Is 25-hydroxycholecalciferol (25-OH-D3) a good biomarker to evaluate bone and cartilage health in pigs?

Introduction

Lameness is both a welfare issue and a source of major economic losses in pigs. Between 2012 and 2021, sow mortality in the US increased from 8.16 to 13.56% (National Hog Farmer, 2021) – with lameness identified as one of the leading reasons for humane euthanasia and mortality.

Lameness is also increasingly seen in finishing pigs. This is because the higher growth rate and lean tissue accretion of modern breeds carries a higher mineral demand which can result in the redirection of minerals away from bone deposition towards muscle growth and immunity.

Lame or unhealthy pigs have been shown to have reduced serum levels of the form of vitamin D₃ known as 25-hydroxycholecalciferol (25-OH-D₃).

It is therefore relevant to consider whether serum 25-OH-D₃ could be used as a good indicator of bone health, to support interventions to avoid lameness. This article explores this by considering the metabolic basis of lameness, how this relates to vitamin D status, and how 25-OH-D₃ supplementation can be used to improve bone health and reduce bone-related disease.

What is lameness?

According to the Merck Veterinary Manual, lameness refers to ‘an abnormal gait……visibly limping, unable to rise, or simply less inclined to move’. Importantly, it results from pain. In most cases, this pain arises from a musculoskeletal problem which can be caused by an infection, trauma, and/or underlying osteochondrosis or metabolic bone disease.

Metabolic bone disease

Metabolic bone disease can be a major contributor to lameness in pigs. It is characterized by a disturbance in the growth, modelling and remodeling of bone and leads to diseases such as rickets, osteomalacia (like rickets but in adult animals), fibrous osteodystrophy (extensive bone resorption and replacement by connective tissue) and osteoporosis/osteopenia (reduced bone mass). Another common clinical feature of lameness and metabolic bone disease is osteochondrosis dessicans (OCD). This is a joint disorder defined by a disturbance in the ossification of cartilage to bone that may affect up to 85–90% of pigs during their lifetime. Risk factors for OCD include heredity, conformation, trauma, and rapid growth, but dietary imbalances can also lead to OCD or a more severe metabolic bone disease.

The major causes of metabolic bone disease include:

• Low levels of circulating 25-OH-D₃ in the blood. Inadequate absorption of phosphorus (P). For example, due to low P in the diet, P being unavailable (e.g. bound to phytate), or ineffective phytase usage
• Inadequate calcium (Ca) in the diet
• Imbalance in the Ca:P ratio in the diet

Diagnosis of metabolic bone disease

Veterinary diagnosticians consider vitamin D status as key for the diagnosis of metabolic bone disease. Specifically, serum 25-OH-D₃ levels. Reference ranges produced in-house by diagnosticians at Iowa State University (Table 1) suggest that serum 25-OH-D₃ levels above 35 ng/ml are sufficient for optimum Ca and P metabolism to support bone health in adult pigs, with some allowances for lower levels in younger animals:

Bone ash, bone density, and bone mineral measurements are also considered essential in diagnostic investigations of lameness. However, bone ash results can be misleading if there’s misunderstanding about the laboratory method used (e.g., whether bones are defatted or not prior to ashing) and the ‘normal range’ that is referenced. The reference range that is frequently used to characterize ‘normal’ bone ash is 58-62%, which is based on information using defatted bone from healthy finishing and adult pigs. Defatting of bone is a time-consuming process that is not often used in diagnostic laboratories. If bone samples are not defatted before determining their ash content, the bone ash results will be considerably lower. Therefore, for bone that has not been defatted, bone ash results <45% are concerning (Darin Madson, Iowa State University, USA, personal communication).

In a summary of cases with rickets diagnosed in growing pigs during 2010–2012, the Iowa State University Veterinary Diagnostic Laboratory observed consistently low serum 25-OH-D₃ (<30 ng/ml) and bone ash percentages (<45%), demonstrating that serum 25-OH-D₃ can be a good indicator of bone health.

Wider diseases and risk factors for lameness

As well as vitamin D status, the incidence of lameness is also associated with health status (e.g., with porcine reproductive and respiratory syndrome virus (PRRSV) and Seneca Valley Virus (SVA) status). In turn, health status is correlated with serum 25-OH-D₃, bone ash, and bone density (Baker et al., 2019; Williams et al., 2023). This again points towards serum 25-OH-D₃ as being an indicator for bone health.

What is the current serum 25-OH-D₃ status of farmed pigs?

If serum 25-OH-D₃ is a good indicator of bone health, this begs the question ‘what is the current vitamin D status of farmed pigs?’

The answer, based on data from the USA (Madson, 2013; see Figure 1) is ‘low’. A significant number of both young (nursery) and more mature (grower- finisher) pigs sampled from commercial farms (raised indoors) in 2013 exhibited low vitamin D status – serum 25-OH-D₃ levels below the reference targets shown in Table 1. This is despite the fact that a separate survey conducted in 2014 showed that vitamin D₃ (cholecalciferol) supplementation levels were, in most cases, well above recommended NRC levels during all growth stages.

Importantly, this low serum 25-OH-D₃ status is not common in pigs reared outdoors, particularly during summer (Jakobsen et al., 2022). This is thought to be because, although vitamin D₃ supplemented as cholecalciferol in the diet has the same biological activity as vitamin D₃ produced in the skin once metabolized, vitamin D₃ produced in the skin using sunlight may have a prolonged half-life in the circulation that results in greater bioavailability (Holick et al., 2008).

The case for supplementing vitamin D₃ in the form of 25-OH-D₃

One way to overcome the difference in bioavailability of vitamin D supplemented as cholecalciferol compared with that produced via sunlight is to supplement directly with 25-OH-D₃ in the diet.

At dsm-firmenich, research over the past 13 years has generated evidence for this, such as:

• Maternal supplementation of 50 µg/kg 25-OH-D₃ as Hy-D® (dsm-firmenich) on top of cholecalciferol vitamin D₃ (2,000 IU/kg) throughout gestation and lactation significantly increased sow serum 25-OH-D₃ at farrowing (from 34.6 to 65.4 ng/ml) as well as bone ash, femur strength and bone density in the newborn piglets, compared with supplementing cholecalciferol alone (Zhou et al., 2017).
• Maternal supplementation of 50 µg/kg 25-OH-D₃ (as Hy-D®) on top of 500 IU/kg of cholecalciferol increased serum 25-OH-D₃ in gilts and sows, as well as their progeny average daily gain during the grower-finisher phase and final bodyweight at slaughter (+4 kg), and tended to improve gain:feed from weaning until harvest, compared with supplementing cholecalciferol alone (Tsai et al., 2023).
• Direct supplementation of 50 µg/kg 25-OH-D₃ (as Hy-D®) to pigs from weaning until harvest (6 to 110 kg BW) increased serum 25-OH-D₃ levels and reduced the incidence of osteochondrosis lesions (associated with lameness) when compared with supplementing cholecalciferol alone at 1,800–1500 IU/kg (Sugiyama et al., 2013).

Collectively, these and other studies have enabled dsm-firmenich to formulate robust reference ranges for serum 25–OH-D₃ levels in pigs. The recommended ranges include:

• 30–60 ng/ml for normal Ca and P metabolism and bone health
• > 60 ng/ml as an optimum level for wider health benefits including muscle growth, reduced farrowing complications, fetal development in-utero, piglet livability and performance.

Conclusion

Lameness is a significant welfare and cost issue in pig production. It is inherently linked to Ca, P and vitamin D status, and all of these can be affected by health status and nutrition. Available data indicate that 25-OH-D₃ is a reliable biomarker for assessing bone health and the risk for lameness, such as diagnosing metabolic bone disease. Pigs reared in modern (indoor) farms without access to sunlight generally have a lower vitamin D status (serum 25-OH-D₃ < 30 ng/ml), regardless of the level of cholecalciferol vitamin D₃ supplementation. In these situations, dietary supplementation with 50 µg/kg 25-OH-D₃ in both the maternal and offspring diets can significantly improve vitamin D status, which in turn will support better bone and joint health and growth performance.