Cross‐sectional associations between healthy eating index and thyroid function in U.S. male Adults, NHANES 2007–2012

Abstract Little is known about whether diet quality is associated with thyroid function. We aimed to examine the relationship between diet quality and thyroid function. Data were from the National Health and Nutrition Examination Surveys, 2007–2012. A total of 3603 males who were at least 20 years old and had dietary recall data were included in the analysis. Thyroid function was assessed by eight indexes, including total and thyroglobulin antibodies, thyroid peroxidase antibodies, free T4 and T3, total T4 and T3, Tg, and thyroid‐stimulating hormone. Multivariable linear regression, subgroup analyses, and interaction terms were employed to test the association between healthy eating index (HEI) and thyroid function. A total of 3603 male participants aged ≥20 years with an average age of 48.17 ± 0.51 years were enrolled. We found a negative association between HEI‐2010 and total T3 (β = −3.41; p = .01) and free T3 (β = −0.06; p = .01). In subgroup analyses, HEI‐2010 was negatively associated with TT3 in male participants aged <65 years old (β = −4.57; p < .01) and FT3 (β = −0.09; p < .001). Higher HEI‐2010 was inversely associated with lower total T3 and free T3. More well‐designed studies are still needed to validate the causal relationship between HEI and thyroid function.

Overall diet quality is critical when it comes to determining the health outcomes of diet since dietary intake is often combined to produce health outcomes (Schwingshackl et al., 2018;Wilson et al., 2016). The healthy eating index (HEI) indicates how closely dietary patterns match Dietary Guidelines for Americans and the food guide pyramid, which can be used to predict chronic diseases (Guenther et al., 2014). Dietary quality measured by HEI-2015 total and component food scores significantly reduced osteoporosis risk in the NHANES (Fan et al., 2022). Based on the results of a metaanalysis, the alternate HEI-2010 was associated with a lower risk of depression (Lassale et al., 2019). HEI-2010 is considered to be one of the reliable indexes for assessing diet quality.
Our study examined whether thyroid function is related to diet quality from a macro perspective using the HEI to determine whether men's diet quality is related to thyroid function. Therefore, determining the relationship between HEI-2010 and thyroid function might bring new views on nutritional management of thyroid condition.

| ME THODS
The National Health and Nutrition Examination Survey (NHANES) survey collected information on demographics, health, and dietary intake in a nationally representative sample of 2-year cycles through a complex, multistage probability design. Informed consent was provided to all participants as part of the NHANES protocol which has been approved by the National Center for Health Statistics Research Ethics Review Board.
An analysis of data from 3 cycles is presented (2007-2008, 2009-2010, and 2011-2012). Male adults aged 20 years and older were included in the study. Those whose thyroid function were unreliable were excluded (n = 187). The analysis also excluded those with missing data for other variables (n = 636). The descriptions are presented in more detail in Figure 1, and final sample size was 3603 male participants aged 20 years and older.

| Assessment of diet quality
According to the HEI-2010, diet quality is determined using both the vegetarian and vegan versions of the USDA Food Pattern as well as the omnivore version of the USDA Food Pattern. In HEI-2010, there are 12 components, including 9 adequacy components (total fruit, whole fruit, total vegetables, greens and beans, whole grains, dairy, total protein foods, seafood and plant proteins, and fatty acids as the ratio of polyunsaturated and monounsaturated fatty acids to saturated fatty acids) and 3 moderation components (refined grains, sodium, and empty calories), with higher scores reflecting better diet quality . With the simple HEI scoring algorithm, the present analysis calculated diet quality for each person based on mean dietary intakes from two 24-hour recalls. Scores from the 12 components are added up for HEI-2010, which has a maximum score of 100.

| Measurement of thyroid outcomes
In this study, thyroid function parameters were examined for total and free thyroxine (T4), total and free triiodothyronine (T3), thyroglobulin (Tg), thyroglobulin antibodies, thyroid peroxidase antibodies, and thyroid-stimulating hormone (TSH). A detailed explanation of the procedure for collecting and processing specimens was provided in the NHANES Laboratory/MedicalTechnologists Procedures Manual (LPM).

| statistical analysis
We used R (version 4.1.2) to perform all statistical analyses. A   In Table 3 and

| DISCUSS ION
This study examined whether thyroid function was associated with general eating patterns, as measured by the HEI, using multivariable models that were adjusted for confounding factors. The nationally representative survey of U.S. male adults showed a negative relationship between HEI-2010 and TT3, FT3, and a negative association between HEI-2010 and FT3/FT4, TT3/TT4 when the ratios of thyroid function measures were evaluated.
Although there is no doubt that iodine intake significantly affects thyroid function, several epidemiological studies have found that diet intake (not related to iodine) could also play a role. It was found by Stella Iacovides et al. that a ketogenic diet decreased T3 levels and increased T4 levels, while a high-carbohydrate, low-fat diet did not affect thyroid function (Iacovides & Maloney, 2022).
Alessio Basolo et al. reported that a high-protein overfeeding led to plasma TSH, FT3, and FT4 decrease, while a low-protein overfeeding resulted in a reduction in plasma TSH and increase in plasma FT3 (Basolo et al., 2019).
Studies about the relationship between diet quality and thyroid function are still scarce. A cross-sectional study indicated that higher adherence to Mediterranean diet (Med-Diet) was found to be TA B L E 4 Subgroup analysis of association between HEI a -2010 and thyroid function stratified by age group. negatively related to free T3 and T4 in a cohort of overweight/obese subjects . Stavroula Lambrinakou et al. found that the frequency of non-home-made meal consumption was positively related to T4. T3 level was confirmed to be positively correlated with vegetables consumption cooked with olive oil, but negatively relationship with consumption of whole grain (Lambrinakou et al., 2017). Another study observed a 10-point increase in HEI associated with 0.6% reduced total T4 among U.S. adults and 0.5% reduced total T4 among male adults (Melough et al., 2022).
Consequently, a balanced diet is an important component of maintaining a healthy thyroid gland function.
A recent study by Roberta Zupo et al. examined the relationship between skeletal muscle mass and TH levels among subjects with overweight and obesity . Nevertheless, subgroup analysis stratified by BMI showed no statistically significant increase between THs and higher HEI in obese subjects. The interaction term test also found no significant difference between HEI and thyroid function among different BMI groups, indicating that the heterogeneities among different subgroups were not statistically significant. As this study indicated, older age had a relatively higher predicted HEI-2010 score. Diet quality increased by age among male participants, which was consistent with results in the study for general adult population . Compared with the lowest education and income categories, significantly higher HEI-2010 scores were observed in the highest categories in our analysis. Age-stratified subgroup analysis indicated that the negative association between HEI and thyroid function became more evident in participants aged <65 years old.
Since older adults have become more health conscious as they age and have improved their diets, they might be more motivated to have a healthy diet and a higher HEI total score. This study has some strengths. In data collection and reporting, NHANES employed standardized and well-controlled protocols.
Second strength was using universalistic data from a broad representative sample from the U.S. adult population. Moreover, the NHANES, which is a comprehensive dataset, includes important covariates such as demographics. Our study does have certain restrictions, though. In terms of dietary patterns, the data used for HEI calculation were collected from a 24-hour dietary recall, not necessarily representative of an individual's habitual diet. Due to the uncertain impact of pregnancy and menstruation on thyroid function in women, we excluded the female participants from our study. As our study is a cross-sectional analysis, we cannot establish causation.
To examine the relationship between chronic eating habits and their long-term impact on thyroid function, a longitudinal cohort study is required.

| CON CLUS ION
This study showed a significant association of higher HEI-2010 score and decreased total T3 in male adults, and the association might become stronger in men <65 years, based on data from NHANES 2007-2012. Our study may indicate that nutritional management might be necessary to ensure normal thyroid function. However, this study is just cross-sectional observational, prospective cohort studies, or randomized controlled trials are warranted to explore the causal relationship between HEI and thyroid function.

ACK N O WLE D G E M ENTS
The authors thank those who took part in the NHANES survey and the Centers for Disease Control and Prevention for their data sharing.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have no competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
The specific dataset associated with this study will be made available by the corresponding author upon reasonable request.

The Centers for Disease Control and Prevention and the Research
Ethics Review Board at the National Center for Health Statistics conducted the consents and have received ethical approval.