Profiling of the glycaemic index of the underutilised vegetables

Thai, Ngoc Ro (2017) Profiling of the glycaemic index of the underutilised vegetables. MRes thesis, University of Nottingham.

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Glycaemic Index is a term used to describe the effect on blood glucose of consuming a carbohydrate rich meal. It is a measure of the rise in blood glucose following consumption of a test food relative to a reference food. Various associations have been found between dietary GI and a number of chronic diseases.

Whilst there has been considerable research into the GI values of commonly consumed foods, very little is known about the GI values of plants that fall into the category of underutilised species. These include numerous fruit, vegetables, legumes, herbs, spices and nuts that are generally grown and consumed by subsistence and small-scale farmers and their communities. Anecdotal evidence suggests that some of these underutilised plants have particular medicinal properties, in particular can be used to control diabetes mellitus.

The total starch and reducing sugar content of white rice plus four selected underutilised species (ginger, turmeric, bitter gourd and fenugreek) were analysed using the DNS method to determine the levels of reducing sugars after acid hydrolysis. The effects of cooking method (raw, boiled or fried) on the rate of starch digestibility were also investigated using in vitro and in vivo digestibility studies.

White rice had the highest starch content (78.47g/100g DM) and lowest (0g) level of reducing sugars present.

Out of the underutilised plants, both ginger and turmeric had high levels of starch (57.54-68.11 g/100g DM) and reducing sugars (2.71-3.12 g/100g DM). The total starch and reducing sugars in bitter gourd were very low (3.52-4.16g/100g DM and 0.86-1.2 g/100g DM respectively) across all three treatments. Mean values for starch and reducing sugars in turmeric and fenugreek respectively were 64.78 g/100g DM and 51.94g/100g DM: and 2.7g/100gDM and 0.7 g/100g DM respectively.

The glucose response of a single meal (individual species) and a mixed meal (rice plus one of the test species) was estimated using an in vitro model of digestion. The samples underwent digestion with α amylase followed by protease and pancreatic amylase. Aliquots of the digesta were removed at 30 minute intervals over a 3 hour period and tested for reducing sugars (using the DNS method) to evaluate the rate of digestion. This was used to estimate the GI response.

There was a significant effect of cooking process (raw, boiled or fried) on the rate of glucose release from all four test samples (p<0.001). The highest values for bitter gourd were in the boiled sample (0.187 mg/ml at 60 minutes) followed by the fried (0.163 mg/ml at 90 minutes) and the raw sample (0.167 mg/ml at 120 minutes). The peak of glucose release from ginger was at 90 minutes in both the boiled and fried samples (0.442 mg/ml glucose and 0.307 mg/ml glucose respectively). Raw ginger responded more slowly, with a peak glucose concentration (0.170 mg/ml) at 180 minutes. Boiled fenugreek seeds had a peak in glucose release (0.128 mg/ml) at 150 minutes.

To evaluate the response to feeding these four underutilised species in a mixed meal based on white rice, in vivo glucose tolerance tests were carried out on a small number (8) of volunteers.

Eight healthy participants (21-33 years old) were recruited from the University of Nottingham Malaysia Campus (UNMC). Ethical approval for the study was granted by the UNMC ethics committee. Volunteers were asked to observe an overnight fast. They took part in the study on two consecutive days. Boiled white rice was used as a reference food and was consumed (3.5g dry weight to provide 1g available carbohydrate) by all participants on day one. Capillary blood glucose was measured at 0 time (before eating) and at 30 minute intervals for 3 hours post prandial. This glucose tolerance test provided the baseline against which the test samples were evaluated for their capacity to moderate the glucose response.

On the second day, a boiled test sample was added to the rice to provide 1g available carbohydrate. The procedure above was repeated. Due to time constraints, only two replicates per sample were taken.

All four underutilised vegetables, when consumed with rice, had the effect of slowing down the digestion and related glucose release into the blood stream. This study did not investigate the mode of action by which this works. Based on these results, the four underutilised species could be classified as low GI foods. This pilot study warrants a larger study to investigate the effects more thoroughly.

Item Type: Thesis (University of Nottingham only) (MRes)
Supervisors: Azam Ali, Susan
Keywords: Underutilised vegetables, diabetes, glycaemic index
Subjects: Q Science > QP Physiology
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Science > School of Biosciences
Item ID: 42379
Depositing User: THAI, NGOC RO
Date Deposited: 26 Sep 2017 10:59
Last Modified: 21 Jul 2019 04:30

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