The Influence of the Subjects' Training State on the Glycemic Index

ORIGINAL ARTICLE The influence of the subjects’ training state on the glycemic index

S Mettler, F Lamprecht-Rusca, N Stoffel-Kurt, C Wenk and PC Colombani

Department of Agricultural and Food Sciences, ETH Zurich, Zurich, Switzerland

Objective: To determine the glycemic index (GI) dependence on the training state of healthy adult males. Subjects and design: Young, adult males of normal body mass index and normal glucose tolerance were tested twice with a 50 g reference glucose solution and twice with a breakfast cereal containing 50 g of available carbohydrates in a randomized order. Ten subjects were sedentary (SE), 12 were moderately trained (MT) and 12 were endurance trained (ET). Blood glucose, insulin and glucagon were measured. Results: The GI differed significantly between SE and ET subjects (P ¼ 0.02, mean difference: 23 GI units, 95% CI ¼ 3–42 GI units). The GI of the MT subjects was intermediary, but did not differ significantly from the SE or ET subjects. The insulin index did not differ significantly between the groups (P ¼ 0.65). Conclusion: The GI of the commercially available breakfast cereal depended on the training state of the healthy males. The training state is the first reported factor influencing the GI that is subject specific rather than food specific. European Journal of Clinical Nutrition (2007) 61, 19–24. doi:10.1038/sj.ejcn.1602480; published online 12 July 2006

Keywords: carbohydrates; glucose; exercise; glycemic index; insulin; insulinemic index

Introduction or absolute glycemic response (Wolever et al., 2003). An important subject specific factor, a subject’s training state or The concept of the glycemic index (GI) was introduced by fitness, however, has not been investigated since the GI’s Jenkins et al. 25 years ago (Jenkins et al., 1981). Since then introduction 25 years ago. Thus, we recently have explored the GI concept has become widely used and even the Food whether the GI might differ according to the training state of and Agriculture Organization/World Health Organisation healthy people, and were surprised to find a substantial recommends the use of the GI (FAO/WHO, 1998). A food’s (25 GI units) variation in a commercially available breakfast GI is a percentage representing the area under the blood cereal’s GI when comparing endurance trained (ET) to glucose response curve (over 2 h and measured at defined sedentary (SE) subjects (Mettler et al., 2006). To our knowl- intervals) relative to the blood glucose response curve of a edge, this was the first report of a subject-specific factor reference food (glucose or white bread) (FAO/WHO, 1998). impacting the GI. As this result would challenge the actual One of the GI’s characteristics is that it describes a specific prescription of the GI determination, and to some extent the food and does not seem to be related to subject-specific existing GI values, it was mandatory to validate the results factors such as age, gender, body mass index (BMI), ethnicity obtained in our recent study. Therefore, this study’s main aim was to repeat the GI determination of a breakfast cereal with SE and ET subjects. Secondary aims were to identify Correspondence: S Mettler, Department of Agricultural and Food Sciences, possible reasons for the GI’s dependence on the training state ETH Zurich, Universita¨tsstrasse 2, ETH Zentrum – LFH A2, Zurich CH-8092, Switzerland. and to include a group of moderately trained(MT) subjects in E-mail: [email protected] order to detect a potential dose–response relation between Study data have not been previously published and are not currently training state and GI. submitted for publication. Guarantor: PC Colombani. Contributors: SM was responsible for and coordinated the execution of the study and laboratory analysis, performed the statistical analysis and drafted Materials and methods the manuscript. SM, CW and PCC obtained the funding, planned and designed the study. SM, FLR and NSK executed the study. PCC edited the Study design manuscript and FLR, NSK and CW commented on the manuscript. Received 6 January 2006; revised 24 May 2006; accepted 8 June 2006; The study was carried out in a random crossover order and published online 12 July 2006 consisted of a duplicate assessment of the reference food Glycemic index and training state S Mettler et al 20 (glucose) and of a commercially available breakfast cereal. At GmbH, Bitz, Germany). The subjects were asked not to have least 7 days before the first of these four trials, which were any large meal within 4 h of the test and not to have any

separated by 3–7 days, each subject performed a VO2max small snack within 2 h of the test. The physically active test. The study’s design was approved by the ethical subjects were asked not to compete the week before the test committee of the ETH Zurich. All participants of the study and not to train vigorously in the 2 days before the test. The gave their written, informed consent. protocol consisted of a 5 min warm-up at 50 W, a start power of 80 and a 15 W increment every 30 s (Buchfuhrer et al., 1983; Zhang et al., 1991). The pedal frequency was not Subjects standardized during the warm-up, but it was set at or above All subjects were young (inclusion criteria: age 18–40 years), 70 (73) r.p.m. throughout the test. Oxygen uptake was non-smoking, normoglycemic (glycosylated hemoglobin measured breath by breath with the Quark b2 System (HbA1c): 4.8–5.9%; fasting blood glucose o5.6 mmol lÀ1) (COSMED, Rome, Italy). The highest oxygen uptake reached, À2 males of normal BMI (19–25 kg m ). None of the subjects’ averaged over 20 s, was designated as the VO2max. All nearest relatives had diabetes. Forty-one subjects who subjects reached a VCO2/VO2 above 1.1 and a subjective fulfilled all inclusion criteria were divided into three groups rating of at least 18 on the BORG scale. according to their reported training volume. Seven subjects dropped out before completing all trials because of illness, unexpected change in their workplace or other personal Study meals reasons, such as not feeling comfortable with the arm vein The reference food for the GI determination was glucose catheter or inaccurate adherence to the standardization (50 g dissolved in 200 ml water) and the commercially procedure. The remaining subjects consisted of 10 SE males available breakfast cereal was Kellogg’s Special K (Kellogg who were neither involved in any exercise-like activity in AG, Zug, Switzerland). The subjects were served 57.6 g of the their leisure time nor engaged in any physically intense cereal with 150 ml of partially skim milk (2.8% fat), so that occupational activity, 12 MT males who exercised two to the test meal provided a total of 50 g available carbohydrates, three times per week and 12 ET males who trained at least 14 g protein, 4.6 g fat and 1.7 g fiber. An additional 200 ml four times per week, and some of them being competitive tap water was served with both meals. The amount of athletes (Table 1). The subjects’ physical activity level had to available carbohydrates was calculated according to nutri- be constant for at least 6 months before the start of and tional information provided by the manufacturer and throughout the study. additionally verified by analysis of the digestible (available) starch and carbohydrate as well as indigestible fibers with Anthropometry. Body mass was measured in the fasted state the AOAC/AACC method (McCleary et al., 1997, 2002) with light clothing (underwear and t-shirt) on a scale (Multi using enzymatic kits of Megazyme International (Wicklow, Range, Sauter GmbH, Albstadt 1-Ebingen, Germany). Height Ireland). was measured in the morning and body fat was measured with a skinfold caliper (Harpenden Skinfold Caliper, British Indicators, West Sussex, UK) at three skinfolds (chest, Experimental procedures abdominal, femoral). Body density and body fat were In addition to maintaining their habitual physical activity calculated according to Jackson et al. (1980) and Siri (1993). level throughout the entire experimental period, each subject’s physical activity level was standardized for 2 days

VO2max. The maximal oxygen uptake (VO2max) was before each trial according to the subject’s habitual lifestyle determined on a cycle ergometer (Ergoline 800S, Ergoline and activity pattern. Thus, the SE subjects refrained from

Table 1 Characteristics of the subjects (mean7s.d.)

SE (n ¼ 10) MT (n ¼ 12) ET (n ¼ 12)

Age (year) 24.374.1 25.073 22.871.9 BMI (kg mÀ2) 21.671.4 22.871.9 22.971.9 Body fat (%) 11.974.7 13.077.0 8.472.9 À1 À1 A A B VO2max (ml min kg ) 42.676.7 47.178.0 55.776.6 Training (sessions weekÀ1)070A 2.570.5B 7.671.7C Total training volume (min weekÀ1)070A 163755B 6597192C Endurance volume (min weekÀ1)070A 82734B 4957187C Fasting capillary glucose (mmol lÀ1) 4.9670.31 4.9370.32 4.8570.32 Glycosylated hemoglobin HbA1c (%) 5.370.3 5.570.4 5.270.2

BMI, Body mass index; ET, endurance trained; MT, moderately trained; SE, sedentary. A,B,CDifferent letters denote a significant difference between the subject groups (Po0.05).

European Journal of Clinical Nutrition Glycemic index and training state S Mettler et al 21 physical activity above their normal level during the 2 days Calculations and statistical methods preceding each trial. In contrast, the MT subjects were not The incremental area under the curve (IAUC) values for allowed to exercise the day before each trial, and the ET glucose and insulin were calculated geometrically, ignoring subjects were not allowed to perform any vigorous exercise areas below the fasting value (Brouns et al., 2005). The IAUC during the 24 h before each trial. was determined for each test. The values of the two reference All subjects semiquantitatively recorded their habitual diet glucose solutions and the two cereal trials were averaged, the day before their first trial by estimating their serving and the GI and insulin indices were calculated by dividing sizes. To replicate their pretrial diet, the subjects received the average cereal IAUC by the average reference glucose a copy of their dietary protocol and were asked to follow solution IAUC. and report it again. A dinner, calculated to provide about Statistical analysis was performed with SAS for Windows one-third of the subject’s daily energy requirements, was (version 8.2, SAS institute Inc., Cary NC, USA) using analysis provided to all subjects (pasta with tomato sauce, apple of variance with Bonferroni adjustment for multiple com- puree and cheese). No food was permitted after dinner until parisons. Data are presented as mean7s.e., unless otherwise the beginning of the trial. Water was allowed ad libitum until stated. A P-value of o0.05 was considered significant. bedtime and restricted to one glass in the morning until 1 h before the trial. Alcoholic drinks were restricted to not more than two glasses and drinks containing caffeine to not more Results than two cups of coffee or 1 l of soft drinks containing caffeine the day before the trial. The fasting values of all parameters (capillary and venous The subjects arrived at the laboratory between 0745 and glucose, insulin, glucagon, subjective well-being and satiety 0830 hours after an overnight fast of at least 10 h, and just rating) were neither statistically different between the after arrival a catheter was placed into an antecubital vein. subject groups nor between the trials. All subjects were Thereafter, adherence to the pretrial standardization was normoglycemic and there was no period effect on any controlled by questionnaire, and subjective well-being and parameter. satiety scores were evaluated with a visual analog scale. At The GI derived from capillary blood, which is the the next step and about 10–15 min after catheter placement, recommended method for GI analysis (Wolever et al., 2003; the fasting blood samples were drawn. Venous blood (6 ml) Brouns et al., 2005), was significantly lower for the ET was taken from the catheter with an ethylenediaminetetraa- subjects than for the SE subjects (P ¼ 0.02, mean difference: cetic acid containing S-Monovette (Sarstedt AG, Sevelen, 23 GI U, 95% CI ¼ 3–42 GI U; Figure 1). The MT subjects’ Switzerland) for glucose, insulin and glucagon analysis. capillary GI value was between the two other groups, but did Capillary blood for glucose analysis was taken from the not differ significantly from either of them (Figure 1). The GI opposite arm simultaneously to the venous blood sampling. calculated from the venous glucose (P ¼ 0.73) and the insulin (This procedure was repeated after 15, 30, 45, 60, 90 and index (P ¼ 0.65) did not differ between the groups (Figure 1). 120 min according to the standard GI measurement proce- Capillary and venous glucose peak time, as well as insulin dure (FAO/WHO, 1998).) Just after the fasting blood samples peak time, did not differ between the groups or the trials. were taken, the reference glucose solution or the breakfast The postprandial glucagon response differed neither be- cereal was served. Both the reference solution and the tween the groups nor the trials (data not shown). The breakfast cereal had to be eaten within 8 min at most. average coefficients of variation (CV) for the duplicate IAUC determinations of the glucose trials were 0.21 (capillary glucose), 0.25 (venous glucose) and 0.23 (insulin). For the Analysis of blood parameters cereal trials, the average CV were 0.26, 0.26 and 0.22. Capillary whole-blood glucose was measured with the Glucotrend 2 system (Roche Diagnostics GmbH, Mannheim, Germany). The 6 ml venous blood sample was stabilized with 300 ml Trasylol (Bayer, Zurich, Switzerland), put on wet ice Discussion and centrifuged for 15 min at 1800 g in a cooled centrifuge (Omnifuge 2.0 RS, Heraeus Sepatech, Osterode, Germany) A breakfast cereal’s GI was determined in young male within 15 min. Then, the plasma was dispersed into poly- subjects who differed in their training states. Similar to our propylene tubes and frozen at À801C. Insulin and glucagon previous study, which found a GI difference of 25 U with a were prepared with a multiplex immunoassay (Linco breakfast cereal between SE and ET subjects (Mettler et al., Research, St Charles, MO, USA) according to the manufac- 2006), the present study found that the cereal’s GI was turer’s standard protocol and were read on a Luminex 100 significantly higher (P ¼ 0.02) with the SE subjects than with (Bio-Rad Laboratories AG, Reinach, Switzerland). Venous the ET subjects (mean difference 23 GI units, 95% CI ¼ 3–42 glucose was measured by an enzymatic colorimetric method, GI units). The results available today suggest that the GI of at using a centrifugal analyzer (Cobas-Mira, Roche, Basel, least some breakfast cereals may be higher with SE subjects Switzerland). than with ET subjects. The GI of the MT, which was between

European Journal of Clinical Nutrition Glycemic index and training state S Mettler et al 22

Figure 1 Capillary and venous glucose and insulin responses to ingestion of a glucose solution and a breakfast cereal (mean7s.e.). SE ¼ sedentary; MT ¼ moderately trained; ET ¼ endurance trained; IAUC ¼ incremental area under the curve. A,B,CDifferent letters indicate significant differences between the groups (Po0.05). *Significant difference of the two trials within a group (Po0.05). ŁSignificant difference between the groups for this time point (Po0.05).

the GI of the SE and the ET, supports the notion that GI Another possible reason that GI could depend on the depends on the subject’s training state. training state could be the insulin response curve’s pattern, rather than the area under the curve. This notion is supported by a recent study, in which the peak time of the Insulin insulin response, but not the area under the curve, was The most obvious reason for a difference in the GI of SE and related to a GI difference between two different breakfast ET subjects could be a different insulin metabolism accord- cereals (Schenk et al., 2003). A divergent insulin secretion ing to the training state. It is often believed that endurance could theoretically be related not only to a food but also to a training leads to improved insulin sensitivity (Rogers et al., subject. A stronger insulin boost in the first postprandial 1990; Nassis et al., 2005), thereby influencing the glycemic minutes with an earlier insulin peak in one of the two trials response. However, improved insulin sensitivity should and in only one of the subject groups could have lead to a improve the glycemic response to both a glucose solution relatively different glucose behavior, and thus a training and a food containing carbohydrates, thus not impacting state-dependent GI in the present study. However, the the relation between the two glycemic responses (i.e., the insulin data do not support this hypothesis, either, and GI). Accordingly, in the present study, the absolute insulin cannot explain the different GI values between the SE and ET response, which was calculated as the insulin area under the subjects. curve according to the GI formula, is significantly higher in the SE subjects than in the MT and ET subjects (Figure 1), whereas the insulin index, as the relative insulin response Design and other issues between the glucose and the cereal, was not significantly This study was designed to represent the habitual lifestyle different (P ¼ 0.65) between the SE, MT and ET subjects of subjects with different physical activity levels. As a (Figure 1). consequence, a different standardization between the sub-

European Journal of Clinical Nutrition Glycemic index and training state S Mettler et al 23 ject groups was necessary. It would not have made sense to Conclusion and future prospects prescribe an exercise session the day before the trials to the usually SE subjects, nor would it have been sound to prohibit This study’s aims were to validate our previous results, which all types of exercise to the more or less daily active ET showed that GI may depend on the subjects’ training state, subjects. However, it is certainly important to standardize and to identify potential reasons for this. The present results within each subject and within each group. seem to confirm the previous results and contribute to the This study cannot determine whether the training actual understanding of the factors affecting the glycemic state’s observed effect is mainly of chronic origin or is response and the GI. From the methodological viewpoint it owing to the last exercise bout only (acute origin). However, seems mandatory to further investigate possible impacts of physical activity of different intensity than usual performed subject-specific factors on the GI. the day before measuring postprandial glycemia does So far, the training state of young male subjects has been not seem to influence postprandial glycemia (Young et al., shown to affect the GI of breakfast cereals. It is now 1989; Campbell et al., 2003). Neither is the within-subject important to expand this knowledge not only to different variation in the postprandial glycemic response reduced foods but also to other population groups such as female or when controlling or not controlling the subject’s diet and elderly subjects, who might have insulin sensitivity or physical activity on the day before measuring the postpran- glycemic responses different from male or young subjects dial glycemia (Campbell et al., 2003). This suggests that (Rogers et al., 1990; Unwin et al., 2002; Borissova et al., 2005). the absolute glycemia is, at least in young and healthy subjects, probably not primarily affected by the acute effects of physical activity. However, because the GI is a relative glycemic response within a specific subject, an Acknowledgements imaginable acute effect of physical activity on the absolute glycemic response between different subjects is secondary We thank Astrid Peterhans, Fabienne Schwitz and Patricia anyway. Kressig for their medical assistance, and the Swiss Founda- As several studies indicate that the GI is independent of a tion for Nutrition Research and the Federal Council of Sports subject’s glucose tolerance or diabetes (Brouns et al., 2005), for partially funding this study. it might appear that physical activity, which may influence the glucose tolerance (Rogers et al., 1990), might also be unrelated to the GI. 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