Bioavailability study using an in-vitro method of iodine and bromine in edible seaweed

Raw edible seaweed harvested in the Galician coast (Northwester Spain), including two red seaweed types (Dulse and Nori), three brown seaweed (Kombu, Wakame and Sea Spaghetti), one green seaweed (Sea Lettuce) and one microalgae (Spirulina platensis) were analysed for total iodine and total bromine, as well as for iodine and bromine bioavailability by in-vitro methods (simulated gastric and intestinal digestion/dialysis). Similarly, a cooked seaweed sample (canned in brine) consisting of a mixture of two brown seaweed (Sea Spaghetti and Furbelows) and a derived product (agar–agar) from the red seaweed Gelidiumm sesquipedale, were also included in the study. All measurements were carried out by inductively coupled plasma–mass spectrometry using tellurium and yttrium as internal standards for iodine and bromine, respectively. An optimised microwave assisted alkaline (TMAH) digestion procedure was used as sample pre-treatment for total iodine and bromine determinations, as well as for the determination of both elements in the non-dialyzable fractions. PIPES buffer solution at a pH of 7.0 and dialysis membranes of 10 kDa molecular weight cut off (MWCO) were used for the intestinal digestion. Accuracy of the method (total bromine and iodine determinations) was assessed by analysing a NIES-09 certified reference material. The accuracy of the in-vitro procedure was established by a mass-balance study which led, after statistical evaluation (95% confidence interval), good accuracy of the whole in-vitro process. The highest dialyzability bromine percentages (36 ± 0.7% and 47 ± 3.0%) were obtained for red seaweed (Dulse and Nori), while higher dialyzability iodine was assessed for the brown seaweed (Kombu), around 17% ± 0.7%.