Evaluation of seaweeds as food sources of iron: mineral content and iron bioavailability using the in vitro digestion/ Caco-2 cell method

Abstract

Iron (Fe) deficiency is the most common nutrient disorder in the U.S. and worldwide, affecting nearly 2 billion people (WHO, 2008). Often Fe deficiency is attributed to poor Fe bioavailability in foods, especially in plant-based diets. With the U.S. Dietary Guidelines suggesting a reduced intake of meat and increased intake in fresh produce and whole grains, the prevalence of Fe deficiency could increase. Thus, there exists a need to discover novel plant foods that supply bioavailable Fe to the diet. Seaweeds have been shown to be rich sources of Fe and are grown and commonly eaten in Hawaiʻi. The purpose of this study was to evaluate the mineral content and Fe bioavailability of various seaweeds to determine their potential nutritive value as a good source of Fe. In this study the mineral content of eleven common and Hawaiian seaweeds (Aonoriko, Dulse, Gorilla Ogo, Green Ogo, Hijiki, Nori, Red Ogo, Rockweed, Sea Lettuce, Wakame, and Wawae'iole) was determined via inductively coupled plasma mass spectrometry (ICP-MS). Of the eleven seaweeds, six (Nori, Red Ogo, Rockweed, Sea Lettuce, Wakame, and Wawaei'ole) were selected on the basis of popularity in western diets, total iron content, and native Hawaiian origin for further analysis of their Fe bioavailability relative to spinach. Relative Fe bioavailability was assessed using a Caco-2 cell/ in vitro digestion method. Results showed no significant difference in bioavailable iron between spinach and Rockweed, Wakame, or Wawaei'ole. Similarly, while Red Ogo contained five times as much Fe as spinach, the bioavailability of that Fe was low even when enhanced with Vitamin C. In contrast, Nori and Sea Lettuce provided 2-4 times more bioavailable Fe per gram dry weight than spinach. When supplemented with vitamin C, an enhancer of Fe absorption, all seaweeds showed a significant increase in Fe bioavailability as expected (except Red Ogo). This suggests that variation between seaweeds may stem from different contents of enhancers (Vit C) and inhibitors (possibly polyphenols or manganese) in the seaweeds themselves. We conclude that Nori and Sea Lettuce provide more bioavailable Fe than spinach and thus should be considered good sources of dietary Fe. Red Ogo is a poor source of Fe possibly due to either high polyphenol or manganese content. It remains to be determined if seaweeds can be recommended in practical amounts to improve Fe status, and if increased consumption will raise concerns for excessive intakes of potentially toxic minerals such as iodine.