The metabolic syndrome (syndrome X) is increasingly being seen as a major risk factor for cardiovascular disease. Metabolic syndrome is a group of metabolic abnormalities that are believed to be caused by insulin resistance. Insulin resistance is characterised by a desensitivity in the insulin receptor, such that the hormone insulin loses its ability to remove glucose from the blood and pass it into cells for energy production. The result is an increase in fasting blood glucose levels. Because insulin resistance develops primarily in hepatocytes and myocytes, the glucose in the blood is still able to pass into adipocytes, where it is converted to triglycerides. Insulin resistance is also characterised by raised plasma levels of insulin, and therefore triglycerides become trapped in the adipocytes, as lipolysis is not possible under these conditions. Understanding the development of insulin resistance is therefore key to understanding obesity and weight gain.
Evidence supports the hypothesis that the interference with the insulin receptor that characterises insulin resistance may originate from excessive storage of lipid droplets in hepatocytes and myocytes. Fructose may the causative agent of this excessive lipid production, as high intakes of fructose, as may occur through excessive soft drink consumption, can overload the liver and cause increased flux through the de novo lipogenesis pathway. The overproduction of lipids leads to their storage in muscle and liver tissue and this may be a direct cause of insulin resistance. The storage of fat in hepatic tissue causes abdominal (visceral) obesity and accumulation of abdominal fat is a known risk factor for cardiovascular disease. Much has been published on the deleterious effects of abdominal fat, but less is known about intramuscular fat and disease. However, evidence also suggests that this type of fat storage may be detrimental to health.
For example, one study1 published in the International Journal of Obesity investigated the metabolic implications of intramuscular fat deposits using magnetic resonance imaging. The subjects were selected because they had fasting glucose measurements above 126 mg/dL, which was indicative of the presence of metabolic syndrome. The results showed that intramuscular fat deposits were associated with fasting serum glucose concentrations and total cholesterol levels. In addition, there was an significant association between intramuscular fat and protein bound glucose, which is a potential biomarker for diabetes. Therefore intramuscular lipid deposits show association with physiological changes and biomarkers that are thought to increase the risk of cardiovascular disease. This is not surprising because intramuscular fat deposits are believed to be formed by the same process that causes visceral fat accumulation.
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