At least 10 minutes of seated rest is required for stabilisation of blood-based hydration biomarkers.

Postural changes elicit well-described haemodynamic effects on plasma volume, affecting blood-based hydration variables. While the time required for plasma volume stabilisation is well-established for supine rest, less is known for blood samples collected in a seated position, as is commonplace in physiology and nutrition experiments. Seventeen healthy participants (9 males; 8 females) stood stationary for 20 min in an upright position before walking on a treadmill at 4 km/h for 20 min (to simulate active travel to blood sampling locations). After walking, participants sat upright (within 7 ± 1 s), and venous blood samples were drawn from a cannula at 0, 5, 10, 20, 30 and 40 min after sitting. Time points were compared to 40 min. At 0 and 5 min, blood (0 min: -3.3%; P < 0.001; 5 min: -1.2%; P = 0.034) and plasma (0 min: -5.9%; P < 0.001; 5 min: -1.7%; P = 0.035) volume were lower than 40 min values. Conversely, plasma osmolality was higher at 0 min (+ 0.61%; P = 0.005) and 5 min (+ 0.55%; P = 0.007), whilst blood/plasma volume and plasma osmolality were not different from 10 min onwards. Plasma sodium, potassium and chloride concentrations were not different from 40 min throughout the sitting period (P ≥ 0.056). These data demonstrate that blood/plasma volume, as well as plasma osmolality were affected by posture and/or walking, but all are not significantly different from 40 min within 10 min of seated rest. Therefore, studies involving blood-based hydration measures taken in an upright seated position should allow at least 10 min of postural control prior to sampling.