Glycemic and Insulinemic Responses to Commonly Consumed Potatoes in Bangladeshi Type 2 Diabetic Subjects

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1 European Journal of Food Research & Review 1(2): 52-60, 2011 SCIENCEDOMAIN international Glycemic and Insulinemic Responses to Commonly Consumed Potatoes in Bangladeshi Type 2 Diabetic Subjects Kaniz Fatema 1*, Farzana Rahman 1, Nurunnahar Sumi 1, Khadizatul Kobura 1 and Liaquat Ali 2 1 Department of Epidemiology & Biostatistics, Bangladesh Institute of Health Sciences (BIHS), 125/1 Darus Salam, Mirpur, Dhaka-1216, Bangladesh; 2 Department of Biochemistry & Cell Biology, Bangladesh Institute of Health Sciences (BIHS), 125/1 Darus Salam, Mirpur, Dhaka-1216, Bangladesh; Research Article Received 25 th January 2011 Accepted 11 th February 2011 Online Ready 24 th March 2011 ABSTRACT Glycemic indices (GIs) and insulin responses are useful for measuring biological effects and consequences of carbohydrates when designing healthy diets, particularly for people with or at risk of developing diabetes and others disorders such as metabolic syndrome. In this study, we investigated GIs and insulin (as measured by C-peptide) responses of patients with type 2 diabetes mellitus (T2DM) to Banglades hi potatoes and sweet potatoes. Using a cross-over design, ten T2DM subjects consumed equivalent carbohydrate amounts (50 g of total carbohydrate) of either the vegetables or white bread (WB) (as reference food). Serum glucose levels were determined after 0, 15, 30, 45, 60, 90, 120, 150 and 180 minutes, and C-peptide levels were determined at 0 and 180 minutes. Glycemic Indices and Glycemic Loads (GLs) were calculated. Both plain and sweet potatoes showed a significantly higher serum glucose response compared to the reference food. The similar glycemic response between plain and sweet potatoes was reflected in their GI values: and , respectively. The GL values were 8 and 11, respectively. The substantially higher glycemic response and GI values of the two potatoes were not the consequence of a suppressed insulin response. Compared to the mean values of the international table, Bangladeshi potatoes and sweet potatoes are very high GI foods. However, based on the dietary practices in our society, potatoes and sweet potatoes may be used as low and medium GL foods, respectively. This work may help create a better food exchange table for diabetic patients. * Corresponding author: kfatema@bihs.edu.bd

2 Keywords: Glycemic Index, Glycemic Load, Insulinemic Index, Potato, Sweet Potato, Type 2 Diabetic Mellitus, Carbohydrate; ABBREVIATIONS T2DM: Type 2 diabetic mellitus; GI: Glycemic Index; GL: Glycemic Load; iauc: incremental area Under the Curve; BMI: Body Mass Index; WB: White Bread; AICP: Absolute Incremental Changes of C-peptide; Mmol: Millimole; Ng: Nanogram; 1. INTRODUCTION In light of the emerging epidemic of type 2 diabetes mellitus (T2DM), even in developing countries (WHO, 2010), the quantitative and qualitative aspects of carbohydrates in diet are the subject of increased attention in the prevention of this disorder and other NCDs (Noncommunicable diseases) related to metabolic syndrome. Postprandial glycemia is influenced by both the amount and the nature of the carbohydrates in foods. The nature of carbohydrates is best described by their glycemic index (GI) (WHO, 2010; Jenkins et al., 1988), defined as the incremental area under the glucose response curve following consumption of a food portion containing 50 grams of carbohydrates relative to that produced by a portion of a control food (either glucose or white bread) conta ining the same amount of carbohydrate. GI is a useful indicator to rank the biological response to dietary carbohydrates and can be converted to a practical tool referred to as the glycemic load (GL) for routine dietary advice. The GI has been recommended to guide food choices. It has been reported that a diet of high GI foods may have adverse health consequences by increasing the risk for chronic disease (Jenkins et al., 1981; Mann, 1980). Recent evidence suggests that high GI/GL diets may increase the risk for cardiovascular disease ( Canadian Diabetes Association, 1980) and T2DM (Wei et al., 2000; Wannamethee et al., 2002; Jenkins et al., 1982). A high GI diet may increase the risk of chronic disease through the stimulation of hyperglycemia and hyperinsulinemia (Mann, 1980). In contrast, a low GI diet has been reported to provide health benefits (Jenkins et al., 1981; Mann, 1980). Epidemiological data indicate that a low GI diet protects against the development of type II diabetes (Wei et al., 2000; Wannamethee et al., 2002; Jenkins et al., 1982), coronary heart disease (Canadian Diabetes Association, 1980) and metabolic syndrome. It is now recognized that the insulin response should be included in analysis when evaluating the biological response to carbohydrate-rich diets because hyperinsulinemia constitutes a major risk factor for cardiovascular diseases (Ludwig, 2002). Due to the problem in differentiating endogenous and exogenous insulin, measuring C-peptide is preferred as a means of determining serum insulin levels. It has been realized that GI, GL and the insulin response need to be studied in different environmental, racial and cultural settings due to the substantial dependence of the chemical composition of and the biological response to carbohydrate-containing foods on these variables. Potatoes are the major source of energy in many societies. Potatoes are a versatile, carbohydrate-rich, high-potassium and low-sodium food that is easily digested, prepared and served in a variety of ways. The GI of potatoes has been studied in populations from Australia, New Zealand, Romania, Canada, Kenya and India, with widely scattered results depending on the variety of potato prepared and the population studied. A high GI 53

3 has been found in Australia, Canada and New Zealand, a medium GI has been found in Romania, and low to high GIs have been found in Kenya and India compared with the reference food, white bread. The insulinemic response to potato has rarely been studied (Galgani et al., 2006). Bangladesh is among the top ten countries in terms of the number of diabetic patients (Burchfiel et al., 1998). Although rice is the staple source of energy in this country, the potato is the most common vegetable consumed either alone or mixed with different types of food items. Sweet potatoes are also consumed in mixed vegetable meals, especially as snacks, but less frequently than the plain potato. The average consumption of potatoes in Bangladesh has been reported as 9.8 kg/capita/year (Roglic et al., 2004), and its cost is almost half that of rice. To ease the increasing demand on rice and to promote food diversity, it is important to further popularize the potato as a staple source of carbohydrate in Bangladesh. In spite of its importance in the Bangladeshi diet, no studies have been conducted on the glycemic and insulinemic response to potatoes of Bangladeshi subjects. The present study was designed to explore these responses for potatoes and sweet potatoes when boiled and consumed as a snack by Bangladeshi type 2 diabetes mellitus subjects. 2. MATERIALS AND METHODS 2.1 STUDY SUBJECTS Subjects were selected from the out-patient department (OPD) of the Bangladesh Institute of Research & Rehabilitation on Diabetes, Endocrine and Metabolic Disorders (BIRDEM). A total of ten T2DM subjects (five males and five females) took part in the study. Diabetes was diagnosed and classified according to WHO criteria. Subjects were requested to maintain their usual daily food intake and activity throughout the study period. The purpose and protocol of the study were explained to the subjects, and written consent was obtained. 2.2 TEST FOODS AND THEIR PREPARATION The study included two test meals (pot ato [Solanum tuberosum] and sweet potato [Ipomoea batatas; SP]), with white bread (WB) serving as the reference food. Both the test foods and the reference food consisted of 50 g available carbohydrates. The masses of food necessary to reach 50 g of available carbohydrate were 65 g, 221 g and 177 g for white bread, potato and sweet potato, respectively. Fresh potatoes and sweet potatoes were purchased from the local Dhaka city market in bulk quantities sufficient to conduct all tests. Washed and weighed (221 g and 177 g, respectively) potatoes with intact skins were boiled in water until soft. Large potatoes were halved before cooking. Sufficient water was added for boiling. After boiling, the water was drained, the skin was removed, and the samples were transferred to a plate. The test meals were served at room temperature. As a reference food, freshly baked white bread was sliced and portioned to the calculated weight (65 g). Each portion was bagged individually and stored frozen. On the days of trial, white bread portions were removed from the freezer 45 minutes before serving and allowed to thaw at room temperature. 54

4 2.3 EXPERIMENTAL PROCEDURE On the first day after selection and consent, detailed sociodemographic data, family history and medical history of the patients were taken using a pre-tested questionnaire, and physical and clinical examinations were conducted. Anthropometric measurements included height, weight, waist circumference and hip circumference. Thereafter, subjects were required to repeat the study protocol on four separate occasions (one trial for each test food and two repeated trials for the reference food) in the morning after a h overnight fasting. Subjects were advised to not smoke or take any kind of medicine the previous day (besides prescribed medications). The reference food test was repeated once to obtain at least two values, and the precision for each subject was improved (Scott et al., 2010). Test and reference meals were given to patients under a cross-over design with a wash-out period of seven days to avoid the second meal effect (Brouns et al., 2005). Patients were advised to rely on the recommended standard carbohydrate diet and were instructed to not eat legumes in the meal preceding the fast. An intravenous cannula was inserted into a superficial vein in the forearm on the day of experiment, drawing the fasting (0 hr.) blood sample of the patient. Subjects were requested to consume the test food with 250 ml plain water (during the protocol of the test potatoes) or 250 ml of glucose water (during the protocol of the reference food) in random order at a comfortable pace within 10 min. Further blood samples were drawn at 15, 30, 45, 60, 90, 120, 150 and 180 minutes after the initial intake of sample. Patients took their prescribed medicine at the beginning of the meal. All the information and data obtained were recorded in a predesigned Case Record Form. Blood samples were centrifuged at 3000 rpm for 15 minutes. The separated plasma was allocated into labeled Eppendorf tubes and preserved at 70 C until biochemical analysis. The C-peptide:glucose ratio was calculated by calculating values of glucose and C-peptide in study participants at 0 and 180 minutes. This ratio evaluated the c- peptide (equivalent to insul in) status of the patients in response to their glucose responses after the ingestion of test foods. GIs and Glycemic Loads (GLs) were calculated by standard formulas (Wolever et al., 1988). 2.4 LABORATORY METHOD Serum glucose was estimated by the glucose-oxidase (GOD -PAD) method using reagents from SERA PAK, USA (Trinder, 1969). Insulin (as measured by c-peptide as a marker of insulin) was determined by ELISA using kits from DRG Diagnostics (Germany). Glycosylated hemoglobin (HbA1c) was measured by high-performance liquid chromatography (HPLC). 2.5 ETHICAL CONSIDERATION The protocol was approved by the Ethical Review Committee of the Diabetic Association of Bangladesh. 2.6 STATISTICAL ANALYSIS All analyses were performed using the statistical package for social science (SPSS) software for Windows. The incremental areas under the curve (iauc) were calculated by the standardized criteria (Wolever et al., 1988), ignoring any area below the baseline. The average iauc for the two white bread tests was used as the reference value, and each subject s individual GI for each food was calculated. To compare differences 55

5 between means, a one-way ANOVA test was performed where appropriate. All parametric variables were expressed as the mean (M) ± SD, and non -parametric data were expressed as a percentage value. P<0.05 was considered to be statistically significant. 3. RESULTS 3.1 CHARACTERISTICS OF THE STUDY POPULATION Ten T2DM subjects (five males and five females; age 44±3 years, [mean±sd]) participated in the study. The mean BMI (± SD) of the study subject was 24 ± 3, and the mean waist-hip ratio was 0.92 ± The average HbA 1 c was for the study subjects. All the participants were of the medium socio-economic class (Table 1). Table 1. Clinical & socioeconomic characteristics of study subjects (n= 10) Parameters Values Age (years, M±SD) 44 3 BMI (M±SD) 24 3 Male: Female 1:1 Waist-Hip Ratio (WHR, M±SD ) 0.92 ± 0.02 Annual income in US $ (median-range) HbA 1 c (M±SD) Values are expressed as Mean ± SD except range and ratio 3.2 GLYCEMIC RESPONSE TO THE FOOD ITEMS Both potatoes and sweet potatoes showed significantly higher serum glucose responses compared to those following consumption of WB (incremental area under the curve: vs and for white bread, potato and sweet potato; p<0.01 and <0.001, respectively). However, although both potato and sweet potato blood glucose dynamics increased, they did not differ significantly from WB at any time point (table-ii). The glycemic response of sweet potato was reflected in higher GI values: ( and for potato and sweet potato, respectively). The GLs, calculated on the basis of Bangladeshi serving size, were 8 and 11 for potato and sweet potato, respectively (Table 2). 3.3 C-PEPTIDE RESPONSE TO THE FOOD ITEMS The baseline values of serum insulin in all the three groups were matched. The substantially higher glycemic response and GI values in potato and sweet potato were not the consequence of a suppressed insulin response (180 min: 2.7 [ ], 1.8 [ ] and 2.8 [ ] for bread, potato and sweet potato, respectively). This was also supported by the c-peptide: glucose ratio at 180 minutes (0.34 [ ], 0.25 [ ] and 0.34 [ ] for bread, potato and sweet potato, respectively). The serum C-peptide value increased at 180 minutes compared to the baseline value in the case of sweet potato, but for the plain potato, there was a lower response that significantly differed from WB (Table 3). Homa%B and Homa%S were calculated, and no significant difference was observed among the groups. 56

6 Table 2. Serum Glucose response of the Subjects (n=10) at different times point after consuming the test foods Test foods Bread (100) Potato (100) Sweet potato (100) Serum glucose (mmol/l) 0/0 min 15 min 30 min 45 min 60 min 90 min 120 min 150 min 180 min (99 3) (103 12) (101 7) (109 10) (131 29) ( (124 10) (143 22) (141 18) ( ) (154 19) (160 15) (138 13) (147 15) (159 13) (119 11) (127 18) (137 13) (105 7) (113 13) (115 16) (97 7) (94 13) (93 11) iauc (mmol/l at 3 hrs) a* a*** GI GL Results expressed as mean± SD; Paired students t, Bonferroni t test were performed where appropriate to calculate statistical difference between groups. Different superscripts in each column indicate significant statistical difference at *p<0.05, **p<0.01 ***p<0.001; Plain potato and Sweet potato has 22.1g/100g, 17.7g/100g of carbohydrate respectively. To calculate GL, the serving size of potato and sweet potato was 22g/serve; n, Number of subjects; iauc, Increment area under the curve Table 3. C-Peptide status of the study subjects (n=10) after feeding different test meals Test foods Serum C-peptide (ng/ml) AICP of C-pep HOMA B% HOMA S% C-peptide: Glucose 00 min 180 min 0 min 180 min 0 min 180 min 0 min 180 min Bread ( ) ( ) ( ) ( ) ( ) Potato ( ) ( ) ( ) a* ( ) ( ) Sweet potato ( ) ( ) ( ) ( ) ( ) Results expressed as mean± SD; Paired students t, Bonferroni t and nonparametric (Mann -Whitney) test were performed where appropriate to calculate statistical difference between groups. Different superscripts in each column indicate significant statistical difference at *p<0.05. AICP; absolute incremental changes of C-peptide over basal values. HOMA %B, Insulin secretory capacity by Homeostasis Model Assessment; HOMA %S, Insulin sensitivity by Homeostasis Model Assessment. 57

7 European Journal of Food Research & Review, 1(2): 52-60, DISCUSSION Popularizing the potato in the Bangladeshi diet is important both from economic and nutritional view points. It will require intensive coordinated effort as it involves changing a deep-rooted cultural habit supporting rice-based diets. However, the impact of such a transition on NCDs needs to also be considered when planning campaigns. There are common beliefs among Bangladeshi people that potatoes are much more diabetogenic than rice, so any public health campaign must incorporate proper evidence generated from local potatoes and the local population on the risks and benefits of these two products. The GI mean value obtained for potato was 162. This is somewhat higher than those previously reported for boiled white potatoes compared with white bread as a reference food ( in Australia (Wolever et al., 1991; Brand-Miller et al., 1998), in Canada (Soh et al., 1999; Wolever et al., 1994), 100 in New Zealand (Wolever et al., 1986), 59 in Romania (Perry et al., 2000) and in India (Ionescu-Tirgoviste et al., 1983). In some cases, the GI range of potatoes was (Perry et al., 2000; Ionescu-Tirgoviste et al., 1983) or (Soh et al., 1999) compared to glucose as a reference food. The GI for sweet potato was 191, which is very high compared to the reported values of 63 in Australia (Kanan et al., 1998), 84 in Canada (Wolever et al., 1994) and 111 in New Zealand (Perry et al., 2000). This discrepancy between studies may be due partly to various cooking methods (such as the addition of salt and the amount of cooking water), the moisture content of the food, the temperature for the degree of starch gelatinization (Thorburn, 1986) and the processes of cutting pieces of potato in different studies. The glycemic loads reported in the current study were 8 and 12 for potato and sweet potato. These findings reflect the GL category as low and the lower limit of medium, respectively, due to existing dietary habits in Bangladesh. However, in various types of diets, vegetables (i.e., potato) are used as a mixed meal, and it has been shown that GL values can be applied to mixed meals or whole diets by calculating the weighted GL value of the meal or diet (Wolever et al., 1986). This is also useful in predicting the acute impact on blood glucose and insulin response within the context of mixed meals (Ludwig, 2002). Furthermore, in the case of the insulin response to the tested root vegetables, this study demonstrated that insulin secretion was downregulated for potato and increased slightly for sweet potato at the expense of blood glucose, which can play a beneficial role in preventing NCDs. This postprandial insulin pattern is also supported by other work (Thorburn, 1986). Therefore, achieving a prescribed goal of carbohydrates (200 g) or the number of exchanges (twelve servings) or to meet the current nutritional advice of increasing intake of starchy foods such as potatoes could contribute to a medium to high glycemic load. This, in turn, may not increase but instead minimize insulin demand, especially for T2DM subjects. This is relevant for epidemiological studies investigating the role of carbohydrates in noncommunicable chronic diseases 5. CONCLUSION In conclusion, when considering economic aspects, crop production, high satiety, therapeutic advantages and food diversity, the potato can be recommended for healthy as well as T2DM patients because of its low-fat and high-micronutrient content and its beneficial effects on the insulin response, as observed in this study. In spite of its high GI, considering the potential sources of carbohydrate current consumption, the potato can be increased from a minor vegetable to the most important vegetable in the diet, either as a mixed meal or as an occasional partial substitute for rice. However, before considering the potato a staple food or 58

8 European Journal of Food Research & Review, 1(2): 52-60, 2011 an acceptable exchange with rice, extensive long-term studies need to be conducted to support its promotion as an alternative. Lastly, the GIs of different varieties of potatoes need to be explored in further studies to identify commercially viable low/medium GI potatoes. ACKNOWLEDGEMENTS The study was supported by the Chemical Science (IPICS), Uppsala University, Sweden and Biomedical Research Group of BIRDEM. We expressed our gratitude to all the subjects who participated in the study. REFERENCES Brand-Miller, J.C., Allwan, C., Mehalski, K., Brooks, D. (1998). The glycaemic index of further Australian foods. Proc. Nutr. Soc. Aust., 22, 110 (abstr). Brouns, F., Bjorck, I., Frayn, K.N., Gibbs, A.L., Lang, V., Slama, G., Wolever, T.M.S. (2005). Glycaemic index methodology. Nutr. Res. Rev., 18, Burchfiel, C. M., et al. (1998). Hyperinsulinemia and Cardiovascular Disease in Elderly Men. The Honolulu Heart Program. Arterioscler Thromb. Vasc. Biol., 18, Canadian Diabetes Association (1980). Guidelines for the nutritional management of diabetes mellitus. J. Can. Diet Assoc., 42, Galgani, J., Aguirre, C., Díaz, E. (2006). Acute effect of meal glycemic index and glycemic load on blood glucose and insulin responses in humans. Nutr. J., 5, Ionescu-Tirgoviste, C., Popa, E., Sintu, E., Mihalache, N., Cheta, D., Mincu, I. (1983). Blood glucose and plasma insulin responses to various carbohydratesin type 2 (non-insulindependent) diabetes. Diabetologia, 24, Jenkins, D.J.A., Wolever, T.M.S., Taylor, R.H., Barker, H., Fielden, H., Baldwin, J.M., Bowling, A.C., Newman, H.C., Jenkins, A.L., Goff, D.V. (1981). Glycemic index of foods: a physiologic basis for carbohydrate exchange. Am. J. Clin. Nutr., 34, Jenkins, D.J.A., Ghafari, H., Wolever, T.M.S. (1982). Relationship between the rate of digestion of foods and postprandial glycemia. Diabetologia, 22, Jenkins, D.J.A., Wolever, T.M.S., Jenkins, A.L. (1988). Starchy foods and glycemic in dex. Diabetes Care, 11, Kanan, W., Bijlani, R. L., Sachdeva, U., Mahapatra, S. C., Shah, P., Karmarkar, M.G. (1998). Glycaemic and insulinaemic responses to natural foods, frozen foods and their laboratory equivalents. Indian J. Physiol. Pharmacol., 42, Ludwig, D. S. (2002). The glycemic index: physiological mechanisms relating to obesity, diabetes, and cardiovascular disease. JAMA., 287, Mann, J. I. (1980). Diet and diabetes. Diabetologia, 18, Perry, T., Mann, J., Mehalski, K., Gayya, C., Wilson, J., Thompson, C. (2000). Glycaemi c index of New Zealand foods. NZ Med. J., 113, Roglic, G.S.W., Green, A. (2004). Global prevalence of diabetes: estimates for the year 2000 and projections for Diabetes Care, 27, Scott, G.J., Bouis, H.E. (2010). Sustainability of Potato Consumption in Developing Countries: The case of Bangladesh. International Potato Center, Program Report, Soh, N.L., Brand-Miller, J. (1999). The glycaemic index of potatoes: the effect of var iety, cooking method and maturity. Eur. J. Clin. Nutr., 53,

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