Skeletal muscle fatty acid handling in insulin resistant men

Multitissue assessment of in vivo postprandial intracellular lipid partitioning in rats using localized 1H-[13C] magnetic resonance spectroscopy

Excess accumulation of lipids in nonadipose tissues such as skeletal muscle and liver has been implicated in the development of obesity-related disorders, but the cause of this ectopic lipid overload remains unknown. The aim of this study was to determine in vivo postprandial lipid partitioning in rat skeletal muscle and liver, using localized 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids. Six rats were measured at baseline and 5 and 24 h after administration of 400 mg [U-13C]-labeled algal lipids. Five hours after administration, fractional 13C enrichments of the lipid pools in muscle and liver were increased 3.9-fold and 4.6-fold (P<0.05), respectively, indicating that part of the ingested lipids had been taken up by muscle and liver tissue. At 24 h, fractional 13C enrichments of muscle and liver lipids were decreased 1.6-fold and 2.2-fold (P<0.05), respectively, compared with the 5 h values. This can be interpreted as a depletion of 13C-labeled lipids from the intracellular lipid pools as a consequence of lipid turnover. In conclusion, the novel application of 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids is applicable for the longitudinal assessment of in vivo lipid partitioning between multiple tissues.

Origins of metabolic complications in obesity: adipose tissue and free fatty acid trafficking

PURPOSE OF REVIEW

Obesity is associated with a number of serious medical complications that are risk factors for cardiovascular disease (e.g., insulin resistance, dyslipidemia, and liver fat accumulation). Alterations in fatty acid trafficking, both between tissues and within cells, represent a key feature in the pathophysiology of the metabolic complications in obese patients. The ways by which fatty acid 're-routing' may affect metabolic function are summarized in this article.

RECENT FINDINGS

Ectopic fat accumulation (i.e., fat accumulation in nonadipose tissues) appears to be a key feature distinguishing metabolically healthy from metabolically abnormal patients. This observation has led to the belief that an imbalance in fatty acid trafficking away from adipose tissue toward nonadipose tissues is a primary cause for the development of metabolic alterations in obese patients. More recently, however, it has become apparent that fatty acid trafficking within nonadipose tissues cells (i.e., toward storage - in the form of triglycerides - and oxidation) may be equally important in determining a person's risk for development of metabolic disease.

SUMMARY

The pathophysiology of the metabolic alterations associated with obesity is probably multifactorial within a complex network of coordinated physiological responses. Only through the integration of multiple concepts, will it be possible to further our understanding in this area and to help prevent the metabolic alterations associated with obesity.

Impaired insulin sensitivity is accompanied by disturbances in skeletal muscle fatty acid handling in subjects with impaired glucose metabolism

OBJECTIVE

To determine insulin sensitivity and skeletal muscle fatty acid (FA) handling at baseline and after a high-fat mixed meal in impaired fasting glucose (IFG), impaired glucose tolerance (IGT), IFG/IGT and normal glucose tolerance (NGT) subjects.

DESIGN

In this multi-center study, insulin sensitivity and β-cell function were assessed (n=102), using a euglycemic-hyperinsulinemic and hyperglycemic clamp with additional arginine stimulation and a 75 g oral glucose tolerance test. Fasting and postprandial skeletal muscle FA handling was examined in a substudy using the forearm balance technique (n=35).

SUBJECTS

A total of 102 subjects with IFG (n=48), IGT (n=12), IFG/IGT (n=26) and NGT (n=16).

RESULTS

IFG, IGT and IFG/IGT subjects had lower insulin sensitivity with no differences between groups, and lower impaired β-cell function compared with NGT controls. The early postprandial increase in triacylglycerol (TAG) concentration was higher (iAUC(0-2 h) IFG: 238.4±26.5, IGT: 234.0±41.0 and NGT: 82.6±13.8 μmol l(-1) min(-1), both P<0.05) and early TAG extraction was increased (AUC(0-2 h) IFG: 56.8±9.0, IGT: 52.2±12.0 and NGT: 3.8±15.4 nmol·100 ml(-1) min(-1), P<0.05 and P=0.057, respectively) in both IFG and IGT subjects.

CONCLUSION

IFG, IGT and IFG/IGT subjects have lower insulin sensitivity and impaired β-cell function compared with age- and BMI-matched NGT controls. The increased postprandial TAG response and higher muscle TAG extraction in both IFG and IGT compared with NGT may lead to ectopic fat accumulation in the skeletal muscle, thereby contributing to insulin resistance.