Free and bound plasma leptin in normal weight and obese men and women: relationship with body composition, resting energy expenditure, insulin-sensitivity, lipid profile and macronutrient preference.
OBJECTIVE
The adipose-borne hormone leptin circulates in free and protein-bound forms but little information is available about their biological significance. Free leptin (FL) levels are related to changes in fat mass, whereas bound leptin (BL) appears to be associated with resting energy expenditure (REE). Our aim was to assess FL and BL levels in normal weight and obese subjects and correlate them with metabolic and nutritional variables.


METHODS
The partitioning of plasma leptin between FL and BL was evaluated in a population (n = 44) including both genders and different degrees of adiposity [body mass index (BMI) range 18.6-79.6 kg/m2].


METHODS
Total leptin and FL and BL concentrations were measured by fast protein liquid chromatography (FPLC) followed by radioimmunoassay (RIA). Body composition, REE, insulin sensitivity, lipid parameters associated with cardiovascular risk and macronutrient preference were also assessed.


RESULTS
The BL/FL ratio was significantly reduced in obese subjects due to a major increase in FL compared with BL. Consequently, the gender difference of the %BL/%FL ratio present in lean subjects (35/65 in women; 65/35 in men) was lost in obese subjects. REE was negatively correlated with total leptin (P < 0.0001) and %FL (P < 0.0001), and positively with %BL (P < 0.001). Total leptin and FL were correlated with the diet carbohydrate content in all subjects.


CONCLUSIONS
FL increases with the amount of fat mass; the prevalence of FL in normal weight women in comparison to men suggests that this fraction is particularly linked to the amount of subcutaneous fat. Moreover, the correlation of BL with REE and the relationship of FL with food intake favours the view of different biological activities for the two circulating forms of leptin.
Magni
Paolo
P
Istituto di Endocrinologia, Università degli Studi di Milano, Milan, Italy. paolo.magni@unimi.it
Liuzzi
Antonio
A
Ruscica
Massimiliano
M
Dozio
Elena
E
Ferrario
Silvia
S
Bussi
Isabella
I
Minocci
Alessandro
A
Castagna
Alessandra
A
Motta
Marcella
M
Savia
Giulio
G
D000273:Adipose Tissue / Q000378:metabolism; D000293:Adolescent; D000328:Adult; D001481:Basal Metabolism; D001798:Blood Proteins / Q000378:metabolism; D001823:Body Composition; D002318:Cardiovascular Diseases / Q000378:metabolism; D016022:Case-Control Studies; D008076:Cholesterol, HDL / Q000097:blood; D008078:Cholesterol, LDL / Q000097:blood; D004040:Dietary Carbohydrates / Q000009:adverse effects; D002149:Energy Intake; D005260:Female; D006801:Humans; D007333:Insulin Resistance; D020738:Leptin / Q000097:blood*; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000097:blood / Q000378:metabolism*; D012307:Risk Factors; D014280:Triglycerides / Q000097:blood
D001798:Blood Proteins; D008076:Cholesterol, HDL; D008078:Cholesterol, LDL; D004040:Dietary Carbohydrates; D020738:Leptin; D014280:Triglycerides