BACKGROUND: Triglyceride (TG) concentration is used as a marker of cardio-metabolic risk. However, diurnal and possibly weekday variation exists in TG concentrations.

OBJECTIVE: To investigate weekday variation in TG concentrations among 1.8 million blood samples drawn between 2008 and 2015 from patients in the Capital region of Denmark.

DESIGN: Plasma TG was extracted from a central clinical laboratory information system. Weekday variation was investigated by means of linear mixed models.

RESULTS: In addition to the profound diurnal variation, the TG concentration was 4.5% lower on Fridays compared to Mondays (P<0.0001). The variation persisted after multiple adjustments for confounders and was consistent across all sensitivity analyses. Out-patients and in-patients, respectively, had 5.0% and 1.9% lower TG concentrations on Fridays compared to Mondays (both P<0.0001). Highest weekday variations in TG concentrations were recorded for out-patients between the age of 9 to 26 years, with up to 20% higher values on Mondays compared to Fridays (all P<0.05).

CONCLUSIONS: Triglyceride concentrations were highest after the weekend and gradually declined during the week. We suggest that unhealthy food intake and reduced physical activity during the weekend increase TG concentrations which track into the week. This weekday variation may carry implications for public health and future research practice.