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Original Research Communications |
1 From the Department of Pediatrics, Laboratory of Nutrition and Metabolism, University Hospital and University of Groningen, Groningen, Netherlands.
2 Supported by grant no. STW-NWO GGN.4487 from the Dutch Foundation of Technical Sciences.
3 Address reprint requests to RJ Vonk, Laboratory of Nutrition and Metabolism, Laboratory Center CMC V, Y2147, University Hospital Groningen, PO Box 30.001, 9700 RB Groningen, Netherlands. E-mail: r.j.vonk{at}med.rug.nl.
| ABSTRACT |
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Objective: To study the characteristics of the fermentation of resistant starch, the digestion of resistant starch in the small intestine has to be quantified. We compared the metabolic fates of highly digestible cornstarch (DCS), Hylon VII (type 2 resistant starch), and Novelose 330 (type 3 resistant starch), which are of corn origin and, therefore, naturally enriched in 13C.
Design: After administration of 40 g starch or glucose to 7 healthy volunteers, glucose and exogenous glucose concentrations in serum and 13CO2 excretion in breath were analyzed for 6 h. 13C abundance in carbon dioxide was analyzed by isotope ratio mass spectrometry (IRMS) and 13C abundance in glucose by gas chromatographycombustion IRMS.
Results: By comparing the area under the curve (2 h) of exogenous glucose concentration in serum (13C glycemic index) after intake of starch or glucose, 13C glycemic indexes for DCS, Hylon VII, and Novelose 330 were calculated to be 82 ± 23%, 44 ± 16%, and 43 ± 15%, respectively. Comparison of 6-h cumulative percentage dose recovery in breath showed that 119 ± 28% of DCS, 55 ± 23% of Hylon VII, and 50 ± 26% of Novelose 330 was digested in the small intestine.
Conclusion: The exogenous glucose response in serum and the 13CO2 excretion in breath can be used to estimate small intestinal digestion of resistant starch, which amounts to
50%.
Key Words: Resistant starch glycemic index glucose digestion stable isotopes healthy subjects adults
| INTRODUCTION |
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| SUBJECTS AND METHODS |
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1 of the 3 starches. Glucose was used as an intraindividual standard. Five volunteers participated in >2 tests. The substrates were studied in a parallel group design. For 6 h, the glucose concentration and the 13C-12C ratio in serum and the 13C-12C ratio in breath carbon dioxide were analyzed. From the serum glucose response, the glycemic index was calculated according to the method of Jenkins et al (17). From the serum exogenous glucose response, the 13C glycemic index was calculated (see Calculations). A digestion index was also calculated from the 13CO2 excretion data (13CO2 glycemic index). The protocol was approved by the Medical Ethical Committee of the Faculty of Medical Sciences of the University of Groningen.
The subjects were asked to not consume 13C-rich foods such as corn products, cane sugar, and pineapple for 2 d before the test. Furthermore, the subjects fasted from 2200 the evening preceding the test until the start of the experiment at 0800 the next day. During the test, they refrained from consuming food and energy-containing drinks and remained seated during the 6-h test period. The experiments were performed with intervals of
48 h between the tests to wash out residual 13C from the previous test. Forty grams of each substrate was dissolved in 200 mL low-fat milk
1 h before the start of the test: DCS, 13C enrichment relative to Pee Dee belemnite
(13CPDB) = -12.19
; Hylon VII,
13CPDB = -12.33
; or Novelose 330, 13CPDB = -11.44
. Glucose derived from corn had a
13CPDB value of -14.93
. Flavorings and sweeteners (without energy) were added to make the test meal palatable. Finally, gelatin was added to the mixture to stiffen it and to get equal consistencies for raw and cooked starch test meals (not described in this article).
Blood sampling and breath collection
Two-milliliter blood samples were collected before, at 15-min intervals during the first hour, every 30 min during the second and third hour after the meal, and every hour until the end of the experimental period (6 h). For this purpose, an intravenous catheter was placed in the cubital vein of one of each subject's arms. Blood was collected in evacuated containers containing 5 mg sodium fluoride and 4 mg potassium oxalate (Becton Dickinson, Meylan Cedex, France). Blood was centrifuged at 900 x g for 10 min and plasma was stored at -20°C until analyzed. Breath samples were collected in triplicate in 10-mL gas collection tubes (Exetainers; Labco, High Wycombe, United Kingdom) at 15-min intervals during the first 2 h and every 30 min thereafter until the end of the experiment. Breath air was blown into the gas collection tubes with a straw.
Analytic procedures
Breath 13CO2 analysis
The procedure to analyze 13C enrichment in breath in our laboratory was described previously (18). During the 6-h period, the total carbon dioxide exhalation rate was measured 3 times at 1.5 h intervals with indirect calorimetry (Oxycon; Dräger, Breda, Netherlands).
Plasma exogenous glucose determination
After serum samples were thawed, one aliquot was analyzed for glucose concentration by applying routine techniques using an ECA-18 glucose analyzer (Medingen, Dresden, Germany) and a second aliquot was prepared for analysis of the 13C-12C ratio. The method for calculating exogenous glucose was first described by Normand et al (13); we used the same method with some modifications. To denature plasma proteins, 100 µL plasma was added to 1 mL ethanol. After mixing and centrifugation (900 x g for 10 min at room temperature), the supernate was transferred to 2-mL vials. After the supernate was evaporated to dryness, sugars were derivatized to the pentaacetate derivative by using 75 µL acetic acid anhydridepyridine (10:5 by vol) and reacting for 1.5 h at room temperature. After evaporation of the reagent, the derivatives were dissolved in 500 µL chloroform.
The 13C-12C ratio of glucose was determined by using gas chromatographycombustion isotope ratio mass spectrometry (GC-CIRMS) with a Delta S/GC instrument (Finnigan MAT, Bremen, Germany). The GC conditions were as follows: 2 µL of the chloroform solution was injected in the splitless mode onto a 25 m x 0.32 mm (0.2-µm film thickness) OV1701 column (CP Sil 19CB; Chrompack, Middelburg, Netherlands) installed in a Varian 3300 gas chromatograph. The oven temperature was programmed from 100°C (1 min) to 275°C (2 min) at a rate of 30°C/min. Helium was used as the carrier gas at a column head pressure of 138 Pa. Eluting compounds were combusted on-line in a platinum-catalyzed cupric oxide oxidation reactor operating at 800°C. Solvent and compounds eluting before 4 min were kept out of the reactor by applying a helium backflush gas flow. Water vapor was removed by nafion tubing (DuPont, Wilmington, DE) and the carbon dioxide pulses formed in the reactor were transferred to the IRMS through an open split interface.
The 13CO2-12CO2 ratio measured by IRMS was corrected for 17O abundance (19) and the final 13C-12C ratio was expressed as
13CPDB (20). The
13CPDB values of the glucose and starch substrates were determined by total combustion using an on-line coupled elemental analyzer.
Calculations
The 13CPDB value was converted to the atom % (AP) value. The AP values after ingestion of substrate were corrected for the baseline abundance. The difference [atom % excess (APE)] was used for further calculations. For breath carbon dioxide, the APE value was related to the carbon dioxide excretion measured by indirect calorimetry to calculate the substrate-derived carbon dioxide exhalation rate. This exhalation rate was calculated as the percentage dose recovered (PDR)/h. Total recovery after 6 h was calculated as the area under the PDR/h-time curve and expressed as the cumulative percentage dose recovered (cPDR). The
13CPDB of the plasma glucose pentaacetate derivative was converted to the 13C AP. The concentration of glucose derived from the [13C]starch (exogenous glucose) in plasma was calculated as follows:
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where [total glucose] is glucose concentration in plasma (mmol/L), AP 13Ct is the AP 13C of plasma glucose pentaacetate at time point t after ingestion of [13C]starch, AP 13Ct = 0 is the AP 13C of plasma glucose pentaacetate before ingestion of [13C]starch, AP 13Csubstrate is the AP 13C of the administered starch, AP 13Cbody is the AP 13C of endogenous glucose, expressed as the basal 13C abundance in breath, and 2.67 is the factor to correct for the dilution of glucose 13C abundance by the 13C abundance of the derivatizing acetate carbon atoms.
Glycemic indexes
The classic way to substantiate starch digestion is by measuring the glycemic index as described previously by Jenkins et al (17). This implies measuring the area under the curve (AUC) of the serum glucose concentration over the first 2 h after administering a starch and dividing this by the serum glucose response after consumption of an equal amount of glucose. The 13C glycemic index was calculated as the ratio of the AUC of the exogenous glucose curve (t = 2 h) after starch consumption and the equivalent area obtained after consumption of glucose, including only the areas above fasting concentrations. The 13CO2 glycemic index was calculated as the ratio of the 6-h cPDR in breath of starch to that of glucose.
Statistics
Statistical evaluation of the differences between group means was made by using Student's t test. P < 0.05 was considered to indicate significance. The statistical analyses were performed by using EXCEL '97 (Microsoft Corp, Redmond, WA).
| RESULTS |
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| DISCUSSION |
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We calculated a 13C glycemic index based on comparison with the response to exogenous glucose. This 13C glycemic index was compared with the classic glycemic index; we found that the SDs for the obtained mean values were smaller with the 13C glycemic index than with the classic glycemic index. This may be because the 13C-glycemic-index method refers to exogenous glucose only and the classic glycemic index test cannot discriminate between endogenous and exogenous glucose. Furthermore, the values resemble the percentage of amylopectin in the starch more closely. A clear difference between the response to DCS, which is a highly digestible starch, and that to resistant starch can be found. DCS was digested an average of 80% and resistant starch
50%. This last value agrees with the data of Champ et al (11), who used retrograded amylose starch. [13C]glucose is ultimately oxidized to 13CO2 and excreted via the breath. By measuring the excretion rate and cumulative excretion, the intestinal digestion of starch and the subsequent absorption of glucose can also be studied. This approach was used by Hiele et al (14, 27) to study the digestion of raw crystalline cornstarch. Also, with 13CO2 breath tests [13C]glucose is used as a reference substrate. The assumption is that [13C]glucose derived from a glucose or a starch load undergoes similar metabolic events.
By using this approach, data about intestinal digestion of DCS, Hylon VII, and Novelose 330 were obtained (Table 1
). These data led to the conclusion that DCS is effectively digested and resistant starch is digested
50%. No difference between Novelose 330 and Hylon VII was observed, despite more processing of the first compound. Many intermediate factors, such as differences in insulin response, tissue glucose clearance, and variable glucose oxidation rates, explain the intra- and interindividual differences in the outcome. The overall similarity in final outcomes via the [13C]glucose method and the 13CO2 method is surprising and indicates that the rate-limiting step in the total process of converting raw starch to carbon dioxide is the hydrolysis step.
To optimize the proposed method for use in future studies to characterize factors involved in small intestinal digestion, the differences between the data obtained with exogenous glucose concentration in serum and 13CO2 excretion in breath were highlighted. The results indicate that the differences are smaller when the starch is digested slower. Obviously, with a rapid influx of glucose from the small intestine, the insulin response will be higher and, consequently, there will be storage in tissues. This relation between exogenous glucose, endogenous glucose, and insulin will be studied in detail in future experiments. The results of the present experiment indicate that the kinetics of starch digestion can be derived from the [13C]glucose response in serum as well as the 13CO2 excretion rate in breath and that these measurements can be used as a first screening of the digestibility of a starch source.
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