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Journal of Bioequivalence & Bioavailability


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ISSN: 0975-0851
 
 
 
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Research Article Open Access
 
Biotechnologically-modified Carrots: Calcium Absorption Relative to Milk
Keli M. Hawthorne1,*, Jay Morris2,1, Tim Hotze1, Kendal D. Hirschi2,1, and Steven A. Abrams1
1United States Department of Agriculture/Agriculture Research Service, Children’s Nutrition Research Center,   Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030
2Vegetable and Fruit Improvement Center, Texas A&M University, College Station, Texas 77845
Corresponding Author : Dr Keli Hawthorne, Children’s Nutrition Research Center,
1100 Bates St., Rm. 7074, Houston, TX 77030,
Phone : 713-798-7085,
Fax      : 713-798-7119,
E-mail : kelih@bcm.edu
 
Received March 13, 2009; Accepted May 15, 2009; Published May 15, 2009
 
Citation: Hawthorne KM, Morris J, Hotze T, Hirschi KD, Abrams SA (2009) Biotechnologically-modified Carrots: Calcium Absorption Relative to Milk. J Bioequiv Availab 1: 034-038. doi: 10.4172/jbb.1000006
 
Copyright: © 2009 Hawthorne KM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
 
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Abstract
 

Background: Biotechnology to increase the nutrient content of fruits and vegetables is an innovative strategy to address insufficient mineral intakes. A novel biotechnologically modified carrot which has higher levels of calcium than control carrots has been developed. Objective: For dietary guidance, it is necessary to understand the relative servings of any specific product that would be needed to provide calcium compared to a standard source, such as milk. Methods: In a crossover study we used stable isotopes to measure calcium absorption from milk in 30 young adults and compared it to calcium absorption from both biotechnologically modified (MOD) and control (CON) carrots. Results: Using a total meal calcium of 300 mg of which 35-40 mg of the calcium is derived from the test product, fractional calcium absorption from milk was slightly higher than from the MOD carrot (50.1 ± 3.0% vs. 42.6 ± 2.8%, Mean ± SEM, p<0.05) but was similar to that from the CON carrot (50.1 ± 3.0% vs. 52.8 ± 3.3%; p=0.7). Conclusions: Biotechnologically-modified carrots have calcium bioavailability levels only slightly below that of milk. Serving sizes of enhanced carrots remain too large to be considered full substitutions for usual sources such as milk, but can supplement these sources effectively. Further biotechnological enhancements of a range of vegetable sources may lead to substantial benefits in nutritional status for minerals such as calcium with significant population- deficient intakes.

 
 
Keywords
 
calcium; absorption; bioavailability; biotechnology; stable isotope; carrot
 
Abbreviations
 
MOD=modified, CON=control
 
Introduction
 
The biotechnological modification of plants to increase their nutritional benefits in the food supply is a rapidly expanding field of nutritional investigation (Freese and Schubert, 2004; DellaPenna, 2007; Sevenier et al, 2002; Mackey, 2002). The terms “biotechnology” and “genetically modified/enhanced” have been used to describe various strategies which implement some form of plant biochemistry modification. Modern biotechnology has been utilized in the United States food supply since the early 1990s (White and Broadley, 2005). Currently, the majority of manufactured foods marketed in the United States contain modified soybean or corn ingredients (Institute of Food Technologists Expert Panel, 2000). Most crops are modified primarily for insect resistance or to improve tolerance to herbicides. However, increasingly, crops are being modified to enhance the nutritional profile of the plant in an effort to decrease nutritional deficiencies, promote health and well being, and to enhance taste. Modified plants have been analyzed for changes in plant metabolism and nutrient composition, however the functional outcomes related to their use has rarely been evaluated.
 
In the United States, milk and other dairy products account for the majority of dietary calcium intake (Huang, 1997). However, persistent inadequacies of calcium intake throughout the life cycle has led to the recognition that alternative approaches are required to provide adequate calcium to the population. These approaches include increasing the bioavailability of calcium (Miller et al, 2001; Wienk et al, 1999; Van Dokkum, 1992) so that more calcium is absorbed and is available for metabolic purposes.
 
It has recently been demonstrated that through biotechnological modification the amount of bioavailable calcium in plants such as potatoes, tomatoes, and carrots can be increased (Park et al, 2004; Park et al, 2005). Carrots are a commonly consumed vegetable in the United States; however, like many vegetables they are a poor source of dietary calcium. A single medium-sized carrot (about 60 g) can only provide about 5% of the calcium of a single 240 mL serving of milk (US Department of Agriculture, 2007).
 
We have recently developed a modified carrot with elevated levels of calcium. In a group of young adults we reported that the total amount of calcium absorbed from the carrot was significantly greater than from a control carrot (Morris et al, 2008). In brief, the calcium levels in the carrots were engineered through high-level expression of a deregulated Arabidopsis calcium transporter called Cation Exchanger 1.
 
However, in order to make specific nutritional recommendations, and to evaluate the level of progress made and future progress needed in this area, it was necessary to compare the total amount of absorbed calcium from such a product with an accepted dietary source of the nutrient. In this report, we describe such a comparison using additional data from the absorption study previously reported (Morris et al, 2008). In this case we evaluated the absorption of calcium from milk and compared it to the absorption of calcium from both modified and control carrots. We hypothesized that one modified carrot (about 60 g) would provide equivalent absorbed calcium as that of an ounce of milk.
 
Materials and Methods
 
Subject Enrollment
 
We enrolled healthy young adults, 21.0-29.9 years of age, whose body mass index (BMI) was < 95th percentile for age and gender. To qualify for the study, subjects had to have an average calcium intake of 600-1200 mg/d based on an assessment made of three nonconsecutive days of 24 hr dietary recalls. Subjects could not regularly be taking any prescription medication other than oral contraceptives. The Institutional Review Board of Baylor College of Medicine and Affiliated Hospitals approved this protocol and written informed consent was obtained from all subjects.
 
Calcium Absorption Study
 
Subjects arrived after an overnight fast to Texas Children’s Hospital (National Institutes of Health, M01-RR00188, General Clinical Research Center) for two outpatient study visits that occurred two weeks apart. Subjects were randomized to receive either the genetically modified (MOD) carrot or the control (CON) carrot at the first visit, receiving the other type of carrot at the second visit. Details of the production and characterization of these carrots have been reported (Morris et al, 2008). Carrots were given with a test meal that consisted of either 65 g MOD carrot or 120 g CON carrot. These carrot weights were chosen so that each serving provided approximately 38 mg calcium, the amount present in 30 g milk. This determination of the relative total amount of calcium in the carrots was based on analysis of previous batches of carrots and we confirmed our test meals with individual analysis of each carrot used in the study. With this meal, we also gave 30 mL of low-fat milk. In addition, the test meal included 170 g of calciumfortified orange juice (Minute Maid, Coca-Cola Co., Atlanta, GA) to insure the test meal provided approximately 300 mg calcium, such as would be found in a typical breakfast. Both types of carrots were intrinsically labeled with42Ca stable isotope and milk was labeled with 48Ca stable isotope. Labeling of the milk was done via the addition of 1 mg 48Ca as the chloride salt approximately 24 hr prior to its being fed. Extrinsic labeling has been shown to be accurate as a measure of milk absorption after allowing adequate time for tracer equilibration (Nickel et al, 1996).
 
After the test meal, subjects received 15 μg 46Ca stable isotope intravenously and then completed a 24 hr urine collection. This urine collection was done at home using timed collection containers.
 
Diets on the study day were designed so that the subjects received approximately 900 mg calcium per day, with onethird of the total daily calcium being provided during the test meal. The remaining intake was divided between lunch and dinner, with one optional snack. Lunch and the snack were provided containing approximately 300 mg calcium total. For dinner, subjects were instructed by the study dietitian about consuming a meal with 300 mg calcium based on a preselected food list. Subjects reported back the following day as to what they consumed. This process was repeated at the second visit when the subject received the reciprocal carrot test meal. Subjects were contacted between the two visits for 24 hr dietary recalls on two nonconsecutive days to insure that their calcium intake was maintained relatively constant during the study.
 
Calculation of Mineral Absorption
 
Serum and urine samples were analyzed for stable isotope enrichment using inductively coupled plasma mass spectrometry to measure the 42Ca and 48Ca enrichment and thermal ionization mass spectrometry to measure the 46Ca enrichment. Sample preparation and analysis were identical to those we have reported previously (Chen et al, 2003; Griffin et al, 2003; Abrams et al, 2007) and have been validated (Abrams, Yergey, and Heaney, 1994). Calcium absorption was calculated as the relative recovery in the urine of the oral isotope divided by the recovery of the intravenous isotope during the 24 hr after isotope administration (from time of the first oral dose until 24 hr after the last oral dose). For the second visit, the proportional recovery of 46Ca from the first visit was used as it was unlikely that a substantial change in distribution kinetics had occurred in otherwise healthy adults over a short time period.
 
Data Analysis and Interpretation
 
Comparison of calcium absorption was done between milk and each of the types of carrots by paired t-tests.
 
Results
 
We enrolled and completed studies in 15 males and 15 females (Table 1). Baseline dietary calcium intake determined from the 24 hr recall data was 865 ± 229 mg/d. Subjects consumed 796 ± 97 mg calcium on the first study day and 797 ± 103 mg calcium on the second study day, two weeks later (p > 0.5 between visits). Between the two study days, subjects consumed 1166 ± 461 mg calcium, maintaining calcium intakes within the inclusion criteria. Fractional absorption results for the test meal days are provided in Table 2.
 
We further calculated the number of servings of each type of carrot that would be needed to achieve the total amount of calcium absorbed as a typical 240 mL (8 ounce) glass of milk. This was done by calculating the total amount of calcium absorbed from 240 mL of milk and then determining the weight of carrots that needed to be consumed to provide the same amount of absorbed calcium. Typical weights of medium sized carrots (65 g) (US Department of Agriculture, 2007) were then used to indicate the number of carrots this would represent. These results are shown in Table 2 and indicate that about 10 MOD or 15 CON carrots would be needed to equal the absorbed calcium from 240 mL of milk.
 
Discussion
 
We evaluated a novel strategy to increase the bioavailability of calcium in the diet by using modified carrots to provide more calcium in each serving of the vegetable. We found that each of these carrots would provide a level of absorbed calcium just below that of about one ounce of milk. While this amount is inadequate to resolve calcium intake deficiencies in the population, it is an important step in enhancing the use of vegetables as sources of minerals deficient in the usual diet, such as calcium. Furthermore, such vegetables would reflect an intrinsic valueadded property to both producer and consumer.
 
The current “Fruits and Veggies Matter” campaign by the National Cancer Institute and Centers for Disease Control and Prevention is designed to promote increased fruit and vegetable consumption (Centers for Disease Control and Prevention, 2008). This campaign defines a standard serving as 1 cup of raw vegetables (e.g., two medium carrots or 12 baby carrots) and promotes consumption based on age, gender and physical activity. However, goal intakes are not widely achieved (Guenther et al, 2006). Therefore, strategies to promote modified vegetables should include focus on overall education to promote vegetable intake (Heaney and Barger-Lux, 1994; Fulgoni et al, 2007).
 
Heffernan and Hillers found that approximately 50% of consumers surveyed would purchase modified foods if fewer pesticides were used in their production compared to natural foods; however 80% reported the importance of considering the environmental impact of such agricultural advances (Heffernan and Hillers, 2002). When consumers were provided with information that the modified food had an enhanced nutritional profile when compared to the control food, acceptability of the modified product increased (Brown and Ping, 2003). Aside from nutritional benefits to increase calcium levels, the use of food biotechnology to increase the calcium content of vegetables could also improve plant productivity, maintain crop firmness during transport, and extend product shelf life (Raz and Fluhr, 1992; Dris and Niskanen, 1999), thus meeting consumer demands for environmental consideration with biotechnology.
 
The World Health Organization (WHO) has reported on the availability of modified vegetables with increased levels of beta-carotene, iron, protein, isoflavones and antioxidants (WHO, 2005). This emerging science may be a step towards diminishing nutritional deficiencies worldwide. Enhanced nutritional benefits of modified foods are likely to increase acceptability as personal risk perception declines. Increased calcium bioavailability could influence people to choose these plants over traditional plants in order to avoid potential calcium deficiency or osteoporosis.
 
Conclusion
 
Our results allow our earlier report about the relative bioavailability of modified and control carrots to be interpreted in terms of a usual calcium source, milk. We found that the modified carrot came close to the fractional absorption of milk. Further research is needed to enhance the calcium content or bioavailability of modified carrots to decrease the number of carrots needed to provide equivalent total calcium compared to a serving of milk.
 
Acknowledgements
 
We would like to thank Penni Hicks, Gloria Orozco, Adrianne Morse, Sevahn Allahverdian, Maria Hamzo and Zhensheng Chen for subject assistance and sample analysis. We are grateful to GCRC nurse Jenell Dancy and staff for help with subject assistance. Funding for this work was supplied by grants from the National Institutes of Health no. IR01 DK 062366 and USDA CSRESS#2005-34402-16401 Designing Foods for Health, both to K. Hirschi. Funding for the General Clinical Research Center was from National Institutes of Health, M01-RR00188, General Clinical Research Center Grant.
 
References

  1. Abrams SA, Griffin IJ, Hawthorne KM (2007) Young adolescents who respond to an inulin-type fructan (ITF) substantially increase total absorbed calcium and daily calcium accretion to the skeleton. J Nutr 137: 2524S- 2526S.   »  CrossRef  »  PubMed  »  Google Scholar

  2. Abrams SA, Yergey AL, Heaney RP (1994) Relationship between balance and dual tracer isotopic measurements of calcium absorption and excretion. J Clin Endocrinol Metab 79: 965-969. »  CrossRef  »  PubMed  »  Google Scholar


  3. Brown JL, Ping Y (2003) Consumer perception of risk associated with eating genetically engineered soybeans is less in the presence of a perceived consumer benefit. J Am Diet Assoc 103: 208-214. »  CrossRef  »  PubMed  »  Google Scholar

  4. Centers for Disease Control and Prevention, Fruits and Veggies Matter.

  5. Chen Z, Griffin IJ, Kriseman YL, Liang LK, Abrams SA (2003) Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: a lowsample- volume acid-equilibration method. Clin Chem 49: 2050-2055.
    »  CrossRef  »  PubMed  »  Google Scholar

  6. DellaPenna D (2007) Biofortification of plant-based food: enhancing folate levels by metabolic engineering. Proc Natl Acad Sci USA 104: 3675-3676. »  CrossRef  »  PubMed  »  Google Scholar

  7. Dris R, Niskanen R (1999) Calcium chloride sprays decrease physiological disorders following long-term cold storage of apple. Plant Foods Hum Nutr 54: 159- 171.
    »  CrossRef  »  PubMed  »  Google Scholar

  8. Freese W, Schubert D (2004) Safety testing and regulation of genetically engineered foods. Biotechnol Genet Eng Rev 21: 299-324. »  PubMed  »  Google Scholar

  9. Fulgoni V, Nicholls J, Reed A, Buckley R, Kafer K, et al. (2007) Dairy consumption and related nutrient intake in African-American adults and children in the United States: Continuing Survey of Food Intakes by Individuals 1994-1996, 1998, and the National Health and Nutrition Examination Survey 1999-2000. J Am Diet Assoc 107: 256-264. »  CrossRef  »  PubMed  »  Google Scholar

  10. Griffin IJ, Hicks PMD, Heaney RP, Abrams SA (2003) Enhanced chicory inulin increases calcium absorption mainly in adolescents with lower calcium absorption. Nutr Res 23: 901-909.

  11. Guenther PM, Dodd KW, Reedy J, Krebs-Smith SM (2006) Most Americans eat much less than recommended amounts of fruits and vegetables. J Am Diet Assoc 106: 1371-1379. »  CrossRef  »  PubMed  »  Google Scholar

  12. Heaney RP, Barger-Lux MJ (1994) ADSA Foundation Lecture. Low calcium intake: the culprit in many chronic diseases. J Dairy Sci 77: 1155-1160. »  CrossRef  »  PubMed  »  Google Scholar

  13. Heffernan JW, Hillers VN (2002) Attitudes of consumers living in Washington regarding food biotechnology. J Am Diet Assoc 102: 85-88. »  CrossRef  »  PubMed  »  Google Scholar

  14. Huang KS (1997) How economic factors influence the nutrient content of diets. Food and Rural Economics Division, Economics Research Service, U.S. Department of Agriculture Technical Bulletin NO. 1864. 20. »  CrossRef   »  Google Scholar

  15. Institute of Food Technologies Expert Panel (2000) Labeling of rDNA biotechnology-derived foods. Food Technol. 54: 62-74.

  16. Mackey M (2002) The application of biotechnology to nutrition: an overview. J Amer Coll Nutr 21: 157S-160S. »  CrossRef  »  PubMed  »  Google Scholar

  17. Miller GD, Jarvis JK, McBean LD (2001) The importance of meeting calcium needs with foods. J Am Coll Nutr 20: 168S-185S. »  CrossRef  »  PubMed  »  Google Scholar

  18. Morris J, Hawthorne KM, Hotze T, Abrams SA, Hirschi KD (2008) Nutritional impact of elevated calcium transport activity in carrots. Proc Natl Acad Sci USA 5: 1431- 1435. »  CrossRef  »  PubMed  »  Google Scholar

  19. Nickel KP, Martin BR, Smith DL, Smith JB, Miller GD, (1996) Calcium bioavailability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labeling techniques. J Nutr 126: 1406-1411. »  CrossRef  »  PubMed  »  Google Scholar

  20. Park S, Cheng NH, Pittman JK, Yoo KS, Park J, (2005) Increased calcium levels and prolonged shelf life in tomatoes expressing Arabidopsis H+/Ca2+ transporters. Plant Physiol 139: 1194-1206. »  CrossRef  »  PubMed  »  Google Scholar

  21. Park S, Kim C, Pike L, Smith R, Hirschi KD () Increased calcium in carrots by expression of an Arabidopsis H+/ Ca2+ transporter. Mol Breeding 14: 275-282.
    »  CrossRef  »  Google Scholar

  22. Raz V, Fluhr R (1992) Calcium requirement for ethylene- dependent responses. Plant Cell 4: 1123-1130. »  CrossRef  »  PubMed  »  Google Scholar

  23. Sevenier R, van der Meer IM, Bino R, Koops AJ (2002) Increased production of nutriments by genetically engineered crops. J Amer Coll Nutr 21: 199S-204S.
    »  CrossRef  »  PubMed  »  Google Scholar

  24. U.S. Department of Agriculture, Agricultural Research Service. 2007. USDA National Nutrient Database for Standard Reference, Release 20. Nutrient Data Laboratory Home Page.

  25. Van Dokkum W (1992) Significance of iron bioavailability for iron recommendations. Biol Trace Elem Res 35: 1-11. »  CrossRef  »  PubMed  »  Google Scholar

  26. Wienk KJH, Marx JJM, Beynen AC (1999) The concept of iron bioavailability and its assessment. Eur J Nutr 38: 51-75.  »  CrossRef  »  PubMed  »  Google Scholar

  27. White PJ, Broadley MR. Biofortifying crops with essential mineral elements. Trends Plant Sci. 10 (12): 586- 593. »  CrossRef  »  PubMed  »  Google Scholar

  28. World Health Organization: Modern food biotechnology, human health and development: An evidence-based study. 2005.

 
 
References

  1. Abrams SA, Griffin IJ, Hawthorne KM (2007) Young adolescents who respond to an inulin-type fructan (ITF) substantially increase total absorbed calcium and daily calcium accretion to the skeleton. J Nutr 137: 2524S- 2526S.   »  CrossRef  »  PubMed  »  Google Scholar

  2. Abrams SA, Yergey AL, Heaney RP (1994) Relationship between balance and dual tracer isotopic measurements of calcium absorption and excretion. J Clin Endocrinol Metab 79: 965-969. »  CrossRef  »  PubMed  »  Google Scholar


  3. Brown JL, Ping Y (2003) Consumer perception of risk associated with eating genetically engineered soybeans is less in the presence of a perceived consumer benefit. J Am Diet Assoc 103: 208-214. »  CrossRef  »  PubMed  »  Google Scholar

  4. Centers for Disease Control and Prevention, Fruits and Veggies Matter.

  5. Chen Z, Griffin IJ, Kriseman YL, Liang LK, Abrams SA (2003) Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: a lowsample- volume acid-equilibration method. Clin Chem 49: 2050-2055.
    »  CrossRef  »  PubMed  »  Google Scholar

  6. DellaPenna D (2007) Biofortification of plant-based food: enhancing folate levels by metabolic engineering. Proc Natl Acad Sci USA 104: 3675-3676. »  CrossRef  »  PubMed  »  Google Scholar

  7. Dris R, Niskanen R (1999) Calcium chloride sprays decrease physiological disorders following long-term cold storage of apple. Plant Foods Hum Nutr 54: 159- 171.
    »  CrossRef  »  PubMed  »  Google Scholar

  8. Freese W, Schubert D (2004) Safety testing and regulation of genetically engineered foods. Biotechnol Genet Eng Rev 21: 299-324. »  PubMed  »  Google Scholar

  9. Fulgoni V, Nicholls J, Reed A, Buckley R, Kafer K, et al. (2007) Dairy consumption and related nutrient intake in African-American adults and children in the United States: Continuing Survey of Food Intakes by Individuals 1994-1996, 1998, and the National Health and Nutrition Examination Survey 1999-2000. J Am Diet Assoc 107: 256-264. »  CrossRef  »  PubMed  »  Google Scholar

  10. Griffin IJ, Hicks PMD, Heaney RP, Abrams SA (2003) Enhanced chicory inulin increases calcium absorption mainly in adolescents with lower calcium absorption. Nutr Res 23: 901-909.

  11. Guenther PM, Dodd KW, Reedy J, Krebs-Smith SM (2006) Most Americans eat much less than recommended amounts of fruits and vegetables. J Am Diet Assoc 106: 1371-1379. »  CrossRef  »  PubMed  »  Google Scholar

  12. Heaney RP, Barger-Lux MJ (1994) ADSA Foundation Lecture. Low calcium intake: the culprit in many chronic diseases. J Dairy Sci 77: 1155-1160. »  CrossRef  »  PubMed  »  Google Scholar

  13. Heffernan JW, Hillers VN (2002) Attitudes of consumers living in Washington regarding food biotechnology. J Am Diet Assoc 102: 85-88. »  CrossRef  »  PubMed  »  Google Scholar

  14. Huang KS (1997) How economic factors influence the nutrient content of diets. Food and Rural Economics Division, Economics Research Service, U.S. Department of Agriculture Technical Bulletin NO. 1864. 20. »  CrossRef   »  Google Scholar

  15. Institute of Food Technologies Expert Panel (2000) Labeling of rDNA biotechnology-derived foods. Food Technol. 54: 62-74.

  16. Mackey M (2002) The application of biotechnology to nutrition: an overview. J Amer Coll Nutr 21: 157S-160S. »  CrossRef  »  PubMed  »  Google Scholar

  17. Miller GD, Jarvis JK, McBean LD (2001) The importance of meeting calcium needs with foods. J Am Coll Nutr 20: 168S-185S. »  CrossRef  »  PubMed  »  Google Scholar

  18. Morris J, Hawthorne KM, Hotze T, Abrams SA, Hirschi KD (2008) Nutritional impact of elevated calcium transport activity in carrots. Proc Natl Acad Sci USA 5: 1431- 1435. »  CrossRef  »  PubMed  »  Google Scholar

  19. Nickel KP, Martin BR, Smith DL, Smith JB, Miller GD, (1996) Calcium bioavailability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labeling techniques. J Nutr 126: 1406-1411. »  CrossRef  »  PubMed  »  Google Scholar

  20. Park S, Cheng NH, Pittman JK, Yoo KS, Park J, (2005) Increased calcium levels and prolonged shelf life in tomatoes expressing Arabidopsis H+/Ca2+ transporters. Plant Physiol 139: 1194-1206. »  CrossRef  »  PubMed  »  Google Scholar

  21. Park S, Kim C, Pike L, Smith R, Hirschi KD () Increased calcium in carrots by expression of an Arabidopsis H+/ Ca2+ transporter. Mol Breeding 14: 275-282.
    »  CrossRef  »  Google Scholar

  22. Raz V, Fluhr R (1992) Calcium requirement for ethylene- dependent responses. Plant Cell 4: 1123-1130. »  CrossRef  »  PubMed  »  Google Scholar

  23. Sevenier R, van der Meer IM, Bino R, Koops AJ (2002) Increased production of nutriments by genetically engineered crops. J Amer Coll Nutr 21: 199S-204S.
    »  CrossRef  »  PubMed  »  Google Scholar

  24. U.S. Department of Agriculture, Agricultural Research Service. 2007. USDA National Nutrient Database for Standard Reference, Release 20. Nutrient Data Laboratory Home Page.

  25. Van Dokkum W (1992) Significance of iron bioavailability for iron recommendations. Biol Trace Elem Res 35: 1-11. »  CrossRef  »  PubMed  »  Google Scholar

  26. Wienk KJH, Marx JJM, Beynen AC (1999) The concept of iron bioavailability and its assessment. Eur J Nutr 38: 51-75.  »  CrossRef  »  PubMed  »  Google Scholar

  27. White PJ, Broadley MR. Biofortifying crops with essential mineral elements. Trends Plant Sci. 10 (12): 586- 593. »  CrossRef  »  PubMed  »  Google Scholar

  28. World Health Organization: Modern food biotechnology, human health and development: An evidence-based study. 2005.

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