What is Phytic Acid and Why is it a Problem?
Phytic acid, or phytate when in its salt form, is the primary storage form of phosphorus in many plants, including cereals, legumes, nuts, and seeds. In the plant, it serves as a phosphorus reserve for germination. However, when consumed by humans and other monogastric animals (those with a single-chamber stomach), it acts as a potent anti-nutrient.
The issue arises because phytic acid has a high negative charge and binds strongly to positively charged molecules, such as essential minerals. In the digestive tract, this leads to the formation of insoluble complexes with minerals like zinc, iron, calcium, and magnesium, making them unavailable for absorption. Beyond minerals, phytic acid can also bind to proteins and digestive enzymes, further hindering overall nutrient digestion. This is particularly relevant for individuals on high-phytate, plant-based diets, and in developing countries where these foods are dietary staples.
The Role and Mechanism of Phytase
Phytase is an enzyme that catalyzes the hydrolysis of phytic acid, a process known as dephytinization. It breaks the phosphate bonds in the phytic acid molecule, sequentially releasing inorganic phosphate and creating lower-inositol phosphate esters. The primary function is to make the bound nutrients, especially phosphorus and other minerals, available for absorption.
Different types of phytases, such as fungal and bacterial phytases, operate with varying efficacy depending on factors like pH stability and resistance to other digestive enzymes. A key factor for effectiveness is the enzyme's rapid action in the acidic environment of the stomach, where phytic acid is most soluble and susceptible to degradation. By breaking down phytic acid early in digestion, phytase prevents the formation of nutrient-chelating complexes that would otherwise interfere with absorption in the small intestine.
Phytase and Mineral Absorption
Research has consistently shown that phytase supplementation significantly enhances mineral bioavailability, with the most robust evidence concerning iron and zinc. Studies have found that phytase can substantially increase the absorption of these minerals, particularly from phytate-rich meals. In addition to iron and zinc, phytase helps release bound calcium, magnesium, and other minerals, contributing to better overall nutritional status. For instance, a review of human intervention trials found that the majority of studies demonstrated improved iron and zinc bioavailability following phytase supplementation or food dephytinization. This is crucial for populations relying on plant-based foods, where mineral deficiencies can be common.
Phytase and Protein Digestion
Phytase's benefits extend beyond mineral release to include improved protein digestibility. Phytic acid can bind to positively charged amino acid residues in the stomach, forming insoluble protein-phytate complexes. These complexes are resistant to enzymatic hydrolysis by proteases like pepsin, which impairs protein digestion. By hydrolyzing phytic acid, phytase disrupts these complexes, making proteins more accessible to digestive enzymes. Furthermore, phytate has been shown to increase the loss of endogenous amino acids (those secreted by the body for digestion). By mitigating this effect, phytase can reduce protein requirements for maintenance and increase amino acid availability for growth.
Comparison: Standard Diet vs. Phytase-Supplemented Diet
| Feature | Standard Plant-Based Diet (High Phytate) | Phytase-Supplemented Plant-Based Diet |
|---|---|---|
| Mineral Bioavailability | Reduced availability of minerals like iron, zinc, calcium, and magnesium due to phytate binding. | Significantly enhanced absorption of minerals as phytase breaks down phytate complexes. |
| Phosphorus Utilization | Low utilization of phytate-bound phosphorus, requiring inorganic phosphate supplementation in many animal feeds. | Efficient utilization of plant-based phosphorus, reducing the need for expensive inorganic phosphate supplements. |
| Protein Digestion | Impaired digestion due to the formation of insoluble protein-phytate complexes and inhibition of digestive enzymes. | Improved protein and amino acid digestibility as phytate complexes are broken down. |
| Gastrointestinal Effects | Increased endogenous losses of amino acids and other nutrients from the intestinal lining. | Reduced endogenous nutrient losses and better intestinal health. |
| Environmental Impact | Higher phosphorus excretion into the environment from undigested phytate in animal waste. | Reduced phosphorus excretion, leading to a smaller environmental footprint. |
Factors Influencing Phytase Efficacy
The effectiveness of phytase can vary based on several factors. The source of the enzyme (microbial vs. plant), the dosage, and the specific composition of the diet all play a role. For example, the presence of high levels of calcium can interfere with phytase activity by increasing the formation of insoluble phytate complexes. Additionally, the efficiency of different commercial phytase products can vary widely due to differences in optimal pH and resistance to digestive proteases. A critical review of phytase efficacy in animal nutrition provides further detail on these variables.
Applications Beyond Animal Nutrition
While widely used in the animal feed industry to improve efficiency and reduce environmental phosphorus pollution, phytase has potential applications in human nutrition and food processing as well. Techniques like soaking, germination, and fermentation utilize natural phytase activity to reduce phytic acid content in foods. For example, studies have shown that adding phytase to bread-making can reduce its phytate content and improve mineral release. Microbial phytases are also being explored for human food fortification to address widespread mineral deficiencies.
Conclusion
Phytase is a powerful enzyme that significantly improves nutrient absorption by neutralizing the anti-nutritional effects of phytic acid. By breaking down phytate, it liberates essential minerals like iron, zinc, and calcium, and enhances protein digestibility. While the benefits are well-established in animal nutrition, its application in human food processing and supplementation offers a promising strategy for combating mineral deficiencies, particularly for individuals on high-phytate diets. Understanding the factors that influence its efficacy is key to maximizing its nutritional benefits. Further research, particularly on long-term human health outcomes, continues to build on the existing evidence supporting phytase as a valuable nutritional tool.
