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Background Histidine is an essential amino acid with health benefits that may warrant histidine supplementation; however, the clinical safety of histidine intake above the average dietary intake (1.52–5.20 g/d) needs to be vetted. Objectives We aimed to determine the tolerance to graded dosages of histidine in a healthy adult population. Methods Healthy adults aged 21–50 y completed graded dosages of histidine supplement (4, 8, and 12 g/d, Study 1) (n  = 20 men and n  = 20 women) and/or a 16-g/d dosage of histidine (Study 2, n  = 21 men and n  = 19 women); 27 participants (n  = 12 men and n  = 15 women) completed both studies. After study enrollment and baseline measures, participants consumed encapsulated histidine for 4 wk followed by a 3-wk recovery period. Primary outcomes included vitals, select biochemical analytes, anthropometry, serum zinc, and body composition (via DXA). Results No changes in vitals or body composition occurred with histidine supplementation in either study. Plasma histidine (measured in subjects who completed all dosages for Studies 1 and 2) was elevated at the 12- and 16-g/d dosages (compared with 0–8 g/d, P  < 0.05) and blood urea nitrogen increased with dosage (P  = 0.013) and time (P  < 0.001) in Study 1 and with time in Study 2 (P  < 0.001). In Study 1, mean ferritin concentrations were lower in 12 g/d (46.0 ng/mL; 95% CI: 34.8, 60.9 ng/mL) than in 4 g/d (51.6 ng/mL; 95% CI: 39.0, 68.4 ng/mL; P  = 0.038). In Study 2, 16 g/d increased mean aspartate aminotransferase from baseline (19 U/L; 95% CI: 17, 22 U/L) to week 4 (24 U/L; 95% CI: 21, 27 U/L; P  < 0.001) and mean serum zinc decreased from baseline (0.75 μg/dL; 95% CI: 0.71, 0.80 μg/dL) to week 4 (0.70 μg/dL; 95% CI: 0.66, 0.74 μg/dL; P  = 0.011). Conclusions Although values remained within the normal reference ranges for all analytes measured, in all dosages tested, the human no-observed adverse effect level was determined to be 8 g/d owing to changes in blood parameters at the 12-g/d dosage. This trial was registered at clinicaltrials.gov as NCT04142294. [ABSTRACT FROM AUTHOR]

Iodine is an essential nutrient for humans as a critical constituent of thyroid hormones; both iodine deficiency and excess have adverse consequences for the thyroid gland. The aims of this research were to compare claimed concentrations of iodine with measured ones in various iodine-supplemented products, estimate the amount of iodine ingested by Italian consumers who use these products, and compare the calculated intakes to the Recommended Daily Allowance and tolerable Upper Level. A convenience sample of 43 food supplements was analyzed for iodine concentration. Analytical values resembled those declared in the label in fewer than half of the examples; in four cases, the maximal daily dose was higher than the tolerable upper level for iodine of 600 μg/day. Labeling of iodine-rich food supplements appears to be unreliable and caution should be exercised in the consumption of food supplements rich in iodine as there is a risk of exceeding the established safe upper level of daily intake. [ABSTRACT FROM AUTHOR]