Introduction. High sodium intake is associated with increased cardiovascular risk factors although underlying mechanisms are unclear. Microvascular dysfunction, including of the glycocalyx, may link sodium intake, insulin resistance and cardiovascular diseases. The effects of changes in sodium intake on (1) function, (2) structure and (3) baseline regulatory mechanisms of the skin and sublingual microcirculation and (4) insulin sensitivity were investigated. Methods. Design: Randomised, double-blind, placebo-controlled, crossover, 7 days low (50 mmol/day-LSI) versus high (250 mmol/day-HSI) sodium intake with a two-week washout in overweight/obese participants free from cardiovascular disease. Outcome measures: Insulin sensitivity and microvascular structure and function. Aims: • Investigate whether low, compared to high, sodium intake improves microvascular structure and function; • Investigate whether low, compared to high, sodium intake improves microvascular structure and function in the presence of hyperinsulinaemic euglycaemia; • Investigate whether supraphysiological, compared to basal, insulin levels alter microvascular structure and function; • Validate the GlycoCheck device against confocal microscopy for assessing the integrity of the glycocalyx in vivo. Results. Recruitment was challenging. 17 participants (7.5% (17/226) of people approached, 46% (17/37) of people screened) completed the study, 15 completed the hyperinsulinaemia days. Urinary sodium increased (69.7 ± 29.8 (mean ± SD) (mmol/day) LSI to 222.2 ± 40.8 HSI, p<0.001, n=17). Interesting and novel findings included: (1) HSI, versus LSI, during hyperinsulinaemia, paradoxically enhanced ACh and SNP responses (peak: ACh (AU) LSI 1.6 ± 0.5, HSI 1.9 ± 0.5, p=0.026, n=15; SNP (AU) LSI 1.4 ± 0.5, HSI 1.8 ± 0.6, p=0.019, n=15). (2) Hyperinsulinaemia + LSI, compared to basal insulin + LSI, unexpectedly reduced perfused boundary region (PBR) (i.e., reduced accessibility of red blood cells towards the endothelium) (hyperinsulinaemia, PBR 2.30 ± 0.30 (mean ± SD) (μm), basal insulin 2.46 ± 0.28, p=0.05, n=8). (3) Hyperinsulinaemia compared to basal insulin enhanced total vessel length, total vessel density, count of vessels crossed at gridlines and count density independent of sodium intake. (4) PBR correlated inversely with confocal microscopy glycocalyx thickness in vivo (r = -0.626, p=0.029) providing validation for the GlycoCheck device. Conclusion. The hypotheses that low sodium improves vascular function and insulin sensitivity in the presence of normal or high insulin levels was not fully supported. Striking evidence of the dominant microvascular effect of insulin, independent of the sodium intake, was reported. My data will inform future study design and considerations, including the challenges in recruiting to clinical research studies and maintaining the study compliance of participants.