Large Iodine Variability in Retail Cows' Milk in the U.S.: A Follow-Up Study among Different Retail Outlets

Abstract

In a previous study, large variability in iodine content was found among samples of store brand retail milk at a single time point in a sampling taken from 24 nationwide U.S. locations for the USDA FoodData Central database, but the sampling plan was not designed to detect differences among locations. This follow-up study was carried out to evaluate iodine levels in retail milk across the U.S. over time. Milk samples (2% fat) were collected bimonthly in fourteen locations for one year and analyzed in duplicate. Control materials were used to support accuracy of results and ensure precision across analytical batches. The overall mean and standard error (SE) for iodine concentration were 82.5 (7.0) µg/240 mL serving, which was comparable to the previous national mean [85.0 (5.5) µg/240 mL]. A similar wide range among individual samples was detected (27.9-282 µg/240 mL). For some locations, the mean iodine concentration differed significantly from others, and differed from the national average by amounts ranging from -47 µg to +37 µg per serving. The between-sample range within location was large for some (up to 229 µg/serving) and minimal for others (as little as 13.2 µg/serving). These findings suggest iodine intake from some retail milk supplies could be over- or underestimated relative to the national average, even if the national average is suitable for population-wide intake estimates.

Keywords: dairy; database; intake; iodine; milk; variability.

Figure 1

Location of the 12 retail outlets identified using the USDA’s national sampling plan [8] that were re-sampled bimonthly (red dots) and two additional locations sampled monthly (black dots).

Figure 2

Iodine concentration (µg per 240 mL serving) in the previous study [5] and current study. Data points represent iodine concentration in individual samples. The vertical bars are 95% confidence intervals for the individual sample values. (×) indicates the observed mean. The lower, middle, and upper horizontal lines in the boxes respectively indicate the first quartile, median, and third quartile.

Figure 3

Iodine content of individual milk samples (○) and the mean ± 95% confidence interval for each location for 14 retail outlets in the U.S. Different capital letters indicate a statistically significant difference between means (p < 0.001), and * denotes a location mean that differs from the overall average of 82.5 µg/240 mL serving (solid line, with a 95% confidence interval of 67.1–101.4 µg/240 mL (dashed lines)).