In the current study, it was observed that food intake scoring was similar among dietary treatments, and no refusals occurred. It was proven that the smell of food also plays a crucial role in indicating nutritional preferences [36
]. Moreover, in the available literature, there is no information about the inclusion of insect species in companion animal diets as an aroma additive and consequently, the palatability. However, Kierończyk et al. [37
] suggested that insects such as BSFL may be attractive to dogs. Thus, the possibility of insect application to dog diets provides the double benefit of an encouraging palatability/aroma and consequently, the food intake as well as an additional high-quality nutrient source.
4.1. Nutrient Digestibility
Organic matter digestibility in our study was similar for dogs offered either PM- or BSFL meal-based diets (83.6%). A similar result was obtained by Penazzi et al. [38
], where organic matter digestibility was similar in dogs fed the control (processed deer protein source up to 40% as fed) and BSFL meal (36.5% as fed). In vitro organic matter digestibility of house crickets was reported to be 88% as observed by Bosch et al. [6
], which was similar to that of PM (85.8%). One factor that could affect organic matter digestibility in pets is the content of crude fiber and its source (soluble or insoluble) as described by Zentek [39
] and De Godoy et al. [40
]. Previously, apparent organic matter digestibility of commercial pet diets was negatively linked with fiber content [41
]. It is well-known that increasing dietary fiber content was associated with reduced organic matter digestibility in pets [42
]. Meyer and Zentek [43
] and Monti et al. [44
] stated that the increase in crude fiber by 1% in the DM of the canine food was accompanied by 1.6% decreased organic matter digestibility due to lower microbial decomposition in the colon as a result of accelerated food passage. In the present study, however, dietary fiber content in the BSFL meal-based diet was higher (39.5 g/kg DM) compared to that of the PM-based diet (14.7 g/kg DM), but with no effect on organic matter digestibility. Chitin, a component of the insect exoskeleton that is recovered in fiber analyses [45
] and that monogastric animals are unable to digest [5
], may explain the increased fiber content of insect-based diets. Chitin has previously been linked to a reduction in insect digestibility in livestock and aquaculture [47
]. However, in the present study, crude fiber content did not seem to dictate apparent organic matter digestibility, implying that our findings may not be biologically significant, or that all diets were below the physiological maximum for fiber.
Protein concentration varies from 18% to 40% in dietary formulas for healthy adult dogs [48
]. High-protein diets may result in greater amounts of undigested protein reaching the colon compared with those of low-protein diets [48
] and consequently affect the digestibility. However, if the aim is to decrease fermentation in the hindgut, the choice of protein source used in the dietary formula is of greater importance than protein concentration [48
]. In the current study, the apparent protein digestibility in both diets ranged between 80.5 and 82.3%. Our results are consistent with previous studies regarding the levels of apparent protein digestibility. According to the European Pet Food Industry Federation [49
], the protein minimum requirement was based on an apparent protein digestibility of 80%, which roughly conforms to our findings. Apparent protein digestibility of canine foods containing BSFL ranged from 73.2 to 87.2%. The typical apparent fecal protein digestibility of conventional dog food is around 80% [23
], implying that insect meals are comparable to traditional protein sources. A similar result was obtained by Penazzi et al. [38
], where apparent protein digestibility was higher in dogs fed the control (processed deer protein source) and BSFL meal. Similarly, Lei et al. [51
] observed that increasing levels of BSFL meal inclusion (at 0, 1, and 2%) in Beagle dog diets raised protein digestibility compared to that of the control diet. As pointed out by Penazzi et al. [38
], compared to that of vertebrate protein meal, collagen is probably limited in insect meal. This could also explain the higher level of protein digestibility of the BSFL meal-based diet compared with that of the PM-based diet.
Additionally, one factor that could affect protein digestibility is dietary crude ash content. In our study, the crude ash content in the PM (as ingredient) was about 147 g/kg DM vs. 68.2 g/kg DM for the BSFL meal (as ingredient). Consequently, the extruded complete foods had differences in crude ash content (82.5–58.9 g/kg DM for PM- and BSFL meal-based diets, respectively), with an influence on protein digestibility. Similarly, according to Meyer and Mundt [52
], higher crude ash content in food possibly leads to insufficient acidification of the chyme, which may result in lower protein digestibility. Siebert [53
] stated that incomplete dissolution of minerals from connective tissue of bone meal resulted in impeded proteolysis, and high crude ash contents in PM are likely to originate from bone fractions, for example. The high ash content of some animal by-product meals negatively affects the quality of their protein, as essential amino acid levels per unit of protein are reduced [54
], limiting their inclusion in diet formulations. Penazzi et al. [38
] speculated that the control diet (processed deer protein source) had a decreased crude protein digestibility compared to that of BSFL meal-based diet in dogs due to the higher crude ash content. Therefore, the low crude ash content of a BSFL meal-based diet represents an advantage over other PM-based diets, which generally have a high mineral content.
Another factor that could influence protein digestibility is dietary fiber content [55
]. In the current study, although crude fiber content in the BSFL meal-based diet was two times more than the level in the PM-based diet (39.5 vs. 14.7 g/kg DM), this had no effect on protein digestibility. The various effects of fiber on digestibility in pets, according to De Godoy et al. [40
], are likely to alter the consequences of fiber levels, type (amount of fermentability), and the dietary matrix. Protein digestion in dogs was not affected by changing the source (beet pulp and maize fibers) or concentration (total dietary fiber 8.40–10.2%) [56
]. In contrast to our results, Siebert [53
] found negative effects of high fiber content in canine food on protein digestibility when adding lignocellulose. Digestibility of crude protein decreased as total dietary fiber consumption increased in dogs [57
]. The microbiome can both trap nitrogen as bacterial protein and liberate nitrogen as ammonia, therefore, fermentable carbohydrates may influence protein digestibility through lower tract metabolism [55
]. Thus, in our study, certain effects on protein digestibility due to crude fiber content can be excluded or rather neglected because of the low contents and/or differences. Overall, PM as an ingredient may present variable amounts of low bioavailable materials, such as residual bone, feathers, feet, and beaks, and can be produced under variable processing conditions [54
], creating variability in the composition and digestibility of the ingredient.
The type of diet (BSFL meal-based diet) used in the canine food in the present study had a significant positive effect on apparent fat digestibility, which could be related to some factors. One of these factors is dietary fat content. In the current study, dietary fat content of the BSFL meal-based diet was about (133 g/kg DM) with a fat digestibility of about 94.5%, while the dietary fat content of the PM-based diet was about (99.3 g/kg DM) with a fat digestibility of about 91.6%. Zuo et al. [58
] found that the fat digestibility increased to about 97% when the amount of dietary fat increased. Hill et al. [59
] noted that the digestibility of fat reached about 99% when the dogs ate diets containing a high amount of fat (about 320 g/kg DM). This increase in fat digestibility is in line with the current study’s findings, which show that dietary fat content promotes fat digestibility. Thus, it can be assumed that the apparent digestibility of fat tends to increase as dietary fat increases. Furthermore, apparent fat digestibility is affected by lipid type and processing conditions [60
]. Notably, the lipid type was the same for both diets (plant source; sunflower oil). However, the lipid type and/or fatty acids in the PM or BSFL meal could not be neglected and hence may have contributed to fat digestibility differences observed in the present study. It is well-known that a high content of saturated fatty acids (especially lauric acid) and monounsaturated fatty acids has been found in BSFL, while the contents of eicosapentaenoic acid (C20:5) and docosahexaenoic acid (C22:6) were low [61
]. The lipids present in the PM ingredient are generally rich in monounsaturated fatty acids (particularly oleic acid) and total n-6 polyunsaturated fatty acids but are low in n-3 polyunsaturated fatty acids, eicosapentaenoic acid (C20:5), and docosahexaenoic acid (C22:6) [64
]. Consequently, the contents of saturated fatty acids and/or total n-6 polyunsaturated fatty acids in diets of dogs could influence fat digestibility.
Another factor affecting fat digestibility in a variable way could be the crude ash content. Actually, in our study, crude ash content in the PM-based diet was higher (+23.6 g/kg DM) than that of the BSFL meal-based diet. However, fat digestibility was significantly higher for dogs offered the BSFL meal-based diet. Similarly, Meyer and Zentek [43
] observed that fat digestibility decreased with an increased level of crude ash content in dog food because of a possible soap formation.
Including the BSFL meal-based diet did not affect the digestibility of the nitrogen free extract, which did not differ significantly from the PM-based diet. This fact may partially be explained by the comparable content of the nitrogen free extract in both diets (428 vs. 442 g/kg DM for PM- and BSFL meal-based diets, respectively). Moreover, the levels of starch and sugar in both diets were identical (328 and 18.4 g/kg DM, respectively).
4.2. Fecal Characteristics
Fecal quality is an important index in the evaluation of dog foods. There are many variables that affect fecal quality, including nutrient digestibility, fiber content, DM intake, and fat tolerance [66
]. In the current study, fecal scores were maintained at acceptable levels with an average of 2.5 for each treatment. The extruded diet with BSFL meal resulted in a fecal consistency score that varied significantly closer to the optimal value (score 2) than that resulting from the PM-based diet. Thus, a clearly positive influence of BSFL meal inclusion compared to PM ingredient on fecal quality could be demonstrated. Our findings are comparable to those of Yamka et al. [22
], who found that all diets containing 20% BSFL meal gave dogs an optimal fecal consistency score. A recent study found that when dogs were fed extruded meals with increasing quantities of cricket meal (8, 16, or 24%), their feces remained well-formed [5
]. Though the number of studies is still limited, it seems that adding insect meals to dry extruded pet foods does not influence intestinal functioning and leads to an acceptable fecal consistency score [6
]. Other factors such as dietary protein content that affect the fecal consistency score should not be neglected either. In the current study, the content of crude protein in the PM-based diet was about 49 g/kg DM higher than that found in the BSFL meal-based diet. Nery et al. [48
] observed a softer fecal consistency at higher protein levels in canine food and explained this by increased fermentative degradation in the colon. Zentek et al. [67
] described an influence of the amount and type of protein source on fecal quality, as the softer fecal consistency was particularly due to a higher collagen content in the protein fraction of the food. Moreover, Weber et al. [68
] stated that the increase in proteolytic fermentations in the hindgut is one of the dietary factors causing greater moisture in feces (and negatively influencing fecal quality).
In the current study, dogs fed the BSFL meal-based diet had a 77 g higher wetter fecal output for five days than that of the dogs fed the PM-based diet. The amount of fecal output may be influenced by food intake, nutrient digestibility, chemical composition of the diet, and physiological state of the animal [69
]. Although the water-holding capacity of the dietary ingredients is a factor, greater nutrient digestibility usually results in lower fecal output [69
], even though the increased fecal output may be explained by an increase in dietary fiber [70
]. Previous studies have shown an increase in wet fecal weight with the increase in dietary fiber [71
]. From another point of view, Jarett et al. [72
] found that diets containing crickets supported the same level of gut microbiome diversity in dogs as a standard healthy balanced diet. This suggests that the increase in wet fecal output associated with higher BSFL meal inclusion was not related to microbial abundance. The DM fecal output is usually unaffected by an increase in wet fecal output, implying that the main contributor is increased fecal water content. Similarly, in this study, the DM fecal output was also not significantly influenced.
Notably, in the current study, the fecal DM content remained significantly low at 28.0% when using BSFL meal in about 30% DM of the basic diet. Many different factors may markedly affect the fecal DM content in pets. Protein digestion and absorption are considered to be one of the dietary factors affecting fecal DM content [68
]. If protein is present but not absorbed, the dietary amino acids in that protein are unavailable to the host and serve as a nitrogen source for proteolytic bacteria, resulting in low fecal quality [73
]. Another factor is fiber fermentation activity. High positive correlations were found in dogs between fermentation activity on the one hand and moisture content of feces on the other hand [39
]. This conclusion could be related to fiber’s “bulking impact”, and it appears to be most strongly linked to insoluble fiber sources that are both poorly fermentable and have high water-binding capacity [74
]. Soluble fiber typically has an increased extent of fermentation by gastrointestinal microbes, yielding short-chain fatty acids (mainly acetate, propionate, and butyrate). Short-chain fatty acids play a variety of physiological roles, including increased water absorption in the gastrointestinal tract [75
]. However, overdosing of butyrate might induce an osmotic effect, resulting in increased fecal moisture content and worse fecal consistency [75
]. Further research is needed to access the water-binding capacity of chitin. Based on the current knowledge, very few studies are available in the literature on the chitin content of BSFL meal. Kroeckel et al. [76
] reported a chitin amount of 96 g/kg DM in defatted BSFL. However, Schiavone et al. [77
] revealed a relevant low chitin content (50 and 69 g/kg DM for partially defatted BSFL meal and highly defatted BSFL meal, respectively). Overall, there are contradictory statements regarding the relationship between fecal DM content and fecal consistency score. In the present study, there was no correlation between these two parameters. According to some authors, however, the correlation is given [35
], but in other studies, this is not comprehensible [39
]. Heide [17
] did not show a significant difference in the DM content of feces, but fecal consistency was found to be significantly firmer when the diet without insect meal was used. Beloshapka et al. [80
] suggested that significant differences in fecal DM content could be achieved when different amounts of a specially processed soy protein were added, but not in terms of fecal consistency.