Phytic acid (Inositol Hexaphosphate- IP6) is a low solubility compound present in broiler feed. It has an anti-nutritive factor as Phosphorus is present in it in a bound form that decreases its availability in the body. To compensate for the deficiency of Phosphorus, additional phosphorus in the form of Dicalcium Phosphate is added in the diet that leads to an increase in feed cost. Moreover, the bound Phosphorus passed from excreta without being absorbed, damages the environment, and depletes the non-renewable resources from which Phosphorus is extracted. Phytic acid gets complexed with protein, starch, and other minerals, thus reducing their availability to the animal. This property to chelate with various important compounds makes the Phytic acid to withstand further digestion. To overcome this problem exogenous phytase in feed is incorporated. But recently Phytase superdosing i.e. use of high doses of enzyme phytase with growth-promoting effects is being widely accepted. In this article, the science behind superdosing and how it helps to increase animal performance is discussed.
What is superdosing and the need of exogenous Phytase?
Application of an intrinsically thermostable highly efficient phytase at high doses i.e., 3-4 times the standard dose (300-500 FTU/kg) developed specifically to target near-complete phytase destruction is called superdosing. 1 FTU equals the amount of enzyme needed to release 1 micromole of inorganic Phosphorus per minute from 5.1 millimole sodium phytate at 37 and pH 5.5 (Williams,2014).
Phytase can chelate with various positively charged mineral cations. This ability increases as pH gradually increase along the digestive tract. Most mineral phytates are soluble in acidic pH (pH 2-3) and since in proximal part of the intestine, the pH is comparatively high (pH 4-5). Therefore, poor substrate solubility is there in the small intestine. This is the major limitation for phytate digestion by the endogenous enzyme (non-specific Phytases or alkaline phosphatases) that usually act in alkaline pH (Peyton Gilbert- 2017). Hence the problem with phytate digestion is not the lack of compatible endogenous enzyme but poor substrate solubility in the small intestine.
Mechanism of action of Phytase at standard doses
Phytate (IP6) in the presence of exogenous phytase gets degraded to IP5 in the acidic medium in crop and proventriculus and releases the bound phosphorus present in the phytate molecule. Higher molecular weight esters such as IP6 and IP5 have higher chelating activity than lower molecular weight esters such as IP4, IP3, IP2 and IP1. Exogenous phytase when added in the feed dephosphorylate IP6 in proximal GIT to lower molecular weight esters of IP. This decreases the anti-nutritive value of IP6 and minimizes the leakage of IP6 and IP5 into the duodenum. In an ideal situation, depending upon the dose of Phytase, there is complete hydrolysis of phytate to yield Inositol molecule andz Phosphorus. However, in the in vivo condition, hydrolysis will be incomplete and there is a mixture of IP5, IP4 and IP3. Therefore, to aid the complete hydrolysis of Phytate, the concept of superdosing comes into play.
How superdosing works in the body?
Phytase at standard dose degrades IP6 and produces IP5, may get further degraded up to IP4 and IP3 depending upon the dose. Thus, diminishes the chelating property and anti-nutritive value of IP6. But the problem doesn’t get solved with the degradation of IP6 to IP5. IP5, IP4, IP3 binds with Zinc much more efficiently and increases its precipitation. Zinc being a cofactor for various pancreatic enzymes gets scarce and unavailable. Thus, there is a reduction in protein digestion. With standard phytase dosing, there would be just replacement of one anti-nutritive compound with the other. If the focus remains only on removing IP6, a considerable amount of IP5, IP4, IP3 would be left in the gut under exploited, and essential nutrients that are to be absorbed would get compromised. Phytase shouldn’t just effectively break IP6 but also should diminish the accumulation of lower esters. Superdosing not only degrades IP6 but also decreases the buildup of IP5, IP4, IP3. Phytase should not only release Phosphorus but should also avail various minerals and protein to which Phytate gets bound. New generation Escherichia coli Phytases target up to 90% destruction of phytate (IP6) and lower esters (IP5, IP4, IP3).
Importance of released INOSITOL
Inositol produced after the removal of Phosphorus from IP1 functions as follows-
Growth promoting effects in chicks.
- Fat metabolism and cell functioning.
- Inositol+ Phosphorus → Phytase (a potent antioxidant in the cell)
It is quite interesting that exogenous phytase is unable to break IP1 and liberate Inositol. Exogenous phytase acts primarily in the acidic environment of crop and degradation of IP1 molecule occurs in the duodenum where the pH is alkaline, therefore, phytase activity is limited to a proximal end of GIT, dephosphorylation of higher IP must occur here. Last IP1-Pi bond is broken down by nonspecific phosphatases (Johanna Hirvonen-2019). Hence, in order to completely destroy the phytase molecule, there should be increased in feed retention time in a crop that allows exogenous phytase to hydrolyze a greater no. of higher esters which in turn increases the efficacy of endogenous phytase on lower IP in the upper small intestine.
Attributes associated with superdosing
- Characteristics of Phytase necessary to drive phytate to near elimination and so maximize Inositol release-
- Thermo stability (withstand high pelleting temperature)
- Type of phytase (bacterial or fungal origin)
- Good gastric tolerance and stability to resist the bird’s own enzymes.
- Acidic pH stability (pH-2-3).
- High rate and extent of Phytate breakdown (sustainable activity even at low phytate concentration)
- Release of additional Phosphorus-
This level of Phytate destruction yields additional Phosphorus compared to standard Phytase dosing. But this has been shown that Phosphorus requirement of superdosed birds also may be higher due to subsequent increase in growth rate and use of Phosphorus to rephosphorylate the additional available Inositol at the cellular level.
Benefits rendered by superdosing of Phytase
Phytase being the enzyme to degrade Phytate avails various cations, minerals and other nutrients that help in the growth of the animal.
Complete growth and development of poultry require proper availability, utilization, and absorption of nutrients. Phytase when added in standard dose just breaks the Phytate molecule to produce other anti-nutritive compounds which further binds with indispensable minerals and nutrients decreasing their availability. Thus, it is nowhere less harmful to the bird’s system. This somewhere makes the addition of Phytase a waste. To draw complete profit out of the addition of phytase and poultry production, Superdosing stands out to be a mandate approach as it culminates with the complete destruction of Phytate and other subordinates. Therefore, superdosing is imperative and should be practiced in order to get complete benefits out of the addition of Phytase.
References are available upon request.
by Dr Pooja Rawal, GBPUAT, Pantnagar