INTRODUCTION
Energy is important for sustaining the life of dogs. Protein, fat, and carbohydrates are considered the major energy sources, and energy requirements should be met according to individual needs. Among them, carbohydrates provide energy in the form of glucose and offer various health benefits, including supporting a healthy digestive tract, maintaining the gut microbiota, and facilitating effective weight management. In commercial dog feed, carbohydrates constitute 30%–60% of dry food and > 30% of wet food [1,2]. Most of these carbohydrates are starch, with digestibility varying based on factors such as carbohydrate ingredients, granule size, amylose-to-amylopectin ratio, microstructure, diet form, processing methods, degree of heat treatment, and other components within the recipe [3–6].
Carbohydrate sources in pet foods include grains (e.g., corn, sorghum, rice, and wheat), legumes (e.g., peas and lentils), tubers (potatoes and tapioca), and by-products or fractions of these ingredients [6]. Digestible carbohydrates are absorbed in the small intestine, whereas indigestible carbohydrates can be fermatable and non/poorly fermentable in the large intestine. As a digestible carbohydrate, starch provides energy after absorption. Moreover, the starch concentration is related to the quality of pet food by increasing the expansion and binding properties of the food matrix during the manufacturing process, which affects the durability and formulation of pet food. Indigestible carbohydrates can be insoluble or soluble. Fibers are insoluble and fermentable and are used by the gut microbiota to modulate microbiota and gut health. Carbohydrates have recently gained attention because of their positive physiological effects on health. However, there is not enough information about the in vivo utilization of carbohydrates as much as that of proteins in dogs.
Corn, rice, brown rice, barley, and mung beans are frequently used as carbohydrate sources in commercial dry dog foods [7]. To gain insight into nutritional characteristics, a comparison of the in vivo digestibility of these carbohydrate sources is essential. Therefore, in this study, we aimed to compare the digestibility of commonly used carbohydrate sources and evaluate the nutrient utilization efficiency of these carbohydrate sources in dogs, which would provide a foundation for designing feeding matrices that strategically and appropriately meet nutrient requirements.
MATERIALS AND METHODS
The animal study was approved by the Institutional Animal Care and Use Committee of the National Institute of Animal Science (NIAS), Korea (approval number: NIAS2022-0584). In this experiment four spayed and four castrated beagle dogs (aged 4.58 ± 0.14 years) were used. The dogs were individually housed in a room (170 cm × 210 cm) with a consistent room temperature (22 ± 1°C) and relative humidity (60 ± 10%) throughout the experimental period. Water was provided ad libitum. The feeding test comprised a three day acclimatization period followed by a four day adaptation period, and fecal collection was conducted over four days. This process was repeated for each diet group. The dog’s health was monitored daily and cared for by a veterinarian as needed.
The experimental diets were prepared as previously described [8]. The composition of the ingredients was formulated as a completely balanced diet based on the minimum nutrient requirements established by the Association of American Feed Control Officials (AAFCO). Five different carbohydrate sources (barley, corn, rice, mung beans, and brown rice) were used in powdered form and mixed with other ingredients, followed by steaming, molding, cutting, and drying to form pellets. All experimental diets were stored at −20°C and allowed to equilibrate at room temperature for 3 h before feeding. Table 1 shows the composition of the ingredients and the experimental diets. The experimental diet was provided to dogs based on their individual metabolic energy requirement (ME; kcal/day 132 kcal × body weight [BW]0.75 kg) according to the recommendation of AAFCO.
Vitamin and mineral premix supplied per kg of diets: 3500 IU vitamin A; 250 IU vitamin D3; 25 mg vitamin E; 0.052 mg vitamin K; 2.8 mg vitamin B1 (thiamine); 2.6 mg vitamin B2 (riboflavin); 2 mg vitamin B6 (pyridoxine); 0.014 mg vitamin B12; 6 mg Cal-d-pantothenate; 30 mg niacin; 0.4 mg folic acid; 0.036 mg biotin; 1,000 mg taurine; 44 mg FeSO4; 3.8 mg MnSO4; 50 mg ZnSO4; 7.5 mg CuSO4; 0.18 mg Na2SeO3; 0.9 mg Ca(IO3)2.
Digestibility of the experimental diets was assessed using the whole feces collection method. Fecal samples were collected twice daily at consistent times over 4 days and were subsequently frozen at −20°C until analysis. The diets and fecal samples were dried in a forced-air oven at 75°C and subsequently homogenized for further analysis. The chemical compositions of both diets and fecal samples were determined using standard methods established by the Association of Official Analytical Chemists [9].
Nutrient digestibility of the experimental diets was calculated using the following equation:
All statistical analyses were performed using R (version 4.2.3). Analysis of variance (ANOVA) was used to evaluate the data, and post-hoc comparisons were performed using Tukey’s test. The general linear hypothesis testing (glht) function from the multcomp package was used to identify statistically significant differences between groups. Statistical significance was set at p < 0.05.
RESULTS AND DISCUSSION
This study aimed to evaluate the digestibility of commonly used carbohydrate sources in commercial dog food. Carbohydrate sources were selected based on a previous study that reported barley, brown rice, corn, mung bean, and rice as the most commonly used carbohydrate ingredients in dog diets [2]. The experimental diets with barley, brown rice, corn, mung bean, and rice were formulated to be equivalent and it was confirmed that they exhibited no significant differences in chemical composition based on the proximate composition analysis of carbohydrate ingredients and nutritional requirements (Table 1). The nitrogen-free extract (NFE) contents of the carbohydrate ingredients were 40.12% for corn, 38.39% for brown rice, 36.98% for rice, 36.46% for barley, and 35.64% for mung beans (Table 2). Crude fiber (CF) content was highest in mung bean (1.12%), followed by brown rice (0.44%), barley (0.37%), rice (0.25%), and corn (0.03%). NFE represents the soluble carbohydrate, and CF represents the insoluble carbohydrate. Corn contained the highest NFE and lowest CF. Mung beans served as both carbohydrate and protein sources, exhibiting the lowest NFE content and the highest CF content (1.12%). Also, metabolizable energy (ME) intake, and feed conversion ratio (FCR) showed no significant differences among the test diets (Table 3). However, the body weight gain (BWG) was highest in dogs fed the rice-based diet at 462.5 ± 93.90a g. In contrast, the BWG for dogs fed the brown rice and mung bean diets were unchanged at 25.0 ± 75.00b g and 25.0 ± 61.96b g, respectively.
ME was calculated using the following equation: ME (kcal/d) = (crude protein × 3.5) + (ether extract × 8.5) + (nitrogen-free extract × 3.5).
The digestibility of dry matter (DM) was 92.95% for corn, 92.45% for rice, and 91.61% for brown rice, whereas barley and mung beans had lower digestibility values of 88.81% and 80.74%, respectively, as determined by apparent total tract digestibility (ATTD) analysis (Table 4). Crude protein (CP) digestibility was the highest in rice (92.43%), followed by corn (92.07%) and barley (91.87%). No significant differences were observed in the digestibility of crude ash. The ether extract digestibility was highest in barley (96.52%) and lowest in corn (93.62%). Compared to the chemical composition of the carbohydrate sources, DM was the highest in corn, and the digestibility of DM was also the highest in the corn diet. CP content was the highest in mung beans. However, CP digestibility was higher in diets containing rice, corn, and barley. The mung bean diet showed the lowest digestibility in dogs.
ME was calculated using the following equation: ME (kcal/kg) = (CP × 3.5) + (EE × 8.5) + (NFE × 3.5).
NFE digestibility was higher for rice (97.08%), corn (96.14%), and brown rice (95.56%). The digestibility of barley (90.10%) was lower than that of rice, corn, and brown rice but significantly higher than that of mung beans (83.38%). Interestingly, the NFE content was higher in barley than in brown rice, and the digestibility of NFE was higher in brown rice than in barley. These findings are consistent with those of previous studies. Murray et al. [10] compared the digestibility of rice, corn, and barley, and found DM digestibility to be 83.9% for rice, 85.4% for corn, and 82% for barley. The authors found that while the DM digestibility of rice and corn was similar, barley exhibited significantly lower DM digestibility than rice and corn [10]. Rice supplementation did not affect the digestibility of the mung bean diet. In a wheat-based diet, substituting brown rice at 15% and 30% resulted in a gradual increase in the digestibility of DM, organic matter (OM), acid-hydrolyzed fat, energy digestibility, ME, and GE [11]. NFE is mostly composed of starch, which is the main source of energy in diets. Although carbohydrate ingredients have a high starch content, their in vivo digestibility does not necessarily correspond to the amount of starch contained in the ingredients that affects the utilization of energy in vivo.
Mung beans are primarily considered a protein source; they also contain various complex carbohydrates and are rich in dietary fiber [6,12,13], serving as both a source of carbohydrates and proteins. The mung bean diet was supplemented with 15% rice to achieve 50% NFE because of its higher CP content. The difference in NFE digestibility between rice (97.08%) and mung beans (83.38%) was 13.6%. Based on the proportion of rice supplemented, the digestibility of the mung bean diet was adjusted to that of the rice diet. Table 4 shows the adjusted daily intake and digestibility.
A legume crop, mung beans contain CP around 20.0%–28.50%, but they also include various complex carbohydrates, with a starch content of 40.6%–48.9% and fiber content of 3.21%–4.18% [14,15]. Legume lentils and peas are also classified as legume crops and have lower DM digestibility than rice and corn, similar to mung beans [2]. Specifically, the DM digestibility was 74.5% for lentils and 76.1% for peas, in contrast to 82.4% for rice and 78.6% for corn. Additionally, starch digestibility was 98.8% and 98.7% for lentils and peas, respectively, compared to 99.3% and 99.1% for rice and corn, respectively. Faba beans also have a lower digestibility of DM, OM, and CP than rice [16], and the ATTD decreases linearly with the inclusion of whole faba beans compared to a diet containing a mix of rice and corn [17]. Therefore, the lower digestibility of the diet with mung beans could be explained by the fact that legumes are generally less digestible than cereals in dogs because of their high fiber and low starch content.
OM digestibility was highest in rice (95.45%) and lowest in mung beans (84.93%). ME digestibility was highest in rice (94.67%), followed by corn (94.22%), brown rice (93.63%), barley (91.15%), and mung beans (83.71%). Additionally, the digestibility of amino acids was higher in rice, corn, brown rice, and barley compared to mung beans, except for methionine (Table 5). The digestibility of methionine was the highest in corn and barley at 96.75% and 95.19%, respectively. Rice and brown rice had 92.23% and 91.49%, respectively, whereas mung beans had the lowest digestibility at 85.68%. Both essential and non-essential amino acids were lowest in mung beans.
Currently, grain-free diets are being developed and offered as a more appropriate nutritional strategy. Despite its high economic value and adequacy as a carbohydrate source in dog food, corn has been devalued and associated with negative perceptions because of its potential to cause allergies and the rising popularity of premium grain- and gluten-free diets [18,19]. Conversely, commercial pet food formulations that replace grains with legumes or tubers as primary carbohydrate sources are gaining popularity [16,20]. However, scientific evidence supporting this assertion is lacking [21]. In this study, rice, corn, and brown rice showed the highest digestibility for CP, NFE, OM, and ME. Moreover, when the proportion of chicken breast was reduced to equalize the protein levels of the other experimental diets, accounting for the protein content of mung beans, the digestibility of CP was significantly reduced with mung beans. In addition, the ATTD digestibility of DM, NFE, OM, and ME also decreased when mung bean was used in dog food as a carbohydrate source, similar to the results of previous studies. However, diets containing legums may be more beneficial for glycemic control than grain-based diets [2]. Among the various carbohydrate sources (rice, barley, corn, and peas), a pea-based diet in adult dogs has been reported to reduce oxidative stress and protect the cardiovascular system by lowering the glycemic response [22].
In commercial dry dog food, carbohydrates constitute the largest portion of nutrients; however, carbohydrate content is not required to be listed in the guaranteed analysis [23,24]. This study showed the effects of different carbohydrate sources on in vivo digestibility. Rice, corn, and brown rice had higher digestibility of energy such as CP, NFE, and ME. If dogs require a higher energy intake, rice, corn, and brown rice would be the proper carbohydrate source. However, if dogs need to control their body weight, it would be better to choose dog food with barley or mung beans.
There are multiple factors to be considered when people choose dog foods, not only as nutritional values but also as functional values. However, it is not easy to choose pet food from a vast number of choices. Our results contribute to our understanding of the digestibility of five carbohydrate sources (barley, brown rice, corn, mung beans, and rice) in healthy adult dogs to develop more appropriate dietary strategies for dogs.
