Phosphorus metabolism and digestibility in pigs

Phosphorus plays a central role in pig nutrition and metabolism, influencing bone development, growth performance, protein synthesis, and overall feed efficiency. However, understanding how phosphorus is absorbed, digested, utilized, and excreted remains one of the major challenges in modern swine nutrition.

As feed costs rise and environmental pressure increases, nutritionists are focusing more closely on phosphorus availability, phytase supplementation, and digestibility systems to formulate more efficient and sustainable diets.

Phosphorus metabolism in pigs

The hydrolysis of phytic acid and phytates occurs progressively in the gastrointestinal tract, releasing free phosphates and intermediate compounds that can be partially absorbed. However, in diets without phytase supplementation, a large proportion of phytate phosphorus remains unavailable to pigs.

Although microbial degradation of phytate occurs extensively in the large intestine, the phosphorus released in this section appears to have limited absorption potential.

The role of phytase in phosphorus utilization

Phytase supplementation has transformed phosphorus nutrition in monogastric animals. According to the article, the stomach is the primary site of phytate hydrolysis when exogenous phytases are included in the diet, largely because gastric pH is more favorable for enzyme activity than intestinal pH.

The greatest differences in phytate degradation between phytase-supplemented and non-supplemented diets are observed in the stomach and duodenum, with effects continuing throughout the small intestine.

Where phosphorus is absorbed

The duodenum and jejunum are considered the primary sites of phosphorus absorption in pigs. This process occurs through specific sodium-phosphate transporters that facilitate active transcellular absorption.

Phosphorus absorption is influenced by:

• Vitamin D and endocrine regulation
• Dietary mineral composition
• The chemical form of phosphorus
• The source of dietary carbohydrates

The large intestine may also contribute to phosphorus recycling and absorption, although to a lesser extent than the small intestine. Interestingly, carbohydrate sources influence phosphorus dynamics:

• Cellulose and pectin are associated with phosphorus secretion
• Starch is associated with phosphorus absorption

Phosphorus excretion and environmental impact

Once absorbed, phosphorus circulates in the bloodstream associated with phospholipids or as inorganic phosphorus, reaching tissues where it may be retained or excreted through urine, feces, or milk.

Excess phosphorus excretion is largely related to:

• Low phosphorus digestibility
• Oversupply of digestible phosphorus
• Undigested phytate phosphorus
• Excess dietary phosphorus beyond animal requirements

The article notes that fecal phosphorus excretion can represent approximately:

• 52% of phosphorus intake in sows
• 46% in weaned piglets
• 55% in growing pigs

Understanding phosphorus homeostasis

Phosphorus homeostasis depends on coordinated regulation involving the intestine, kidneys, bones, and multiple hormones.

The most important regulators include:

• Parathyroid hormone (PTH)
• Vitamin D (calcitriol)
• Fibroblast Growth Factor 23 (FGF23)
• Klotho

These regulators control phosphorus absorption, renal excretion, and bone mobilization to maintain metabolic balance.

Bioavailability vs digestibility

Historically, phosphorus nutrition relied heavily on the concept of “available phosphorus,” which attempted to estimate the proportion of phosphorus that animals could utilize.

The Relative Biological Value (RBV) method was traditionally used to evaluate phosphorus sources by measuring variables such as:

• Bone strength
• Bone ash weight
• Bone ash percentage

However, this system presented important limitations and could overestimate phosphorus utilization in certain ingredients.

ATTD, STTD and true phosphorus digestibility

Modern pig nutrition increasingly relies on digestibility systems such as:

• ATTD – Apparent Total Tract Digestibility
• STTD – Standardized Total Tract Digestibility
• TTTD – True Total Tract Digestibility

ATTD accounts for phosphorus intake and fecal excretion but does not separate endogenous phosphorus losses from undigested dietary phosphorus.

STTD corrects this limitation by accounting for basal endogenous phosphorus losses, making phosphorus values more additive and practical for feed formulation.

Final considerations

Phosphorus remains one of the most important and complex minerals in swine nutrition. Optimizing phosphorus utilization requires balancing animal performance, feed efficiency, economic sustainability, and environmental responsibility.

Advances in phytase supplementation, digestibility systems, and precision formulation are allowing nutritionists to improve phosphorus utilization while reducing waste and environmental losses.