Solving the paradox of protein digestion in weaned piglets

By Marisabel Caballero, Category Manager Monogastrics, and Jes Klausen, Nutritionist, Hamlet Protein

Weaned piglets need highly digestible protein, but their immature digestive systems leaves a good part of this dietary protein undigested. When this protein reaches the hindgut, it supports undesirable fermentation contributing to impaired gut health. Feed formulation should therefore consider not only crude protein and amino acid digestibility, but also where and how quickly protein is digested, and how much remains undigested.

After weaning, protein is both necessary and problematic. Piglets require a substantial amino acid supply to support rapid growth, yet the paradox is that they have poor protein-digestion capacity, and excess or poorly digestible protein triggers gut imbalances and post-weaning diarrhea (PWD), negatively affecting digestive health in piglets.

The key issue is not crude protein alone, but how much of that protein is digested and absorbed before reaching the hindgut.

Beyond crude protein: Understanding protein digestibility and protein fractions in piglet diets

In practical feed formulation, protein evaluation has traditionally focused on two primary criteria:

• Crude protein (CP) content
• Standardized ileal digestibility (SID) of amino acids (AA)

While these parameters are essential, they are not sufficient to fully characterize the biological and functional effects of protein in piglet diets. Protein in feed ingredients consist of different fractions with distinct physicochemical properties, digestion kinetics, and physiological effects. Even ingredients with similar CP and SID AA profiles can have different impacts on gut health and performance.

From a nutritional perspective then, dietary protein can be divided into several functional fractions:

1. Fast Protein: Referring to soluble or easily hydrolyzed proteins. Fast protein, also referred to as fast digestible protein, is absorbed in the proximal small intestine and represents an efficient AA source.
2. Slow Protein: Consists of structurally complex proteins, which require extensive enzymatic breakdown. They are important as they contribute to the sustained release of amino acids during the digestive process. But in newly weaned piglets, which have a limited digestion capacity, slow protein may remain largely undigested.
3. Resistant Protein: All protein passing the distal ileum is considered undigestible; for instance, cell wall proteins (bound to fiber), heat-damaged proteins, and proteins associated with antinutritional factors. In the hindgut, these proteins are substrates for fermentation, promoting the production of toxic metabolites, such as ammonia.
4. Additionally, some proteins contain bioactive components, such as functional peptides and immunoglobulins, which support gut and immune functions, contributing to intestinal health and immune development.

Why resistant protein challenges piglet gut health after weaning

Protein that is not digested and absorbed in the small intestine becomes available for microbial fermentation in the hindgut. This can influence gut integrity, microbial balance, and the risk of post-weaning digestive disturbances.

Our trials indicate that 14 days after weaning, an average piglet has consumed around 120 grams of resistant protein, but it can reach more than 200 grams, depending on feed intake and the protein sources in the diet.

Poorly digestible protein is consistently associated with PWD: osmotic diarrhea ensues, as unabsorbed peptide and protein particles remain in the intestine, and create a high-solute environment, which pulls water from the tissues. The increase in fluid volume in the lumen exceeds the reabsorption capacity, resulting in loose feces.

Additionally, undigested protein feeds opportunistic bacteria such as enterotoxigenic Escherichia coli (ETEC), often resulting in pathogenic post-weaning diarrhea.

In short, undigested protein leads to reduced gut barrier integrity, inflammation, dysbiosis, and proliferation of intestinal pathogens (Figure 1), with the subsequent burden on piglet health, welfare, performance, and the profitability of the operation.

Figure 1: Effects of undigested protein in weaning piglets

Why fast protein matters after weaning

To address the challenges discussed, the pig production industry is increasingly shifting focus from protein quantity to protein quality, digestibility, and kinetics.

That is why fast protein is a piglet’s best friend after weaning. Research has shown that diets high in fast digestible protein are better utilized for growth and performance, while improving piglet gut health and reducing protein excretion.

For instance, when using enzyme-treated soybean meal (ESBM, Hamlet Protein), an increase in fast protein intake of 3 to 4 grams/piglet/day can be achieved by substituting part of the SBM in the diet, which continues to include other high-quality proteins such as whey and milk powder. This results in around 6% higher body weight in piglets weaned under 6 and 7 kg, and 2.8% in piglets weaned over 7 kg (Figure 2). FCR was also improved, reducing at least 10 points (around 8%) in piglets weaned under 6 and 7 kg (Figure 3).

Figure 2: Average daily gain (ADG) of piglets after post-weaning feeding phase 1.

Figure 3: Feed conversion rate (FCR) of piglets after post-weaning feeding phase 1.

These results highlight the importance of fast protein in challenged (small) piglets during the critical period of weaning, moreover, the intake of resistant protein in the experiments can be reduced by up to 2 grams/piglet/day when including ESBM.

Depending on the diet, this represents a decrease in resistant protein consumption of 5% to 20%. resulting in lower gut health issues. In fact the diarrhea rate, an index combining the number of piglets affected and its duration, could be reduced by an average of 67%.

Weaned piglets need more than nutrient supply: considering nutrients’ absorption kinetics and their interaction with gut physiology and microbiota, influences weaning stress and subsequent health and growth.

How nutritionists can apply protein digestibility and protein fraction concepts

To reduce PWD, low-protein diets supplemented with essential amino acids are a good strategy: lower protein intake immediately means lower undigested protein intake. But there is more to consider; nutrient dynamics, particularly digestion kinetics and synchronization, are critical; moreover, during critical periods, the role of each ingredient in supporting gut health is crucial. The concept of protein fractions becomes even more relevant when viewed through the lens of gut health:

• High fast digestible protein promotes early amino acid absorption and improved protein digestibility
• Low resistant protein minimizes nitrogen flow to the hindgut and reduces protein fermentation

The impact can be even higher, for instance, in low-protein diets, which may lead to suboptimal growth due to incomplete nutrient utilization because the high levels of synthetic amino acids utilized are not synchronized with the non-added AAs.

Moreover, the FP:RP ratio is proposed as a simple functional index describing the balance between protein that nourishes the piglet and protein that potentially nourishes pathogenic bacteria. When an ingredient or a diet delivers much more rapidly available protein while leaving much less substrate available for hindgut fermentation (i.e., a higher ratio), we can say that the protein functions more efficiently in the animal.

The actions to take in feed formulation before a complete protein-fraction system is widely available can be strategic for the near-term future of pig production:

• Reduce crude protein levels while maintaining amino acid balance
• Prioritize ingredients rich in fast-digestible protein fractions
• Minimize undigested and heat-damaged protein
• Distinguish digestible from effectively utilized

Want to evaluate protein quality beyond crude protein?

Explore our research on protein digestion in weaned piglets or contact our swine nutrition team to discuss your formulation priorities.