The metabolic syndrome and inflammation: association or causation?

The aim of this editorial was to discuss evidence indicating a role for low-grade inflammation as a pathogenetic event of the metabolic syndrome. The metabolic syndrome has emerged as an important cluster of risk factors for atherosclerotic disease. Common features are central (abdominal) obesity, insulin resistance, hypertension, and dyslipidemia, namely high triglycerides and low high-density lipoprotein cholesterol. According to the clinical criteria developed by ATP III, it has been estimated that 1 out of 4 adults living in the United States merits the diagnosis. The presence of the metabolic syndrome is highly prognostic of future cardiovascular events. Chronic inflammation may represent a triggering factor in the origin of the metabolic syndrome: stimuli such as overnutrition, physical inactivity, and ageing would result in cytokine hypersecretion and eventually lead to insulin resistance and diabetes in genetically or metabolically predisposed individuals. Alternatively, resistance to the anti-inflammatory actions of insulin would result in enhanced circulating levels of proinflammatory cytokines resulting in persistent low-grade inflammation. A generally enhanced adipose tissue derived cytokine expression may be another plausible mechanism for the inflammation/metabolic syndrome relationship. The role of adipose tissue as an endocrine organ capable of secreting a number of adipose tissue-specific or enriched hormones, known as adipokines, is gaining appreciation. Although the precise role of adipokines in the metabolic syndrome is still debated, an imbalance between increased inflammatory stimuli and decreased anti-inflammatory mechanisms may be an intriguing working hypothesis. The proinflammatory state that accompanies the metabolic syndrome associates with both insulin resistance and endothelial dysfunction, providing a connection between inflammation and metabolic processes which is highly deleterious for vascular functions.