Pass the (iodized) salt: Iodine deficiency drives the risk of weakness in people with diabetes

Pass the (iodized) salt: Iodine deficiency drives the risk of weakness in people with diabetes

A five-year study in Shanghai shows that using iodized salt can prevent frailty in older adults with diabetes - particularly those with underactive thyroids.

In a recently published study inThe Journal of Nutrition, Health and AgingResearchers examined whether iodine concentrations (UIC) and iodized salt consumption were associated with frailty in people with type 2 diabetes. Their findings point to the importance of maintaining adequate iodine intake, primarily through iodized salt, to reduce the risk of weakness while living with diabetes, particularly among those with thyroid disorders.

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Slow gait speed stood out as the frailty component being most affected by low iodine, while other symptoms such as fatigue and grip strength showed no measurable association.

The prevalence of diabetes is rapidly increasing worldwide; In 2021, they affected 537 million adults, a number that is expected to rise to 783 million by 2045. As physiological functions decline, individuals may experience frailty, a condition that disproportionately affects people with diabetes with an estimated prevalence of 50%. Frailty increases the risk of mortality, hospitalization, heart disease, and disability in diabetic populations, but may be reversible.

People with type 2 diabetes also have lower UIC than healthy individuals. Previous studies suggest that lower levels of UIC have been linked to a higher risk of diabetes and could worsen health outcomes in people with the condition. Low UIC has been implicated in high cholesterol, blood pressure and blood sugar.

Iodine is crucial for the production of thyroid hormones and the regulation of growth and metabolic functions, but up to two billion people around the world are iodine deficient despite intensive salt iodization efforts.

While previous research has found that thyroid dysfunction, particularly hypothyroidism, increases the risk of frailty in older individuals, these two conditions have not been studied in people with diabetes.

About the study

In this study, the research team examined how frailty among an urban population was influenced by UIC and iodized salt intake, and also assessed whether these relationships were measured by thyroid functions, as measured by thyroxine (FT4) and thyroid stimulating hormone (TSH).

They used data from a cohort study of metabolic conditions in Shanghai, China, which followed 957 people with type 2 diabetes in 11 communities between 2018 and 2023. 850 people provided complete data.

People with diabetes may intentionally avoid iodized salt despite its protective effects shown in this study.

Their iodine intake was measured by asking about the type of salt they consumed (non-iodized, mixed, or iodized) and by measuring UIC (urinary iodine concentration) levels in morning urine samples using a mass spectrometer.

People were considered frail if they had three or more frailties related to slow walking speed, poor measured grip strength, low levels of physical activity, fatigue, and weight loss.

Other clinical tests and questionnaires were used to collect medical history (including thyroid disorders, diabetic kidney disease, cardiovascular disease, dyslipidemia, and hypertension), physical measurements (including blood pressure and body mass index or BMI), demographic data (including lifestyle, education, sex, and age), and blood markers (including for diabetes, vitamin D, kidney function, cholesterol, and thyroid hormones).

Statistical comparisons were used to compare frail and non-frail groups of individuals, while regression models including subgroup analyzes were performed to examine the relative risks of frailty across different iodine levels.

Results

Participants in the study were 65.6 years old and had been diagnosed with diabetes for an average of approximately 9.1 years. Their mean UIC was 139.7 μg/L, with 50.5% reporting use of non-iodized salt, 21.9% reporting both iodized and non-iodized salt, and 27.6% using iodized salt.

In five years, 12.9% or 111 people showed at least three signs of frailty. On average, frail patients consumed more non-iodized salt, had a higher BMI, and were older. They also had lower UIC levels, although this difference was not statistically significant (p = 0.230).

Thyroid hormones FT4 and TSH influenced how much iodine status affected frailty, suggesting a biological interaction between iodine and thyroid function.

Compared to those given iodized salt, those who were not had a 9% higher risk of frailty, while lower UIC was associated with a 10% higher risk. When analyzed by UIC tertiles, low UIC levels were associated with a higher risk of frailty, but there was no significant dose-response trend or nonlinear relationship observed in restricted cubic spline models.

Notably, slow walking speed was the only frailty component significantly tied to iodine status: specifically, consumption of non-iodized salt was associated with a 32% increased risk of slow walking speed, while lower UIC was associated with a 26% increased risk. Other components of frailty – tinnitus, weight loss, fatigue and low physical activity – showed no significant associations.

Older individuals and women were at higher risk of frailty if they had low UICs and consumed non-iodized salt. However, this association was not statistically significant. People with high TSH and low FT4 levels - indicators of hypothyroidism - prompted stronger associations between iodine deficiency and frailty, although the risk of frailty did not increase.

Conclusions

While China has implemented a universal salt iodine program since 1996, concerns remain. People with diabetes in this study had lower median UIC levels than the country's general population. They were also more likely to consume non-iodized salt, possibly due to concerns about the risk of thyroid disease, as diabetes is associated with higher rates of thyroid disease.

Iodized salt remains the most effective and affordable method of preventing iodine deficiency, even in urban coastal areas. The results suggest that iodized salt use may help reduce the risk of frailty, particularly in patients with high TSH or low FT4 levels. Despite some limitations of the study, including the possibility of reverse causality (although the researchers adjusted for baseline physical activity to partially account for this) and lack of generalizability to other regions, these results highlight the importance of maintaining adequate iodine intake, particularly for individuals with thyroid dysfunction.

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