The Postprandial Glycemic and Insulinemic Effects of Three Cooked Vegetables
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The Postprandial Glycemic and Insulinemic Effects of Three Cooked Vegetables: Corchorus Olitorius, Spinacia Oleracea, and Daucus Carota on Steamed White Rice Ahmad Faqih*1 and Buthaina Al-Khatib2 Abstract Background: Eatingcooked vegetables with rice is quite common in Jordan and worldwide. Dietary fibers of vegetables are expected to play a role in the glycemic control of meals. Aim: To study the postprandial glycemic and insulinemic effect of three cooked vegetables: mulukhiyah leaves (Corchorus olitorius), spinach (Spinacia oleracea) and carrots (Daucus carota). Methods: The postprandial glycemic and insulinemic effect of the threecooked vegetables on steamed rice were studied by runningtheoral glucose tolerance tests on apparently healthy young adults, each of who served as his own control, using white bread as the reference. Insulin sensitivity was measured by calculating the composite insulin sensitivity index. Results: The glycemic index (GI) of rice (84.2±10.5%) ingested with chicken broth was significantly lowered only by eating 120 g of mulukhiyah leaves (ML) and not by either carrots or spinach. The insulinemic index (II) of steamed white rice eaten with broth was significantly lowered by120 g of ML (61.7± 9.2%) and 150 g of spinach (42.9± 9.0%). Insulin sensitivity was only improved by spinach. All results are expressed as means ± SEM and are considered statistically significant at P<0.05. The results also suggested that there was no significant difference between the calculated relative GI and relative II responses of the three cooked vegetables at the two assigned levels, each eaten with rice and broth compared to the corresponding GI or the II values of the individual foods from which they were composed of. Conclusions: Eating a relatively high portion of mulukhiyah leaves improves the glycemic response to white rice whereas only higher levels of carrots tended to improve the glycemic response of white rice, and spinach at either level had no apparent effect. While insulin level was lowered by the three tested vegetables, its sensitivity was improved by eating spinach only. Keywords: Glycemic index, insulinemic index, mulukhiyah leaves, spinach, carrots, insulin sensitivity. (J Med J 2013; Vol. 47 (2):161- 175) Received Accepted February 2, 2012 December 16, 2011 1. Faculty of Agriculture, The University of Jordan. 2. Msc in Dietetics and Human Nutrition, Faculty of Agricultrue, The University of Jordan. * Correspondence should be addressed to: Prof. Ahmad Faqih E-mail: [email protected] © 2013 DAR Publishers⁄ University of Jordan. All Rights Reserved. The Postprandial Glycemic and Insulinemic Effects … Ahmad Faqih and Buthaina Al-Khatib Introduction under the glucose response curve after an amount of carbohydrate from a test food Dietary fibers which by definition cannot be relative to that of a control food that is digested by humans play an important role in consumed.7 The area under the curve for improving the glycemic control. Water- glucose (AUCg) is calculated to reflect the soluble dietary fibers are more effective than total rise in blood glucose level after the test insoluble fibers.1, 2 Part of this control can be food. The glycemic index is calculated by ascribed to glucagon-like peptide-1(GLP-1) dividing the AUCg resulting from eating a which is secreted by intestinal L-cells and to fixed amount of carbohydrates (50 g) of the ghrelin which is secreted by the stomach. It is test food by the AUCg resulting from eating a well established that GLP-1slows the gastric similar amount of carbohydrates from the emptying rate and consequently slows down reference food and multiplying the answer by the rate of glucose absorption into the blood 100.7The use of a reference food is essential stream which plays an important role in for reducing the confounding influence of glucose control and diabetes treatment3. It differences in the physical characteristics of also stimulates glucose-dependent insulin the subject.8 Jenkins and his co-workers 8 first secretion in addition to decreasing the introduced the concept of the glycemic index production of glucagon. Furthermore, insulin (GI) in 1981. sensitivity can be increased by some fibers.4, 5 Ghrelin, an appetite-stimulating hormone is Recent studies show a positive impact of low- down regulated by soluble fibers by delaying GI diets on hunger ratings,9 resting energy gastric emptying.6 Glucose-dependent expenditure, nitrogen balance, fat loss,10 and insulintropic polypeptide (GIP) is secreted by weight loss.6,11,12 In 2007, the World Health K-cells of the intestine and as the name Organization (WHO) and Food and indicates its function is to stimulate insulin Agriculture Organization (FAO) 13 updated the secretion. GIP and GLP-1 are called incretins 1998 recommendations about carbohydrates in and thus both have the incretin effect, human nutrition by recommending to people meaning that a higher amount of insulin is from industrialized countries to base their diets secreted when an equivalent amount of on foods with low GI (< 55%), in order to glucose is administered orally than prevent the most common diseases of intravenously.5 Tahrani and colleagues affluence, notably obesity, coronary heart summarize the difference between GIP and disease, and diabetes. GLP-1 as follows, “unlike GLP-1, GIP has no effect on α-cells that secrete glucagon and has The development of insulinemic indices (IIs) no impact on food intake, satiety, gastric for foods has also been reported as necessary emptying, or body weight.”5 to supplement the GI tables used in the dietary management of diabetes mellitus.14 It has been The postprandial glycemic and insulinemic widely assumed that the insulin response is effect of foods can be studied in terms of their proportional to the glucose response, and effect on the glycemic index (GI) and therefore the glycemic response is an accurate insulinemic index (II) on cooked rice as in our predictor of the insulin response.15 The II of study. The GI refers to the incremental area foods can also be determined from the 162 J Med J 2013; June Vol. 47 (2) http:⁄⁄dar.ju.edu.jo⁄jmj The Postprandial Glycemic and Insulinemic Effects … Ahmad Faqih and Buthaina Al-Khatib corresponding incremental blood insulin chopped into small square pieces (about 3 areas.16 cm2), mulukhiyah leaves were chopped into Traditionally, many Middle Eastern dishes are small pieces, and carrots of medium length prepared from cooked rice along with (about 80 g for each) were peeled and chopped vegetables in different amounts and various to round slices (about 5mm). All vegetables methods. For this report, carrots, spinach, and were weighed, put into polyethylene bags, and mulukhiyah leaves were selected. The GI and kept frozen until used within three weeks. II have been widely determined from single food items, which may not necessarily predict Chicken broth (B) was prepared separately by their GI and II in mixed dishes.17 Besides, using young chickens (age 42 days, about 2 people do not usually eat single food items.16 kg) without skin and bones, using two liters of Thus, additional research is needed to water and 30 grams of salt per one kilogram of determine the glycemic index of mixed dishes chicken and were boiled for a period of 20 rather than the single foods that comprise minutes. Chicken broth was served in 200 mL them. The glycemic and insulinemic response disposable plastic cups along with steamed elicited by a meal is termed the relative white rice and with each vegetable dish at two glycemic response (RGR) and the relative different levels. insulinemic response (RIR), respectively. Wolever and Bolognesi18 developed an Rice (R; American white rice, medium-grain; equation to estimate the RGR and RIR of a trade mark: Sun White) was weighted by an meal from the individual GI, II, and the electronic balance,put in a household steam amount of carbohydrate content of the meal cooker, covered with aluminum foil, and left components. The equation was found to for a period of four hours until it became explain 90% of the variation in mean glucose tender but firm. Sunflower oil (5 g) and table responses conducted by mixed meals in normal salt (5 g) were added to each serving of white subjects.18 rice, the average weight of which before steaming (and after steaming) was as follows: The objective of the present study was to 60g (150g), 55g (138g ), 50g (125g), and 40g determine the glycemic and insulinemic (100g). indices of mulukhiyah leaves (Corchorus olitorius), spinach (Spinacia oleracea), or Each vegetable dish was prepared either at a carrots (Daucus carota) when eaten at two low level equivalent to one cup of raw different levels with steamed white rice and vegetables or half a cup of cooked vegetables chicken broth. or at a high level to about twice or three times the low level per serving. The average amount Materials and Methods of one serving and the method of preparation of each dish were determined by a pilot study Preparation of Test Foods: White rice, of five neighboring housewives. chicken, mulukhiyah leaves, spinach, and carrots were purchased from a local market in Each serving of frozen vegetables was Amman, Jordan. All vegetables were washed prepared by adding 40 mL of filtered water and with water. Spinach (medium size leaves) was 5g of salt and boiled for 20 min. for the low 163 J Med J 2013; June Vol. 47 (2) http:⁄⁄dar.ju.edu.jo⁄jmj The Postprandial Glycemic and Insulinemic Effects … Ahmad Faqih and Buthaina Al-Khatib level. For the high level, the amount of water “x”, “x” is subjected to the first OGTT for the and salt were doubled for carrots and tripled reference food. The area under the curve for for spinach and mulukhiyah leaves andthen glucose (AUCg) is assigned the value of 100 cooked for 20 min.