of the DRD technique, and 3) a differential dilution of [2H4]retinol in plasma by the 3 supplementation levels of unlabeled vitamin A leading to a spurious overestimate of the pool size on day 20.
The first reason seems unlikely given that the subjects were randomly assigned to treatment groups and there were no differences in serum retinol concentrations at baseline. It is highly improbable that a true increase in the total body pool of vitamin A could have explained the differences in isotopic ratios on day 20 when it is assumed that the total vitamin A pool sizes were similar in each group initially. As described above, the DRD technique supposedly estimates total-body vitamin A stores on the day the test dose of isotope is administered rather than on the day that the isotopic ratio is measured. If we consider, for the moment, that the technique is estimating total-body vitamin A stores on the day that the isotope ratio is measured rather than on the day the isotope is administered, the estimates of vitamin A pool size are still higher than can be explained by the supplemental vitamin A that was provided during the 20-d period. If 50% of the supplemental vitamin A that was consumed was retained (as is typically assumed; 1) and 0.5% of the total pool was irretrievably disposed of each day, the pool sizes could have increased by only 0.048 and 0.095 mmol during the first 20 d of supplementation in the groups that received 5.2 and 10.5 µmol RE vitamin A/d. However, the observed increments in vitamin A pool size, as estimated by the DRD technique, were 0.120 and 0.243 mmol for the vitamin A supplemented groups (Table 5). Thus, using these commonly applied assumptions regarding vitamin A retention and disposal, the DRD test provided a 2.5-fold overestimate of the maximum potential increment in pool sizes. Even if all of the vitamin A that was consumed was retained (an assumption that is extremely unlikely), the DRD test would have provided overestimates of the likely true pool size. Thus, the high estimates of total body stores of vitamin A in the vitamin A–supplemented groups cannot be explained by the amounts of vitamin A provided by the diet and supplements during the 20-d period. It is more likely that the vitamin A pool sizes of the vitamin A supplemented groups were overestimated because of the diluting effect of the supplemental unlabeled vitamin A on [2H4]retinol in plasma. In particular, the newly absorbed unlabeled vitamin A of dietary origin probably had a greater diluting effect on the plasma pool of [2H4]retinol than was anticipated by the prediction equation.
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In vitamin A–deficient populations, the usual intake of vitamin A is likely to be low and constant; thus, it would be preferable in future studies to conduct baseline and postintervention studies of pool size of such individuals while they consume their usual level of intake, before and after any supplemental vitamin A is provided. However, in populations with variable amounts of vitamin A in their usual diet, it might be preferable to conduct baseline and postintervention studies of vitamin A pool size while subjects are consuming controlled diets to ensure that daily intake of vitamin A is constant during the equilibration period.
When the DRD technique is used to evaluate the efficacy of vitamin A supplementation programs by comparing vitamin A pool sizes before and after an intervention, the evaluation should be designed to avoid problems in interpreting the data because of the observed rebound effect of the plasma isotopic ratio of labeled to nonlabeled retinol. After the supplementation period, subjects from populations with low dietary intakes of vitamin A should resume their usual (preintervention) vitamin A intakes before the second dilution study is initiated to estimate the postsupplementation vitamin A pool size. This will allow the subjects to adjust to their usual vitamin A intakes so that the plasma isotopic ratios that are measured to estimate the postsupplementation pool size will not be affected by an abrupt change in dietary vitamin A intake. The amount of time that subjects should consume their usual amounts of vitamin A before initiating the second dilution study has not yet been determined. In round 1 of the present study, supplementation with unlabeled vitamin A was discontinued on day 75. Thereafter, plasma isotopic ratios were measured on day 90 only, before the second isotope dilution study was initiated. Thus, the point at which the mean plasma isotopic ratios attained a new equilibrium, or plateau, after day 75 could not be determined. Nevertheless, it is clear that abrupt changes in dietary vitamin A intake can affect plasma isotopic ratios and estimates of total-body stores of vitamin A when the DRD technique is used.
It was possible to estimate total body stores of vitamin A in subjects participating in the second round of the study by using the DRD technique because these subjects continued to receive their assigned supplement throughout the 129-d study period. The estimated mean total-body vitamin A stores were significantly different across the 3 treatment groups and varied in relation to the amount of supplementary vitamin A that was provided. The ratio of observed to theoretical difference in pool size for the 2 vitamin A–supplemented groups was 1.08 and 1.17, and neither ratio was significantly different from 1.0. Thus, the DRD technique detected a differential increment in the total-body vitamin A pool size in the vitamin A–supplemented groups that was very similar to the expected difference, based on theoretical estimates.
Dietary vitamin A intake was strictly controlled during the study period, although it is conceivable that the subjects consumed additional food sources of vitamin A when they returned to their homes at night. However, this is unlikely because the typical Bangladeshi diet consists of rice and lentils and small amounts of curried meats and vegetables and is generally low in vitamin A content (11). Moreover, there was no significant change in the mean plasma retinol concentration or estimated pool size within the placebo group during the study, which suggests that vitamin A intake remained fairly constant throughout the observation period.
In summary, these data show that the DRD technique can detect changes in total-body vitamin A stores in response to supplementation in populations with low vitamin A status. The data therefore suggest that the DRD technique can be used to evaluate the impact of vitamin A intervention programs. However, note that the subjects in this study consumed controlled diets and remained healthy throughout the study period. Dietary vitamin A intake during the equilibration period, morbidity, and greater variation in vitamin A status in free-living populations may affect the ability of the technique to detect changes in vitamin A pool sizes in response to an intervention.
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ACKNOWLEDGMENTS |
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REFERENCES |
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-tocopherol and retinol in plasma or red cells by high pressure liquid chromatography. Am J Clin Nutr 1979;32:2143–9.This article has been cited by other articles:
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  A. L. Escaron, M. H. Green, J. A. Howe, and S. A. Tanumihardjo Mathematical Modeling of Serum 13C-Retinol in Captive Rhesus Monkeys Provides New Insights on Hypervitaminosis A J. Nutr., October 1, 2009; 139(10): 2000 - 2006. [Abstract] [Full Text] [PDF]
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S. M. Ahmad, M. J. Haskell, R. Raqib, and C. B. Stephensen Markers of Innate Immune Function Are Associated with Vitamin A Stores in Men J. Nutr., February 1, 2009; 139(2): 377 - 385. [Abstract] [Full Text] [PDF]
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S. M. Ahmad, M. J. Haskell, R. Raqib, and C. B. Stephensen Men with Low Vitamin A Stores Respond Adequately to Primary Yellow Fever and Secondary Tetanus Toxoid Vaccination J. Nutr., November 1, 2008; 138(11): 2276 - 2283. [Abstract] [Full Text] [PDF]
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 J. D Ribaya-Mercado, C. C Maramag, L. W Tengco, G. G Dolnikowski, J. B Blumberg, and F. S Solon Carotene-rich plant foods ingested with minimal dietary fat enhance the total-body vitamin A pool size in Filipino schoolchildren as assessed by stable-isotope-dilution methodology Am. J. Clinical Nutrition, April 1, 2007; 85(4): 1041 - 1049. [Abstract] [Full Text] [PDF]
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M. J Haskell and K. H Brown Reply to M van Lieshout and S de Pee Am. J. Clinical Nutrition, April 1, 2005; 81(4): 945 - 946. [Full Text] [PDF]
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J. D Ribaya-Mercado, N. W Solomons, Y. Medrano, J. Bulux, G. G Dolnikowski, R. M Russell, and C. B Wallace Use of the deuterated-retinol-dilution technique to monitor the vitamin A status of Nicaraguan schoolchildren 1 y after initiation of the Nicaraguan national program of sugar fortification with vitamin A Am. J. Clinical Nutrition, November 1, 2004; 80(5): 1291 - 1298. [Abstract] [Full Text] [PDF]
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M. J Haskell, K. M Jamil, F. Hassan, J. M Peerson, M I. Hossain, G. J Fuchs, and K. H Brown Daily consumption of Indian spinach (Basella alba) or sweet potatoes has a positive effect on total-body vitamin A stores in Bangladeshi men Am. J. Clinical Nutrition, September 1, 2004; 80(3): 705 - 714. [Abstract] [Full Text] [PDF]
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 L. E. CAULFIELD, S. A. RICHARD, and R. E. BLACK UNDERNUTRITION AS AN UNDERLYING CAUSE OF MALARIA MORBIDITY AND MORTALITY IN CHILDREN LESS THAN FIVE YEARS OLD Am J Trop Med Hyg, August 1, 2004; 71(2_suppl): 55 - 63. [Abstract] [Full Text] [PDF]
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J. D Ribaya-Mercado, F. S Solon, L. S Fermin, C. S Perfecto, J. A. A Solon, G. G Dolnikowski, and R. M Russell Dietary vitamin A intakes of Filipino elders with adequate or low liver vitamin A concentrations as assessed by the deuterated-retinol-dilution method: implications for dietary requirements Am. J. Clinical Nutrition, April 1, 2004; 79(4): 633 - 641. [Abstract] [Full Text] [PDF]
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M. J Haskell, J. L Lembcke, M. Salazar, M. H Green, J. M Peerson, and K. H Brown Population-based plasma kinetics of an oral dose of [2H4]retinyl acetate among preschool-aged, Peruvian children Am. J. Clinical Nutrition, March 1, 2003; 77(3): 681 - 686. [Abstract] [Full Text] [PDF]
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J. D Ribaya-Mercado, F. S Solon, G. E Dallal, N. W Solomons, L. S Fermin, M. Mazariegos, G. G Dolnikowski, and R. M Russell Quantitative assessment of total body stores of vitamin A in adults with the use of a 3-d deuterated-retinol-dilution procedure Am. J. Clinical Nutrition, March 1, 2003; 77(3): 694 - 699. [Abstract] [Full Text] [PDF]
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, 0 mg retinol equivalents (RE)/d; 
, 1.5 mg RE/d; 
, 3.0 mg RE/d. Isotopic ratios were significantly different by treatment group on day 4 and subsequently, P < 0.001. Inset: within-group isotopic ratios were significantly different for day 75 compared with day 90 (a and b, P < 0.01; c and d, P < 0.02).