Nutrient requirements to feed the immune system: Its not just about quantity, quality is key!

Introduction

The removal of antibiotics from swine diets has led to a substantial increase in the use of vaccines. Vaccination stimulates an immune response which requires specific nutrients that are not then available for use in muscle deposition.

Nutrients are also required to support immune responses to other challenges (bacteria, viruses, parasites) that are routinely encountered during the normal growth cycle. Mounting an effective response to these challenges can impact negatively on growth performance.

Alongside this, improvements in sow prolificacy have increased the numbers of piglets with low birth weight and intrauterine growth restriction (IUGR). Increased farrowing time has reduced the ‘shot’ of immunity (IgG) that is passed on to piglets via colostrum. These pressures can affect the long term immunocompetency of fatteners, even into finisher phase¹.

The capacity of pigs to address these challenges without losing performance will depend on their adaptability. Specifically, their ability to repartition available nutrients between the (competing) functions of lean muscle development, the immune system and bone health.

This article explores how immune challenge affects the partitioning of nutrients and how dietary supplementation with 25-OH-D3 (hydroxycholecalciferol) is one way that can be used to support immune competency without loss of performance.

Immune challenge alters nutrient partitioning and feed efficiency

Figure 1 illustrates how immune challenge can impair growth in fattening pigs². Different sources of challenge can reduce the mean growth response by up to 40%. Separation of the relative contributions to these reductions reveals that, for most sources of challenge, at least some of the reduction is not due to reduced feed intake (red part of bars) but to feed efficiency (blue part of bars). This implies the reallocation of nutrients away from growth towards immunity under conditions of challenge.

Research at the level of the gut and surrounding tissues indicates that this repartitioning of nutrients involves an initial reduction in protein digestibility (pre-absorptive effect) followed by a re-partitioning of free amino acids away from protein deposition in muscle towards the production of inflammatory proteins (post-absorptive effect)^3,4.

Nutrient requirements for immunity are different than those for growth

When considering whether and how diets can be optimized to prevent this nutrient repartitioning affecting growth, it needs to be recognized that nutrient requirements for immunity differ fundamentally from those required for growth.

For example, the amino acid profiles of acute phase proteins (the first proteins produced in response to inflammation) are completely different from that of lean-mass muscle protein (Table 1). This is illustrated by the example of threonine in fattening pigs where it has been observed that the requirement to support a serum IgG response to ovalbumin injection is ~12% greater than that required for average daily gain⁴. The implication is that if the amino acid composition of the diet is balanced for growth then this may not be adequate to support an effective immune response.

Are we currently feeding correctly to meet the nutrient requirements of the immune system?

If the immune response requires different nutrients to those needed for muscle deposition and the aim is to maintain muscle deposition in the face of immune challenge, this generates the question: ‘Are we feeding correctly to meet the nutrient requirements of the immune system?'

The answer is likely no, because diets are formulated based on recommended nutrient requirements derived from animals reared in isolation, in clean conditions, without immune challenge. Emerging evidence from routine observations on-farm is indicating a rise in the incidence of swine inflammation and necrosis syndrome (SINS), ear and tail necrosis in fattening pigs without obvious diet deficiencies or environmental or health stressors.

25-OH-D3 (hydroxycholecalciferol) as a potential tool to support nutrient requirements for immunity

Vitamin D supplementation represents one potential tool by which to support nutrient requirements for immunity so that lean muscle deposition is maintained.

1. Rationale for using 25-OH-D3 to support nutrient allocation and immunity

The rationale behind the use of 25-OH-D3 to support immunity and growth comes from existing knowledge of vitamin D biology in both humans and animals. In particular, the key involvement of vitamin D in both the regulation and maintenance of skeletal muscle and in immune system integrity.

This two-way relationship is illustrated in Figure 2.

The dual role of vitamin D is illustrated by observations in humans that vitamin D deficiency impairs the development of skeletal muscle in conditions of oxidative stress by disrupting mitochondrial function, leading to reduced protein synthesis and muscle atrophy⁶.

The beneficial effects of vitamin D on the immune system are two-fold, acting on both the innate and adaptive immune systems via interactions with vitamin D receptors on immune cells. In humans, research has shown that when levels of the more biologically active form of vitamin D known as 25-OH-D3 (hydroxycholecalciferol) reach sufficient levels (e.g. through dietary supplementation), it exerts an antibacterial effect by inducing monocyte and macrophage activity to kill bacteria and separately decreases inflammation and autoimmunity by inducing dendritic cells to release a cascade of cytokines⁷.

2. Examples of immune-benefits of 25-OH-D3 (Hy-D®) supplementation in pigs

dsm-firmenich’s 25-OH-D3 (Hy-D®) additive has been developed to support nutrient allocation and metabolism in response to routine immune challenges that are encountered during the normal growth cycle. Some examples of its proven beneficial activity are given below.

Example 1: Increasing supplementation of 25-OH-D3 in the range 5.5 to 155.5 μg/kg alleviated diarrhea in piglets infected with porcine epidemic diarrhea virus (PEDV)⁸. The beneficial effect was mediated via a reduction in serum diamine oxidase activity indicating reduced gut permeability and increased maintenance of intestinal tight junction structure, likely via an antioxidant effect⁸.

Example 2: Supplementation of 25-OH-D3 in place of classical vitamin D reduced levels of the acute phase protein haptoglobin in the blood after 18-19 weeks in boars and gilts showing signs of SINS, ear and tail necrosis. It also reduced the percentage of animals with SINS lesions⁹. These changes were accompanied by improved final body weight (+2.3%) and reduced mortality (1.7% vs. 4.3%), which was due to improved availability of nutrients for growth because of the beneficial effects of the 25-OH-D3 on the immune system response (manuscript under preparation).

Example 3: Long term (84 days) supplementation of 25-OH-D3 in growing pigs increased antioxidant gene expression as well as the content of individual amino acids (tryptophan, tyrosine and asparagine) and total protein in muscle⁹.

Example 4: Supplementation of 25-OH-D3 in piglets fed a diet contaminated with mycotoxin (DON) and challenged with endotoxin (LPS) improved vitamin D status, calcium metabolism and increased the immune response to deal effectively with the endotoxin challenge.¹⁰

Conclusion

Today’s fattening pigs are exposed to increased immune challenge from a variety of sources. These challenges affect the partitioning of nutrients across growth, immunity and bone health and can result in a reduction in growth performance if nutrient availability is not adequate. Supplementation with 25-OH-D3 as Hy-D® can be effective in supporting immunocompetency, enabling growth to be maintained in the face of challenges such as PEDV, PRRS, SINS and endotoxins.