Calcium and phosphorus are the framework for bone and eggshell development, resulting in muscle growth and egg production – the essentials for poultry profitability.
Calcium is necessary in the body for a wide range of functions including nerve transmission, muscle contraction and blood clotting. Normally, these functions demand relatively small amounts of Ca. The total Ca requirement is mostly driven by the need for bone growth and maintenance as well as by eggshell formation in layers and breeders. Blood Ca levels are strictly regulated. When these levels drop relative to the minimum blood level, the hormonal mechanisms increase Ca recovery from the kidney, Ca mobilization from the bone and the efficiency of Ca absorption from the diet. Conversely, when blood Ca levels are high, the hormonal mechanisms increase Ca excretion from the kidney, stimulate bone formation and downregulate the active Ca transport mediated by vitamin D from the gut.
In growing animals, Ca requirements largely reflect the need for bone growth and maintenance. Since bone growth is proportionately faster in young birds and it decreases with age, Ca requirements as a percentage of the diet tend to drop as broilers age. Normally, in broilers, Ca is required at a ratio of approximately 2:1 to available P. As long as this ratio is maintained, there will be a margin of Ca in the diet on which bone quality is optimized.
The role of Vitamin D
Vitamin D plays a critical role in Ca and P homeostasis in poultry, and it is unique among vitamins in that it can be synthesized in the skin of animals when it is exposed to ultraviolet light, i.e., sunlight. However, even in open barns, there is limited exposure to sunlight, thus, poultry generally require dietary supplementation with vitamin D.
Traditionally, vitamin D has been supplemented in poultry diets in its crystalline form as cholecalciferol, which is an inactive precursor of the form that the body is able to utilize. This form of vitamin D is absorbed in the digestive tract, and it is converted to an intermediate form, 25-hydroxycholecalciferol (25-OH vitamin D3 or 25-OHD) in the liver. This natural metabolite of vitamin D is the main source of vitamin D activity in the body, even in birds that have only received vitamin D in the diet.
The active form of vitamin D is 1,25 (OH)2 vitamin D3. The conversion of 25-OHD to the active form occurs in many organs, but mainly in the kidney. Because of its toxicity, it is also strictly regulated in the bird. 25-OHD is absorbed in the gut more efficiently than vitamin D because its absorption is less dependent on micelle-mediated absorption, thus overcoming the low absorption efficiency of fats and fat-soluble vitamins that occurs at an early age in young birds. On the other hand, in very young chicks and in birds whose liver function may be impaired (for example, due to mycotoxins or diseases), the conversion of vitamin D to its active form may be lower and, consequently, the absorption of Ca and P will be limited. This does not occur when the 25-OHD metabolite is supplemented because no metabolic process is necessary in the liver. Nowadays, this natural metabolite is commercially available as a dietary supplement (Hy-D®).
The absorption of Ca
The absorption of Ca in the digestive tract involves both active and passive mechanisms. When Ca demands are low and dietary Ca levels are high relative to the bird´s requirements, Ca is u sed mainly for paracellular transport that is not energy-dependent and occurs through the junctions between epithelial cells to absorb Ca from the intestine. On the other hand, when demands are high relative to the dietary Ca content, 25-OHD is converted in the kidney to the active form of vitamin D3 (1,25 (OH)2D3). This active form of vitamin D upregulates several mechanisms to increase the active transport of Ca through the intestinal cell itself (transcellular absorption).
Phosphorous, besides being a structural component of cell membranes, participates in several physiological processes, as muscle tissue formation, enzyme activation processes, osmotic maintenance, and the formation of the adenosine-triphosphate molecule. It is also crucial for the formation of collagen and skeletal mineralization. Phosphorous is one of the nutrients with the highest economic impact in the animal production industry, considering the animals’ requirements and the cost involved in diet formulation. Phosphorous absorption can occur through two different pathways: sodium-dependent and sodium-independent. The sodium-dependent transport is not influenced by the Ca concentration present in the enterocyte membrane. Therefore, the transport of Ca and P appears to occur separately. On the other hand, the sodium-independent transport seems to be related to the amount of Ca present. Thus, the Ca:P ratio influences phosphorus absorption through this pathway.
It is well known that after feed intake, the rise in P concentration favors paracellular transport. However, transcellular transport, which involves energy expenditure, is stimulated by the presence of vitamin D, and it occurs through sodium-dependent co-transporters (type I, II and III). The type II Na-P co-transporters (IIa, IIb and IIc) seem to more related with the transcellular transport of P in birds. Another important factor in the absorption process is the structural form in which P is found in the intestinal lumen. The main form of phosphorous reserves in plants is phytin, solubilized in the proventriculus and the gizzard in a pH range between 2.5 and 3.0. It is called phytic acid, and it is free and reactive. However, with the gradual increase in pH as the feed reaches the posterior segments of the GI tract, between approximately 5.0 to 6.0, the precipitation of phytic acid and its binding to other minerals and amino acids, in addition to phosphorous, are favored, making them unavailable to monogastric animals. To mitigate the negative effects of the presence of phytic acid in the GI tract, exogenous phytase is widely used nowadays in animal production, with the purpose of hydrolyzing phytic phosphorous and releasing the nutrients trapped in the molecule, thus favoring their absorption and that of other compounds in the diet.
The benefits of using phytases
Phytases are a diverse group of enzymes covering a range of sizes, structures and catalytic mechanisms. The reaction promoted by the phytase is the dephosphorylation of the myo-inositol- 1,2,3,4,5,6-hexakiphosphate molecule, characterized by the removal one by one of the phosphate groups attached to the myo-inositol ring. The following are the main benefits of using phytase: reducing the use of inorganic phosphates in the diet, by increasing the availability of phytic phosphorous from plant ingredients; reducing the antinutritional factors of phytic acid, by lowering the endogenous losses and favoring the utilization of minerals, amino acids and energy. An increase in the availability of P and other nutrients also plays an important role in terms of the environment, since reducing the excretion of these elements into the environment is extremely important, especially in regions where animal production is highly concentrated. Therefore, the effective use of phytase can reduce the need for both Ca and available P supplementation.
Adding 25-OHD to the diet
Along with higher concentrations of circulating Ca and P and increased blood levels of 25-OHD (as assessed by the DBS test), improvements in meat yield have also been reported with the use of 25-OHD in broiler diets. Many cell types have vitamin D receptors (VDR) and the binding of active vitamin D to these receptors leads to an altered gene expression in several tissues. Thus, supplying 25-OHD as a partial or total replacement of vitamin D increases meat yield (mainly breast meat) due to the upregulation of the vitamin D receptors in the breast muscle cells, as well as of the genetic control factors of muscle protein synthesis. Many types of immune cells also express the vitamin D receptor. Adding 25-OHD to the diet protects broiler growth from the effects of systemic inflammation. This protective effect has been associated with a decrease in Interleukin 1, which is an important pro-inflammatory cytokine that causes decreased appetite, higher metabolic rates and the use of skeletal muscle for the protein synthesis of the immune response.
Several studies have also shown that broilers supplemented with 25-OHD have a higher ratio of intestinal villus height to crypt depth, compared to broilers supplemented solely with vitamin D. This seems to indicate that the surface area of intestinal absorption is maintained or is greater, while the rate of intestinal cell turnover is lower. Therefore, maintaining intestinal digestive efficiency with reduced metabolic cost (due to lower cell turnover) and, in turn, a greater area of nutrient absorption, may contribute to the increase in feed efficiency often observed when birds are supplemented with 25-OHD.
Hy-D® supplementation can partially or completely replace vitamin D in poultry diets. Additionally, to fulfill the basic functions of vitamin D, 25-OHD has an impact greater than vitamin D on several metabolic systems, including the intestinal, skeletal, muscular, immune and reproductive systems. It has also been fully demonstrated that phytases decrease the requirement for both Ca and available P supplementation in poultry diets. RONOZYME® HiPhos and HiPhorius™ have exceptional phytic acid degradation, leading to greater availability of dietary P and less P excreted into the environment. Therefore, a combination of feed additives that favor the metabolism and the availability of minerals, diminish their effect on the environment and reduce the cost of the diet, will undoubtedly be an adequate strategy for modern broilers.