Circadian rhythmicity of salivary leptin in healthy subjects

In humans, circulating leptin correlates with body weight and fat mass. The main source of leptin is adipose tissue although placenta has been shown to produce leptin. More recently leptin and its receptor have been found in gastric mucosa and salivary gland. In the present study, we set to confirm the presence of leptin in human saliva and salivary gland, and if present, investigate whether salivary leptin exhibited circadian rhythmicity. Saliva and plasma were collected at 2 h intervals for 24 h in 12 healthy volunteers (6 females; 6 males). As previously described in human saliva and salivary gland, we have confirmed the presence of leptin at both mRNA and protein level. Leptin concentrations were significantly higher in plasma than saliva (p=0.002), and a strong correlation between salivary and plasma leptin was demonstrated (r=0.76; p<0.001). Salivary leptin, like plasma leptin, showed circadian variations in both men and women, with a peak around 2400 h and a nadir at 1000 h. Both, mean 24-h salivary leptin and plasma leptin concentrations were significantly higher in women than men (p<0.05; p<0.01, respectively), but the ratios of salivary and plasma leptin concentrations were higher in men (p<0.05), independent of other variables measured. In this study we have confirmed that leptin is synthesised and secreted by the human salivary gland, and have demonstrated for the first time that salivary leptin shows circadian variation. The precise role of salivary leptin needs to be elucidated. Our findings suggest that there may be differential regulation of leptin (salivary) between men and women. Finally, measurement of leptin in saliva is a simple, non-invasive and may be an acceptable alternative to plasma sampling.