Ferritin, Polyphenols and Phytate: The Complex Business of Estimating Iron Absorption
Iron absorption is notoriously difficult to estimate. In what is sure to be an influential article, Armah, Carriquiry and Reddy estimate total population iron absorption and compare it to a value widely used to reflect iron absorption when creating Dietary Reference Intakes (DRIs). Can their results explain the widespread iron deficiency anemia that is found in well-nourished populations?
The estimation of iron absorption is complex: there are different types of iron in the diet that are absorbed differently, components in foods consumed at the same time affect absorption, and absorption is also greater in people who have low iron stores. The way that iron absorption in a large group of people is estimated takes into account all three aspects of iron absorption to come to a single figure used to approximate the population’s overall iron absorption.
The two broad types of iron are heme iron from meat, fish and poultry, and non-heme iron from plants, cereals and iron used as an enrichment agent. Heme iron absorption is considered to be fairly predictable and not influenced by other dietary components with around 25% absorption, however there is still some variation around this percentage in the literature. Monsen states that 15% to 35% of heme-iron is absorbed depending on the iron status of the individual. Non-heme iron is much more influenced by dietary factors and its absorption is able to be regulated either up or down depending on body iron stores. Absorption of non-heme iron varies more than 10-fold, from less than 1% to over 15% depending on various factors. Common inhibitors of non-heme iron absorption include phytate from grains and seeds, tannins and other polyphenols from tea and coffee, and interactions with other minerals including zinc and calcium. . For example, Layrisse and co-workers could vary iron absorption from breakfast between 3% and 15% by changing the form of iron used, phytate and polyphenol content. On the other hand, acids such as vitamin C increase absorption of iron.
The authors note that iron deficiency anemia is still an issue in the US population despite wide consumption of iron-enriched foods and heme iron. Perhaps a cause is that the single study used as a basis for recommendations, and which therefore determines the amount of iron added to foods and dietary supplements, is not representative of the true iron absorption?
Using nationally representative data from NHANES and supplemented by estimates of the phytate and polyphenol content of the US diet, the authors calculate total iron absorption. They used two scenarios: in the first scenario, the authors used data from all participants and mimicked a situation where individuals had no iron stores; in the second scenario, absorption was estimated only in subjects with iron deficiency.
There main finding was that iron absorption in both scenarios was around 15%, somewhat lower than the 18% currently used for determining the DRIs. This means that less iron is absorbed than expected, and could explain why iron deficiency anemia is still found to a significant degree in the US. To give an example, the Micronutrient Calculator estimates that the average iron intake for young women aged 19 to 30 years is 13.1 mg per day. Using an absorption of 18%, around 2.4 mg per day is absorbed. When the lower estimate is used, only 2.0 mg is likely to be absorbed. The lower absorption means that a greater proportion of women are unlikely to be meeting iron absorption requirements than originally thought, as menstruating women are estimated to require anywhere between 1.5 mg and 3.4 mg per day to replace blood loss. There may be a need to reassess the calculations used to develop DRIs for iron.
Armah SM, Carriquiry AL, Reddy MB. Total Iron Bioavailability from the US Diet Is Lower Than the Current Estimate. J Nutr. 2015 Sep 16. pii: jn210484. [Epub ahead of print]. http://www.ncbi.nlm.nih.gov/pubmed/26377760
Gulec S, Anderson GJ, Collins JF. Mechanistic and regulatory aspects of intestinal iron absorption. Am J Physiol Gastrointest Liver Physiol. 2014 Aug 15;307(4):G397-409. doi: 10.1152/ajpgi.00348.2013. Epub 2014 Jul 3. http://www.ncbi.nlm.nih.gov/pubmed/24994858
Layrisse M, García-Casal MN, Solano L, Barón MA, Arguello F, Llovera D, Ramírez J, Leets I, Tropper E. Iron bioavailability in humans from breakfasts enriched with iron bis-glycine chelate, phytates and polyphenols. J Nutr. 2000 Sep;130(9):2195-9. http://www.ncbi.nlm.nih.gov/pubmed/10958812
Monsen ER. Iron nutrition and absorption: dietary factors which impact iron bioavailability. J Am Diet Assoc. 1988 Jul;88(7):786-90. http://www.ncbi.nlm.nih.gov/pubmed/3290310