Introduction

A balanced diet is crucial for maintaining the health of companion animals. The appropriate diet for a companion animal depends on various factors, including the animal’s breed, health status, diet format, and the nutritional profile of the food (Do et al., 2021). Commercial pet foods are formulated to meet nutritional requirements based on specific life stages or physiological needs, such as growth, maintenance, or pregnancy (Montegiove et al., 2022). To meet the nutritional requirements of the diet, various ingredients which contain proteins, fats, carbohydrates, vitamins, and minerals are used (Zicker, 2008). Among these nutrients, fat sources are rarely added separately because the protein sources in pet food contain sufficient fat to meet the minimum fat requirement (AAFCO, 2014). When fat sources are added separately, it is related to the formulation of the diet and improvement of palatability (İnal et al., 2020; Samant et al., 2021). Also, dietary fats serve as an energy source and provide fat-soluble vitamins and essential fatty acids such as linoleic (omega-6) and α-linolenic (omega-3) acids, which dogs cannot synthesize endogenously (İnal et al., 2020; Richards et al., 2023).

Both animal and vegetable oils can be used in pet food (Dhakal and Aldrich, 2022). Animal fats commonly include mammalian and poultry fats, while vegetable oils such as corn, sunflower, safflower, and soy oils are frequently used (İnal et al., 2020). Chicken oil, which is mainly used for animal fat, has the highest content of unsaturated fatty acids among other animal fat sources and is also rich in polyunsaturated fatty acids (Peña-Saldarriaga et al., 2020; Hwang et al., 2021). Sunflower oil, a plant-based fat, is predominantly composed of linoleic acid and oleic acid (Rauf et al., 2017). Linoleic acid is known to be an essential nutrient for dogs, playing an important role in forming skin barriers and suppressing inflammation (Watson et al., 2018). Recent studies indicate that dogs may prefer vegetable fats with high linoleic acid content over animal fats (İnal et al., 2020).

Despite their frequent use in pet food, the scientific data of animal- and plant-origin ingredients especially fat sources are limited. Therefore, this study was conducted to analyze and compare palatability, fecal score, nutrient digestibility, and blood profile for the nutritional value of chicken and sunflower oils in dogs.

Materials and Methods

The protocol for this study was reviewed and approved by the Institutional Animal Care and Use Committee of the National Institute of Animal Science (NIAS), Rural development Administration, Korea (approval number: NIAS2022-0584).

Animals and experimental design

All animals used in this study were owned by the NIAS. To evaluate in vivo digestibility, a total of eight healthy Beagle dogs (4.58 ± 0.14 years old; 4 neutered males and 4 spayed females) were used. The dietary treatment groups were as follows: 1) Chicken oil; a chicken breast powder-rice based diet with chicken oil; 2) Sunflower oil; a chicken breast powder-rice based diet with sunflower oil. The experimental diet was designed to meet the AAFCO (2016) nutrient requirements for adult dogs (Table 1) and was prepared according to the method described by Seo et al. (2021). The fatty acid composition of the experimental diet is shown in Table 2. Each experimental diet had a 3-day adaptation period, a 4-day experimental diet feeding period and a 4-day sampling period for collecting feces. The feed intake was restricted to the maintenance energy requirement of each animal with reference to the AAFCO (2016) equation (132 kcal × kg body weight^0.75 per day). To test palatability additional ten healthy Beagle dogs were used. All dogs were housed in an independent space (170 × 210 cm·dog^-1) with a constant indoor temperature (22 - 23℃) and lighting (16 h light and 8 h dark cycle). Water was provided ad libitum, and feed was fed once a day based on the calculated value of maintenance energy requirements. All dogs had approximately 3 h of outdoor activity per day. The health of the dogs was monitored daily and checked by a veterinarian at NIAS, if needed.

Sampling and measurements

Palatability

Palatability testing was conducted with a two-bowl test by Beagle dogs (n = 10) with each experimental diet. After providing a chicken oil diet and a sunflower oil diet at the same time, an analysis was conducted on which feed was approached first (first smelling), which feed was eaten first (first touch), and which feed was completely consumed first (intake ratio).

Fecal score

Fecal scores were evaluated on a 5-point fecal score scale according to the Waltham feces scoring system (Moxham, 2001) every day during the entire experimental period. The fecal score was evaluated from 1 to 5, and the criteria for each score are as follows: 1, dry, crumbly feces; 2, well-formed, easy to pick up, and left no marks; 3, left a mark when removed from a dry surface, tacky to the touch, soft centered; 4, moist and poorly formed with a consistency of putty or porridge; 5, diarrhea.

Nutrient digestibility

The total fecal collection method was used to analyze the nutrient digestibility. Fecal samples were collected for 4 days and weighed daily. The collected feces and diet samples were stored at -20℃ until analyzed. To calculate nutrient digestibility, dry matter (DM), crude protein (CP), ether extract (EE), ash, and crude fiber (CF) contents of diet and feces were analyzed according to the method of AOAC International (2006). The nitrogen-free extract (NFE) was calculated using the analyzed moisture, CP, ash, EE, and CF contents. The apparent total tract digestibility (ATTD) was calculated according to the method of Donadelli and Aldrich (2019) as illustrated in Eq. (1).

where, D : digestibility (%)

Nd : nutrient in diet (%)

FI : feed intake (g)

Nf : nutrient in fecal (%)

FO : fecal output (g)

Blood profile

Blood was collected from the jugular vein on the last day of fecal sampling for each experimental diet. The collected blood was immediately transferred to an EDTA vacutainer tube and a serum separating tube. Blood collected in EDTA vacutainer tubes was analyzed for complete blood count (CBC) using an automatic hematology analyzer (IDEXX Laboratories, Inc., USA) within one hour after collection. Blood in the serum separating tube was centrifuged at 4℃ and 3,000 rpm for 10 minutes to separate serum and then stored at -80℃ until analysis. Serum biochemical parameters were analyzed using an automatic biochemical analyzer (Cobas C 111, Roche, Switzerland). The analyzed serum biochemical parameters are as follows: alanine transaminase (ALT), alkaline phosphate (ALP), aspartate transaminase (AST), glucose (GLU), lactate dehydrogenase (LD), cholesterol (CHO), triglyceride (TG), urea nitrogen (UN), calcium (CA), lipase (LIP), creatinine (CREA), and creatine kinase (CK).

Statistical analysis

All data except for palatability were analyzed through the general linear model procedure in JMP Pro 16.0 (SAS, 2021). Differences among the treatments were separated by a t-test. The palatability was compared with a chi-square test, using the FREQ procedure of JMP Pro (SAS, 2021). Variability in the data is expressed as the standard error, and a probability level of p < 0.05 was considered to be statistically significant.

Results

Palatability

There was no difference (p > 0.05) between chicken oil and sunflower oil in the palatability (first smelling, first touch, and intake ratio; Table 3).

Fecal score

During the entire experimental period, the average fecal scores between chicken and sunflower oils were not significantly different (p > 0.05; Table 4). Both groups remained within ideal fecal scores throughout the entire experimental period.

Nutrient digestibility

There was no difference (p > 0.05) between chicken oil and sunflower oil in the nutrient digestibility (DM, CP, EE, ash, and NFE; Table 5).

Blood profile

Neither the CBC in blood nor the serum biochemical parameters showed any significant difference (p > 0.05) between chicken oil and sunflower oil (Tables 6 and 7). All blood parameters in both groups were within normal ranges.

Discussion

This study investigated the effects of chicken oil and sunflower oil as the main fat source in dog food, focusing on palatability, fecal score, nutrient digestibility, and blood profiles. The results showed that there were no significant differences in all measured parameters between the two dietary treatments. Also, although it was not shown in the data, the body weight of the chicken oil group at the end of the experiment was 12.69 kg, and the body weight of the sunflower oil group was 13.10 kg, and the body condition score (BCS) was 5.38 and 5.88, respectively, showing no significant difference between the two treatment groups. These results suggest that both oils can serve as functionally equivalent fat sources when balanced in fatty acid composition and energy density in dog food formulations.

Palatability is an important factor in measuring the performance of all commercial pet food products (Watson et al., 2023). Traditionally, animal fats have been shown to be more effective than vegetable oils in enhancing palatability (NRC, 2006). Also, it has been reported that the palatability of fat is influenced by the saturation state of fatty acids (İnal et al., 2020). However, the results of this study showed that there was no significant difference in feed intake or palatability between diets containing chicken and sunflower oils. This finding is consistent with recent studies suggesting that advances in processing techniques and fat application methods can minimize flavor differences between animal and vegetable oils (Kim et al., 2023). Also, the cabbage and seaweed contained in the experimental diet of this study may act as natural antioxidants (Morales-López et al., 2017; Michalak et al., 2022). Appropriate antioxidants in diets can alter dogs’ preferences and minimize the occurrence of off-flavors that could negatively affect taste (İnal et al., 2020).

Fecal score, which includes fecal form and viscosity, is an important indicator of gastrointestinal health and dietary tolerance (Lee et al., 2021). In this study, there was no difference in fecal scores between the chicken oil and sunflower oil groups, and both groups were within the ideal fecal score range of 2 - 3. These observations are consistent with previous studies showing that fecal quality in dogs is more influenced by dietary fiber content, protein digestibility, and overall nutrient balance than by the specific type of fat present in the diet (Kilburn et al., 2020; Kahraman et al., 2022). Although vegetable oils are often high in polyunsaturated fatty acids, which could theoretically influence microbial fermentation and excretion (Vastolo et al., 2022; Maina and Cox, 2025), the results of this study suggest that the dog gut microbiota is at least resilient to changes in these dietary fat sources when the diet is balanced and appropriately formulated.

Previous studies have demonstrated that dogs can digest a variety of dietary fats, with ATTD of EE often exceeding 90%, regardless of the fat source (Donadelli and Aldrich, 2019; Kahraman et al., 2022). In this study, the EE digestibility of chicken oil (rich in saturated and monounsaturated fatty acids) and sunflower oil (rich in polyunsaturated fatty acids, especially linoleic acid) was 95.23% and 94.04%, respectively, which were similar to those in previous studies. Also, the digestibility of DM, CP, EE, ash, and NFE did not show significant differences between the groups, and the two fat sources showed similar digestibility. Pancreatic lipase efficiently hydrolyzes both animal and vegetable triglycerides, and absorption is more affected by chain length and saturation than by the fat source itself (Carrière et al., 1997). This suggests that when diets are carefully formulated to be isocaloric and balanced in essential fatty acids, the fat source does not have a negative effect on overall nutrient absorption and utilization.

Blood profile results showed no significant differences in hematological and biochemical parameters between the chicken oil and sunflower oil groups. Additionally, all blood values ​​were found to be within normal range. This suggests that both chicken oil and sunflower oil are safe and nutritionally interchangeable fat sources in dog diets when consumed in appropriate amounts. Similar to the results of this study, it has been reported that adding sunflower oil to cat diet did not have any negative effects on nutrient digestibility or blood biochemical parameters (Paßlack et al., 2024). Also, when chicken oil was included in diets with varying main protein sources for senior dogs, all blood biochemical parameters remained within the normal range (Sechi et al., 2022). Previous studies have shown that serum CHO and TG levels in dogs are more affected by total fat intake and fatty acid ratios (e.g., omega-6 to omega-3 ratio) than by specific fat sources (Geary et al., 2024; Maina and Cox, 2025). In fact, in this study, although there was a difference in the fatty acid content in the diet according to chicken oil and sunflower oil, there was no difference in the body weight, BCS, nutrient digestibility, and blood profiles of the Beagle dogs between the two treatment groups. According to Phungviwatnikul et al. (2020), when comparing the treatment group that used only chicken oil as a fat source and the treatment group that used a combination of chicken oil, fish oil, and coconut oil, there was no significant difference in body weight and BCS, and CHO, TG, and leptin in blood were all within the normal range. This suggests that the total fat content in the diet has a greater impact on dog metabolism than differences in the specific fatty acid content of the diet, regardless of the fat source used in the diet. Also, this supports the conclusion that neither animal nor vegetable oils have a negative impact on systemic health indicators in dogs, provided essential fatty acid requirements are met.

Conclusion

In conclusion, this study supported that chicken oil and sunflower oil can be used interchangeably in pet food without negatively affecting palatability, fecal score, nutrient digestibility, or blood profiles. These results can be used as basic data for selecting fat sources in pet food formulations.