Effects of increased free fatty acid availability on adipose tissue fatty acid storage in men

Abstract

Context: A portion of free fatty acids (FFA) released from adipose tissue lipolysis are re-stored in adipocytes via direct uptake. Rates of direct adipose tissue FFA storage are much greater in women than men, but women also have greater systemic FFA flux and more body fat.

Objective: We tested the hypotheses that experimental increases in FFA in men would equalize the rates of direct adipose tissue FFA storage in men and women.

Design: We used a lipid emulsion infusion to raise FFA in men to levels seen in post-absorptive women. Direct FFA storage (μmol · kg fat(-1) · min(-1)) rates in abdominal and femoral fat was assessed using stable isotope tracer infusions to measure FFA disappearance rates and an iv FFA radiotracer bolus/timed biopsy.

Setting: These studies were performed in a Clinical Research Center.

Participants: Data from 13 non-obese women was compared with that from eight obese and eight non-obese men.

Intervention: The men received a lipid emulsion infusion to raise FFA.

Main outcome measures: We measured the rates of direct FFA storage in abdominal and femoral adipose tissue.

Results: The three groups were similar in age and FFA flux by design; obese men had similar body fat percentage as non-obese women. Despite matching for FFA concentrations and flux, FFA storage per kg abdominal (P < .01) and femoral (P < .001) fat was less in both lean and obese men than in non-obese women. Abdominal FFA storage rates were correlated with proteins/enzymes in the FFA uptake/triglyceride synthesis pathway in men.

Conclusion: The lesser rates of direct FFA adipose tissue in men compared with women cannot be explained by reduced FFA availability.

Figure 1.

The average plasma concentrations of oleate, linoleate, palmitate, and stearate are shown. Blood samples were collected before (time, 0–30 min) and during the last 30 minutes (time, 150–180 min) the infusion of a lipid emulsion designed to modestly increase total FFA concentrations in men to achieve concentrations comparable to those seen in non-obese women.

Figure 2.

Relationships between fatty acid storage factors (CD36 content and ACS and DGAT activities) and direct FFA storage rates per kg adipose tissue lipid in UBSQ fat in men (closed circles) and women (open circles). A, Direct FFA storage fat relative to acyl-CoA synthetase activity. B, Direct FFA storage relative to DGAT activity. C, Direct FFA storage relative to CD36 content. Regression lines indicate statistically significant correlations for men only (see Results).

Figure 3.

Relationships between fatty acid storage factors (CD36 content and ACS and DGAT activities) and direct FFA storage rates per kg adipose tissue lipid in LBSQ fat in men (closed circles) and women (open circles). A, Direct FFA storage fat relative to acyl-CoA synthetase activity. B, Direct FFA storage relative to DGAT activity. C, Direct FFA storage relative to CD36 content. None of the correlations reached statistical significance.