https://orcid.org/0000-0001-9668-1004
ABSTRACT
Sugar types, sugar amounts, phenolic contents constituents were determined in fresh fruits of 17 Saudi date cultivars and the obtained values were correlated with the corresponding vales of glycemic index. Glucose and fructose were presented in all tested cultivars in the range 104–395 g kg−1 and 82–361 g kg−1, respectively. Glucose/fructose ratio constituted between 1.03–1.27. Sucrose was considered the main sugar in four date cultivars. Total phenolics presented in the range 2.15–3.86 g kg−1. Antioxidant activities varied between tested cultivars and ranged from 2.46 to 9.80 µmol Trolox g−1 as assayed by DPPH method and from 5.90 to 17.78 µmol Trolox g−1 as assayed by FRAP method. No significant correlation was found between the levels of glucose, fructose, sucrose, total phenolics and antioxidant activity with the corresponding glycemic index values.
Introduction
Date (Phoenix dactylifera L.) is the main fruit in the Kingdom of Saudi Arabia. In the ancient date production countries, the date fruits consumed more as a sugar source than as a fruit for their high sugar content (440–880 g kg−1) (Al-Shahib and Marshal, Citation2003; Assirey, Citation2015). In the most cultivars of dates, the major sugars present was glucose and fructose in almost equal amounts (Fareha et al., Citation2016; Vayalil, Citation2012). Whereas, sucrose was found to be the dominant sugar in many Saudi Arabian date cultivars such as Sukkary and Nabtat Ali (Al-Humaid et al., Citation2010).
Besides nutritional value, date fruit was rich in phenolic compounds possessing antioxidant activity. Higher level of phenolic contents and antioxidant activity was reported for date fruits produced in Kuwait (Vayalil, Citation2002), Algeria (Freha et al., Citation2016; Mansouri et al., Citation2005), Oman (Al-Farsi et al., Citation2005), Iran (Biglari et al., Citation2008), and Bahrain (Al-Laith, Citation2008). Moreover, date seeds serve as a good source of phenolics and natural antioxidants (Al AL Juhaimi et al., Citation2018; El-Mergawi et al., Citation2016; Juhaimi et al., Citation2011; Özcan and Fahad, Citation2015). The importance of antioxidant compounds has been increasing because of their high efficiency in scavenging free radicals related to coronary heart disease, cardiovascular disease, cancer, aging, neurodegenerative, and diabetes diseases (Tang et al., Citation2013).
Regular consumption of sugars is the main cause of the development of hyperglycemia-induced chronic diseases (Koh et al., Citation2010). High consumption of dates (annual consumption 25 kg per capita) may be associated with the high rate of prevalence of diabetes disease in Saudi Arabia (24 %) (Alhadd et al., Citation2007; Al-Laith, Citation2008). The glycemic index (GI) used as a tool in planning diets for diabetic patients and the prevention of diabetes (Miyashita et al., Citation2006). The GI is based on the measurement of blood glucose within 2 h from the consumption of a test food containing 15 g of available carbohydrates and comparing it to the blood glucose response to a similar amount of glucose (Jenkins et al., Citation1981). The GI of foods depended upon their contents from carbohydrate and other chemical constituents (Bjorck et al., Citation1994). The GI values of common sugars depended on the type of sugars (fructose ˂ sucrose ˂ glucose) (Bantle, Citation2009). High GI foods cause a rapid and large release of glucose and are associated with the hyperglycemia and subsequently diabetes mellitus (Schulze et al., Citation2004). Phenolic compounds may affect glycemia and GI through different mechanisms, including the inhibition of glucose absorption in the gut or of its uptake by peripheral tissues (Pandey and Rizvi, Citation2009). Very limited information is available on GI values of different date cultivars, and their relation with other fruit constituents (Miller at al., Citation2003). In a previous study, we determined the GI of 17 Saudi Arabian date cultivars and we found that GI levels varied greatly between tested cultivars (AlGeffari et al., Citation2016). In this study, we determined the sugar types and phenolic contents as well as antioxidant activities of such 17 date cultivars and correlated the values of these constituents with the previous obtained GI data. The results of this study may be useful for health professionals in dietary recommendations for judging on consuming dates in regions, which consumed high amounts through traditional diets.
Material and Methods
Preparation of Fruits for Analysis
Fruits from 17 date cultivars that consumed in Saudi Arabia at tamer stage were tested: 16 cultivars grown in Al-Qassim and one (Ajwah) was most popular type in Al-Madinah Al-Munawarah. Mature fruits of uniform size and that were free of physical damage, injury from insects, and fungal infestation were selected for the study.
Determination of Sugar Constituents by HPLC
Fresh fruits (about 1 g) was homogenize in 40 mL of ethanol: water (80% v/v) extraction solution. The mixture sonicated for 15 min at 60°C and cooled at room temperature. The solution then completed to 50 mL total volume with the extraction solution and filtered through 0.42 μm filter. The chromatographic analysis was done in a Shimadzu High Performance Liquid Chromatography (LC-20) equipped with a double pump, a 7725 Rheodyne manual injector (Cotati, CA, USA) with a 20 μL loop, a RID-6A Shimadzu refractive index detector and a C-R6A Chromatopac integrator. Chromatographic separation was achieved with a Tracer carbohydrates column (5 μm particle size; 250 mm × 4.6 mm i.d.), and an NH2 precolumn (13 mm× 3 mm i.d.), both from Tracer (Teknokroma, Barcelona, Spain). Chromatographic separation undertaken with an isocratic elution mobile phase of acetonitrile–water (75:25, v/v) and degassed before use (Chavez-Servin et al., Citation2004). The flow-rate of this eluent is 1.8 ml/min and the volume of the sample injected was 20 μL (filling the loop completely). Column temperature maintained at 25°C. Peaks identified by comparing retention times with sugar standards. The respective peak areas used for the quantitative analysis. Calibration curves for each sugar was prepared at seven levels, from 0.5 to 10 mg/ml for fructose and glucose, and sucrose.
Determination of Total Phenolic
Total phenol was determined in 70% acetone extracts of fresh fruits by using the Folin-Ciocalteu reagent and gallic acid as a standard (Singleton and Rossi, Citation1965). The results expressed in g gallic acid kg−1 FW (g kg−1).
Determination of Antioxidant Activity
Antioxidant activity was determined in 70% acetone extracts of fresh fruits using 2, 2-Diphenyl-2-picrylhydrazyl (DPPH) method as described by Brand-Williams et al. (Citation1995) and Ferric reducing antioxidant power assay (FRAP) as described by Benzie and Strain (Benzie and Strain, Citation1999). A standard curve was done using 25 and 800 μmol Trolox, the results expressed in μmol Trolox g−1 FW.
Statistical Analysis
Conventional statistical methods used to calculate means and standard deviations. Analysis of variance and comparison of treatment means (LSD, 5% level) performed using Stat Graphics Plus Version 5.1. Correlations among sugar types, phenolic compounds, antioxidant activity, and estimated glycemic index of date cultivars obtained through Pearson’s correlation coefficient (r). The results analyzed by Analysis of Variance (ANOVA).
Results and Discussion
Sugar Types
As shown in , sugars in different date cultivars consisted from glucose, fructose and sucrose. Among 17 cultivars, 13 cultivars found to be contain glucose and fructose in high amounts, but sucrose not detected or detected in a very low concentration. Levels of glucose and fructose significantly varied among tested cultivars and ranged between 104–395 g kg−1 and 82–361 g kg−1, respectively. The highest glucose content found in Khudri (395 g kg−1), Medjool (395 g kg−1) and Sellaj (393 g kg−1) cultivars. Whereas, the highest fructose content was recorded for Shaqra (361 g kg−1), Maktoomi (359 g kg−1), Nabtat-Seyf (35.2 g kg−1) and Khudri (35.1 g kg−1) cultivars. Sucrose considered the main sugar in four date cultivars i.e. Sukkary (458 g kg−1), Nabtat Ali (408 g kg−1), Wannanah (38.2 g kg−1) and Ruthana (245 g kg−1). These four cultivars had the lowest concentrations of glucose (10.4–23.4 g kg−1) and fructose (82–201 g kg−1). Sugar in the fruits of Rashodia cultivar divided between glucose (282 g kg−1), fructose (251 g kg−1) and sucrose (210 g kg−1). As for glucose/fructose ratio, values of this ratio ranged between 1.03 and 1.27 for 17 examined cultivars. Such ratio reached to the highest levels in Um-Kabar and Sukkary cultivars and to the lowest levels in Maktoomi and Shaqra cultivars. These results were consistent with the results described in literature for Omani date cultivars (Ali et al., Citation2009), Algerian cultivars (Freha et al., Citation2016), Tunisian cultivars (Mrabet et al., Citation2008) and Pakistani cultivars (Haider et al., Citation2014). These results were confirmed with the previous study on 22 Saudi Arabian date cultivars by El-Mergawi (Citation2017). Who found that, except the higher sucrose content in Qurawia cultivar (494 g kg−1), glucose was considered the main sugar in all tested cultivars (ranged from 266 to 431 g kg−1), followed by fructose (ranged from 259 to 395 g kg−1). Presence of high amount of soluble sugars (glucose, fructose, and sucrose) in date fruits led to suggest that the date has been consumed more as a sugar source than as a fruit (Al-Shahib and Marshall, Citation2003; Assirey, Citation2015). High consumption of date fruits may affect blood sugar levels, so it is important to control their consumption in quantity and quality. Occurrence of high amounts of glucose is readily absorbed during the digestion and can lead to a rapid elevation of blood sugar and glycemic index (Schulze et al., Citation2004). On the other side, the dates contain almost one-half of the amounts of sugars in the form of fructose, which is twice as sweet as glucose and can induce a feeling of satiety (Al-Farsi and Lee, Citation2008). Moreover, it is likely that fructose present in dates, which may be released in small to moderate amounts into the blood stream, could behave catalytically to attenuate postprandial glycemia considerably in both hyperglycemic patients and healthy individuals (Vayalil, Citation2012). The low GI values of some date cultivars might be due to the high fructose content (Ali et al., Citation2009). Consequently, presence of high fructose in dates is recommended for diabetes mellitus and insulin resistant conditions (Watford, Citation2002).
Table 1. Sugars (g kg−1) in 17 Saudi Arabian date cultivars.
Phenolic Content
Date fruits of 17 tested cultivars had a relatively high phenolic content as shown in . The different date cultivars presented phenolics in the range 2.15–3.86 g kg−1 FW. The highest phenolic content was found in cultivars of Maktoomi (3.86 g kg−1) followed by Ajwah (3.82 g kg−1), Nabtat-Ali (3.64 g kg−1) and Shaqra (3.57 g kg−1). Whereas, the lowest phenolic contents possessed Osilah (2.15 g kg−1), Sukkary (2.48 g kg−1), Rashodia (2.62 g kg−1) and Ruthana (2.67 g kg−1) cultivars. These phenolic values are in agreement with those obtained by Al-Farsi et al. (Citation2005) for Omani cultivars (2.17–3.43 g kg−1), by Saafi et al. (Citation2009) for Tunisian cultivars (2.09–4.48 g kg−1), by Khanavi et al. (Citation2009) for Iranian cultivars (1.03–2.77 g kg−1) and by Al-Laith (Citation2008) for Bahraini cultivars (2.50–3.76 g kg−1). These results indicated that the date fruit had a high phenolic content compared with other fruits, such as strawberries (0.04–0.33 g kg−1 FW), raspberries (0.06–2.28 g kg−1 FW), apples (0.01–0.48 g kg−1 FW) and tomato (0.01–0.30 g kg−1 FW) (Proteggente et al., Citation2002). Moreover, Wu et al. (Citation2004) suggested that the fruits of some date cultivars had much higher phenolic content than many fruits and vegetables consumed in the US. Some of these phenolic constituents were identified such as phenolic acids, that is, p-coumaric acid, ferulic acid, and sinapic acids as well as flavonoids, that is, luteolin, quercetin, and apigenin (Biglari et al., Citation2008; Mansouri et al., Citation2005).
Table 2. Total phenolics and antioxidant activities of 17 Saudi Arabian date cultivars.
Antioxidant Activity
Antioxidant activity of 17 date cultivars was measured using DPPH and FRAP methods, the results presented in . Antioxidant activities varied between tested cultivars and ranged from 2.46 to 9.80 µmol Trolox g−1 FW as assayed by DPPH method and from 5.90 to 17.78 µmol Trolox g−1 FW as assayed by FRAP method. The highest value of antioxidant activity obtained by Nabtat-Ali (9.80 and 17.78 µmol Trolox g−1, respectively), Ajwah (6.14 and 14.45 µmol Trolox g−1, respectively) and Rashodia (6.11 and 11.96 µmol Trolox g−1, respectively). However, the lowest values produced by Osilah (2.46 and 5.90 µmol Trolox g−1, respectively). In line of these findings, Al-Laith (Citation2008), evaluated the antioxidant activity of sixteen Bahraini date cultivars at different ripening stages using FRAP method. Who mentioned that the highest total antioxidant activity was found at rutab (soft and ripped) stage (5.5–32.0 µmol Trolox g−1 FW) followed by tamer (dried fruit) stage (6.5–11.8 µmol Trolox g−1 FW). Different methods were used to measure antioxidant activity of date fruits (5 methods), which make the quantitative comparison between data of different studies invalid (Al-Farsi and Lee, Citation2008). However, Guo et al. (Citation2003) reported that dates had the second-highest antioxidant value of 28 fruits commonly consumed in China.
Correlation between the GI values and values of Different Sugar Types, Phenolic Contents, and Antioxidant Activity
In a previous study (AlGeffari et al., Citation2016), we determined the total carbohydrate content for 17 date cultivars grown in Saudi Arabia. Moreover, we used 19 healthy volunteers (9 females and 10 males) to determine the GI for these cultivars. Levels of GI were found to be 74.6, 71.0, 61.7, 59.9, 58.7, 56.6, 55.9, 55.3, 54.9, 54.4, 52.5, 50.9, 50.9, 50.2, 44.6, 43.4, and 42.8 for Sellaj, Maktoomi, Khudri, Nabtat-Ali, Um-Kabar, Osilah, Ajwah, Medjool, Sabaka, Nabtat-Seyf, Ruthana, Rashodia, Wannanah, Shishi, Sag‘ai, Sukkary, and Shaqra cultivars, respectively. In the present study, the correlation coefficients (r) between the previous obtained GI values for 17 date cultivars with their corresponding values from glucose, fructose, sucrose, total phenolic contents, and antioxidant activity were calculated; the (r) values presented . No significant correlation was found between the levels of glucose, fructose and sucrose, with the corresponding GI levels. In line of these results, Ba-Jaber et al. (Citation2006) reported that the level of glucose or fructose had no significant effect on the GI for the nine date cultivars grown in Saudi Arabia. Also, Ali et al. (Citation2009) found that the glucose values had no significant correlation with the corresponding GI values for three Omani date cultivars, however, an inverse correlation (r2 = −0.693) was observed between their fructose contents and GI values. Very limited, inconsistent and contradictory information is available on GI values of different date cultivars, and their relation with sugar types (Miller et al., Citation2003). Theoretically, GI levels of foods depended mainly on the sugar type, since fructose has a very low glycemic index of 19, compared with 100 for glucose and 61 for sucrose (Bantle, Citation2009). The present results indicated that the ratio glucose and fructose in all tested cultivars of date tended to originate from a one-to-one basis, however the GI values appeared great variations (from 24.8 to 74.6). Hence, there is no apparent explanation for the insignificant correlation between GI and different forms of sugars possessed date fruits either in this study or in the previous studies (Ali et al., Citation2009; Ba-Jaber et al., Citation2006). In addition, insignificant correlation was found between GI values and the corresponding values of total phenolic or antioxidant activity of tested cultivars. Studies pertaining to the relation between GI values of date fruits and their phenolic contents and antioxidant activity are still insufficient. Although, the present results were inconsistent with the results obtained by Moraes et al. (Citation2015) on sorghum flour samples, who reported that the levels of phenolic compounds and antioxidant activity were all negatively correlated to GI. Further studies are required, however, to compare the relation between GI of date cultivars and their nutritional and metabolic characteristics, in both normal and diabetic subjects.
Table 3. Correlation coefficients (r values) between GI values of 17 date cultivars and their sugars and phenolic contents and antioxidant activities.
Conclusions
High consumption of dates may be associated with the high rate of prevalence of diabetes disease in Saudi Arabia. In this study, the analysis of 17 Saudi Arabian date cultivars indicated that date fruits contained high amounts of sugars and cultivars possessed great variations in their contents from the three main sugars, glucose, fructose and sucrose. Also, date fruits varied in their contents from phenolics and antioxidant activities. Glycemic index (GI) used as a tool in planning diets for diabetic patients and the prevention of diabetes. In this study, we found that among 17 Saudi Arabian date cultivars, no significant correlation was found between the GI levels with the corresponded levels from glucose or fructose or sucrose or phenolic contents or antioxidant activity. Further studies are required to study the relation between GI of date cultivars and their nutritional and metabolic characteristics, in both normal and diabetic subjects.
Acknowledgments
The authors thank Saleh Kamel Chair for Date Palm Research, Qassim University, Saudi Arabia for financial support.
Disclosure statement
The authors declare that there is no conflict of interest.
References
- AL Juhaimi, F., M.M. Özcan, O.O. Adiamo, O.N. Alsawmahi, K. Ghafoor, and E.E. Babiker. 2018. Effect of date varieties on physio-chemical properties, fatty acid composition, tocopherol contents and phenolic compounds of some date seed and oils. J. Food Processing and Preservation. 42(7):e13584.
- Al-Farsi, M.A., C. Alasalvr, A. Morris, M. Baron, and F. Shahidi. 2005. Comparison of antioxidant activity, anthocyanins, carotenoids and phenolics of three native fresh and sun-dried date (Phoenix dactylifera L.) cultivars grown in Oman. J. Agric. Food Chem. 53:7592–7599. doi: 10.1021/jf050579q.
- Al-Farsi, M.A., and C.Y. Lee. 2008. Optimization of phenolics and dietary fibre extraction from date seeds. Food. Chem. 108:977–985. doi: 10.1016/j.foodchem.2007.12.009.
- AlGeffari, M.A., E.S. Almogbel, H.T. Alhomaidan, R. El- Mergawi, and I.A. Barrimah. 2016. Glycemic indices, glycemic load and glycemic response for seventeen varieties of dates grown in Saudi Arabia. Ann. Saudi Med. 36(6):397–403. doi: 10.5144/0256-4947.2016.397.
- Alhadd, T.A., A.A. Al-Amoudi, and A.S. Alzahrani. 2007. Epidemiology, clinical and complications profile of diabetes in Saudi Arabia: A Review. Ann. Saudi Med 27:241–250.
- Al-Humaid, A., H.M. Mousa, R. El-Mergawi, and A.M. Abdel-Salam. 2010. Chemical composition and antioxidant activity of dates and dates-camel milk mixtures as a protective meal against lipid peroxidation in rats. American J. Food Technol. 5:22–30. doi: 10.3923/ajft.2010.22.30.
- Ali, A., M. Yusra, Y.S. Al-Kindi, and F. Al-Said. 2009. Chemical composition and glycemic index of 3 cultivars of Omani dates. Inter. J. Food Sci. Nutr. 60:51–62. doi: 10.1080/09637480802389094.
- Al-Laith, A.A. 2008. Antioxidant activity of Bahraini date palm (Phoenix dactylifera L.) fruit of various cultivars. Int. J. Food Sci. Technol. 43:1033–1040. doi: 10.1111/j.1365-2621.2007.01558.x.
- Al-Shahib, W., and R.Marshall. 2003. The fruit of the date palm: Its possible use as the best food for the future. Int. J. Food Sci. Nutr. 54:247–259. doi: 10.1080/09637480120091982.
- Assirey, E.A.R. 2015. Nutritional composition of fruit of 10 date palm (Phoenix dactylifera L.) cultivars grown in Saudi Arabia. J. Taibah Univ. Sci. 9:75–79. doi: 10.1016/j.jtusci.2014.07.002.
- Ba-Jaber, A.S., E. Fares, M. Al-Rakban, H. Al-Kahtani, and A. Dafallah. 2006. Glycemic Index (GI) of some popular Saudi dates and the effect of their composition on the GI. Arab J. Food Nutr 15:6–13.
- Bantle, J.P. 2009. Dietary fructose and metabolic syndrome and diabetes. J. Nutr. 139:1263S–1268S. doi: 10.3945/jn.108.098020.
- Benzie, I.F.F., and J.J. Strain. 1999. Ferric reducing/antioxidant power assay: Direct measure of total antioxidant activity of biological fluids and modified version for simultaneous measurement of total antioxidant power and ascorbic acid concentration. Methods Enzymol 299:15–27.
- Biglari, F., F.M. Abbas, and M.E. Azhar. 2008. Antioxidant activity and phenolic content of various date palm (Phoenix dactylifera) fruits from Iran. Food. Chem. 107:1636–1641. doi: 10.1016/j.foodchem.2007.10.033.
- Bjorck, I., Y. Granfeldt, and H. Liljeberg. 1994. Food properties affecting the digestion and absorption of carbohydrates. American J. Clin. Nutr. 59:S699–S705. doi: 10.1093/ajcn/59.3.699S.
- Brand-Williams, W., M.E. Cuvelier, and C. Berset. 1995. Use of free radical method to evaluate antioxidant activity. Lebensmittel Wissenschaft Und Technol. 28:25–30. doi: 10.1016/S0023-6438(95)80008-5.
- Chávez-Servín, J., A. Castellote, and M. López-Sabater. 2004. Analysis of mono- and disaccharides in milk-based formulae by high-performance liquid chromatography with refractive index detection. J. Chrom. A. 1043:211–215. doi: 10.1016/j.chroma.2004.06.002.
- El-Mergawi, R.A. 2017. Evaluation of sugars, phenolics, carotenoids and antioxidant activities in 22 Saudi Arabian date cultivars. Int. J. Pharma and Biol. Sci 8((2)(B)):342–347.
- El-Mergawi, R.A., A. Al-Humaid, and D. El-Rayes. 2016. Phenolic profiles and antioxidant activity in seeds of ten date cultivars from Saudi Arabia. J. Food, Agric. Environ 14(2):38–42.
- Freha, G., M. Fatma, D. Meriem, and H. Cherifa. 2016. Effect of Algerian varieties dates on glycemic, arterial blood pressure and satiety responses. Asian J. Pharm. Res. Health Care. 8(2):52–61. doi: 10.18311/ajprhc/2016/852.
- Guo, C., J. Yang, J. Wei, Y. Li, J. Xu, and Y. Jiang. 2003. Antioxidant activities of peel, pulp and seed fraction of common fruits as determined by FRAP assay. Nat. Res 23:1719–1726.
- Haider, M.S., I.A. Khan, M.J. Jaskani, A.N. Summar, and M.M. Khan. 2014. Biochemical attributes of dates at three maturation stages. Emirate J. Food Agric. 26(11):953–962. doi: 10.9755/ejfa.v26i11.18980.
- Jenkins, D.J.A., T.M.S. Wolever, and R.H. Taylor. 1981. Glycemic index of foods: A physiological basis for carbohydrate exchange. American J Clin. Nutr. 34:362–366. doi: 10.1093/ajcn/34.3.362.
- Juhaimi, A.L., F. K. Ghafoor, and M.M. Özcan. 2011. Physical and chemical properties, antioxidant activity, total phenol and mineral profile of seeds of seven different date fruit (Phoenix dactylifera L.) varieties. Int. J. Food Sci. Nutr. 63(1):84–89. doi: 10.3109/09637486.2011.598851.
- Khanavi, M., Z. Saghari, A.I. Mohammad, R. Khademi, A. Hadjiakhoondi, and M. Abdollahi. 2009. Comparison of antioxidant activity and total phenols of some date varieties. Daru 17(2):104–108.
- Koh, L.W., L.L. Wong, Y.Y. Loo, S. Kasapis, and D. Huang. 2010. Evaluation of different teas against starch digestibility by mammalian glycosidases. J. Agric. Food Chem. 58:148–154. doi: 10.1021/jf903011g.
- Mansouri, A., G. Embarek, E. Kokkalou, and P. Kefalas. 2005. Phenolic profile and antioxidant activity of the Algerian ripe date palm fruit (Phoenix dactylifera). Food. Chem. 89(3):411–420. doi: 10.1016/j.foodchem.2004.02.051.
- Miller, C.J., E.V. Dunn, and I.B. Hashim. 2003. The glycemic index of dates and date/yoghurt mixed meals. Are dates ‘the candy that grows on trees’? Eur. J. Clin. Nutr. 57(3):427–430. doi: 10.1038/sj.ejcn.1601596.
- Miyashita, Y., M. Ebisuno, M. Ohhira, K. Endoh, and A. Saiki. 2006. Enhancement of serum lipoprotein lipase mass levels by intensive insulin therapy. Diabetes Res. Clin. Practical. 72:61–67. doi: 10.1016/j.diabres.2005.09.016.
- Moraes, E.A., R.S. Marineli, S.A. Lenquiste, C.J. Steel, C.B. Mnezes, V.A. Queiroz, and M.R. Junior. 2015. Sorghum flour fractions: Correlations among polysaccharides, phenolic compounds, antioxidant activity and glycemic index. Food. Chem. 180:116–123. doi: 10.1016/j.foodchem.2015.02.023.
- Mrabet, A., A. Ferchichi, N. Chaira, B.S. Mohamed, M. Baaziz, and T.M. Penny. 2008. Physico-chemical characteristics and total quality of date palm varieties grown in the Southern of Tunisia. Pakistan J. Biol. Sci. 11:1003–1008. doi: 10.3923/pjbs.2008.1003.1008.
- Özcan, M.M., and F.Y. AlJuhaimi. 2015. Effect of date (Phoenix dactylifera L.) seed extract on some oxidative parameters of olive Oil. J. Food Sci. Technol. 52(2):1218–1222. doi: 10.1007/s13197-013-1071-2.
- Pandey, K.B., and S.I. Rizvi. 2009. Plant polyphenols as dietary antioxidants in human health and disease. Oxi. Med. Cell. Longev. 5:270–278. doi: 10.4161/oxim.2.5.9498.
- Proteggente, A.R., A.S. Pannala, G. Paganga, L. Van Buren, E. Wagner, S. Wiseman, F. Van De Put, C. Dacombe, and C.A. Rice-Evans. 2002. The antioxidant activity of regularly consumed fruit and vegetables reflects their phenolic and vitamin composition. Free Rad. Res. 36:217–233. doi: 10.1080/10715760290006484.
- Saafi, E.B., A. El Arem, M. Issaoui, M. Hammami, and L. Achour. 2009. Phenolic content and antioxidant activity of four date palm (phoenix dactylifera) fruit varieties grown in Tunisia. Int. Food Sci. Technol. 44:2314–2319. doi: 10.1111/j.1365-2621.2009.02075.x.
- Schulze, M.B., S. Liu, E.B. Rimm, J.E. Manson, and W.C. Willett. 2004. Glycemic index, glycemic load, and dietary fiber intake and incidence of type 2 diabetes in young and middle-aged women. American J. Clin. Nutr. 80:348–356. doi: 10.1093/ajcn/80.2.348.
- Singleton, V.L., and J.A. Rossi. 1965. Colorimetry of total phenolics with phosphomolybdic –phosphtungstic acid reagents. American J. Enol. Vitic 16:144–158.
- Tang, Z., L. Shi, and S.M. Aleid. 2013. Date fruit: Chemical composition, nutritional and medicinal values products. J. Sci. Food Agric. 93:2351–2361. doi: 10.1002/jsfa.6154.
- Vayalil, P.K. 2002. Antioxidant and antimutagenic properties of aqueous extract of date fruit (Phoenix dactylifera L. Arecaceae). J. Agric. Food Chem. 50(3):610–617. doi: 10.1021/jf010716t.
- Vayalil, P.K. 2012. Date fruits (Phoenix dactylifera Linn): An emerging medicinal food. Crit. Rev. Sci. Nutr. 52:249–271. doi: 10.1080/10408398.2010.499824.
- Watford, M. 2002. Small amounts of dietary fructose dramatically increase hepatic glucose uptake through a novel mechanism of glucokinase activation. Nutr. Rev. 60:253–257. doi: 10.1301/002966402320289377.
- Wu, X., G. Beecher, J. Holden, D. Haytowitz, S. Gebhardt, and R. Prior. 2004. Lipophilic and hydrophilic antioxidant capacities of common foods in the United States. J. Agric. Food Chem. 52:4026–4037. doi: 10.1021/jf049696w.