Net carbon flux across subcutaneous adipose tissue after a standard meal in normal-weight and insulin-resistant obese subjects

OBJECTIVE: To elucidate the alterations in subcutaneous adipose tissue metabolism in obesity which affect substrate storage in the fed state, by describing as completely as possible the 'metabolic balance sheet' for adipose tissue. SUBJECTS: 13 lean people (age: 24-64 years, BMI: 19.3-29.4 kg/m(2)) and 9 insulin-resistant obese subjects (age: 38-57 years, BMI: 32.4-56.0 kg/m(2)). MEASUREMENTS: Net flux of eight major substrates and CO2 across subcutaneous adipose tissue in vivo before and after eating a standard mixed meal (3.1 MJ, 41% from fat). RESULTS: The net flux of total carbon atoms in the 8 substrates and CO2 was negative (net export from the tissue) in both groups in the fasting state. After the meal it changed rapidly to a positive balance (net substrate storage) in the control group and remained positive until 5 h, whereas in the obese group it remained negative throughout. The overall carbon balance in adipose tissue reflected predominantly the movement of fatty acids in and out of the tissue. CONCLUSIONS: The fact that fat storage failed to occur despite elevated plasma concentrations of triacylglycerol, glucose and insulin indicates an alteration in metabolic regulation at the cellular level which might be called insulin resistance. Insulin resistance of fat metabolism may be an adaptation which limits further fat deposition, but is maladaptive in terms of risk factors for atherosclerosis.