Rye Bread Decreases Postprandial Insulin Response but Does Not Alter Glucose Response in Healthy Finnish Subjects
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European Journal of Clinical Nutrition (1999) 53, 262±267 ß 1999 Stockton Press. All rights reserved 0954±3007/99 $12.00 http://www.stockton-press.co.uk/ejcn Rye bread decreases postprandial insulin response but does not alter glucose response in healthy Finnish subjects K Leinonen,1* K Liukkonen,2 K Poutanen,2 M Uusitupa1 and H MykkaÈnen1 1Department of Clinical Nutrition, University of Kuopio, P.O.Box 1627, 70211 Kuopio, Finland; and 2VTT Biotechnology and Food Research, P.O. Box 1500, FIN-02044 VTT, Finland Objective: To compare if postprandial glucose and insulin responses to wholekernel rye bread are lower than to wheat bread, and to see if these responses to two types of rye breads are different. To explore starch digestion in more detail, rate of starch hydrolysis of same breads was measured in vitro. Design: Subjects were given test breads (43±61 g available carbohydrates by analysis) with standardized breakfast in a random order after a fast. Eight postprandial blood samples were collected during the following three hours. Rate of starch hydrolysis was determined by an in vitro enzymatic hydrolysis method. Subjects: 10 men and 10 women, aged 32 Æ 3 and 27 Æ 5 y, BMI 24.5 Æ 2.2 and 20.3 Æ 1.1 kg=m2, respectively, all had normal glucose tolerance. Results: Plasma insulin response to wholekernel rye bread was lower than to wheat bread (45 min P 0.025, 60 min P 0.002, 90 min P 0.0004, 120 min P 0.050, 150 min P 0.033), but there was no difference in glucose responses. In comparison of two types of rye breads, glucose response to wholemeal rye bread at 150 and 180 min was higher (P 0.018 and P 0.041, respectively) and insulin response at 60 min was lower (P 0.025) than those to wholemeal rye crispbread. Total sugar pro®les in vitro were similar for all breads. When free reducing sugars were subtracted, starch in wholekernel and wholemeal rye breads appeared to be hydrolysed slower than starch in wholemeal rye crispbread and wheat bread. Conclusions: Wholekernel rye bread produces lower postprandial insulin response than wheat bread, but there is no difference in glucose response. The latter is in accordance with in vitro results. Postprandial glucose and insulin may also be affected by type of rye bread. Characteristics of different types of rye breads must be further investigated to develop health properties of rye breads. Sponsorship: Vaasa Bakeries Ltd, P.O. Box 105, 00240 Helsinki, Finland and Fazer Bakeries Ltd, P.O. Box 40, 15101 Lahti, Finland. Descriptors: rye bread; wheat bread; glucose response; insulin response; starch hydrolysis Introduction variables not only in diabetes and hyperlipidemia, but also in healthy subjects (BjoÈrck et al, 1994; Englyst & Hudson, Wholemeal sour rye breads are an essential part of the 1997). The glycemic index (GI) concept, although subject to Finnish diet. Rye products comprise approximately 36% some critique (Wolever, 1997), is a widely used method for of the total intake of cereals in Finland (Kleemola et al, classi®cation of different foods according to their effect on 1994). The energy and macronutrient contents of rye grain postprandial glucose levels. The glucose and insulin are similar to those of other cereals. Typical rye products responses of different rye breads and other rye products vary from country to country, variation being caused by have been reported to be variably lower than those of wheat variable ¯our yield and coarseness, the amount of other bread, as reviewed by Foster-Powell & Brand Miller (1995). ingredients (wheat ¯our, sour dough cultures), and the The lowest GI-values (66±80) have been reported for baking process. In Finland the traditional and still popular pumpernickel-type breads containing intact kernels (Jenkins rye breads are based on wholemeal ¯our, and are thus rich et al, 1985, 1986; Wolever et al, 1987, 1994). There is a in dietary ®bre (DF). The DF content of rye ¯our is consensus that intact botanical structure protects the encap- 15±17%, the main chemical constituents being arabin- sulated starch of the kernel against the hydrolysis (Brand et oxylans (8±10%), beta-glucan (2±3%) and cellulose al, 1990; Granfeldt et al, 1992). The amount of whole (1±3%) (HaÈrkoÈnen et al, 1997; AÊ man et al, 1995). kernels in the bread has been concluded to be more effective Mainly due to its high DF content, wholemeal rye bread in reducing the glucose and insulin responses than the high may reduce the health risks associated with coronary heart ®ber content as such (Jenkins et al, 1985, 1986; Wolever et disease (Pietinen et al, 1996) and colon, breast and prostate al, 1994; Liljeberg et al, 1992). cancer (Consensus meeting, 1997; Zhang et al, 1997). Heat processing is generally considered to increase the Slowly digestible carbohydrates have been suggested to GI of foods, mainly due to increased enzymatic accessi- be nutritionally most desirable, improving metabolic bility of the gelatinized starch. The best known exception to this rule is pasta manufacture, most pasta products having a *Correspondence: K. Leinonen, Department of Clinical Nutrition, GI-value of 50±70 (Foster-Powell & Brand Miller, 1995). University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland. Received 30 June 1998; revised 5 November 1998; accepted 16 November Low temperature and long-time baking may slow the 1998 digestion of bread by increasing the retrogradation of Rye bread decreases postprandial insulin response K Leinonen et al 263 amylose and hence the amount of resistant starch (RS) in was forbidden. Subjects were also asked to avoid heavy the product (BjoÈrck & Liljeberg, 1997). RS passes the small eating and heavy exercise one day before the tests and on intestine without digestion and is available as energy only the test day. A dietitian gave detailed written and oral after colon fermentation. Traditional Finnish rye bread is instructions on the dietary and study regimen. Dietary mostly baked on sour dough, which contains organic acids analysis of nutrient intake was calculated using the and their salts. The latter are supposed to lower postpran- Micro-Nutrica1 software package (Social Insurance Institu- dial glucose and insulin responses (Liljeberg & BjoÈrck, tion, Helsinki, Finland 1993) (Rastas et al, 1993). 1994; Liljeberg et al, 1995; Liljeberg & BjoÈrck, 1996) In the test morning an intravenous catheter was inserted either by interfering the action of hydrolytic enzymes in the in the antecubital vein of the arm. After the fasting blood small intestine or by delaying gastric emptying (BjoÈrck & sample was drawn, the subject had to eat the test meal Liljeberg, 1997). within 15±20 min. The mean times of eating breads were The majority of the studies concerning glycemic 8 min for wheat bread, 9 min for wholekernel rye bread, responses of rye bread have been conducted in diabetic 10 min for wholemeal rye bread and 11 min for wholemeal patients. In this study we wanted to determine in healthy rye crispbread. Blood samples were taken through the subjects whether the postprandial glucose and insulin catheter at 15, 30, 45, 60, 90, 120, 150 and 180 min after responses to rye bread (wholekernel bread) are lower than the start of eating the test meal. those to wheat bread. We also wanted to ®nd out if various Plasma glucose was analysed by the glucose oxidase types of rye breads give different glucose and insulin method (Glucose Auto & Stat, Model GA-110, Daiichi, responses (wholemeal crispbread vs wholemeal bread). Kyoto, Japan) and plasma insulin by the RIA-method To explore the starch digestion in more detail, the rate of (Phadaseph Insulin RIA 100, Pharmacia Diagnostica, starch hydrolysis of the same test breads was measured in Uppsala, Sweden). Maximal glucose and insulin responses vitro. were calculated by subtracting the highest value of blood glucose or insulin from the fasting value of blood glucose or insulin. The areas for glucose and insulin were calcu- Methods lated from initial rise of blood glucose or insulin, respec- Subjects tively, above the baseline (Ha et al, 1992). Twenty healthy Finnish adults (10 men and 10 women, Statistical signi®cances of differences in the postpran- aged 32 Æ 3 and 27 Æ 5 y, respectively) with normal glu- dial glucose and insulin responses between the test bread cose tolerance participated in the study. The body mass (wholekernel rye bread vs wheat bread, wholemeal rye index (BMI) of men was 24.5 Æ 2.2 kg=m2 and that of bread vs wholemeal rye crispbread) were assessed by the women 20.3 Æ 1.1 kg=m2. According to the four day food non-parametric Wilcoxon's test. records kept before the study, the mean energy intake by Approval of the human study was given by the Ethics the men was 9668 kJ=d (2302 kcal=d) (range 6145± Committee at the Kuopio University Hospital. 12243 kJ=d) and by the women 7510 kJ=d (1788 kcal=d) (range 5998±9731 kJ=d). The subjects were asked to main- Chemical composition of the test breads tain their body weight and their energy intake unchanged The moisture content was determined by oven drying at during the study. 130C for 1 h. The protein content was determined by the Kjeldahl method (nitrogen 6 6.25). The fat was determined Test breads and test meals gravimetrically by extraction in diethyl ether and petroleum The test breads were three commercial wholemeal sour rye ether after hydrolysis with acid (AOAC Of®cial Method breads and a white wheat bread as a reference. The breads 922.06, 1995). The dietary ®ber (DF) content was deter- were received from two Finnish bakeries (Fazer Bakeries mined according to Asp et al (1983).