| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Original Research Communications |
1 From the Unilever Research Vlaardingen, Netherlands; the Division of Human Nutrition and Epidemiology, Wageningen Agricultural University, Wageningen, Netherlands; the Department of Obstetrics and Gynaecology, University Hospital Nijmegen St Radboud, Nijmegen, Netherlands; and the Department of Epidemiology, Catholic University, Nijmegen, Netherlands.
See corresponding editorial on page 179.
2 Address reprint requests to KH van het Hof, Unilever Research Vlaardingen, PO Box 114, 3130 AC Vlaardingen, Netherlands. E-mail: hof{at}unilever.com.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Objective: To assess the bioavailability of ß-carotene and lutein from vegetables and the effect of increased vegetable consumption on the ex vivo oxidizability of LDL.
Design: Over 4 wk, 22 healthy adult subjects consumed a high-vegetable diet (490 g/d), 22 consumed a low-vegetable diet (130 g/d), and 10 consumed a low-vegetable diet supplemented with pure ß-carotene (6 mg/d) and lutein (9 mg/d).
Results: Plasma concentrations of vitamin C and carotenoids (ie, 
-carotene, ß-carotene, lutein, zeaxanthin, and ß-cryptoxanthin) were significantly higher after the high-vegetable diet than after the low-vegetable diet. In addition to an increase in plasma ß-carotene and lutein, the pure carotenoid–supplemented diet induced a significant decrease in plasma lycopene concentration of -0.11 µmol/L (95% CI: -0.21, -0.0061). The responses of plasma ß-carotene and lutein to the high-vegetable diet were 14% and 67%, respectively, of those to the pure carotenoid– supplemented diet. Conversion of ß-carotene to retinol may have attenuated its plasma response compared with that of lutein. There was no significant effect on the resistance of LDL to oxidation ex vivo.
Conclusions: Increased vegetable consumption enhances plasma vitamin C and carotenoid concentrations, but not resistance of LDL to oxidation. The relative bioavailability of lutein from vegetables is higher than that of ß-carotene.
Key Words: Vegetables • carotenoids • bioavailability • antioxidants • low-density-lipoprotein oxidation • humans • LDL • lutein • ß-carotene
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Various studies have shown a significant correlation between habitual vegetable intake and plasma carotenoid concentrations (8–10). In addition, increased vegetable consumption results in increased blood carotenoid concentrations (11–13). However, the effectiveness of vegetables as a source of carotenoids has been questioned because studies have shown that the bioavailability of ß-carotene from vegetables is less than previously thought (14, 15). This was shown most conclusively for green leafy vegetables (16). The bioavailability of ß-carotene may, however, vary among types of vegetables because a difference was also found between fruit and green leafy vegetables (17). Green leafy vegetables are not the only type of vegetables in many diets and it is therefore of interest to determine the relative bioavailability of ß-carotene from a mixed-vegetable diet.
More information is needed on the bioavailability of carotenoids other than ß-carotene as well because evidence is accumulating that these may also have important health benefits (5, 18, 19). Lutein is a major carotenoid in vegetables and has been implicated in the etiology of age-related macular degeneration (5). No information is available on the relative bioavailability of lutein from vegetables.
We performed a 4-wk, controlled-intervention dietary study in which we investigated the relative bioavailability of ß-carotene and lutein from mixed vegetables compared with purified ß-carotene and lutein supplements. In addition, we determined the effect of increased vegetable consumption on the resistance of LDLs to oxidation ex vivo. Witztum and Steinberg (20) suggested that oxidative modification of LDL is an important step in the etiology of atherosclerosis. Although the effect of ß-carotene supplementation on the susceptibility of LDL to oxidation ex vivo was shown to be limited (21–23), increased vegetable consumption may be more effective because a range of antioxidants is supplied.
|
SUBJECTS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Study design
In a strictly controlled 4-wk dietary intervention study, 22 subjects received a low-vegetable diet (130 g vegetables/d; control group), 23 subjects received a high-vegetable diet (490 g/d; vegetable group), and 10 subjects received a low-vegetable diet supplemented with pure ß-carotene and lutein (carotenoid-supplemented group). Because this experiment was designed to also investigate the bioavailability of folate from fruit and vegetables, an additional group of 22 subjects was included to receive supplemental folic acid. This part of the study will be reported separately. The treatment groups were stratified for total energy intake, sex, and number of vegetarians. Fasting blood samples were taken before the start and at the end of the study for analysis of plasma concentrations of retinol, carotenoids, and other antioxidants (ie, vitamins C and E), total antioxidant activity, and resistance of LDL to oxidation ex vivo. For practical reasons, we limited the number of subjects in the carotenoid-supplemented group. Power calculations based on data from previous studies showed that n = 10 would be sufficient to show a 33–50% difference in plasma responses of ß-carotene and lutein and a 15% difference in LDL oxidizability (lag phase) ( = 0.05, ß = 0.20). The study was performed at the Department of Human Nutrition and Epidemiology of Wageningen Agricultural University from November to December 1996. The study protocol was approved by the medical ethical committee of the Department.
Diets
During the intervention period, most (90% of energy intake) of the diet was supplied to the subjects. Diets were individually tailored to meet each volunteer's energy requirement (±0.5 MJ/d), which was estimated by questionnaire before the start of the study (24). Body weight was measured twice each week and, if necessary, energy intake was adjusted to prevent further changes in body weight. Subjects were allowed to choose a limited number of additional food items that were low in carotenoids, vitamin C, vitamin E, and fat. From the food diaries that were kept during the study, it was calculated that these foods provided on average 11 ± 1.3% of the total energy intake (25).
The diets were provided as a 6-d menu cycle and comprised conventional foods and drinks. All subjects received the same basic diet, which was supplemented with a fixed amount of additional vegetables and fruit, independent of the subjects' total energy requirements. The control and carotenoid-supplemented groups were provided with the same additional products (consisting of, depending on the day of the menu cycle, a rice or pasta salad, a soup containing little or no vegetables, a pear or apple, and apple juice or grape juice). For the carotenoid-supplemented group, purified ß-carotene [ß-carotene, 30% fluid suspension in vegetable oil (E160a), >95% as all-trans ß-carotene; Hoffmann-La Roche, Basel, Switzerland] and lutein (Flora GLO all-trans lutein, 21% suspension in safflower oil also containing 1.4% all-trans zeaxanthin; Kemin Foods LC, Des Moines, IA) were added to the salad dressing provided. The vegetable group received, in addition to the basic diet, 185 g cooked vegetables/d (depending on the day of the menu cycle, green beans, broccoli, spinach, green peas, Brussels sprouts, or a vegetable mix) and a vegetable-based salad and soup. Instead of an apple or pear they received an orange or 2 tangerines, and instead of the apple or grape juice they were given orange juice. The low-vegetable diet provided on average 130 g vegetables/d, whereas the total daily amount of vegetables in the high-vegetable diet was 490 g. The amount of vegetables provided by the low-vegetable diet is comparable with the average vegetable intake in the Dutch population (26), whereas the amount of vegetable in the high-vegetable diet (490 g/d) was chosen as a high but acceptable amount for consumption during a 4-wk period. The additional vegetables provided to the vegetable group were frozen, obtained from Birds Eye Walls (Flowestoft, United Kingdom), Langnese-Iglo (Heppenheim, Germany), Frudesa (Valencia, Spain), and Sagit (Rome). The fruit juices were from Albert Heijn (Zaandam, Netherlands) and fresh vegetables and fruit were obtained from a local supermarket.
Hot meals (including the additional cooked vegetables) were consumed under supervision at lunchtime at the university from Monday to Friday. Foods for the rest of the day (including additional salad, soup, fruit juice, and fruit) and for the weekend were taken home by the subjects. Volunteers were carefully instructed in how to prepare these foods. Compliance was checked by food diaries kept by the subjects.
Analysis of diets
Duplicate portions of the diets were prepared on each day of the 6-d menu cycle. One pooled sample was prepared and stored at -20°C for analysis of fat, protein, carbohydrate, and dietary fiber. To assess the amount of carotenoids and vitamin C in the diets, one sample from each day of the menu cycle was analyzed and results were averaged per treatment. For these analyses, samples were stored at -80°C. For vitamin C, 5% metaphosphoric acid was added for stabilization. Carotenoid content was determined by reversed-phase HPLC. Samples were extracted with methanol:tetrahydrofuran (1:1 by vol). An aliquot of the filtrate was saponified in boiling ethanolic 2 mol KOH/L after addition of a mixture of sodium ascorbate (10%) and sodium disulfide:glycerol (2:1 by vol). After cooling, the saponification mixture was extracted with diisopropylether. The extract was washed 3 times with water. The solvent was evaporated and the residue was dissolved in diisopropylether. Carotenoids and -tocopherol were separated on a Hypochrome column (Bisschoff Chromatography, Stuttgart, Germany) filled with nucleosil 120–3C18 (Machery-Nagel, Duren, Germany) with acetonitrile:methanol:methylene chloride:ammonium acetate (900:50:40:10 by vol) as the mobile phase at a flow rate of 1 mL/min and at room temperature. External standards were used for calibration. Tests of this method showed 86–103% recovery of the carotenoids and the CV ranged between 5.4% and 15.3% depending on the type of carotenoid. For analysis of vitamin C content, the samples were extracted with metaphosphoric acid:acetic acid (60:80 by vol). Vitamin C content was subsequently determined fluorimetrically as ascorbic acid plus dehydroascorbic acid (27). The composition of the diets is shown in Table 1
. Because we planned to match the vegetables in the high-vegetable diet with other fiber-rich foods (eg, rice or pasta), the difference in fiber content between the high- and low-vegetable diets was only 0.7 g/MJ. Differences in carotenoid and vitamin C contents between the diets were generally as expected. However, the lycopene content of the low-vegetable and carotenoid-supplemented diets was higher than that of the high-vegetable diet. Two of the ready-to-eat soups that were provided in the 6-d menu cycle to the low-vegetable and carotenoid-supplemented groups but not to the high-vegetable group apparently contained more lycopene than we expected. The diets were calculated to provide 
500–600 µg preformed vitamin A/d (25). The ß-carotene, -carotene, and ß-cryptoxanthin in the control, high-vegetable, and carotenoid-supplemented diets hypothetically provided an additional 300, 1000, and 1219 µg retinol equivalent/d, respectively (based on data in Table 1 and the assumption that 6 µg ß-carotene or 12 µg -carotene or ß-cryptoxanthin equals 1 µg retinol equivalent).
|
Vitamin C concentration in trichloroacetic acid–treated plasma was determined fluorimetrically as ascorbic acid plus dehydroascorbic acid (27). Plasma concentrations of carotenoids, retinol, and -tocopherol were assessed by reversed-phase HPLC on a Vydac column (model 201TP54; Separations Group, Hesperia, CA) with retinyl acetate as the internal standard. After extraction with n-heptane:diethyl ether (1:1 by vol) and evaporation of the solvents, the residue was dissolved in eluent and compounds were separated at a flow rate of 0.8 mL/min and a column temperature of 20°C by using a step gradient: 0–80 min methanol:ammonium acetate (19:1 by vol), and 80–85 min methanol:tetrahydro-furan (19:1 by vol). Peak areas were measured spectrophotometrically at 292 nm for -tocopherol, at 325 nm for retinol, at 450 nm for -carotene, ß-carotene, lutein, zeaxanthin, and ß-cryptoxanthin, and at 470 nm for lycopene. The percentage recovery of the internal standards varied between 86% and 99.9% for the different compounds and the detection limit was 
0.02 µmol/L for carotenoids, 0.03 µmol/L for retinol, and 0.65 µmol/L for -tocopherol. Intraassay variation ranged from 0.6% to 5.1%.
Total cholesterol and triacylglycerol concentrations were measured in plasma by using enzymatic colorimetric methods (Boehringer Mannheim, Mannheim, Germany). The antioxidant activity of plasma was assessed as its ferric-reducing ability (29). LDL was isolated from thawed plasma by discontinuous density-gradient ultracentrifugation for 24 h at 4°C (30). EDTA was removed as described by Puhl et al (31). Immediately thereafter, LDL protein content was determined by using bovine serum albumin (Fraction V; Sigma, St Louis) as the standard (32). Subsequently, resistance to copper-mediated oxidation of the EDTA-free LDL fraction was determined as described by Princen et al (21). Intraassay variations were 10% for the lag phase and 4% for the maximum rate of oxidation.
Statistical evaluation
Differences in changes during the experiment between the 3 groups were compared by one-way analysis of variance. Significance of the differences was assessed by Tukey's procedure ( = 0.05). For plasma concentrations and changes in plasma concentrations of ß-carotene, data were log-transformed to minimize correlation between mean values and SE. Data for ß-carotene are therefore presented as geometric means with the SE as a percentage of the geometric mean. Other data are shown as means with their SE, or as means and SDs in the case of descriptive measures.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
.
|
). Plasma concentration of lycopene was significantly reduced during consumption of thehigh-vegetable diet, reflecting the 50% lower lycopene content of this diet compared with the low-vegetable diet (Table 1
).
|
).
In comparison with the changes in the carotenoid-supplemented group, consumption of the high-vegetable diet induced significantly smaller increases in plasma concentrations of ß-carotene and lutein. The high-vegetable and carotenoid-supplemented diets contained slightly different amounts of these carotenoids. We therefore calculated the relative plasma carotenoid responses for the high-vegetable and carotenoid-supplemented groups by dividing the changes in plasma carotenoid concentrations by the carotenoid intake (mg/d), both corrected for those observed in the low-vegetable group. This revealed that the relative plasma ß-carotene response to the high-vegetable diet was substantially less than that to the carotenoid supplement (Table 4). From the ratio of the 2 responses, a measure of relative bioavailability can be obtained by dividing the relative plasma response to vegetable ß-carotene by that to synthetic ß-carotene. This gave a figure of 14% for the relative bioavailability of ß-carotene from vegetables (Table 4). For lutein, the difference in the relative plasma carotenoid responses between the high-vegetable and the carotenoid-supplemented groups was not as large as for ß-carotene, and the relative bioavailability of lutein from vegetables was calculated to be 67% (Table 4).
|
-tocopherol remained unchanged in all 3 groups and no significant differences were found between the groups (Table 3). No significant differences in changes between the groups were found in plasma concentrations of total cholesterol and triacylglycerol (results not shown).
Despite the significant increases in plasma concentrations of vitamin C and carotenoids in the vegetable- and carotenoid-supplemented groups, the total antioxidant activity of plasma, measured as ferric-reducing ability, remained unchanged (Table 3). Furthermore, consumption of the diets supplemented with vegetables or carotenoids did not enhance protection of LDL against copper-induced oxidation ex vivo. Neither the changes in lag time before onset of oxidation nor the maximum rate of oxidation were significantly different from those found in the control group (Table 3).
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Bioavailability of ß-carotene and lutein
The relatively low bioavailability of ß-carotene from vegetables compared with pure ß-carotene was reported previously. Our results confirm the assumption that the matrix in which ß-carotene is located is a major limiting factor for its bioavailability. The relative bioavailability of 14% for ß-carotene from the mixed-vegetable diet lies within the range of relative bioavailabilities from different vegetable types that have been reported by others. It is higher than the 7% availability reported from green leafy vegetables (16) but lower than the 19–30% from carrots and 22–24% from broccoli (33–35). In the present study, spinach was the only green leafy vegetable in the menu cycle and it contributed 43% of the total ß-carotene intake (36). It seems that the low bioavailability of ß-carotene from spinach was compensated for by the higher bioavailability of ß-carotene from other types of vegetables in the menu cycle.
Until recently, the major focus has been on the health benefits of ß-carotene. However, it is now being recognized that other carotenoids present in vegetables may also be crucial for optimal health (5, 18, 19). For this reason, the carotenoid-supplemented group received not only ß-carotene but also lutein so that we could calculate the relative bioavailability of this carotenoid. For lutein in vegetables the bioavailability was found to be 67%. This suggests that the bioavailability from vegetables of the more hydrophilic lutein is 5 times greater than that of ß-carotene. On the other hand, the difference in relative plasma response between ß-carotene and lutein may not reflect the true differences in absorbability. Part of the absorbed ß-carotene is cleaved and converted to retinyl esters before entering the blood stream. In the vegetable group, a relatively larger percentage of the absorbed ß-carotene may have been converted than in the ß-carotene-supplemented group because of the large difference in absorbed ß-carotene between these 2 groups. Because this phenomenon does not occur for lutein, which has no provitamin A activity, this may have resulted in an underestimation of the relative bioavailability of ß-carotene from vegetables compared with lutein.
Interestingly, the plasma response of lutein after supplementation with pure lutein and ß-carotene was substantially smaller than that of ß-carotene. There are several explanations for this finding. First, less lutein may have been absorbed per milligram ingested. The solubility of lutein in oil is lower than that of the more lipophilic ß-carotene (37) and a larger portion of lutein may have been present as crystals in the salad dressing that contained the carotenoids. It has been suggested that the crystalline form of carotenoids is less bioavailable (38). In addition, a relatively smaller uptake of lutein may have resulted from competition for absorption. As more ß-carotene was released from the food matrix after ingestion of the purified carotenoids, the ratio of released ß-carotene to lutein was larger than in the case of the vegetable-supplemented diet, in which part of the ß-carotene was still locked in the cellular compartments of the vegetables. Kostic et al (39) showed that simultaneous ingestion of purified lutein and ß-carotene decreases the bioavailability of lutein. Second, a difference in plasma response between different carotenoids may not reflect a difference in true absorption because the rate and extent of tissue uptake and subsequent metabolism may vary between carotenoids. Faster serum clearance of lutein than of ß-carotene has been observed in preruminant calves (40). On the other hand, ß-carotene has provitamin A activity and the rate and extent of ß-carotene metabolism may thus be higher. This would imply, in contrast with our findings, however, a lower ß-carotene response compared with that for lutein.
Interaction between ß-carotene, lutein, and lycopene
The increases in plasma concentrations of other carotenoids and vitamin C and the decrease in plasma concentration of lycopene in the vegetable group were anticipated on the basis of the composition of the diets (Table 1). However, the decrease in plasma lycopene concentration during consumption of the ß-carotene- and lutein-supplemented diet was surprising. The lycopene contents of the control and carotenoid-supplemented diets were similar (Table 1). Other studies have been equivocal with respect to the effect of ß-carotene supplementation on plasma or LDL concentrations of lycopene, whereas no information is available on the effect of lutein. Some studies also showed a reduction in lycopene concentrations during supplementation with ß-carotene (41, 42) whereas others found no effect (43, 44) or even an enhancing effect (45, 46). The enhancing effect in the single-dose study of Johnson et al (46) was attributed to increased solubility of lycopene in the suspension that contained ß-carotene, whereas during the long-term study reported by Wahlqvist et al (45), a sparing effect of lycopene as an antioxidant may have occurred. However, the results of the present study and those of others (41, 42) suggest that carotenoid supplements compete with lycopene for absorption or transport in plasma. This phenomenon may be particularly important for the risk of prostate cancer because an inverse association with lycopene intake has been reported (18).
Effects on antioxidant capacity
Consumption of the high-vegetable diet increased plasma ß-carotene concentrations by 50% to 0.55 µmol/L, whereas the increases in the carotenoid-supplemented group were even higher (Table 3). These concentrations and the plasma concentrations of total carotenoids are beyond the thresholds that were suggested in relation to reducing risk of cardiovascular disease (47). Despite these substantial increases in plasma antioxidant concentrations in the vegetable and carotenoid-supplemented groups, we found no significant effect on the total antioxidant activity of plasma or on the susceptibility of LDL to oxidation ex vivo. Oxidative modification of LDL has been proposed as an important step in the etiology of atherogenesis (20). Recently, Hininger et al (48) reported that increased fruit and vegetable consumption significantly enhanced the resistance of LDL to oxidation both in smokers and nonsmokers. However, the effect reported may have been due to external factors because they did not include a control group that received a lower amount of fruit and vegetables. The decrease in plasma concentrations of lycopene we observed in the high-vegetable and carotenoid-supplemented groups may also have outweighed a possible protective effect of the other carotenoids, thus explaining the lack of effect of vegetable or carotenoid supplementation in the present study. On the other hand, previous studies showed that vitamin E supplementation in particular is effective in increasing the resistance of LDL to oxidation (21, 49), whereas studies on the benefits of synthetic ß-carotene only have been equivocal (21, 22, 50, 51). The increased antioxidant concentrations may have had an effect on other oxidative stress-related variables that were not assessed in this study, such as isoprostanes (52). This should be addressed in future research.
Conclusion
When designing future studies on health benefits of carotenoids or formulating recommendations on carotenoid intake, the variation in the bioavailability of ß-carotene in particular should be taken into account. The present study clearly showed that vegetable consumption induces a more moderate increase of ß-carotene in plasma than does purified ß-carotene (Table 4). Five milligrams ß-carotene from vegetables would equal only 0.7 mg ß-carotene from a supplement. This may be less important for lutein because the differences in plasma responses between vegetables and purified lutein were less pronounced.
In conclusion, the present study showed that increased vegetable consumption (ie, an additional 360 g/d) enhances the plasma concentrations of vitamin C and carotenoids substantially but not the resistance of LDL to oxidation. The relative bioavailability of ß-carotene and lutein from mixed vegetables compared with purified carotenoids is 14% and 67% respectively.
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
-carotene, ß-carotene and other phytochemicals in the etiology of lung cancer. J Natl Cancer Inst 1996;88:612–5.-carotene in the ferret model. J Am Coll Nutr 1996;15:84–91.[Abstract]
-tocoph-erol on the oxidative modification of low density lipoprotein. J Lipid Res 1992;33:899–906.[Abstract]
This article has been cited by other articles:
|
|
 |
|
  N. C. Khan, C. E West, S. de Pee, D. Bosch, H. D. Phuong, P. J. Hulshof, H. H. Khoi, H. Verhoef, and J. G. Hautvast The contribution of plant foods to the vitamin A supply of lactating women in Vietnam: a randomized controlled trial Am. J. Clinical Nutrition, April 1, 2007; 85(4): 1112 - 1120. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Wang, S. L Connor, E. J Johnson, M. L Klein, S. Hughes, and W. E Connor Effect of dietary lutein and zeaxanthin on plasma carotenoids and their transport in lipoproteins in age-related macular degeneration Am. J. Clinical Nutrition, March 1, 2007; 85(3): 762 - 769. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Granado, B. Olmedilla, C. Herrero, B. Perez-Sacristan, I. Blanco, and S. Blazquez Bioavailability of Carotenoids and Tocopherols from Broccoli: In Vivo and In Vitro Assessment Experimental Biology and Medicine, December 1, 2006; 231(11): 1733 - 1738. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Paterson, M. H. Gordon, C. Niwat, T. W. George, L. Parr, S. Waroonphan, and J. A. Lovegrove Supplementation with Fruit and Vegetable Soups and Beverages Increases Plasma Carotenoid Concentrations but Does Not Alter Markers of Oxidative Stress or Cardiovascular Risk Factors J. Nutr., November 1, 2006; 136(11): 2849 - 2855. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. A. Novotny, A. C. Kurilich, S. J. Britz, and B. A. Clevidence Plasma appearance of labeled {beta}-carotene, lutein, and retinol in humans after consumption of isotopically labeled kale J. Lipid Res., September 1, 2005; 46(9): 1896 - 1903. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. A Thurmann, W. Schalch, J.-C. Aebischer, U. Tenter, and W. Cohn Plasma kinetics of lutein, zeaxanthin, and 3-dehydro-lutein after multiple oral doses of a lutein supplement Am. J. Clinical Nutrition, July 1, 2005; 82(1): 88 - 97. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Brevik, S. E. Vollset, G. S Tell, H. Refsum, P. M. Ueland, E. B. Loeken, C. A Drevon, and L. F. Andersen Plasma concentration of folate as a biomarker for the intake of fruit and vegetables: the Hordaland Homocysteine Study Am. J. Clinical Nutrition, February 1, 2005; 81(2): 434 - 439. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. D. Ribaya-Mercado and J. B. Blumberg Lutein and Zeaxanthin and Their Potential Roles in Disease Prevention J. Am. Coll. Nutr., December 1, 2004; 23(suppl_6): 567S - 587S. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
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]
|
|
|
|
|
|
C. Chitchumroonchokchai, S. J. Schwartz, and M. L. Failla Assessment of Lutein Bioavailability from Meals and a Supplement Using Simulated Digestion and Caco-2 Human Intestinal Cells J. Nutr., September 1, 2004; 134(9): 2280 - 2286. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H.-Y. Chung, H. M. Rasmussen, and E. J. Johnson Lutein Bioavailability Is Higher from Lutein-Enriched Eggs than from Supplements and Spinach in Men J. Nutr., August 1, 2004; 134(8): 1887 - 1893. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. L Molldrem, J. Li, P. W Simon, and S. A Tanumihardjo Lutein and {beta}-carotene from lutein-containing yellow carrots are bioavailable in humans Am. J. Clinical Nutrition, July 1, 2004; 80(1): 131 - 136. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Blakely, A. Herbert, M. Collins, M. Jenkins, G. Mitchell, E. Grundel, K. R. O'Neill, and F. Khachik Lutein Interacts with Ascorbic Acid More Frequently than with {alpha}-Tocopherol to Alter Biomarkers of Oxidative Stress in Female Zucker Obese Rats J. Nutr., September 1, 2003; 133(9): 2838 - 2844. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. E. West, A. Eilander, and M. van Lieshout Reply to Russell et al. J. Nutr., September 1, 2003; 133(9): 2917 - 2917. [Full Text] [PDF]
|
|
|
|
 |
|
 V. Tyssandier, E. Reboul, J.-F. Dumas, C. Bouteloup-Demange, M. Armand, J. Marcand, M. Sallas, and P. Borel Processing of vegetable-borne carotenoids in the human stomach and duodenum Am J Physiol Gastrointest Liver Physiol, June 1, 2003; 284(6): G913 - G923. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Potischman Biologic and Methodologic Issues for Nutritional Biomarkers J. Nutr., March 1, 2003; 133(3): 875S - 880. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. van Lieshout, C. E West, and R. B van Breemen Isotopic tracer techniques for studying the bioavailability and bioefficacy of dietary carotenoids, particularly {beta}-carotene, in humans: a review Am. J. Clinical Nutrition, January 1, 2003; 77(1): 12 - 28. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Miller, J. Humphrey, E. Johnson, E. Marinda, R. Brookmeyer, and J. Katz Why Do Children Become Vitamin A Deficient? J. Nutr., September 1, 2002; 132(9): 2867S - 2880. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. H. Allen and M. Haskell Estimating the Potential for Vitamin A Toxicity in Women and Young Children J. Nutr., September 1, 2002; 132(9): 2907S - 2919. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. E. West, A. Eilander, and M. van Lieshout Consequences of Revised Estimates of Carotenoid Bioefficacy for Dietary Control of Vitamin A Deficiency in Developing Countries J. Nutr., September 1, 2002; 132(9): 2920S - 2926. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
