Evaluation of clams as a food source of iron: Total iron, heme iron, aluminum, and in vitro iron bioavailability in live and processed clams

Clams are listed on prominent healthcare websites as an excellent source of iron (Fe). However, the amount, type and bioavailability of iron in clam products are poorly understood, as is their content of environmental metals such as aluminum (Al). In this study, live Manila and processed clams were assayed for Fe, Al, and heme-iron content. Results were compared to beef. Relative iron bioavailability was assessed by an in vitro digestion/Caco-2 cell method using FeSO4 as a reference iron source. Inhibitory effects of Al and tea flavonoids on iron bioavailability were also evaluated. Mean iron content expressed as mg/reference amount (RA, 56 g edible portion) ranged from 0.57 in minced clams to 17.2 mg/RA in whole baby clams; Al ranged from 0.29 to 26.4 mg/RA, probably reflecting grit content. Beef contained 3.0 mg Fe and 0.07 mg Al per RA of 85 g. Heme iron was very low in clams (<0.4 mg/RA) relative to beef (2.3 ± 0.2 mg/RA). One serving (56 g) of live or minced clams provided less than one-third as much bioavailable iron as 9.8 mg FeSO4; whereas one serving of whole baby clams provided the same amount of bioavailable iron as 9.8 mg of FeSO4. Flavonoids reduced iron solubility and bioavailability; Al reduced only iron bioavailability. We conclude that live and minced clams should not be considered good sources of bioavailable iron, and the high Al content of whole baby clams raises concerns about recommending these clams as an iron source.