Original Communication

European Journal of Clinical Nutrition (2003) 57, 427–430. doi:10.1038/sj.ejcn.1601535

The glycaemic index of dates and date/yoghurt mixed meals. Are dates 'the candy that grows on trees'?

C J Miller1, E V Dunn1 and I B Hashim2

  1. 1Department of Family Medicine, Faculty of Medicine and Health Sciences, United Arab Emirates University, Al Ain, United Arab Emirates
  2. 2Department of Food Sciences, Faculty of Food Systems, United Arab Emirates University, Al Ain, United Arab Emirates

Correspondence: C J Miller, Department of Family Medicine, Faculty of Medicine and Health Sciences, UAE University, PO Box 17-666, Al Ain, United Arab Emirates. E-mail: cjmiller@uaeu.ac.ae

Guarantor: CJ Miller.

Contributors: CJ Miller led the research project and was involved in all aspects from conception to writing the article. EVD was involved in study design, data analysis, and writing of the article. IBH coordinated all laboratory analysis and was involved in finalizing the writing of the article.

Received 10 January 2002; Revised 6 May 2002; Accepted 17 June 2002.

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Abstract

Objective: To determine the glycaemic index of one variety of date, alone and in mixed meals with plain full-milk yoghurt.

Design: An experimental study involving the measurement of the glycaemic responses of participants to the ingestion of dates, alone and with yoghurt

Setting: Within the Department of Family Medicine, Faculty of Medicine and Health Sciences, UAE University

Subjects: Healthy male and female volunteers aged 32–67 y.

Interventions: Glycaemic responses to date and date/yoghurt meal ingestion were plotted for each subject. Glycaemic indexes were calculated as the ratios of the incremental areas under these response curves to those for glucose ingestion. Results are presented using means and standard deviations. Glycaemic indexes were compared using paired t-tests.

Results: Mean glycaemic indexes of the dates were 47.2, 45.3, 35.5, 37.3, 28.9 for rutab, traditionally stored, commercial, rutab/yoghurt and commercial tamer/yoghurt preparations, respectively. There was a significant difference between the results for rutab vs commercial tamer dates (P<0.05), but other comparisons failed to reach statistical significance.

Conclusions: Khalas dates, when eaten alone or in mixed meals with plain yoghurt have low glycaemic indexes. The consumption of dates may be of benefit in glycaemic and lipid control of diabetic patients. The consumption of dates in mixed meals with yoghurt appears to have, at most, a minimal effect on the glycaemic index.

Sponsorship: This study was not financed by any grant or institution.

Keywords:

glycaemic index, dates, yoghurt, mixed meal

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Introduction

Dates were cultivated in the Middle East and Egypt over 6000 years ago. Their spread to Spain occurred probably as early as the Moorish conquest in the eighth century, and thence to Europe and to the New World in the eighteenth century (Krueger, 1998). The modern date cultivation industry is large, employing an estimated one million people worldwide. Dates, in their many varieties and ripening stages, are a dietary staple in many countries.

Date consumption is, of course, high among people of Arabic origin, many of whom are Muslims. Dates are one of the few foods directly referred to in the Muslims' holy book, the Qur'an (Koran). Among the 29 references to dates is advice to Mariam (Mary), during the birth of Jesus, to eat dates for her labour pains: 'shake towards thyself the trunk of the palm-tree: it will let fall fresh ripe dates upon thee' (Ali, 1992). The Hadith (the sayings of the Holy Prophet Mohamed) contains numerous references to dates, including the protective benefits of date consumption and their use as taxes and for tithes to the poor: 'He who eats seven dates every morning will not be affected by poison or magic on the day he eats them' (Bukhari, 2001b). 'From all the trees there is one tree that resembles the faithful believer. It is the date palm' (Bukhari, 2001a).

In Arabic countries dates are very commonly eaten with coffee or yoghurt and, during Ramadan, the Muslim month of fasting, it is usual to break the daily fast with dates.

Diabetes mellitus type 2 has recently become endemic in developing Arabic countries and in people of Arabic descent living in developed countries. This is probably related, in part, to the rapid social and lifestyle changes experienced by these people particularly in the past two to three decades. The prevalence of diabetes has now reached very high levels in many Arabic countries (King & Rewers, 1993; Abdella et al, 1995; Al Mahroos & McKeigue, 1998; Pugh et al, 1998): In urban Saudi Arabia and the United Arab Emirates, the age-adjusted prevalence of diabetes in the 30–65 y age group is over 20% and in the sixth decade it is nearly 50% (El Mugamer et al, 1995; Al Nuaim, 1997).

Dietary control is one of many strategies and methods urgently needed to control the disease. The high consumption of dates in these traditional cultures raises the question, should the population, and especially diabetic patients, eat dates as did their forefathers? As suggested by a recent internet article, are dates 'the candy that grow on trees'?

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Glycaemic index

The glycaemic index (GI), as a system of classifying food items by glycaemic response, was first reported in 1981 (Jenkins et al, 1981). There is evidence to support its application to mixed meals in terms of benefits for lipid and glycaemic control (Brand et al, 1991; Wolever et al, 1992; Fontvieille et al, 1992; Brand Miller, 1994; Frost et al, 1999; Buyken et al, 2001; Ford & Liu, 2001). Gilbertson et al recently showed improved glycaemic control and quality of life for children with type 1 diabetes who had consumed a flexible low-GI diet compared with those on a measured carbohydrate exchange diet (Gilbertson et al, 2001).

A search of the medical literature and the internet revealed only one article which included the GI of dates (Lock et al, 1988). That study was carried out only on pregnant women. Date variety and stage of ripening were not reported and it is not clear whether the methods used to determine GI were those standardized by Wolever et al (1991).

The purpose of this study was to determine the GI of the khalas variety of date, when eaten alone and in mixed meals with full milk plain yoghurt.

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Methods

The khalas variety of date, very popular in the United Arab Emirates, was chosen for study. The same batches of dates were used for all tests. The same commercial brand of full milk plain yoghurt was used for the mixed meals. Because of the duration of the study, it was not possible to use the same yoghurt batch for all tests. However, the yoghurt batches were processed to commercial standards of composition consistency. The compositions of the various forms of dates and of the yoghurt were analysed using standard methods (Association of Official Analytical Chemists, 1990).

The dates were tested on volunteer subjects who had no known medical conditions affecting glucose metabolism, including diabetes mellitus. Three potential subjects were found to have abnormal glucose response curves following the 50 g glucose load and were excluded from the study. Subjects were tested using five preparations: the soft, early ripened (rutab) dates, the sun-dried dates (tamer) stored in the traditional Bedu manner, the commercial dark brown dry dates (tamer), familiar to consumers worldwide, and the rutab and commercial tamer dates in mixed meals with a commercially available full-milk plain yoghurt.

The whole dates were consumed (with stones removed) mixed with the yoghurt as one might have in a breakfast meal.

The GI was calculated according to Wolever et al (1991). Capillary blood samples were taken using One Touch II® Lifescan glucometers. Two glucometers were used for the study, each of which was tested for accuracy and precision against a Beckman Synchron CX7 laboratory analyser which uses the glucose oxidase method of analysis.

After overnight fasting, each subject underwent three tests using 50 g of glucose as the standard food and one test for each of the date preparations in amounts equivalent to 50 g of available carbohydrate. The two rutab/yoghurt and commercial tamer/yoghurt meals each comprised 125 g of yoghurt (equivalent to 5.25 g available carbohydrate) mixed with 91.1 g of rutab dates and 59.7 g of tamer dates, respectively (each date component equivalent to 44.75 g of available carbohydrate). Both mixed meals, therefore, involved the consumption of 50 g available carbohydrate. The incremental areas under the glycaemic-response curves for each date meal were expressed as a percentage of the mean area under the three glucose curves for the same subject. The resulting values for all subjects were averaged to calculate the GI for each meal. Areas under the glycaemic-response curves were computed using an Excel spreadsheet. Comparisons between the GIs for the different date preparations were made using only those results from subjects who had completed both the test and glucose meals.

The data are presented as means and standard deviations. The glycaemic indexes of the different date preparations, including the rutab and tamer alone vs the mixed meals, were compared using paired t-tests.

The study conformed to the requirements of the Declaration of Helsinki and the study was approved by the Research Ethics Committee of the Faculty of Medicine and Health Sciences, United Arab Emirates University. Subjects signed written informed consent to the study protocol.

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Results

Testing of the two glucometers for accuracy using samples across high, mid and low glucose values revealed the following: for the high value sample (305.0 mg/dl by analyser), the accuracy was within +5.73% and +0.6% for glucometers A and B, respectively; for the mid range value (130.5 mg/dl), the accuracy was within -3.83% for both glucometers, and at the low range value (91.0 mg/dl), the accuracy was within -15.93% and -19.78% for glucometers A and B, respectively. The precision studies showed coefficients of variation of 2.20–2.89% for both glucometers using three testing samples across the low, mid and high glucose range.

Eleven subjects (six females, five males, mean age 48 y, mean BMI 23.6 kg/m2) were tested to calculate the GI of the three forms of dates alone. Nine subjects (five females, four males, mean age 45 y, mean BMI 23.8 kg/m2) were tested using the rutab/yoghurt mixed meal (these included eight of the same subjects tested with dates alone). Eight subjects (four females, four males, mean age 48 y, mean BMI 23.9 kg/m2) were tested using the commercial tamer/yoghurt mixed meal (these included seven of the same subjects tested with dates alone). Potential subjects with abnormal responses to the 50 g glucose load were excluded.

Table 1 outlines the analyses of the date preparations. The rutab dates have a much higher moisture content and lower percentage available carbohydrate than the other date forms. The crude fat content of the yoghurt reflects the full milk variety used for the study.


Table 2 outlines the GI results for the three forms of khalas dates. There were differences in the GIs with the commercial tamer dates having the lowest value. The differences in means were significant between the rutab and the commercial forms (paired t-test; t=2.704, P<0.05), but not between the traditionally prepared and the commercial dates (P=0.23).


GI results for the rutab/yoghurt and commercial tamer/yoghurt mixed meals were 37.3 and 28.9, respectively. When the GIs for rutab and commercial tamer dates alone and the mixed meals were compared, the mixed meals had lower glycaemic indexes, however the differences failed to reach statistical significance (paired t-test; t=1.998, P=0.07).

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Discussion

The range of glycaemic indexes calculated in this study for the dates alone (35.5–47.2) places dates among low GI food items and comparable with or lower than many other fruits. For example, from international tables, mean GIs for fresh apples, bananas, oranges and kiwifruit are quoted as 36, 53, 43 and 52, respectively (Foster-Powell & Miller, 1995) We have not confirmed the one date GI result of 61.6 we found in the literature, however, as stated above, methodological details are unclear in that study (Lock et al, 1988).

The low GI level is likely to be associated with the high fructose content of the dates. Ahmed et al has reported a glucose:fructose ratio of 1.2 and 1.1 in khalas rutab and tamer dates respectively (Ahmed et al, 1995). The significantly higher GI of the rutab dates compared with the commercial dates suggests that the much greater level of hydration of rutab dates affects gastric emptying, intestinal absorption or insulin secretion.

The lowering of the GI of the rutab and tamer dates when eaten, albeit in a slightly lower amount, in mixed meals with yoghurt is consistent with other evidence that the presence of fat and protein in test meals affects postprandial glucose and insulin responses (Gulliford et al, 1989). However our results failed to reach statistical significance. This effect of protein and fat has been challenged in the context of normal mixed meals on the grounds of fat and protein dose response (Wolever & Bolognesi, 1996).

The implications of these results are that diabetic patients can be reassured that consumption of dates, in similar quantities as were used in this study, will not result in rapid and large fluctuations in blood sugar even when eaten alone. Six to eight tamer and eight to 10 rutab dates were used in the study date meals (and marginally less in the mixed meals), which are similar amounts to what many Middle Eastern Arabic people would normally consume at a single sitting. Patients are currently commonly advised to either refrain from eating dates or to eat very modest quantities of the fruit. It would appear that this advice is without foundation as well as being impractical among those for whom dates are a dietary staple.

The evidence for the beneficial effects of low GI meals on serum lipids has been accumulating recently (Fontvieille et al, 1992; Brand Miller, 1994; Frost et al, 1999; Buyken et al, 2001; Ford & Liu, 2001). Clearly, in the context of high and rising levels of diabetes and ischaemic heart disease, as is the case in many countries where dates are a dietary staple, any lowering of risk factors is potentially beneficial. The consumption of dates, with or without the also commonly consumed cultured dairy products, may turn out to be of such benefit. The results from this study support the view that those treating diabetic patients should give dietary advice based on the growing evidence in favour of low GI diets.

It would seem that dates are not 'the candy that grows on trees', at least in terms of the glycaemic response to their ingestion, and this is good news for diabetic patients.

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References

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Acknowledgements

Professor T Wolever, University of Toronto, is thanked for the Excel glycaemic index calculator, Al Ain date factory for the donation of the dates, Modern Pharmaceutical Company for the donation of glucose testing strips, Central Laboratories Unit, UAE University and Food Environment Control Centre, Abu Dhabi Municipality for analyses of the dates and yoghurt, John Cherian, the technician, who performed the capillary blood tests on the subjects, and, most importantly, the subjects themselves who endured numerous fasts and blood glucose tests.

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