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Low-glycaemic study


C. J. K. Henry, K. J. Newens & H. J. Lightowler

Nutrition and Food Research Group, School of Life Sciences, Oxford Brookes University, Oxford, UK

Background: Previous research has shown that substitution of high-glycaemic index (GI) bread for low-GI bread can favourably alter the 24-h glucose profile. Given the high beverage consumption in the UK, the present study aimed to evaluate the impact of consuming a beverage containing a low-GI, naturals weetener at mealtimes compared to a sucrose-based beverage, on the 24-h glucose profile.

Methods: In a randomized crossover design, six subjects (aged 41 ± 16 years; body mass index = 25.8 ± 4.1 kg m)2) were provided with a diet including low-GI or control (sucrose) beverages on two nonconsecutive days. On each study day, subjects consumed the low-GI or control beverage at breakfast, lunch and dinner. Interstitial glucose concentrations were measured over 24 h using a continuous glucose monitoring system.
Results: Compared to the control beverage, the low-GI beverage significantly reduced mean glucose concentration over 24 h (P < 0.05). Similarly, 24-h and daytime incremental area under the curves for glucose were significantly lower (P = 0.001).

Conclusions: The substitution of a sucrose-based beverage for a beverage containing a low-GI, natural sweetener at mealtimes is a simple dietary change that can significantly reduce 24-h glucose levels and may be beneficial for many individuals.

The glycaemic index (GI), first introduced in 1981 (Jenkins et al., 1981), is a classification of the blood glucose raising potential of carbohydrate foods. Subsequently, research on GI has been extensive, with studies demonstrating the potential of low-GI diets in reducing the risk of certain chronic diseases (Barclay et al., 2008). Low-GI foods have also been shown to be more satiating than high GI foods and help to limit subsequent food intake
(Warren et al., 2003). Moreover, recent studies have shown that overweight and obese people may be able to lose weight and increase body fat loss on low-GI diets, at the same time as having the added benefit of improving lipid profiles and reducing cardiovascular risk (McMillan-Price et al., 2006; Thomas et al., 2007; Philippou et al.,2008).

Despite the growing evidence of the benefits of low-GI foods for all individuals, the GI is still not widely adopted as a dietary concept. For consumers, the complexity of the issue may play a part because there are many factors that determine the GI value of a food, such as particle size, cooking and food processing, other food components (fat, protein, dietary fibre) and starch structure (Bjorck et al., 1994). This, when taken together with various
other nutritional considerations, such as limiting calories, and saturated fat and salt, may put too much pressure on the consumers’ food choice. A potential simple dietary change, such as the inclusion of a low-GI product during the day, may be a quick and easy way to gain the benefits of a low-GI diet. The continuous glucose monitoring system (CGMS) is an innovative instrument that records interstitial glucose levels every 5 min, providing a detailed view of the glucose profile, to show potentially when and to what extent an intervention affects the glycaemic excursions. The other
major advantage with this new technology is that the monitor can be worn unobtrusively, allowing the subject to continue his/her daily living. This gives a unique insight into how diet affects glucose profile at different times of the day.
Previous research has shown that the substitution of high-GI bread for low-GI bread at breakfast, lunch and dinner can favourably alter glucose profile (Henry et al., 2006). Given the high beverage consumption in the UK (Swan,2004), the present study aimed to evaluate the impact of consuming a beverage containing a low-GI, natural sweetener at mealtimes compared to a sucrose-based beverage, on the 24-h glucose profile.

Materials and methods

Subjects were randomly selected from a pool currently participating in studies on the glycaemic response to different foods. Subjects were asked to complete a health screening questionnaire to check against ill health, including clinically abnormal glucose metabolism (fasting blood glucose >6.1 mm L)1) and any medical conditions or medications that might affect glucose regulation or appetite. Six subjects (two female and four male), mean age 41 ± 16 years and a mean body mass index of 25.8 ± 4.1 kg m)2, successfully completed the study. Ethical approval for the study was obtained from the University Research Ethics Committee at Oxford Brookes University. All subjects provided their written informed consent prior to participation.

Dietary intervention
In a randomized crossover design, subjects were provided with a diet including 3 · 330 mL low-GI or control (sucrose-based) beverages on two nonconsecutive days. The low-GI beverage was formulated using a natural plant-derived sweetener (Fruit Up; Rudolph Wild, Heidelberg-
Eppelheim, Germany), blackcurrant juice concentrate, citric acid and flavourings. A beverage of equal calories and carbohydrate content was used as a control; the only difference was a substitution of the natural plant-derived sweetener with sucrose. Meals were identical on the two study days, with the exception of the beverage, and were designed to reflect usual eating patterns in the UK. Breakfast consisted of cereals and toast; lunch consisted of sandwiches, potato crisps and a piece of fruit; dinner consisted of chilli con carne, white basmati rice, low-fat yogurt and a piece of fruit; snacks (cereal bar and dried apricots) were provided both mid-morning and mid-afternoon. In addition, subsubjects were instructed to drink either the low-GI or control beverage at each meal event. Subjects ate and drank only the food and drinks provided, except for water, which could be taken in moderation. In addition to the study day, meals were also provided for the evening before (jacket potato, cheese, baked beans, apple and yoghurt) to
minimize the ‘second-meal’ effect (Wolever et al., 1988).

Measurement of 24-h glucose response
Interstitial glucose was measured using the Medtronic MiniMed (Northridge, CA, USA) CGMS, which records glucose levels every 5 min, giving a total of 288 readings over 24 h. The CGMS sensor was inserted under the skin in the abdominal region at 15.00 h before each study day
and removed at 10.30 h after the study day. The sensor was calibrated using finger-prick capillary blood glucose measurements taken by the subjects at set times over the study period. Interstitial glucose values have been shown to closely match plasma glucose values (Monsod et al., 2002).

Statistical analysis
Statistical analysis was performed using SPSS software, version 14 (SPSS Inc., Chicago, IL, USA). Results are given as the mean ± SD. Continuous glucose data were analysed over 24 h (06.00–06.00 h), during the day (06.00–22.00 h) and at night (22.00–06.00 h). The incremental
area under the curve (IAUC) was calculated geometrically for each subject’s glucose response; the 06.00 h glucose result was taken as the baseline. Prior to statistical analysis, the normality of the data was assured using the Shapiro–Wilks statistic. Differences in mean glucose concentration,
and IAUC between the two beverages were assessed using paired t-tests. P < 0.05 was considered statistically significant.

Table 1 presents the mean glucose concentrations for the different time periods. When the low-GI beverage was consumed, there was a significantly lower mean glucose concentration compared to the control beverage over 24 h (P = 0.012). A lower mean glucose concentration was also seen during the day (P = 0.078) and at night (P = 0.211). To standardize the data, the mean change in glucose levels from baseline was determined (Fig. 1) and used to calculate the IAUC (mm min L)1) for the three time periods (Table 1). When the low-GI beverage was consumed, the IAUC over 24 h and during the day was significantly lower compared to the control beverage (P = 0.001 and P = 0.001, respectively). The IAUC at night was alsoDiscussion
An intervention using beverages to lower 24-h glucose levels is very applicable for the UK population due to their high beverage consumption (Swan, 2004). Such high beverage consumption has also been reported in North America and several European countries. The present study investigated the impact of substituting three sucrose-based beverages for beverages formulated with a natural fruit derived sweetener. As the natural plantderived
sweetener has a lower GI value than sucrose, the hypothesis was that the glucose response to usual eating patterns over 24 h would be reduced with the inclusion of the low-GI beverage.

The results obtained in the present study demonstrate that the substitution of a sucrose-based beverage for a beverage containing a low-GI, natural sweetener at mealtimes led to a reduction in glucose response. There was a consistent lowering effect of the low-GI beverage throughout the
day on glucose levels and a trend of lower glucose values during the night. These results may have important clinical significance. The blood glucose-lowering effect lasts longer than the initial post-prandial period and may confer important clinical benefits to those who are managing diabetes by
diet control (Frost & Dornhorst, 2000). For individuals with a normal glucose range, maintaining low blood glucose levels reduces the risk of developing insulin resistance, which is a major contributor to type 2 diabetes (Willett et al., 2002). In addition, cardiovascular disease risk is associated with increased blood glucose concentration in diabetic (Muggeo et al., 1997; Barzilay et al., 2004), prediabetic (Levitzky et al., 2008) and nondiabetic (Bjørnholt et al., 1999; Lawes et al., 2004; Levitan et al., 2004) individuals. Major supermarket chains in the UK are aware of the potential health benefits of consuming a low-GI diet. Many have commissioned the determination of the GI value of foods in order to classify their products as low,
medium or high GI. Replacing traditional sugars or sweeteners in soft drink beverages with a fruit-derived low-GI sweetener may be an easier way to optimize glycaemic response for the consumer. The present study demonstrates the benefit of substituting three sucrosebased
beverages for beverages formulated with a low-GI, natural fruit-based sweetener. Further work is needed to ascertain whether similar results could be obtained when consuming fewer beverages (less than three per day) or altering time of consumption (e.g. outside meal times).

In conclusion, our findings have shown that beverages containing a low-GI, natural sweetener reduce 24-h glucose response compared to sucrose-based beverages. As an increasing number of health benefits of low-GI diets have been identified, a simple modification to the diet
(i.e. the exchange of a high-GI beverage to low-GI beverage), if adopted in the long-term, may be beneficial for many individuals