Diurnal Variation of Tear and Meibum Lipids in Healthy Subjects

To determine whether the lipid composition of tears and meibum demonstrates diurnal variation in healthy individuals, with emphasis on the relative distribution of lipid classes.

Fifteen non-contact lens wearers (10 females, 5 males; mean age 29.5 ± 11.6 years) provided morning (09:00-10:00 h) and evening (16:00-17:00 h) tear and meibum samples. Tears were collected with Schirmer strips without anaesthesia, and meibum was expressed from the lower lid margin using a Korb expressor and collected with a sterile spatula under slit-lamp guidance. Lipids were extracted in chloroform and analysed by mass spectrometry. Relative molar proportions of wax esters (WE), cholesteryl esters (CE), triglycerides (TG) and polar lipid classes were quantified.

Non-polar lipids comprised most tear lipids in the morning (98.8%) and evening (99.0%), with WE being most abundant, followed by CE and TG. No significant diurnal differences were observed in tear lipid class proportions. Meibum lipid composition was also dominated by non-polar species (99.4% morning; 99.2% evening), with WE as the principal component. TG levels were higher in evening meibum, while diacylglycerol and phosphatidylserine were significantly elevated among polar lipids (p = 0.04 and p = 0.03, respectively).

The overall distribution of major non-polar lipids in tears and meibum remains stable throughout the day in healthy individuals. Minor changes in specific lipid classes suggest potential molecular-level modulation, the functional relevance of which warrants further investigation.