Stable isotope methods for the study of beta-carotene-d8 metabolism in humans utilizing tandem mass spectrometry and high-performance liquid chromatography

A review of vitamin A equivalency of β-carotene in various food matrices for human consumption

Vitamin A equivalency of β-carotene (VEB) is defined as the amount of ingested β-carotene in μg that is absorbed and converted into 1 μg retinol (vitamin A) in the human body. The objective of the present review was to discuss the different estimates for VEB in various types of dietary food matrices. Different methods are discussed such as mass balance, dose-response and isotopic labelling. The VEB is currently estimated by the US Institute of Medicine (IOM) as 12:1 in a mixed diet and 2:1 in oil. For humans consuming β-carotene dissolved in oil, a VEB between 2:1 and 4:1 is feasible. A VEB of approximately 4:1 is applicable for biofortified cassava, yellow maize and Golden Rice, which are specially bred for human consumption in developing countries. We propose a range of 9:1-16:1 for VEB in a mixed diet that encompasses the IOM VEB of 12:1 and is realistic for a Western diet under Western conditions. For a 'prudent' (i.e. non-Western) diet including a variety of commonly consumed vegetables, a VEB could range from 9:1 to 28:1 in a mixed diet.

An LC/MS/MS method for stable isotope dilution studies of β-carotene bioavailability, bioconversion, and vitamin A status in humans

Isotope dilution is currently the most accurate technique in humans to determine vitamin A status and bioavailability/bioconversion of provitamin A carotenoids such as β-carotene. However, limits of MS detection, coupled with extensive isolation procedures, have hindered investigations of physiologically-relevant doses of stable isotopes in large intervention trials. Here, a sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) analytical method was developed to study the plasma response from coadministered oral doses of 2 mg [(13)C10]β-carotene and 1 mg [(13)C10]retinyl acetate in human subjects over a 2 week period. A reverse phase C18 column and binary mobile phase solvent system separated β-carotene, retinol, retinyl acetate, retinyl linoleate, retinyl palmitate/retinyl oleate, and retinyl stearate within a 7 min run time. Selected reaction monitoring of analytes was performed under atmospheric pressure chemical ionization in positive mode at m/z 537→321 and m/z 269→93 for respective [(12)C]β-carotene and [(12)C] retinoids; m/z 547→330 and m/z 274→98 for [(13)C10]β-carotene and [(13)C5] cleavage products; and m/z 279→100 for metabolites of [(13)C10]retinyl acetate. A single one-phase solvent extraction, with no saponification or purification steps, left retinyl esters intact for determination of intestinally-derived retinol in chylomicrons versus retinol from the liver bound to retinol binding protein. Coadministration of [(13)C10]retinyl acetate with [(13)C10]β-carotene not only acts as a reference dose for inter-individual variations in absorption and chylomicron clearance rates, but also allows for simultaneous determination of an individual's vitamin A status.

An LC/MS method for d8-β-carotene and d4-retinyl esters: β-carotene absorption and its conversion to vitamin A in humans

The intestinal absorption and metabolism of β-carotene is of vital importance in humans, especially in populations that obtain the majority of their vitamin A from provitamin A carotenoids. MS has provided a better understanding of the absorption of β-carotene, the most potent provitamin A carotenoid, through the use of stable isotopes of β-carotene. We report here an HPLC-MS method that eliminates the need for complicated sample preparation and allows us to detect and quantify newly absorbed d8-β-carotene as well as its d4-retinyl ester metabolites in human plasma and chylomicron fractions. Both retinoids and β-carotene were recovered in a single simple extraction that did not involve saponification, thus allowing subsequent quantitation of individual fatty acyl esters of retinol. Separation of d8-β-carotene and its d4-retinyl ester metabolites was achieved using the same C30 reversed-phase liquid chromatography followed by mass spectrometry in selected ion monitoring and negative atmospheric pressure chemical ionization modes, respectively. Total time for the two successive runs was 30 min. This HPLC-MS method allowed us to quantify the absorption of intact d8-β-carotene as well as its extent of conversion to d4-retinyl esters in humans after consumption of a single 5 mg dose of d8-β-carotene.

Synthesis and use of stable isotope enriched retinals in the field of vitamin A

The role of vitamin A and its metabolites in the life processes starting with the historical background and its up to date information is discussed in the introduction. Also the role of 11Z-retinal in vision and retinoic acid in the biological processes is elucidated. The essential role of isotopically enriched systems in the progress of vision research, nutrition research etc. is discussed. In part B industrial commercial syntheses of vitamin A by the two leading companies Hoffmann-La Roche (now DSM) and BASF are discussed. The knowledge obtained via these pioneering syntheses has been essential for the further synthetic efforts in vitamin A field by other scientific groups. The rest of the paper is devoted to the synthetic efforts of the Leiden group that gives an access to the preparation of site directed high level isotope enrichment in retinals. First the synthesis of the retinals with deuterium incorporation in the conjugated side chain is reviewed. Then, 13C-labeled retinals are discussed. This is followed by the discussion of a convergent synthetic scheme that allows a rational access to prepare any isotopomer of retinals. The schemes that provide access to prepare any possible isotope enriched chemically modified systems are discussed. Finally, nor-retinals and bridged retinals that give access to a whole (as yet incomplete) library of possible isotopomers are reviewed.

A minute dose of 14C-{beta}-carotene is absorbed and converted to retinoids in humans

Our objective was to quantify the absorption and conversion to retinoids of a 1.01-nmol, 3.7-kBq oral dose of (14)C-beta-carotene in 8 healthy adults. The approach was to quantify, using AMS, the elimination of (14)C in feces for up to 16 d after dosing and in urine for up to 30 d after dosing. The levels of total (14)C in undiluted serial plasma samples were measured for up to 166 d after dosing. Also, the levels of (14)C in the retinyl ester (RE), retinol (ROH), and beta-carotene fractions that were isolated from undiluted plasma using HPLC were measured. The apparent digestibility of the (14)C was 53 +/- 13% (mean +/- SD), based on the mass balance data, and was generally consistent with the area under the curve for zero to infinite period of (14)C that was eliminated in the feces collections made up to 7.5 d after dosing. Metabolic fecal elimination, calculated as the slope per day (% (14)C-dose/collection from d 7.5 to the final day), was only 0.05 +/- 0.02%. The portion of the (14)C dose eliminated via urine was variable (6.5 +/- 5.2%). Participants [except participant 6 (P6)] had a distinct plasma peak of (14)C at 0.25 d post-dose, preceded by a shoulder at approximately 0.1 d, and followed by a broad (14)C peak that became indistinguishable from baseline at approximately 40 d. Plasma (14)C-RE accounted for most of the absorbed (14)C early after dosing and P1 had the longest delay in the first appearance of (14)C-RE in plasma. The data suggest that plasma RE should be considered in estimating the ROH activity equivalent of ingested beta-carotene.

Bioaccessibility of carotenoids and vitamin E from their main dietary sources

Vitamin E and carotenoids are fat-soluble microconstituents that may exert beneficial effects in humans, including protection against cancer, cardiovascular diseases, and age-related eye diseases. Their bioavailability is influenced by various factors including food matrix, formulation, and food processing. Since human studies are labor-intensive, time-consuming, and expensive, the in vitro model used in this study is increasingly being used to estimate bioaccessibility of these microconstituents. However, the ability of this model to predict bioavailability in a healthy human population has not yet been verified. The first aim of this study was to validate this model by comparing model-derived bioaccessibility data with (i) human-derived bioaccessibility data and (ii) published mean bioavailability data reported in studies involving healthy humans. The second aim was to use it to measure alpha- and gamma-tocopherol, beta-carotene, lycopene, and lutein bioaccessibility from their main dietary sources. Bioaccessibility as assessed with the in vitro model was well correlated with human-derived bioaccessibility values (r = 0.90, p < 0.05), as well as relative mean bioavailability values reported in healthy human groups (r = 0.98, p < 0.001). The bioaccessibility of carotenoids and vitamin E from the main dietary sources was highly variable, ranging from less than 0.1% (beta-carotene from raw tomato) to almost 100% (alpha-tocopherol from white bread). Bioaccessibility was dependent on (i) microconstituent species (lutein > beta-carotene and alpha-carotene > lycopene and alpha-tocopherol generally > gamma-tocopherol), (ii) food matrix, and (iii) food processing.

Plasma appearance of labeled beta-carotene, lutein, and retinol in humans after consumption of isotopically labeled kale

The bioavailability of carotenoids from kale was investigated by labeling nutrients in kale with 13C, feeding the kale to seven adult volunteers, and analyzing serial plasma samples for labeled lutein, beta-carotene, and retinol. Ingested doses of labeled carotenoids were 34 micromol for beta-carotene and 33 micromol for lutein. Peak plasma concentrations, areas under the plasma concentration-time curves (AUCs), and percentages of dose recovered at peak plasma concentrations were calculated. Average peak plasma concentrations were 0.38, 0.068, and 0.079 microM for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. Average AUC values (over 28 days) were 42.8, 13.6, 13.2 microM h for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. Percentages of dose recovered at peak plasma concentrations were 3.6, 0.7, and 0.7% for [13C]lutein, [13C]beta-carotene, and [13C]retinol, respectively. A positive relationship was observed between baseline plasma retinol levels and [13C]retinol plasma response. It is possible that this relationship was mediated either through some aspect of beta-carotene absorption or via the common pathways of metabolism for postdose and endogenous retinoid.

Challenges to understanding and measuring carotenoid bioavailability

Carotenoids are an excellent example of where poor understanding of food structure, complexity of behaviour during digestion, and inter-individual differences in response, lead to misinterpretation of study results. Four challenges associated with understanding and measuring carotenoid bioavailability are discussed: release of carotenoids from food structure and processing into an absorbable form (bioaccessibility), passage of carotenoids from gut lumen into the body (absorption), interpreting plasma response and inter-individual variation. Bioaccessibility of carotenoids is governed by characteristics of the food matrix, which affect the efficiency of physical, enzymic and chemical digestion. Carotenoids used as colorants are likely to be better absorbed because of the form in which they are dispersed in food. Extent of absorption of carotenoid supplements will depend on the proximity of dosing to the consumption of a fat-containing meal. Release of carotenoids from food plants occurs only when the plant cell is fractured and this occurs only during food preparation, processing and/or mastication, not during digestion. Following release from the food matrix, the major limiting factor is solubility of carotenoids in digesta. Absorption studies are best carried out by measuring chylomicron carotenoid excursion, with modelling of chylomicron turnover rate. In this way, inter-individual differences in lipoprotein metabolism can, in part, be taken into account before formulating conclusions on the rate and extent of absorption.

Both free and esterified plant sterols reduce cholesterol absorption and the bioavailability of beta-carotene and alpha-tocopherol in normocholesterolemic humans

BACKGROUND

Plant sterols reduce cholesterol absorption, which leads to a decrease in plasma and LDL-cholesterol concentrations. Plant sterols also lower plasma concentrations of carotenoids and alpha-tocopherol, but the mechanism of action is not yet understood.

OBJECTIVES

The aims of this clinical study were to determine whether plant sterols affect the bioavailability of beta-carotene and alpha-tocopherol in normocholesterolemic men and to compare the effects of plant sterol esters and plant free sterols on cholesterol absorption.

DESIGN

Twenty-six normocholesterolemic men completed the double-blind, randomized, crossover study. Subjects consumed daily, for 1 wk, each of the following 3 supplements: a low-fat milk-based beverage alone (control) or the same beverage supplemented with 2.2 g plant sterol equivalents provided as either free sterols or sterol esters. During this 1-wk supplementation period, subjects consumed a standardized diet.

RESULTS

Both of the milks enriched with plant sterols induced a similar (60%) decrease in cholesterol absorption. Plant free sterols and plant sterol esters reduced the bioavailability of beta-carotene by approximately 50% and that of alpha-tocopherol by approximately 20%. The reduction in beta-carotene bioavailability was significantly less with plant free sterols than with plant sterol esters. At the limit of significance (P = 0.054) in the area under the curve, the reduction in alpha-tocopherol bioavailability was also less with plant free sterols than with plant sterol esters.

CONCLUSIONS

Both plant sterols reduced beta-carotene and alpha-tocopherol bioavailability and cholesterol absorption in normocholesterolemic men. However, plant sterol esters reduced the bioavailability of beta-carotene and alpha-tocopherol more than did plant free sterols.

Isotopic labeling and LC-APCI-MS quantification for investigating absorption of carotenoids and phylloquinone from kale (Brassica oleracea)

The ability to study bioavailability of nutrients from foods is an important step in determining the health impact of those nutrients. This work describes a method for studying the bioavailability of nutrients from kale (Brassica oleracea var. Acephala) by labeling the nutrients with carbon-13, feeding the kale to an adult volunteer, and analyzing plasma samples for labeled nutrients. Results showed that conditions for producing atmospheric intrinsically labeled kale had no detrimental effect on plant growth. Lutein, beta-carotene, retinol, and phylloquinone were analyzed using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry. Analysis of plasma samples showed that labeled lutein peaked in plasma at 11 h (0.23 microM), beta-carotene peaked at 8 (0.058 microM) and 24 h (0.062 microM), retinol peaked at 24 h (0.10 microM), and phylloquinone peaked at 7 h (3.0 nM). This method of labeling kale with (13)C was successful for producing clearly defined kinetic curves for (13)C-lutein,(13)C-beta-carotene, (13)C-retinol, and (13)C-phylloquinone.

Short-term (intestinal) and long-term (postintestinal) conversion of beta-carotene to retinol in adults as assessed by a stable-isotope reference method

BACKGROUND

Quantitative information on the conversion of beta-carotene to vitamin A in humans is limited.

OBJECTIVE

We determined the short- and long-term conversion of labeled beta-carotene to vitamin A by using a stable-isotope reference method.

DESIGN

[(2)H(8)]beta-Carotene (11,011 nmol, or 6 mg) in oil was given with a liquid diet (25% of energy from fat) to 22 adult volunteers (10 men, 12 women). Three days after the [(2)H(8)]beta-carotene dose, the volunteers each took a dose of [(2)H(8)]retinyl acetate (8915 nmol, or 3 mg) in oil with the same liquid diet. Blood samples were collected over 56 d.

RESULTS

The 53-d area under the serum [(2)H(4)]retinol response curve (from the [(2)H(8)]beta-carotene dose) was 569 +/- 385 nmol. d, and the 53-d area under the serum [(2)H(8)]retinol response curve (from the [(2)H(8)]retinyl acetate dose) was 1798 +/- 1139 nmol. d. With the use of [(2)H(8)]retinyl acetate as the vitamin A reference, the [(2)H(4)]retinol formed from [(2)H(8)]beta-carotene (11,011 nmol) was calculated to be equivalent to 3413.9 +/- 2298.4 nmol retinol. The conversion factor of beta-carotene to retinol varied from 2.4 to 20.2, and the average conversion factor was 9.1 to 1 by wt or 4.8 to 1 by mol. This conversion factor was positively correlated with body mass index (r = 0.57, P = 0.006). The postabsorption conversion of beta-carotene was estimated as 7.8%, 13.6%, 16.4%, and 19.0% of the total converted retinol at 6, 14, 21, or 53 d after the [(2)H(8)]beta-carotene dose, respectively.

CONCLUSION

The quantitative determination of the conversion of beta-carotene to vitamin A in humans can be accomplished by using a stable-isotope reference method. This approach provides in vivo metabolic information after a physiologic dose of beta-carotene.

Isotopic tracer techniques for studying the bioavailability and bioefficacy of dietary carotenoids, particularly beta-carotene, in humans: a review

Vitamin A deficiency is a serious health problem in many developing countries. Provitamin A carotenoids in fruit and vegetables are the major source of vitamin A for a large proportion of the world's population. However, the contribution of plant foods is substantial only when both the consumption and provitamin A content of such food is high and, at the same time, the bioefficacy of the provitamin A is high. With respect to provitamin A carotenoids, the term bioefficacy is defined as the product of the fraction of the ingested amount that is absorbed (bioavailability) and the fraction of that which is converted to retinol in the body (bioconversion). Isotopic tracer techniques can meet the need for accurate and precise estimates of the bioavailability, bioconversion, and bioefficacy of dietary carotenoids in humans. Use of such techniques will enable proper evaluation of food-based approaches to eliminating vitamin A deficiency. In addition, the putative antioxidant capacities of carotenoids can be better understood if their bioavailability is known. Here, we discuss how tracer techniques can be applied to obtain reliable and representative data. A step-by-step discussion of aspects related to these techniques is provided, including study design, choice of isotopic tracers, dosing regimen, collection of samples, chemical analysis of samples, and data analysis.

Carotenoid bioavailability and bioconversion

The possible role of carotenoids and their metabolites in disease prevention is far from fully understood, because the bioavailabilities of carotenoids are complicated by multiple factors that affect their absorption, breakdown, transport, and storage. Rapid progress in developing sophisticated methodologies, including use of stable-isotope dilution methods, now allows for an accurate determination of the true vitamin A activity of provitamin A carotenoids. The recent identification of specific enzymes, which catalyze the breakdown of beta-carotene as well as nonprovitamin A carotenoids, is providing a better understanding of the functions of carotenoids at the molecular level. The pathways and possible mechanisms of carotenoid breakdown and factors affecting the bioavailability of carotenoids, such as carotenoid type, food matrix, interaction with other carotenoids and other food components, nutritional status, aging, and infection, are discussed in this review.

Analysis of lipophilic antioxidants in human serum and tissues: tocopherols and carotenoids

Tocopherols and carotenoids are naturally occurring lipophilic micronutrients, suggested to play a role in the prevention of several degenerative diseases. Thus, methods for the quantification of these nutrients in human samples have been developed during recent years. Blood and tissue levels of tocopherols and carotenoids are influenced by a variety of parameters related to disease, age, diet and lifestyle. This review summarizes general aspects of chromatographic analysis of tocopherols and carotenoids in human samples and deals with information on the outcome of human studies, in which such measurements were applied.

Quantitative determination of 13C-labeled and endogenous beta-carotene, lutein, and vitamin A in human plasma

Quantitative procedures employing liquid-chromatography/particle beam-mass spectrometry (LC/PB-MS) and gas chromatography-mass spectrometry (GC-MS) were applied to the determination of the endogenous and 13C-labeled beta-carotene, lutein, and retinol in plasma of a subject who consumed kale (Brassica oleracea) that had been grown in a 13CO2-enriched atmosphere. All compounds were analyzed in the negative chemical ionization (NCI) mode using methane as the moderating reagent gas. Beta-carotene and lutein were analyzed using LC/PB-MS applying reversed-phase high-performance liquid chromatography (HPLC) separation procedures to resolve the analytes. The concentrations of the beta-carotene isotopomers in the plasma over a several-week period were determined using 2H8-beta-carotene as an internal standard. The total plasma concentrations of all trans-lutein were quantified by HPLC analysis with a photodiode array detector using beta-apo-8'-carotenal as an internal standard, and the ratio of the 13C:12C isotopomers of lutein was determined by PB-MS. The retinol isotopomers were collected from individual HPLC fractions of the plasma extract and then analyzed as the trimethylsilyl ethers by GC-MS in the NCI mode. The 13C- and 12C-retinol isotopomers were quantified using 2H4-retinol as an internal standard. These methods demonstrate the application of highly sensitive procedures employing NCI MS for the quantitative determination of carotenoids and vitamin A for the purpose of conducting metabolism studies of phytonutrients.

A liquid chromatography-mass spectrometry method for the quantification of bioavailability and bioconversion of beta-carotene to retinol in humans

A method based on high-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (APCI LC-MS) was developed for the quantification of the bioavailability of retinyl palmitate and beta-carotene and the bioconversion of beta-carotene to retinol in humans. Following oral administration of [8,9,10,11,12,13,14,15,19,20-13C10]-retinyl palmitate and [12,13,14,15,20,12',13',14',15',20'-13C10]-beta-carotene at physiological doses to children between 8 and 11 years of age, blood samples were drawn and serum was prepared. Retinol and beta-carotene were extracted from 0.2- and 1.0-mL serum samples, respectively, and analyzed using reversed-phase HPLC with a C30 column interfaced to an APCI mass spectrometer. Unlike other LC-MS assays for carotenoids, no additional purification steps were necessary, nor was any derivatization of retinol or beta-carotene required. APCI LC-MS showed a linear detector response for beta-carotene over 4 orders of magnitude. Using selected ion monitoring to record the elution profile of protonated circulating beta-carotene at m/z 537 and [13C10]-beta-carotene at m/z 547, the limit of detection was determined to be 0.5 pmol injected on-column. To assess the ratio of labeled to unlabeled retinol, selected ion monitoring was carried out at m/z 269, 274, and 279. These abundant fragment ions corresponded to the loss of water from the protonated molecule of circulating retinol, [13C5]-retinol (metabolically formed from orally administered [13C10]-beta-carotene), and [13C10]-retinol (formed by hydrolysis of [13C10]-retinyl palmitate). The ratios of labeled to unlabeled retinol and the ratio of labeled to unlabeled beta-carotene were calculated. Combined with standard HPLC measurement of beta-carotene and retinol concentration and a mathematical model, these results showed that this simple LC-MS method can be used to quantify beta-carotene bioavailability and its bioconversion to retinol at physiologically relevant doses.

Green and yellow vegetables can maintain body stores of vitamin A in Chinese children

BACKGROUND

Vitamin A activity of plant provitamin A carotenoids is uncertain.

OBJECTIVE

The objective was to determine whether plant carotenoids can sustain or improve vitamin A nutrition during the fall season in kindergarten children in the Shandong province of China.

DESIGN

The serum vitamin A concentration of 39% of the children was <1.05 micromol/L and of 61% of the children was > or = 1.05 micromol/L. For 5 d/wk for 10 wk, 22 children were provided approximately 238 g green-yellow vegetables/d and 34 g light-colored vegetables/d. Nineteen children maintained their customary dietary intake, which included 56 g green-yellow vegetables/d and 224 g light-colored vegetables/d. Octadeuterated and tetradeuterated vitamin A were given before and after the interventions, respectively, and their enrichments in the plasma were determined by gas chromatography-mass spectrometry. Serum retinol and carotenoid concentrations were measured by HPLC.

RESULTS

Carotenoid nutrition improved after consumption of green-yellow vegetables. Serum concentrations of retinol were sustained in the group fed green-yellow vegetables but decreased in the group fed light-colored vegetables (P < 0.01). The isotope-dilution tests confirmed that total-body vitamin A stores were sustained in the group fed green-yellow vegetables, but decreased 27 micromol (7700 microg retinol) per child, on average, in the group fed light-colored vegetables (P < 0.06).

CONCLUSION

Green-yellow vegetables can provide adequate vitamin A nutrition in the diet of kindergarten children and protect them from becoming vitamin A deficient during seasons when the provitamin A food source is limited.

Protocol development for biological tracer studies

Improved instrumentation and the increased availability of labeled compounds have democratized the application of isotope-dilution (tracer) methodology in nutrient metabolism. Still, the most challenging aspects of tracer experimentation reside in the steps that precede the measurement of an isotopically labeled tracer, i.e. the design of a suitably labeled tracer and its isolation and purification from complex biological matrices. Construction of useful mathematical models of nutrient dynamics require methodologies that guarantee that the integrity of the tracer is maintained across the entire sampling and analyte isolation protocol. The ability to provide accurate and reliable data highlights a need for analytical chemists to play a central role in these studies. In this regard, examples and discussion of issues relevant to stable-isotope experimentation are provided.

Compartmental models of vitamin A and beta-carotene metabolism in women

We have developed compartmental models of vitamin A and beta-carotene (beta C) metabolism in women living under controlled conditions on diets with known concentrations of vitamins and carotenoids. Fourteen healthy adult women were given either retinyl-d4 acetate, or beta C-d8 before breakfast. Natural and stable-isotopes of retinol and beta C were collected in serum for up to 95 days or 20 days, respectively. Stable isotopes were separated from other components and measured by GC-MS or HPLC-UV. Preformed retinyl-d4 acetate metabolism in all women tested can be accurately described by a simple four-compartment model. However, the model did not fit one women initially, when she had marginal vitamin A status. We tested the hypothesis that dietary changes of beta C intake have important roles on the kinetics of vitamin A metabolism. Dietary changes of beta C intake did not influence the turnover rate of retinol in any compartment. However, it did result in changes in steady-state masses and residence times of retinol in several compartments. A working compartmental model for beta C metabolism was developed. The kinetics of retinol-d4 formed from beta C is more complicated than the pre-formed retinol-d4. Results suggest that beta C-d8 readily converts into retinol-d4 with high inter-individual variability.

Bioavailability and bioconversion of carotenoids

Factors that influence the bioavailability of carotenoids and their bioconversion to retinol are species of carotenoids, molecular linkage, amount of carotenoids consumed in a meal, matrix in which the carotenoid is incorporated, effectors of absorption and bioconversion, nutrient status of the host, genetic factors, host related factors, and mathematical interactions. In this paper, current knowledge of these factors is examined. Although data are not sufficiently comparable to allow an extensive systematic comparison of results, a number of conclusions can be drawn from the information available.

Ion trap mass spectrometry for kinetic studies of stable isotope labeled vitamin A at low enrichments

The role of beta-carotene in chemoprevention of cancers and other chronic diseases generated controversy when subpopulations taking beta-carotene supplements showed increased mortality in clinical trials. Determination of the dynamics of beta-carotene in individual human subjects has emerged as a high priority. Stable isotope labeled beta-carotene tracers can be employed to determine rates of conversion to retinol (vitamin A), but tracer doses must be small to minimize perturbation of endogenous retinoid and carotenoid pools. In such cases, ratios of labeled tracer/endogenous retinol are often low, and quantitative analysis at enrichments of < 1 mol% are unreliable owing to ion-molecule reactions that generate ions at the same mass as the labeled tracer even when no tracer is present. The current study demonstrates improved gas chromatography/mass spectrometry quantification of retinol-d4 and unlabeled retinol, as their tert-butyldimethylsilyl ethers, at low enrichments using an ion trap mass spectrometer operated in selected ion storage mode. Electron ionization of analyte takes place in the ion trap using conditions that eject ions outside the range m/z 390-420, and molecular ions at m/z 400 and 404 from retinol and retinol-d4 are quantified. Using this approach, unlabeled retinol yields a signal close to values calculated from natural isotopic abundances (approximately 0.13%), whereas several quadrupole instruments operated using selected ion monitoring yielded 2-5 times greater signal when no labeled retinol was present.

Development of a method for quantitation of retinol and retinyl palmitate in human serum using high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry

A method for the quantitative analysis of the vitamin A compounds all-trans-retinol and all-trans-retinyl palmitate was developed using high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (APCI-LC-MS). Unlike previous quantitative mass spectrometric methods for vitamin A, HPLC separations were carried out using a C30 reversed-phase column instead of GC separation. Because no sample hydrolysis or derivatization was necessary, retinyl palmitate was preserved for analysis instead of being hydrolyzed to retinol. Human serum was analyzed following simple hexane extraction without saponification or any additional purification. A comparison of APCI and electrospray ionization showed that only APCI produced a linear response over all four orders of magnitude of retinol and three orders of magnitude of retinyl palmitate concentrations. Selected ion monitoring of the fragment ion of m/z 269 was used for APCI quantitation of both retinol and retinyl palmitate, since it was the base peak and the only abundant ion in the mass spectra of both compounds and the internal standard, retinyl acetate. The ion of m/z 269 corresponded to loss of water, loss of palmitic acid, or elimination of acetic acid from the protonated molecules of retinol, retinyl palmitate and retinyl acetate, respectively. The limit of detection of APCI-LC-MS for all-trans-retinol and all-trans-retinyl palmitate was determined to be approximately 34 fmol/microliter and 36 fmol/microliter (0.670 pmol all-trans-retinol and 0.720 pmol all-trans-retinyl palmitate injected in 20 microliters on-column), respectively. The limit of quantitation was approximately 500 fmol/microliter and 250 fmol/microliter (10 pmol and 5 pmol injected in 20 microliters on-column) for retinol and retinyl palmitate, respectively.