Global meat consumption continues to rise: according to the OECD-FAO, it is projected to increase by around 1.6% a year from 2013 to 2022. Poultry meat uptake per person will increase faster than that for pork or beef, growing by some 9% between 2013 and 2022. Increasing meat yield is essential to meet the extra demand.
Legislative constraints on poultry production, such as the environment, hygiene and welfare, have increased production costs and pushed up profitability thresholds. Further commitments under quality schemes have also increased costs and so margins for broiler producers largely depend on final product characteristics, such as meat quality, the live weight of the birds, the feed conversion rate and meat yields.
When the birds are processed, the meat products – such as the whole carcass, the parts and the further processed products – are sold and distributed. Nowadays, the global broiler industry has advanced to the point that every gram of meat has value, with the most valuable part of the broiler generally being the breast in many countries. Breast meat yield may therefore offset ‘losses’ incurred in feed conversion rate and/or daily gain.
In broilers, increased serum levels of 25-hydroxycholecalciferol (25-OH-D3, tradename Hy·D®), the first metabolite of vitamin D3, have been positively correlated with increased body weight, improved feed conversion and higher breast yield. Figure 1 demonstrates the increase of breast meat yields obtained in several experimental and field trials with broilers when fed with HyD®, compared with a diet supplemented only with vitamin D3. The outcome of 26 trials revealed that the addition of Hy·D® to the broiler diet can enhance breast meat production by almost 1% compared with standard production methods in which the feed is supplemented with vitamin D3 as the sole source of vitamin D.
25-OH-D3 promotes muscle formation and development.
The impact of 25-OH-D3 on skeletal muscle development is an exciting and interesting area of research. Some preliminary studies have attempted to shed light on the action of this metabolite of vitamin D3 on the myogenesis to explain the excellent results obtained at the processing plants.
Berri et al. (2013) and Hutton et al. (2014) have conducted similar studies to evaluate the effect of 25-OH-D3 on skeletal muscle growth and on the involvement and activity of the satellite cells in broiler chickens.
It is well known that, during the embryonic phase, the cells called myoblasts proliferate and differentiate, thanks to growth factors and hormones, so as to generate the muscle fibres. During this process, the fibres increase in number (hyperplasia) and then their number is fixed at birth. After hatching, the muscle cells can only grow in size (hypertrophy) if additional DNA is available. The satellite cells are the source of the DNA that enables cell proliferation.
Nowadays, the global broiler industry has advanced to the point that every gram of meat has value, with the most valuable part of the broiler generally being the breast in many countries.
In the studies of Berri and Hutton, the gene expression of certain myogenic factors were analysed to determine if there is a link between them and the circulating levels of 25-OH-D3. The results showed a greater proportion of proliferative satellite cells in the breast muscles of the birds fed with a combination of vitamin D3 and 25-OH-D3, compared with the broilers fed only with vitamin D3. Moreover, the number of vitamin D receptors (VDRs) was increased in chicken supplemented with Hy·D®, suggesting that this metabolite can enhance the metabolic activity of vitamin D and modulate the expression of specific genes linked to muscle development. These preliminary results may help to explain the enhancement of broiler breast meat yields at the processing plant.
Vignale et al. (2015) carried out another study to determine the effect of 25-OH-D3 on protein synthesis through the activation of the mechanistic target of the rapamycin (mTOR) pathway.
The mTOR is a protein that is a key regulator of cell growth and proliferation in response to nutrient availability and growth stimuli, and is related to protein synthesis.
In this trial, four different feed treatments were tested:
The analysis of the expression of the mTOR, the gene related to protein synthesis, showed a significantly higher expression of this gene in treatment (3), birds fed 25-hydroxycholecalciferol for the full cycle. In the same group of birds, a significantly higher protein synthesis rate was also found. Moreover, in agreement with previous results, the broilers supplemented with HyD® for the full period also showed a markedly higher concentration of VDRs, suggesting again a link between the circulating levels of 25-OH-D3, the concentration of VDRs, and the gene transcription and activation.
As shown in Figure 2, the birds whose diet was supplemented with Hy·D® also performed better in terms of breast meat production. Surprisingly, birds fed with a higher amount of vitamin D3 showed the same meat production as did broilers fed half the dose of D3.
Collectively, these findings indicate that improving the vitamin D status by feeding 25-hydroxycholecalciferol to broilers can stimulate the activity of the satellite cells, which are muscle fibre support cells necessary for increasing fibre hypertrophy (size) and which also stimulate protein synthesis. It is important as well to notice that levels of 25-OH-D3 in the blood are not affected by doubling the vitamin D3 content of the diet (Figure 3). More studies are probably necessary to elucidate all the aspects of the metabolic effect of 25-OH-D3, but the research conducted so far indicates a strong correlation between dietary supplementation with 25-OH-D3 and improved breast meat production in broilers.
References available upon request