Tempeh and tofu start as the same thing. Same soybeans. Same iron content, same zinc, same calcium sitting inside the same raw legume.
One of them delivers those minerals to your bloodstream. The other lets most of them pass straight through your gut, unused. The difference isn't about how much iron is inside each food.
How Phytase in Tempeh Improves Iron Bioavailability
Soybeans store their minerals in a molecular cage. A compound called phytate, found in every legume, wraps itself around iron, zinc, and calcium and holds on tight. Your digestive system cannot break that grip. The minerals travel through your gut still locked inside their cage, and you absorb almost none of them.
Fermentation rewrites that story. During the days tempeh spends developing its dense, sliceable texture, fungi growing through the soybeans produce an enzyme called phytase. Phytase does one specific job: it dismantles phytate, piece by piece, converting it into smaller molecular fragments that can no longer hold onto minerals. By the time tempeh reaches your plate, much of the iron that was trapped is now free and available for absorption.
During tempeh fermentation, fungi produce an enzyme called phytase that breaks down phytate, the compound in soybeans that locks iron, zinc, and calcium in place. By dismantling phytate before you eat, fermentation makes those minerals available for absorption. The iron content stays the same. What changes is your body's ability to access it.
— Yarlina et al. 2026 · npj Science of Food (Nature) · 36 studies reviewed
A 2026 scoping review in npj Science of Food, covering research across 25 years, confirmed this pathway. Lab digestion models and animal studies consistently show that fermented legumes produce significantly better mineral solubility than their unfermented counterparts. The enzyme does not add iron. It removes the barrier that was keeping the iron from reaching you.
Here is what that evidence does not include: a single human trial. No controlled feeding study has tracked iron from a block of tempeh through a living person's digestive tract and measured how much arrived in their blood. The enzyme chemistry is mapped. The lab evidence is consistent across multiple research models. The human confirmation does not exist yet.
Still, the mechanism itself is worth understanding, because phytate is not unique to soybeans. It sits inside every legume, every whole grain, every nut and seed you eat. Wherever plant-based iron lives, phytate is likely standing guard.
Fermentation is one way past that guard. Vitamin C, garlic, and fermented soy sauce each work through an entirely different mechanism. Stack three of them in a single bowl, and three independent pathways to iron absorption converge on one plate, each one freeing minerals that phytate would otherwise keep locked away.
