Dietary roles of phytate and phytase in human nutrition: A review

Abstract

Phytate is the primary storage form of both phosphate and inositol in plant seeds. It forms complexes with dietary minerals, especially iron and zinc, and causes mineral-related deficiency in humans. It also negatively impacts protein and lipid utilisation. It is of major concern for individuals who depend mainly on plant derivative foods. Processing techniques, such as soaking, germination, malting and fermentation, reduce phytate content by increasing activity of naturally present phytase. Supplementation of phytase in diets results in increase in mineral absorption. Apart from negative effects, its consumption provides protection against a variety of cancers mediated through antioxidation properties, interruption of cellular signal transduction, cell cycle inhibition and enhancement of natural killer (NK) cells activity. It has therapeutic use against diabetes mellitus, atherosclerosis and coronary heart disease and reduces kidney stone formation, HIV-1 and heavy metal toxicity; however, information on the dosage for humans for eliciting beneficial effects is limited.

Introduction

Plant-based food products are the main staple food for human beings in many parts of the world. They constitute an important source of carbohydrates, protein, dietary fibre, vitamins and non-nutrients (Katina et al., 2005). Among all the antinutritional components, phytic acid is of prime concern for human nutrition and health management. The chemical description for phytic acid is myoinositol (1,2,3,4,5,6) hexakisphosphoric acid. The unique structure of phytic acid offers it the ability to strongly chelate with cations such as calcium, magnesium, zinc, copper, iron and potassium to form insoluble salts. It therefore adversely affects the absorption and digestion of these minerals by animals (Raboy, 2001). Salts of phytic acid are designated as phytates (myo-inositol-1,2,3,4,5,6-hexakisphosphates) which are mostly present as salts of the mono- and divalent cations K⁺, Mg²⁺ and Ca²⁺. Phytate accumulates in the seeds during the ripening period and is the main storage form of both phosphate and inositol in plant seeds and grains (Loewus, 2002). Phosphorus, in this form, is not utilised by human beings, dogs, pigs, birds or agastric animals because they lack the intestinal digestive enzyme phytase (Holm, Kristiansen, & Pedersen, 2002). Phytate works in a broad pH-region as a highly negatively charged ion and therefore its presence in the diet has a negative impact on the bioavailability of divalent and trivalent mineral ions such as Zn²⁺, Fe^2+/3+, Ca²⁺, Mg²⁺, Mn²⁺ and Cu²⁺ (Fredlund et al., 2006, Lopez et al., 2002, Lönnerdal, 2002). Besides, phytate has also been reported to form complexes with proteins at both low and high pH values. These complex formations alter the protein structure, which may result in decreased protein solubility, enzymatic activity and proteolytic digestibility. Hitherto, massive investigations have been carried out on the negative aspects of phytate that have offered overwhelming evidence that dietary phytate is an antinutrient component. As a solution, the phytate-degrading enzyme, phytase, is in vogue for degradating phytate during food processing and in the gastrointestinal tract. Major efforts have been made to reduce the amount of phytate in foods by different processes and/or the addition of exogenous enzymes. In spite of many negative aspects on human health, the consumption of phytate, however, has been reported to have some favourable effects. The outcome of surveillance of populations consuming vegetarian-type diets has shown lower incidence of cancer, which suggests that phytate has an anticarcinogen effect (Shamsuddin, 2002, Vucenik and Shamsuddin, 2003). The metal binding characteristics of phytate endow it an anti-oxidant function, inhibiting the production of hydroxyl radicals that normalise cell homeostasis (Minihane & Rimbach, 2002) and it also acts as a natural food anti-oxidant (Raboy, 2003). Dietary phytate may have health benefits for diabetes patients because it lowers the blood glucose response by reducing the rate of starch digestion and slowing gastric emptying (Thompson, 1993). Likewise, phytate has also been shown to regulate insulin secretion (Barker & Berggren, 1999). It is believed that phytate reduces blood clots, cholesterol and triglycerides and thus prevents heart diseases (Jariwalla et al., 1990, Onomi et al., 2004). It is also suggested that it prevents renal stone development (Grases et al., 2000a, Grases et al., 2000b, Selvam, 2002). It is used as a complexing agent for removal of traces of heavy metal ions (Wise, 1982). In vitro studies have indicated that phytic acid incubated with HIV-1 infected T cells inhibits the replication of HIV-1 (Otake et al., 1999, Otake et al., 1989). Hitherto, many literature reviews, primarily focussing on the antinutritional aspects of phytate, have been published but information on the beneficial effect of phytate is still very scarce and scattered. The purpose of this review is to discuss both negative and prophylactic and therapeutic effects of phytate and the mechanisms responsible for these effects.