Phytate has direct anti-nutritional effects on poultry and swine, causing reductions in performance by lowering amino acid and mineral absorption and increasing endogenous losses.
PHYTIN (a deposited complex of inositol hexaphosphate with potassium, magnesium and calcium) - more recognized in animal nutrition as phytate - is the major storage form of phosphorus present in seeds.
Originally recognized as a source of phosphorus during germination, the presence of phytin in the seed during the germination process is now also known to play an important role in preventing oxidative stress and, thus, preventing embryo death. This is thought to be related to the ability of phytic acid to chelate minerals such as iron, calcium and zinc, which reduces the formation of reactive oxidative compounds that could be created during germination, when vegetable cells pass through a high anabolic/catabolic process.
In animal nutrition, phytate is important as a possible source of phosphorus for poultry and swine. Ruminants have a fermentation process during which phytate can be broken down by bacteria, releasing phosphorus to be absorbed by the animal. However, phytate has long been recognized as a non-available phosphorus "source" for poultry and swine because monogastrics do not have enzymes to hydrolyze phytate.
Actually, researchers have now shown that the low absorption of phytic phosphorus isn´t related to a lack of ability of poultry and swine to absorb this phosphorus source because low mineral diets result in phytic phosphorus absorption as high as 70%.
Rather, the phytic acid molecule interacts with other minerals and proteins present in the intestinal tract, making phytic phosphorus less available (or digestible) for poultry and swine.
When reacting with other nutrients present in the intestinal tract of poultry and swine, phytate not only reduces the digestibility of these nutrients but also causes an endogenous response by the animal. Both of these effects are associated with the anti-nutritional effects of this molecule.
A better understanding of the effect of phytate on nutrient digestibility and its anti-nutritional effects opens up an opportunity to improve animal performance and goes beyond viewing phytate solely as a possible phosphorus source for the animal.
Phytate is a complex molecule with 12 different pKa values (acid dissociation constants), two for each phosphate group. This complex subject can be simplifi ed by saying that even at a low pH(below 2.0), the phytate molecule will still be negatively charged and will become more negative as the pH increases.
This is important because when phytate passes through the digestive tract, it goes from a lower pH (in the stomach/gizzard) to a neutral pH in the lower intestine. Thus, as pH increases, phytate increases its negative charge and, as a consequence, its desire to react with cations (primarily divalent cations such as calcium, zinc and copper). As a result,stable salts are formed that precipitate out of solution (illustrated in Figure 1).
Even though phytate has a higher affinity for cations such as copper and zinc, it is the affinity to calcium that causes major concerns in animal nutrition due to the higher concentration of this mineral in animal feeds.
In vivo trials have shown that highcalcium diets reduce phytic-phosphorus absorption in broilers, and an increased dietary phytate concentration also increases the animal´s requirement for calcium.
Once the amount of calcium and phytate in solution exceeds a critical concentration, salt formation and precipitation occur, reducing the amount of calcium available in the intestine forabsorption. The calcium requirement for broilers increases from 0.60% to 0.95% when the phytic phosphorus level in the diet increases from 0% to 0.25% (the latter being the approximate phytic phosphorus concentration in a standard corn/soybean diet).
In some feeds, primarily piglet prestarter and starter diets, copper and zinc may also be important cations, where high inclusion rates of these minerals are used as growth promoters.
Blood concentrations can be an indication of animal mineral status, and a recent study showed that serum zinc concentrations were reduced in the presence of high phytate levels, suggesting that high dietary phytate may reduce the availability of zinc and potentially copper (Figure 2).
This is already an important issue in human nutrition, where mineral supplementation may be necessary for diets based on a high cereal/vegetable content.
In addition to piglets, animals that have a longer production period, such as from this interference, which causes suboptimal levels of minerals, even though these animals often receive a high level of mineral supplementation.
Protein digestive processes start in the stomach, where secreted pepsinogen is activated to pepsin, the active endogenous enzyme that initiates hydrolysis of feed protein. In vitro assays have shown that the presence of phytate reduces pepsin activation between pH 0.8 and 2.8. This may result in less protein being initially digested in the acid phase of the digestive process in poultry and swine and, as a consequence, may affect overall protein digestibility.
Besides reducing pepsin activation,which could be overcome by higher pepsinogen production in the stomach, phytate presence also directly reduces protein solubility and consequent digestibility. The initial hypothesis to explain this reduction in digestibility is that at a low pH, most of the feed proteins, particularly those with high concentrations of basic amino acids, will be below their isoeletric point and, thus,will be positively charged. This would attract the protein molecule to phytate, which is negatively charged at a low pH, causing an initial phytateprotein link and, consequently, a proteinprotein link, thus reducing protein solubility and digestibility. A new hypothesis proposes that this phytate-protein link does not necessarily occur because high phytate solubility at a low pH would be expected to increase rather than reduce protein solubility. This hypothesis claims that the presence of phytate in solution changes the water conformation of the solution, moving water molecules closer to phytate and farther away from the protein molecules (Figure 3, page 20). This reduced amount of water around the protein molecule would result in a reduction in protein solubility and, consequently, reduced digestibility.
Digestive secretions are regulated by intrinsic and extrinsic stimuli. Gastric secretions of hydrochloric acid and pepsinogen, for example, will be regulated by factors such as visual and odor stimulus, stomach distension (which is more important in swine than in poultry) and the presence of specific components in the gut.
The presence of undigested protein in the lower part of the intestinal tract will stimulate hormone secretions such as gastrin and cholecystokinin, which will stimulate hydrochloric acid and pepsinogen secretion in the stomach while reducing gastric empting. An increased concentration of peptides and amino acids in the lower part of the gut will have the opposite effect.
Whatever the cause, phytate can reduce protein solubility and consequent digestibility in the gut. This will result in more undigested protein reaching the duodenum, stimulating gastrin and, thus, hydrochloric acid and pepsinogen secretion in the stomach.
It has been shown that the pH in the gizzard at seven and 21 days of age is lower in broilers fed a higher-phytate diet, which is presumably related to higher hydrochloric acid and pepsinogen production in the proventriculus.
Another consequence of this increased hydrochloric acid and pepsinogen production is an increase in the animal´s endogenous losses. This is because higher hydrochloric acid and pepsinogen production has an irritant effect on the gut mucosa, which is compensated for by increased production of mucus as a protective layer (Figure 4, page 20). Also, once the digesta gets to the duodenum, the pancreas needs to secrete a greater amount of sodium bicarbonate to increase pH and to compensate for the lower stomach pH.
This excess sodium usage can compromise the absorption of amino acids that depend on active transportation via the sodium/potassium pump. Phytate has been demonstrated to increase sodium and sialic acid (a mucus marker) excretion, as described in Figure 3.
Animals, particularly swine and poultry,have always been fed diets with different phytate concentrations and, thus, have suffered from the anti-nutritional effects of phytate. The main problem in studying this anti-nutritional effect is how to vary the concentration of phytate in diets and determine its effect on animal physiology and performance.
Trials evaluating the anti-nutritional effects of phytate through the use of high-phytate ingredients such as rice bran or comparing a vegetable diet with diets containing high levels of animal byproducts have already been done.
The problem with this approach is the difficulty with isolating one factor and being sure that any difference in animal performance is related to the phytate concentration of the diet rather than other differences in the composition of the feed (e.g., fiber, mineral and aminoacid digestibility) when these different ingredients are included.
One possibility is to include synthetic phytic acid in the diet, which artificially increases phytate without significantly changing diet composition (Figure 5).
When using a synthetic compound, one important factor to take into account is the choice of phytic acid source.
Some phytic acids have low solubility, making them less likely to cause the anti-nutritional effects seen with the natural phytic acid present in vegetable feedstuffs. Natural phytate can also vary in solubility at a low pH. Thus, the antinutritional effects will depend on the ingredient supplying the phytate.
Another possibility is to use a higher concentration of phytase in the diets, which aims for a quick reduction in phytate concentration in the stomach and, thus, a reduction in reducing the anti-nutritional effects.
In this approach, as phytate hydrolysis will liberate phosphorus molecules that can be absorbed by animals, it is important to feed diets that are not limiting in phosphorus to be sure any improvement in performance is related to the reduction of the antinutritional effects of phytate and not to an increase in phosphorus digestibility. This approach is the opposite of what is normally recommended when comparing or evaluating phytases.
In this case, the enzyme should be included in a reduced-phosphorus diet, with enzyme activity evaluated by the improvement in performance and bone parameters resulting from the higher phosphorus digestibility.
Phytate has direct anti-nutritional effects on poultry and swine, causing reductions in performance by lowering amino acid and mineral absorption and increasing endogenous losses. Evaluating this anti-nutritional effect is not simple because any change in the diet to reduce the phytate concentration will result in other modifications in diet composition that may also interfere with animal performance.
One alternative is to use high levels of phytase in the diet, with the goal that the phytase will hydrolyze and, thus, reduce the anti-nutritional effects of phytate. In this case, it is important to ensure that the diet is not limited in phosphorus and to use a phytase product with a higher capacity to degrade intact phytate, which is not necessarily related to the ability to release phosphorus or even related to enzyme activity determination in vitro.
This article was originally published in Feedstuffs, Vol. 84, No. 04, January 23, 2012.