Measurements of elemental iodine in soy sauces in Taiwan using a modified microplate method.

Background: Soy sauce is widely used in a variety of Asian dishes to enhance flavor. Soybean and most soybean products, including soy sauces, are listed as prohibited foods in a low iodine diet. However, the iodine content in soy sauces is largely unknown. The aim of this study was to determine the iodine content in domestic soy sauces in Taiwan. Methods: Twenty-five different kinds of soy sauces were diluted with distilled water and with a dilution factor of fifty or above. Iodine concentrations of the diluted samples were measured colourimetrically based on the Sandell-Kolthoff reaction by a modified microplate method. All the measurements were repeated twelve times on three different days for determination of mean and standard deviation (SD), and coefficients of variance (CV). Serial dilution and recovery tests were also performed for validation. The results were confirmed by an inductively coupled plasma mass spectrometry (ICP-MS) method. Results: Among the twenty-five surveyed soy sauces, most of them (n=22) were iodine-free (<16 ug/L, and thus un-detectable). The iodine concentrations (mean ± SD) of the three iodine-containing soy sauces were 2.7 ± 0.1, 5.1 ± 0.2, and 10.8 ± 0.6 mg/L, respectively. The inter-assay, intra-assay and total CVs were all <5.3% for the modified microplate method. The results obtained by ICP-MS were consistent with those of the modified microplate method. The recovery rates in the serial dilution test and recovery test ranged from 94.7% to 118.6%. Two of the three iodine-containing soy sauces were supplemented with kelp extract, while the other one without kelp extract had the highest amount of salt among the three iodine-containing soy sauces. Therefore, we postulate that iodized salt instead of kelp extract is the source of higher iodine content in that sauce. Conclusion: The results suggest that most soy sauces are iodine-free and may be allowed during low iodine diets.

Analytical validation of an inductively coupled plasma mass spectrometry method for urinary iodine concentration measurements in Taiwan.

BACKGROUND: Urinary iodine concentration (UIC) measured by Sandell-Kolthoff spectrophotometric method has been used in the Nutrition and Health Surveys in Taiwan but this method is time consuming and produces toxic waste from arsenic trioxide. The aim of this study was to develop and validate an inductively coupled plasma mass spectrometry (ICP-MS) system to determine UIC in Taiwan. METHODS: Samples and iodine calibrators were diluted 100-fold into an aqueous solution containing Triton X-100, 0.5% ammonia solution, and tellurium (128Te) as an internal standard. Digestion prior to analysis was not necessary. Precision, accuracy, serial dilution, and recovery tests were performed. A total of 1243 urine samples covering a wide range of iodine concentrations were measured by both Sandell-Kolthoff method and ICP-MS. Passing-Bablok regression and Bland-Altman plots were used to compare values across methods. RESULTS: The limit for detection and quantification by ICP-MS was 0.95 µg/L and 2.85 µg/L, respectively. The intra-assay and inter-assay coefficients were <10%, with a recovery range of 95%-105%. The results obtained by ICP-MS and the Sandell-Kolthoff method were highly correlated (Pearson's correlation: r = 0.996, 95% confidence interval [CI]: 0.9950-0.9961, p < 0.001). For UIC between 20 and 1000 µg/L, the y-intercept for the Passing-Bablok regression was -1.9 (95% CI: -2.5599 to -1.3500) and the slope was 1.01 (95% CI: 1.0000-1.0206). CONCLUSION: This validated ICP-MS system can be used for measuring UIC.

Iodine nutritional status of lactating women in northern Taiwan in 2019.

BACKGROUND: Pregnant and lactating women are vulnerable to iodine deficiency. This study was conducted to evaluate the iodine nutritional status of lactating women in northern Taiwan. METHODS: Women recruited from Taipei Veterans General Hospital (TVGH) in 2019 provided a spot urine sample and completed a Food Frequency Questionnaire. The urinary iodine concentration (UIC) was measured by inductively coupled plasma mass spectrometry. RESULTS: The overall median UIC in 198 women was 120.4 µg/L, indicating a sufficient iodine status. Univariate analysis revealed a lower median UIC in women of younger age (p = 0.004), who were not taking multivitamins (p = 0.004), not on a postpartum nourishment diet (p = 0.04), and whose infant received more breast milk (p = 0.004). The median UIC was <100 µg/L in the group aged 20 to 29 years (UIC: 74.4 µg/L) and in women whose infants' diet was composed of >50% breast milk (UIC: 86.1 µg/L). A postpartum nourishment diet was followed by 73.7% (n = 146) of the women. Nevertheless, a significant decrease in the intake frequency of iodine-containing foods, including seaweeds (p < 0.001), seafood (p < 0.001), dairy products (p = 0.009), and multivitamins (p < 0.001) was observed compared with the intake noted in a previous survey of pregnant women in TVGH. Following multivariate analysis, only younger age (20-29 vs ≥30 years; odds ratio [OR]: 3.38; 95% confidence interval [CI]: 1.49-7.65), no use of multivitamin (OR: 1.89; 95% CI: 1.03-3.48), and infant diet composition (>50% breast milk vs <50% breast milk; OR: 2.93; 95% CI: 1.37-6.25) were independently associated with UIC < 100 µg/L. CONCLUSION: The results suggest that the iodine status in lactating women in northern Taiwan is adequate. However, iodine deficiency may continue to be present in certain subgroups, such as women of younger age and those who do not take multivitamins.

Iodine nutritional status of pregnant women in an urban area of northern Taiwan in 2018.

Pregnant women are considered as one of the most vulnerable groups for iodine deficiency. The Nutrition and Health Survey in Taiwan 2013 revealed that the median urinary iodine concentration (UIC) of non-pregnant women of child-bearing age of 15-44 years was 124 µg/L, which was adequate in general, but insufficient according to pregnancy criteria. The aim of this study was to determine the iodine nutritional status of pregnant women in an urban area of Northern Taiwan. A hospital-based cross-sectional survey was conducted in Taipei Veterans General Hospital. Random spot urine samples were collected from January to October, 2018 and UIC was determined by inductively coupled plasma mass-spectrometry. A food frequency questionnaire was also delivered to the participants. The overall median UIC was 225.3 µg/L (IQR: 109.1-514.2 µg/L) for 257 pregnant women ranging from 21-47 years-old. The distribution of UIC was as follows: 35.4% with UIC <150 µg/L, 17.1% with UIC within 150-249 µg/L, 21.8% with UIC within 250-499 µg/L, and 25.7% with UIC ≥500 µg/L. The use of prenatal multivitamin was very common among the participants: 79.4% (n = 204) took multivitamin either every day or less frequently, with 52.5% (n = 135) taking one pill every day, and only 20.6% (n = 53) never took multivitamin during their pregnancy. Other commonly consumed iodine-containing foods were dairy products and fish. Our results indicate that the iodine status in the studied women is adequate. However, efforts are still needed to avoid iodine deficiency as well as iodine excess.