What is Phytic Acid? The Key to Understanding Bran's Effect
Phytic acid, or phytate, is a naturally occurring compound found primarily in the hulls of nuts, seeds, grains, and legumes. It serves as the main storage form of phosphorus in these plants. From a nutritional standpoint, phytic acid is often referred to as an "anti-nutrient" because of its strong ability to bind to certain minerals, including calcium, zinc, and iron. When phytic acid binds with these minerals in the digestive tract, it forms insoluble compounds called phytates, making the minerals unavailable for the body to absorb.
The Mechanism: How Phytic Acid Binds Calcium
The binding of phytic acid to calcium is a chemical process that occurs in the gastrointestinal tract. At the slightly acidic to neutral pH levels of the small intestine, phytic acid's negatively charged phosphate groups can readily chelate (bind) with positively charged mineral ions like calcium ($Ca^{2+}$). Once this stable phytate-mineral complex is formed, it cannot be broken down by the human digestive system. This is because humans lack the enzyme phytase, which is necessary to hydrolyze (break down) phytic acid and release the bound minerals. The mineral-phytate complex is then simply passed through the body and excreted, taking the calcium with it.
The Specific Impact of High-Concentration Wheat Bran
Research has shown that the effect on calcium absorption is most notable with high-concentration sources, particularly 100% wheat bran. When consumed at the same time as a calcium-rich food, such as milk on a 100% wheat bran cereal, the phytates in the bran can significantly reduce the amount of calcium your body absorbs from the milk. For instance, studies have found that consuming 40 grams of a cereal with 16 grams of wheat bran can abolish the usual dose-dependent relationship of calcium absorption, keeping it low across varying calcium loads. This is a key distinction from other foods containing phytates. While legumes also contain phytates, the interference is often limited to the calcium within the bean itself, whereas high-concentration wheat bran can affect the calcium from other foods in the same meal.
The Milder Effect of Bran in Other Foods
For most people eating a balanced and varied diet, the bran content in common foods like breads and other baked goods is not a concern. The concentration of phytic acid is simply not high enough to have a noticeable impact on overall calcium status. Furthermore, food preparation methods like milling and fermentation, which are part of making bread, can help reduce the phytic acid content. This means you can enjoy whole-grain breads as part of a healthy diet without worrying about a major deficit in your mineral absorption.
Is Bran a Concern for Everyone?
The risk of mineral deficiency due to phytate intake is most relevant for those with imbalanced diets, particularly those heavily reliant on unprocessed grains and legumes. Vegetarians and vegans, whose diets are often high in plant-based, phytate-rich foods, may also need to pay more attention to mineral intake and preparation methods. For individuals with existing mineral deficiencies like osteoporosis or anemia, monitoring and adjusting dietary habits might be prudent. For those with adequate dietary diversity and no pre-existing conditions, the benefits of the fiber and other nutrients in bran often outweigh the minimal impact on mineral absorption.
Comparison: Phytates (Bran) vs. Oxalates (Spinach)
| Feature | Phytates (in Bran) | Oxalates (in Spinach, Rhubarb) |
|---|---|---|
| Primary Location | Outer layer of whole grains, seeds, legumes | Green leafy vegetables, beets, nuts |
| Mechanism of Action | Binds to minerals (calcium, iron, zinc) to form insoluble salts | Binds to calcium specifically to form insoluble calcium oxalate |
| Effect on Other Foods | High-concentration wheat bran can reduce absorption from other foods in the same meal | Calcium from foods eaten with high-oxalate foods is affected |
| Mitigation | Soaking, sprouting, fermentation | Cooking (reduces some oxalate), consuming calcium-rich foods concurrently |
| Health Impact Concern | Primarily for those with nutrient-poor or limited diets | Concern for those prone to kidney stones and those with mineral deficiencies |
Strategies to Mitigate the Effect of Phytic Acid
There are several effective ways to reduce the impact of phytic acid on calcium absorption without giving up nutrient-dense bran and whole grains:
- Timing is Key: If you rely on supplements or have concerns, take calcium supplements at least two hours before or after consuming high-phytate foods like 100% wheat bran.
- Soaking: Soaking grains and legumes for several hours before cooking can significantly reduce their phytic acid content. Drain and use fresh water for cooking.
- Sprouting: Germinating or sprouting seeds, grains, and legumes activates enzymes that help break down phytic acid, improving mineral bioavailability.
- Fermentation: The fermentation process, such as making sourdough bread, can promote phytate degradation.
- Vary Your Diet: The simplest strategy is to eat a wide variety of foods. This ensures you are not relying too heavily on any single source for minerals and helps balance out the effects of anti-nutrients.
Conclusion
While it is true that high-concentration wheat bran can interfere with calcium absorption due to its phytic acid content, this effect is highly manageable and typically not a major issue for individuals with a varied, balanced diet. Understanding the interaction between phytic acid and minerals allows for smart dietary choices, such as timing supplement intake or using simple food preparation techniques. The health benefits of dietary fiber and other nutrients in bran mean it should not be eliminated from a healthy diet, but rather consumed mindfully alongside other calcium-rich sources.
For more detailed information on dietary factors affecting bone health, consult resources from the Bone Health and Osteoporosis Foundation. Bone Health & Osteoporosis Foundation
