Subcutaneous adipose tissue free fatty acid uptake measured using positron emission tomography and adipose biopsies in humans

Abstract

Positron emission tomography (PET) radiopharmaceuticals can noninvasively measure free fatty acid (FFA) uptake into adipose tissue. We studied 29 volunteers to test whether abdominal and femoral subcutaneous adipose tissue FFA uptake measured using [1-¹¹C]palmitate PET agrees with FFA storage rates measured using an intravenous bolus of [1-¹⁴C]palmitate and adipose biopsies. The dynamic left ventricular cavity PET images combined with blood sample radioactivity corrected for the ¹¹CO₂ content were used to create the blood time activity curve (TAC), and the constant (K_i) was determined using Patlak analysis of the TACs generated for regions of interest in abdominal subcutaneous fat. These data were used to calculate palmitate uptake rates in abdominal subcutaneous adipose tissue (µmol·kg^-1·min^-1). Immediately after the dynamic imaging, a static image of the thigh was taken to measure the standardized uptake value (SUV) in thigh adipose tissue, which was scaled to each participant's abdominal adipose tissue SUV to calculate thigh adipose palmitate uptake rates. Abdominal adipose palmitate uptake using PET [1-¹¹C]palmitate was correlated with, but significantly (P < 0.001) greater than, FFA storage measured using [1-¹⁴C]palmitate and adipose biopsy. Thigh adipose palmitate measured using PET calculation was positively correlated (R² = 0.44, P < 0.0001) with and not different from the biopsy approach. The relative differences between PET measured abdominal subcutaneous adipose tissue palmitate uptake and biopsy-measured palmitate storage were positively correlated (P = 0.03) with abdominal subcutaneous fat. We conclude that abdominal adipose tissue FFA uptake measured using PET does not equate to adipose FFA storage measured using biopsy techniques.

Keywords: Patlak analysis; [1-C]palmitate; [U-C]palmitate.

Fig. 1.

Relationships between adipose tissue positron emission tomography (PET) [1-¹¹C]palmitate uptake and biopsy-measured [1-¹⁴C]palmitate storage. Palmitate storage in abdominal and thigh subcutaneous fat measured using a bolus/biopsy approach is plotted versus adipose palmitate uptake measured using PET with [1-¹¹C]palmitate. A: abdominal subcutaneous palmitate uptake measured with biopsy versus PET calculated using the left ventricle (LV) hybrid time activity curve for the input function. B: abdominal subcutaneous palmitate uptake measured with biopsy versus PET calculated using the aorta max-hybrid time activity curve for the input function. C: abdominal subcutaneous palmitate uptake measured with biopsy versus PET calculated using the aorta average-hybrid time activity curve for the input function. D: femoral subcutaneous palmitate uptake measured with biopsy versus PET calculated using the ratio of the standardized uptake values in thigh versus abdominal adipose tissue.

Fig. 2.

Abdominal fat and measures of abdominal adipose fatty acid uptake/storage. The amount of abdominal subcutaneous fat measured by a single slice computed topography (CT) of the abdomen is plotted versus the relative differences between palmitate storage in abdominal subcutaneous fat measured using a bolus/biopsy approach and adipose palmitate uptake measured using positron emission tomography (PET) with [1-¹¹C]palmitate.