Oligosaccharides in Monogastric Diets

The main aim of this article is to explore a category of carbohydrates referred to as oligosaccharides, particularly emphasizing their presence in soybean meal when considering dietary considerations for broiler chickens.

The main oligosaccharides present in soybean meal are raffinose and stachyose. Both are formed by short chains of galactose, glucose, and fructose that bond together. Overall, these compounds beneficially influence the intestinal microbiota, although they can cause negative effects as they are considered anti-quality factors.

While corn typically serves as the predominant raw material in diets by volume, soybean meal accounts for approximately 35% (Rostagno et al., 2017) of feed formulations designed for broiler chickens.

Worldwide, during the 2022/23 harvest, slightly over 369 million tons of soybeans were produced, with Brazil taking the lead in production, followed by the United States (USDA, 2023).

As per the National Supply Company (Conab, 2023), in the 2022/23 harvest, the production reached around 155 million tons, with notable soybean-producing states being Mato Grosso, Paraná, Goiás, and Rio Grande do Sul (Table 1).

Table 1. Production indices of soybean grains in the main states of Brazil.

Adapted from Conab (2023).

In a study by Ibánez et al. (2020), the authors assessed research investigating the chemical composition and protein quality of soybean meal from 2002 to 2018 across several countries, including major producers like Brazil, the United States, Argentina, and India. Their findings demonstrated variations in the chemical composition of soybean meal, including oligosaccharides, depending on the geographic origin and processing methods, as outlined in Table 2.

Table 2. Nutritional composition and protein quality of soybean meal from different geographical origins.

1 FDN – Neutral detergent fiber 2 KOH – Protein solubility in KOH 3 TIA – Trypsin inhibitor activity 4 AU – Urease activity

[register]By definition, oligosaccharides are carbohydrates that consist of 2 to 10 monosaccharide units, which can be linear or branched and connected by α and/or β-glycosidic bonds (Patel et al., 2014). They serve as nutrients for beneficial microorganisms in the gastrointestinal tract, such as bacteria of the genera Bifidumbacterium and Lactobacilus (Patel and Goyal, 2011).

These bacteria are capable of converting soluble oligosaccharides into organic acids, primarily short-chain fatty acids (acetate, propionate, and butyrate), which improve gastrointestinal health (Tuohy et al., 2005).

Table 3. Impact of dietary supplementation with oligosaccharides derived from non-starch polysaccharides on the composition of cecal microbiota in avian species.

Adapted from Jahan et al. (2022).

Raffinose is produced through the sequential addition of glycosyl groups to a sucrose marker within soybeans as they undergo ripening (Amuti & Pollard, 1977).

• Raffinoses fulfill significant physiological functions, including enhancing resistance to drought and low temperatures, facilitating respiration during germination, and prolonging the shelf life of legumes (Gu et al., 2018).

Although raffinose is considered an anti-quality factor for production animals, recent studies have shown that this compound can be used as a functional substance with prebiotic activity (Amorim et al., 2020).

The absence of the enzyme alpha-galactosidase, responsible for the hydrolysis of raffinose, in the upper gastrointestinal tract of monogastric animals causes raffinose to remain intact until the lower portion of the intestine where absorption occurs (Fleming, 1981; Kaczmarek, 2014; Zartl et al., 2018).

The fact that raffinose is not digested in the upper gastrointestinal tract has sparked scientific interest in this compound.

Soybean meal can contain from 4 to 6% of raffinose in its composition (on a dry matter basis) and, besides being a heat-resistant anti-quality factor (Zang et al., 2019), it can also contribute to an increase in digesta viscosity, thus interfering with nutrient digestion (Kasprowicz-Potocka et al., 2022).

Supplementing broiler diets with raffinose holds promise, although few studies have been conducted to investigate its impact on performance.

Another study, however, conducted with pigs, evaluated the inclusion of 0.2 and 0.5% raffinose in the diet and observed a reduction in performance in the growth phase, decreased feed intake and nutrient utilization, while an improvement in intestinal morphology was observed at the same time (Zeng et al., 2021).

Stachyose is an alpha-galacto-oligosaccharide consisting of two units of:

• α-galactose,
• one α-glucose
• one β-fructose
  (Muland Perry, 1994).

Like raffinose, stachyose is also considered an anti-quality factor resistant to thermal processing, and in the case of soybean meal, it can represent 5 to 6% (Sorensen et al., 2011).

According to Teague et al. (2023), the use of both raffinose and stachyose in broiler chicken diets can influence not only the performance of the animals but also intestinal integrity and quality, as well as microbiota composition.

The authors reported that the use of these two oligosaccharides together had an effect on the feed conversion rate of broiler chickens in the growth phase (0 to 21 days), with birds fed 0.9% and 1.8% showing an improvement in feed conversion compared to birds that did not receive supplementation in the diet.

Regarding alpha, which represents the number and abundance of bacterial species within a community, and beta diversity, which relates to the composition of species and their abundances between communities, the study by Teague et al. (2023) shows that increasing levels of raffinose and stachyose influenced the alpha and beta diversity of the microbiota (Figure 1), with emphasis on the abundance of bacteria from the Ruminococcus and Bifidobacterium genera represented in the graph by the light blue and dark yellow fractions (Figure 2).

Figure 1. Increasing levels of supplementation with raffinose and stachyose (GOS) and their effect on the composition of cecal microbiota. Adapted from Teague et al. (2023).

Adapted from Teague et al. (2023).

Figure 2. Alpha diversity represented by the Shannon index of the intestinal microbiota community of broiler chickens fed with increasing levels of raffinose and stachyose.

Adapted from Teague et al. (2023).