The results from both epidemiological and physiological studies have demonstrated
a strong association between excess abdominal adipose tissue and the presence of metabolic
risk factors for coronary heart disease (CHD), including insulin resistance, impaired
glucose tolerance, type 2 diabetes, dyslipidemia, and increased circulating inflammatory
proteins (1
–3). Abdominal adipose tissue is a complex organ and is composed of multiple distinct
compartments and subcompartments, including subcutaneous fat and intra-abdominal fat,
which can be further subdivided into retroperitoneal and intraperitoneal fat, which
can be divided again into mesenteric and omental fat masses. Intraperitoneal fat,
which is also known as visceral adipose tissue (VAT), is considered a particularly
important marker of metabolic risk (4
–6).
It has been hypothesized that increased VAT is directly involved in the pathogenesis
of metabolic dysfunction because VAT releases free fatty acids (FFAs) and inflammatory
proteins into the portal vein, which are delivered to the liver (7). However, most
FFAs in the portal circulation are derived from subcutaneous adipose tissue, and <20%
of total FFAs delivered to the liver or skeletal muscle originate from lipolysis of
VAT in obese people (8,9). Moreover, most inflammatory adipokines in the portal vein
are likely derived from subcutaneous fat, which releases adipokines into the systemic
circulation that enter the portal vein through the splanchnic bed (10). In addition,
increased VAT itself is not associated with insulin resistance or dyslipidemia without
a concomitant increase in intrahepatic triglycerides (11). Therefore, these data do
not support an obvious causal link between intraperitoneal fat and metabolic disease.
Surgical removal of the greater omentum makes it possible to evaluate the importance
of VAT in the pathophysiology of obesity in people. In fact, the results from two
randomized controlled studies (12,13) have already been reported that evaluated the
effect of surgical removal of the greater omentum on insulin action in obese patients
undergoing bariatric surgery. Unfortunately, the data and conclusions from these studies
are contradictory. In one study (12), subjects randomized to adjustable gastric banding
plus omentectomy had a greater improvement in oral glucose tolerance and insulin sensitivity,
assessed by using an intravenous insulin tolerance test, than subjects randomized
to adjustable gastric banding alone. However, the omentectomy group also experienced
more weight loss than the banding-alone group, which could have contributed to the
observed differences in insulin action. In the second study (13), the prevalence of
hyperglycemia and hyperinsulinemia 2 years after surgery were not different in subjects
randomized to roux-en Y gastric bypass (RYGB) surgery plus omentectomy or RYGB alone,
but insulin sensitivity was not directly assessed.
In this issue of Diabetes Care, Herrera et al. (14) report the results of a 1-year
randomized controlled trial that evaluated whether omentectomy provided additional
therapeutic effects on selected metabolic variables and circulating inflammatory proteins
and adipokines in obese patients who undergo RYGB surgery (15). Twenty-two subjects
were randomized to have RYGB surgery or RYGB surgery plus omentectomy. The rate of
weight loss was the same in both groups throughout the study and reached a maximum
of ∼30% weight loss at 1 year. The amount of VAT removed (∼0.8 kg), which presumably
represents at least 25% of total VAT, is greater than diet-induced reductions in VAT
that is associated with a 25–50% increase in skeletal muscle and liver insulin sensitivity
(15
–17). In their subjects, surgery-induced weight loss resulted in considerable improvement
in most metabolic and inflammatory outcomes (plasma glucose, insulin, adiponectin,
and C-reactive protein concentrations; lipid profile; blood pressure; impaired glucose
tolerance; and diabetes) but had mixed results in plasma concentrations of several
other adipokines. However, there was no significant difference in any outcome measure
between groups. In addition, performing an omentectomy was not trivial and had adverse
effects; omentectomy increased the duration of surgery by >1 h and caused a serious
complication in one subject.
The results from the study by Herrera et al. have important implications regarding
the role of VAT in the pathophysiology of obesity and suggest that increased VAT does
not directly cause metabolic dysfunction. However, limitations in study design leave
two important questions unanswered. First, is it possible that the overwhelming effect
of weight loss induced by RYGB surgery masked the potential therapeutic effects of
removing VAT? Second, were the outcome measures sensitive enough to detect metabolic
improvements, particularly in insulin sensitivity, which is probably the most common
metabolic abnormality associated with increased VAT? Additional studies are still
needed that use more sensitive methods to assess insulin action and that evaluate
the effect of omentectomy alone without concomitant weight loss surgery.