The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2659S-2662S

Measurement of Postprandial Incremental Glucose and Insulin Changes in Healthy Dogs: Influence of Food Adaptation and Length of Time of Blood Sampling^1,2

Patrick Nguyen³, Henri Dumon, Vincent Biourge^*, and Etienne Pouteau

Department of Nutrition, École Nationale Vétérinaire de Nantes, 44307 Nantes Cedex 03, France and ^* Royal Canin Research Centre, 56007 Vannes Cedex, France

KEY WORDS: dog foods · glycemic response · insulinemic response · dogs

	INTRODUCTION

Introduction References

Variations in the blood glucose and insulin responses to different foods have been studied in dogs (Holste et al. 1989, Nguyen et al. 1994), and different trends in postprandial concentrations have been observed. These differences arose in the extent of the variations (areas under the curves and maximal increments) and the time from meal to peak increases. The main purpose of these studies was to rank foods on the basis of the incremental glucose responses that they produced and to relate these responses to foods characteristics. It is known that methodologic variables can markedly modify the interpretation of the glycemic response. In particular, this concerns the length of time of blood sampling (Gannon and Nuttall 1987), short-term (Wolever and al. 1988) and long-term (Cannon and al. 1980) remnant effects of the previous meal, blood sampling (Jackson and al. 1983) and fasting blood glucose values (Nielsen and Nielsen 1989).

In dogs, the variations of the glycemic response have been evaluated with (Holste et al. 1989) or without (Nguyen et al. 1994) an adaptation period to the tested meals. As in human beings, the plasma concentrations were measured over a period of 3 or 4 h even though the gastrointestinal transit time is shorter in dogs than in humans.

View larger version (24K): [in this window] [in a new window]   Fig 1. Postprandial glucose response curves from time-0 values before and after a 15-d adaptation period in healthy dogs fed two different foods (n = 6).

The purpose of our study was to examine whether an adaptation time (inducing digestive changes and modifications in basal insulin secretion and glucose tolerance) modifies the postprandial response to meal feeding in normal dogs. We also studied the effect of length of time of blood sampling on the consistency of the response value expressed by the areas under the glycemic and insulin curves, the peak incremental values and the times from meal to peaks.

Materials and methods.  Animals. Twelve adult (older than 15 mo) beagle dogs, allotted to two groups, were studied, according to the French Ministry of Agriculture and Fisheries regulatory rules for animal welfare. None of the dogs was obese (13.7 kg mean body weight) and all were clinically normal. Their basal plasma glucose (Experiment 1: 5.41 ± 0.54 mmol/L; Experiment 2: 5.27 ± 0.55 mmol/L) and their response to the intravenous glucose tolerance test (performed after a 24-h period of food deprivation), using a glucose dose of 500 mg/kg body weight, infused as 50% glucose solution in 30 s, were also normal. These dogs were accustomed to the experimental procedure. They were commonly used for digestibility trials in the cages used in this study and had been previously subjected to repeated venipuncture. Therefore, their responses were due to the experimental variables and not to stress.

Experimental diets. Two experiments were performed according to the two objectives.

Experiment 1. Five experimental foods (A1, A2 and B1-B3 foods) were tested. Foods A1 and A2 were canned foods, whereas B1-B3 were dry. Foods B1 and B3 were given as is, with separate water, whereas food B2 was hydrated before feeding. These test foods were intended to be representative of foods currently used for maintenance in adult dogs. They were designed to vary in macronutrient composition [27.2-62.6% crude protein (CP),⁴ 10.9-25.7% ether extract (EE), 1.1-3.6% crude fiber (CF), 4.4-50.5% nitrogen-free extract (NFE) and 3.2-12.4% total dietary fiber (TDF), on a dry matter basis]. The energy profile was as follows: 25-54% energy derived from protein, 25-51% from fat and 4-48% from nitrogen-free extract, according to standard energy conversion factors, 14.6 kJ/g CP or NFE, 35.6 kJ/g EE). The composition of the test meals is shown in Table 1. The daily chromium intake was not <100 µg per dog. (A requirement for chromium to maintain normal glucose tolerance has not been shown in dogs. However, a daily allowance of 100 µg per dog per day was assumed to be required according to a human recommendation of 50-200 µg/d.)

Experiment 2. Eleven experimental foods (C1-C5 and D1-D6 foods) were tested. Foods D1-D6 were dry foods, whereas C1-C5 were canned. These test foods were intended to be representative of foods currently used for maintenance or for clinical purposes in adult dogs. They were designed to vary in macronutrient composition [16.7-62.6% CP, 7.9-28.4% EE, 3.2-32.7% TDF and 0.4-35.9% starch (ST), on a dry matter basis]. Details of the diets consumed in this experiment were described by Nguyen and al. (1998). The daily chromium intake was not <100 µg per dog.

Design and procedures. Before the initiation of the study, the dogs consumed the same dry food in a single daily meal consisting of 29.3% CP, 11.2% EE, 11.0% TDF and 27.8% ST on a dry matter basis for at least 2 wk.

Experiment 1. On d 1, after a 24-h period of food deprivation, the dogs were again given one of the five experimental foods on the morning of the test day in a single meal (Table 1). Each dog was placed in a separate cage 30 min before feeding time, and a basal blood sample was obtained from the jugular vein. Each dog consumed all of the food in <5 min. Further jugular vein blood samples were collected at the end of the meal (time 0) and 5, 10, 15, 20, 30, 45, 60, 90 and 120 min after. Then, the dogs were offered daily, for 15 d, the same amount of the same experimental diet. The experimental procedure was repeated on d 15.

Experiment 2. The procedure was the same as that in Experiment 1 but used the eleven mentioned foods. In all cases, the size of the meal was determined by the energy requirement of the individual dog calculated using an allowance of 552 kJ metabolizable energy per kilogram metabolic body weight (BW^0.75).

In both experiments, blood samples were taken in 5-mL heparinized vacuum collecting tubes and immediately refrigerated on ice. They were centrifuged at 600 × g for 15 min to separate the plasma, which was stored at 20°C until analyses. Plasma glucose concentrations were determined by an enzymatic kit (Glucose GOD-PAP, Boehringer-Mannheim, Germany). Plasma insulin was measured by RIA using a commercially available kit (human insulin as standard; Insik-5, Sorin Biomedica, Saluggia, Italy).

Calculations and statistics. Changes in serum glucose and insulin concentration were calculated separately for each postmeal period by using the plasma concentration before the meal as a baseline. Postprandial responses were compared for maximum increase, time to peak increase and incremental area under the glucose (AUCG) and insulin (AUCI) curves for each food. The integrated area under the postprandial glucose and insulin response curves was calculated by the trapezoidal method. Area increments under the curves for a given food were determined for the 3-h period after the meal.

The Multifit 2.01 software (Day Computing, Cambridge, UK) was used for the area under the curve computing. Correlation between the glycemic and insulin responses at different times were assessed by linear regression analysis. Glycemic and insulin response means were determined for all foods. Statistical comparisons were performed by two-way Anova. When the F-test indicated a significant effect, the differences between the means were analyzed by a protected least significant difference (LSD) test (Fisher 1949). Differences were considered significant at P < 0.05. The statistical software used was SuperAnova, version 1.11, (Abacus Concepts, Berkeley, CA).

Results. 

Experiment 1.  As an example, the glucose response curves to foods A1 and B3 before and after a 15-d adaptation period are shown in Figure 1. The 0-120 min AUCG and AUCI before and after a 15-d adaptation period are shown in Table 2. The canned foods induced a rapid decrease in blood glucose. The subsequent AUGC was slightly positive because of the positive time-0 value and because the calculation method takes into account only the area above the basal value. The foods B1 and B3 induced a transitory decrease in blood glucose followed by a persistent high increment. The food B2 induced an initial increment followed by a decrease under the basal value. There was no significant difference for the same food in area under the glycemic or insulinemic curve between d 1 and 15; neither was there a significant difference for the same food in glucose maximal increment or time to peak or in insulin maximal increment or time to peak between d 1 and 15.

Experiment 2.  The trend of the glucose response curve was the same as in Experiment 1 for canned and dry foods. Characteristics of plasma glucose and insulin responses 90, 120 and 180 min after meal feeding are shown in Table 3. The 90, 120 and 180 min areas under the curve (AUC) were correlated for glucose (90 vs. 120 min, r = 0.99; P < 0.01; 90 vs. 180 min, r = 0.94; P < 0.01) and for insulin (90 vs. 120 min, r = 0.97; P < 0.01; 90 vs. 180 min, r = 0.85; P < 0.01). Values were also correlated for 90 vs. 120 min and for 120 vs. 180 min maximal increment in glucose (r = 0.96 and 0.98, respectively; P < 0.01) and in insulin (r = 0.86 and 0.96, respectively; P < 0.01) and for time to glucose (r = 0.92 and 0.69, respectively; P < 0.01) and insulin (r = 0.87 and 0.82, respectively; P < 0.01) peak.

Discussion.  Feeding the same food to healthy adult dogs for 15 d did not induce any significant difference in areas under the glycemic and insulin curves measured at the start and the end of the period. In contrast, previous meals have been shown to have some effects in humans. The trend of the curves, especially the characteristics of the maximal increments, was also not significantly different between the first and the second measurements. Short- and long-term studies have suggested a sustained metabolic effect of slowing the absorption of carbohydrates. Low glycemic index carbohydrate eaten at dinner reduces the glycemic response to the subsequent breakfast (Wolever et al. 1988). The long-term effect was suggested from results of studies in which diabetic patients were treated with viscous fiber with consequent reduction in fasting blood glucose levels and insulin requirements (Ray et al. 1983). Rapidly absorbed carbohydrates strongly stimulate the insulin secretion, which induces a rapid decrease in blood glucose. This decrease could induce the secretion of counterregulatory hormones (such as glucagon, growth hormone or cortisol), the release of fatty acid and, hence, an impaired glucose tolerance and an insulin resistance. However, day-to-day variation of the glycemic response in diabetic subjects has been shown to be negligible, and this permits the estimation of the glycemic response after a single meal when near-normal fasting blood glucose concentration is ensured (Rasmussen 1993). More than day-to-day variations, the preprandial blood glucose values have been shown to be of great importance in the interpretation of the area under the glucose response curve (Nielsen and Nielsen 1989).

The absence of long-term effects in dogs may be due to their feeding behavior and gastrointestinal transit characteristics. Feeding dogs a large meal once daily is consistent with their original carnivorous predator status. The gastrointestinal transit time in dogs is shorter than that in humans. Thus, dogs must face sudden large flows of food and nutrients, and, in the long term, they do not adapt their digestive and metabolic pathways to the quality of these flows.

The 90, 120 and 180 min AUC were correlated for glucose as well as for insulin. Thus, a duration of blood sampling >90 min appears to be unnecessary to characterize the glycemic and insulin responses to meals in normal dogs.

The effects of the length of time of blood sampling on the relative area under the glucose curve of foods in humans has been reviewed (Gannon and Nuttall 1987). Long-time measurements tend to reduce the differences in AUCG between foods, especially between foods resulting in high peak rises followed by a rapid decrease, and foods for which the glucose response presents a lower peak but tends to remain above the baseline for a prolonged period of time. If measured for too long a time, the area under these types of curves may be nearly the same, despite markedly different effects on insulin and counterregulatory hormone responses. These factors are related to the acute rate of carbohydrate absorption, which in turn is related to the incremental area under the early part (120 min) of the glucose response curve in normal subjects (Wolever et al. 1991).

A shorter duration of blood sampling could be sufficient in normal dogs because of their high gastrointestinal transit rate. The relatively rapid gastric emptying and digestive processes allow an early expression of the acute effects of foods on the glucose response curve. Increasing the period of blood sampling beyond 90 min did not modify the discrimination between foods.

We concluded that the diet consumed before the initiation of the study had not influenced the response to test foods and that an adaptation time appears to be unnecessary. The postprandial glycemic and insulin responses of normal dogs may be obtained after a single meal. We also conclude that a 90-min blood sampling period was sufficient to characterize the glycemic and insulin responses in normal dogs.

	FOOTNOTES

	LITERATURE CITED

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