Can Biofortification or Reduction of Phytic Acid in Beans Increase Iron Uptake?
Iron deficiency anemia is the most widespread micronutrient deficiency, according to the World Health Organisation. Eliminating iron deficiency anemia is a world-wide priority, and a three-pronged approach is suggested including increasing intakes of iron, controlling infectious disease with better hygiene and immunization, and improving overall nutrient intakes. Measures to increase iron intakes need to focus not only on increasing the iron content of the diet but also the absorption of iron. One factor that has a considerable impact on the absorption of iron and other minerals such as zinc and calcium is the phytic acid content of the diet. Phytic acid is the storage form of phosphorus in seeds and is found in the outer husk: when the seed germinates, the phosphorus is released to nourish the growing seedling. As a result, all edible seeds (think cereals, beans and pulses) contain a certain amount of phytic acid (see table below). Phytic acid also binds certain minerals, which leads to a lower bioavailability.
Various methods have been developed to reduce the phytic acid content of seeds, such as milling to remove the bran on seeds where, germinating the seeds before they are consumed so that the seeds release some of the phytic acid, and the addition of phytase, an enzyme that breaks down phytic acid (Troesch et al.). Another approach is to breed crops that naturally have a lower phytic acid content. A good candidate is the bean. This crop is nutritionally valuable as it has a high protein content, the amount of phytic acid in the seed is high, and there is considerable genetic diversity in the seed lines, making selection for low phytic acid content using conventional plant breeding techniques easier.
As a result, low phytic acid beans have been developed. These beans have a phytic acid content around 10% of standard crops. In a recent clinical trial, Petry and co-workers looked at the effect of this reduced phytic acid bean on iron absorption in 25 iron-deficient women living in Rwanda. They compared iron absorption of the low phytic acid bean with a high iron bean and a control bean. The high iron bean had around double the iron content of the control bean, and a similar phytic acid content, while the iron content of the low phytic acid bean was 50% higher than the control bean. Bean meals were consumed at the study site twice a day for 5 consecutive days per bean type. Stable isotopically-labelled iron sulfate was used to measure iron absorption from the meals.
The authors found that the amount of iron absorbed from the high iron bean and the low phytic acid bean was significantly higher than from the control bean (425 µg per meal compared to 280 µg). Even so, the percent iron absorbed was similar between all three beans. The low phytic acid content of the bean tested did not result in greater absorption.
Why did this occur? It turns out that there were some difficulties in preparing the low phytic acid bean. Even after soaking for 7 hours and simmering for 5 hours, it did not reach the normal level of tenderness of the other two beans. The reasons for this were unclear, but it could relate either to the reduced phytic acid content, or to harvest and storage conditions. The poor cooking quality likely resulted in incomplete inactivation of phytohemagglutinin in the beans. This protein is a toxin that is naturally present in un(der)cooked kidney beans. It causes transient gastrointestinal symptoms such as nausea, diarrhea and vomiting. Most of the subjects who consumed the low phytic acid bean experienced these symptoms.
Even so, the results from the biofortified bean were encouraging. The 30% increase in iron absorbed from the high iron bean has the potential to provide a meaningful increase in iron availability in the diets of people who consume large quantities of beans.
While most deficiency diseases are a result of poor health status and a limited diet, and are linked to resource-poor environments, staple food biofortification can be an important weapons in the fight against micronutrient deficiencies. This study highlights the effect of biofortified beans on iron absorption, and potential pitfalls in the development of biofortified foods.
Nicolai Petry, Fabian Rohner, Jean Bosco Gahutu, Bruno Campion, Erick Boy, Pierrot L Tugirimana, Michael Bruce Zimmerman, Christian Zwahlen, James P Wirth, and Diego Moretti. In Rwandese Women with Low Iron Status, Iron Absorption from Low-Phytic Acid Beans and Biofortified Beans Is Comparable, but Low-Phytic Acid Beans Cause Adverse Gastrointestinal Symptoms. J. Nutr. jn223693; first published online March 30, 2016. doi:10.3945/jn.115.22369. http://jn.nutrition.org/content/early/2016/03/30/jn.115.223693.abstract
Food and Drug Administration. Bad Bug Book. Foodborne Pathogenic Microorganisms and Natural Toxins Handbook: Phytohaemagglutinin. 2015. http://www.fda.gov/Food/FoodborneIllnessContaminants/CausesOfIllnessBadBugBook/ucm071092.htm
Hurrell RF, Reddy MB, Juillerat MA, Cook JD. Degradation of phytic acid in cereal porridges improves iron absorption by human subjects. Am J Clin Nutr. 2003 May;77(5):1213-9. http://www.ncbi.nlm.nih.gov/pubmed/12716674
Troesch B, Egli I, Zeder C, Hurrell RF, de Pee S, Zimmermann MB. Optimization of a phytase-containing micronutrient powder with low amounts of highly bioavailable iron for in-home fortification of complementary foods. Am J Clin Nutr. 2009 Feb;89(2):539-44. doi: 10.3945/ajcn.2008.27026. Epub 2008 Dec 23. http://www.ncbi.nlm.nih.gov/pubmed/19106242
World Health Organisation. Micronutrient Deficiencies: Iron Deficiency Anemia. 2016. http://www.who.int/nutrition/topics/ida/en/
content (mg/100 g)
|Corn, flour, whole grain||733|
|Corn, flour, nixtamalized||596|
|Beans, pinto, raw||2380|
|Wheat, flour, whole grain||733|
|Wheat, flour, white||500|
|Soy, soya bean meal, defatted||1500|
|Rice, white, polished, uncooked||661|
|Rice, brown, uncooked||920|
|Peanuts, dry roasted||952|