Can saliva and tears replace blood tests for vitamin D and glucose?

In a small but revealing study, scientists discovered that tears strongly mirror blood glucose and saliva may reflect vitamin D status. This could lead to less invasive ways to monitor chronic health.

Study: Tears and Saliva as Biological Matrices for Vitamin D and Glucose Assessment: A Pilot Study. Image credit: Shotmedia/Shutterstock.com

A new study published in the journal Physiologia explores the feasibility of saliva and tears as alternative biological fluids for measuring vitamin D and glucose levels in patients with type 2 diabetes.

Background

Vitamin D, or calciferol, is an essential fat-soluble micronutrient with two main representatives: vitamin D2 and vitamin D3. It can be ingested through diet or synthesized in the human skin using high-energy ultraviolet radiation from the sun. 

Vitamin D is essential for intestinal calcium absorption and maintaining calcium and phosphate levels in the blood to facilitate bone growth and mineralization. It also plays crucial roles in reducing inflammation, promoting immune functions, and is linked to glucose metabolism.

Vitamin D deficiency has been linked to a range of health conditions, including cardiovascular disease, autoimmune diseases, diabetes, metabolic syndrome, and microbial infections. However, in most cases, vitamin D levels are primarily measured in blood samples, not other biological fluids.

In the current study, researchers explored the feasibility of using other biological fluids, including saliva and tears, to assess vitamin D and glucose levels in patients with type 2 diabetes.

Study design

The study population included six adult patients with type 2 diabetes and eight healthy adults. Three diabetic patients were taking vitamin D supplements, while none of the healthy volunteers were.

After a minimum of three hours of fasting, blood, tears, and saliva samples were collected from the participants and analyzed using the electrochemiluminescence method to assess vitamin D and glucose levels.

Key findings

The biochemical assessment revealed similar vitamin D levels in blood samples collected from diabetic and healthy participants. A slightly higher blood vitamin D level in diabetic patients might be the effect of supplementation. However, no effect of supplementation was observed on vitamin D levels in saliva and tears samples.

The correlation analysis between collected samples revealed that a higher blood vitamin D level is significantly associated with a lower salivary vitamin D level. A similar but non-significant negative correlation was observed between vitamin D levels in blood and tears.

The study found a significantly positive association between blood and tears samples regarding glucose levels.

Study significance

The study suggests that saliva and tears indicate potential feasibility as alternative biological matrices for assessing vitamin D and glucose levels in diabetic patients and healthy adults.

Existing evidence reports low vitamin D levels in blood and saliva samples collected from newly diagnosed diabetic patients. The current study reports a negative correlation between blood and salivary vitamin D levels in diabetic patients diagnosed with the disease for at least five years. These findings, therefore, suggest that salivary levels may reflect blood levels of vitamin D only in certain conditions.

The study also finds a tendency toward a negative correlation between blood and tears vitamin D levels, with tears samples containing higher levels of this vitamin than blood samples in both diabetic and healthy adults. This observation indicates potential, but it has not yet been established as a reliable matrix for vitamin D detection.

However, in contrast to the current study findings, previous studies involving healthy volunteers have shown a positive correlation between blood and tear levels of vitamin D. This difference in findings suggests that individuals’ health conditions may potentially influence vitamin D levels in different biological samples.   

The study finds a significant positive correlation between blood and tear samples regarding glucose levels in diabetic patients. This suggests that tear sampling may serve as a potentially dependable, noninvasive alternative approach for glucose level monitoring in diabetic patients.

Overall, the study highlights the potential feasibility of tear samples for glucose measurement and raises the possibility of saliva samples reflecting serum vitamin D under certain conditions. Still, it emphasizes that these findings are preliminary.

However, researchers explain that it is vital to establish specific reference intervals for each biological fluid before clinical application, as differences in protein content, pH, enzymatic activity, and viscosity between these matrices are known to affect analyte distribution and assay performance.

Since the study included a small number of participants, further large-scale studies are required to confirm the observed correlations and develop an accurate equation for estimating these biochemical parameters in these alternative matrices.

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