A day in the life of phosphorus in pigs: Part I

Phosphorus metabolism in Pigs

» Phytase-free diets

The hydrolysis of phytic acid and phytates occurs gradually, producing free phosphates and several intermediate products that can be absorbed to a limited extent.

The degradation of phytate in the gastrointestinal tract is almost complete, even in diets lacking intrinsic feed phytase, as there appears to be considerable microbial breakdown of phytate in the large intestine. However, the phosphorus released from phytate in the large intestine does not appear to be absorbed.

» Diets with phytase

» Phosphorus absorption

The duodenum and jejunum are the main sites of phosphorus absorption in pigs and other monogastric species. This process occurs via specific sodium/phosphate transporters in the small intestine, facilitating active and transcellular absorption.

This absorption is regulated by endocrine factors (such as vitamin D and triiodothyronine) and nutritional factors, including dietary minerals and the chemical form of phosphorus.

In piglets, absorption can reach 97%.

• Cellulose and pectin are associated with net phosphorus (P) secretion.
• Starch is associated with net phosphorus (P) absorption.

In practical diets, greater phosphorus (P) absorption in the large intestine has been observed compared to semi-purified diets.

» Phosphorus excretion

Once absorbed, phosphorus (P) travels through the bloodstream bound to phospholipids or as inorganic P, reaching various tissues. Depending on the body’s needs, it is either retained in tissues or excreted through milk, feces, or urine—with urine being the main route of elimination. The regulation of P metabolism involves the cooperative interaction of the intestine, bones, and kidneys, as well as the parathyroid hormone, calcitriol, and calcitonin.

Phosphorus (P) excretion in pigs is mainly due to the low digestibility of P and the excessive digestible P content in the diet.

Fecal P is composed of:

• → Undigested portions of phytate-bound phosphorus (phytic phosphorus, PP) and non-phytate phosphorus (NPP) from plant sources.
• → Undigested portions of phosphorus (P) from animal by-products and mineral supplements.
• → The surplus of bioavailable phosphorus (P) that exceeds the animal’s requirements.

These factors result in significant phosphorus (P) excretion in feces.

Fecal P excretion is influenced by three main factors:

• Unavoidable phosphorus (P) losses, which depend on the body weight of the pigs.
• Dietary phosphorus (P) availability, which is related to the source of P in the diet.
• Regulatory phosphorus (P) excretion due to adaptation in absorption and/or endogenous secretion based on the level of P supply.

Urinary phosphorus (P) excretion also depends on the pig’s metabolic status regarding its phosphorus level.

» Phosphorus homeostasis

Phosphorus (P) homeostasis is regulated through intestinal absorption, mobilization of P from bones, and renal excretion via various hormonal and metabolic mechanisms

Parathyroid hormone (PTH), also known as parathormone or parathyrin: secreted by the parathyroid glands, this hormone increases phosphorus (P) reabsorption in the renal tubules, renal excretion, and mobilization from bones into the bloodstream.

Vitamin D or calcitriol: stimulates phosphorus (P) absorption in the small intestine. It is activated through its conversion in the liver to 25-hydroxycholecalciferol and then in the kidneys to 1,25-dihydroxycholecalciferol (calcitriol), its most active form.

Fibroblast growth factor 23 (FGF23): inhibits renal phosphorus (P) reabsorption and suppresses circulating calcitriol levels, thereby reducing intestinal P absorption and promoting its renal excretion.

• *25-hydroxycholecalciferol: the main circulating metabolite of vitamin D₃, formed in the liver. It acts as a precursor that is converted in the kidneys into the active form of vitamin D₃..
• **1,25-dihydroxycholecalciferol (calcitriol): the most active metabolite of vitamin D₃, produced in the kidneys. It increases phosphorus (P) absorption in the small intestine, facilitates the release of P from bones, and reduces renal P excretion.
• 24,25-dihydroxycholecalciferol, a less active metabolite of vitamin D₃, is also produced in the kidneys. Its exact function is not fully understood, but it is considered less effective in regulating phosphorus (P) homeostasis compared to calcitriol.

Phosphorus availability

» Available phosphorus, bioavailability, or phosphorus availability 

Bioavailability (available P) is an abstract concept used to describe the extent to which phosphorus (P) in a feed is absorbed, metabolized, and utilized by the animal, but it has often been used without due caution in many publications.

It reflects the net effects of digestion, absorption, and post-absorptive utilization of P by tissues and organs for maintenance, growth, or reproduction.

To estimate phosphorus requirements in pigs, the concept of available P has been widely used and, as previously mentioned:

• ♦ Phytic phosphorus (PP) was previously considered unavailable.
• ♦ Non-phytic phosphorus (NPP) was considered available to the pig.

The relative bioavailability (RBV) of phosphorus (P) is not additive in mixed diets for pigs.

Figure 3. Description of the Slope Ratio Method.

Phosphorus (P) availability depends on several factors.

• The chosen response variable for its evaluation, such as the pig’s body weight, bone ash content, or femur breaking strength.
• Inorganic phosphate selected as the reference.
• Diet composition.
• The calcium-to-phosphorus (Ca:P) ratio, which varies among different raw materials.
• Pig age or development stage (piglet, finisher, or sow).
• Trial duration

Phosphorus Digestibility

Factorial estimation is based on estimating the requirements fo:

→ Maintenance (replacement of unavoidable or basal endogenous losses in feces and urine).

→ Phosphorus (P) retention in the body and in the products of conception.

→ Phosphorus (P) excretion in milk.

To estimate maintenance requirements, endogenous phosphorus (P) losses must first be calculated, which can be divided into basal and specific losses.

Basal endogenous losses

Losses associated with the inherent characteristics of the animal’s physiological needs, and are included in nutritional requirements.

These losses are calculated using:

♦ Diets that cause little to no specific phosphorus (P) loss, such as semi-synthetic P-free diets or phosphorus-free diets.

♦ Techniques such as extrapolation to “zero P intake.”

♦ Diets with radioactively labeled phosphorus (P).

Specific endogenous losses

Losses associated with dietary fiber, considered a characteristic of feed ingredients that are implicitly accounted for in the digestibility coefficient.

» Phosphorus digestibility

In theory, phosphorus (P) digestibility in an ingredient can be calculated by multiplying its relative bioavailability (RBV) by the digestibility of P in the standard mineral P used for comparison, although this conversion has been questioned by comparative studies.

To measure P digestibility, several operational decisions are required:

» Phosphorus (P) digestibility: choosing between ileal and fecal sampling

It has been shown that the large intestine does not play a significant role in phosphorus (P) digestion, and there is no difference in true P digestibility between ileal and total digestive tract levels.

This justifies determining fecal phosphorus (P) digestibility, which is characterized by its accuracy and ease of operation.

» Apparent phosphorus digestibility or ATTD of phosphorus (P) 

The term “apparent” in phosphorus digestibility (ATTD, Apparent Total Tract Digestibility) refers to the calculation where both undigested dietary phosphorus and the pig’s endogenous phosphorus losses are subtracted from the dietary phosphorus supply.

ATTD of phosphorus (P) % = [(P intake – P excreted) / P intake] x 100

Some studies have found that ATTD values of phosphorus (P) measured for ingredients are not always additive when used in pig diet formulation. For example:

→ ATTD values for phosphorus (P) in corn vary significantly in studies with pigs of different weights.

→ In contrast, the ATTD of phosphorus (P) in monocalcium phosphate does not change with increasing inclusion rates.

→ ATTD values for phosphorus (P) in soybean meal and canola meal also do not vary with inclusion level.

» Standardized phosphorus digestibility or STTD of phosphorus (P)

The term “standardized” (STTD, Standardized Total Tract Digestibility) refers to the calculation in which both undigested dietary phosphorus (P) and the pig’s specific endogenous P losses are subtracted from the dietary P supply, while the unavoidable basal endogenous P loss is considered part of the pig’s nutritional requirement.

Basal endogenous losses are the inevitable phosphorus losses that occur in the pig’s digestive tract, unrelated to the type of feed. They are considered a constant.

STTD of phosphorus (P) (%) = [(P intake – (P excreted – Basal endogenous P losses)) / P intake] × 100

→ The NRC (2018) calculated the STTD of phosphorus (P) for different feed ingredients using basal endogenous losses of 190 mg/kg of dry matter intake to correct the ATTD values. However, Stein et al. (2011) used a value of 200 mg/kg to correct ATTD values in their calculations.

→ Almeida and Stein (2010) indirectly demonstrated that STTD values for phosphorus (P) are additive, as they observed no significant differences among four diets formulated using STTD values of P.

→ She et al. (2018) demonstrated that the STTD of phosphorus (P) in corn, soybean meal, and canola meal is more additive than their ATTD equivalents in pigs.

» True phosphorus digestibility or TTTD of phosphorus (P)

The term “true” in phosphorus digestibility (TTTD, True Total Tract Digestibility) refers to the actual capacity of a feed to provide phosphorus (P), based on a calculation in which only the undigested P derived from the feed is subtracted from the dietary P supply.

Different methods have been used to measure true phosphorus (P) digestibility:

Petersen et al. (2011) estimated specific endogenous phosphorus (P) losses and true P digestibility in inorganic P sources based on a phosphorus-free diet and, along with other authors, tested the additivity of TTTD values of P in different plant feed ingredients.

The main challenge of TTTD is the difficulty in measuring the specific endogenous phosphorus (P) loss associated with each ingredient and considering it as part of the pig’s requirements.

In contrast, the STTD system, which adjusts apparent digestible phosphorus (P) by basal endogenous losses, appears to be the most practical option.

These methodologies estimate requirements but do not capture the pig’s responses, such as transient deficiencies or compensatory retention that occur in practice.