Comparison of adipocytokine levels in age, sex and BMI matched vegetarian versus non-vegetarian adults
White adipose tissue (WAT) stores energy in the form of triglycerides during nutritional abundance and releases energy as free fatty acids during nutritional deprivation.1 This has advantages for times of starvation but disadvantages during times of nutritional plenty. WAT is also a major endocrine organ producing a range of biologically active molecules called adipocytokines. These molecules appear to influence the function as well as the structural integrity of other tissues.2 Included in these are adiponectin, leptin and resistin.
Leptin and resistin concentrations both positively correlate with obesity and insulin resistance (IR) while adiponectin is inversely correlated. Importantly, while serum adiponectin levels are inversely related to adiposity amongst adults and children the differences are more marked amongst women compared to men.3-5
Despite this difference, serum adiponectin levels are associated with adiposity, type 2 diabetes and cardiovascular disease in both genders.6 Adiponectin is negatively regulated in obesity and negatively correlated with serum leptin concentration, fasting insulin, insulin resistance,7 body mass index, systolic and diastolic blood pressure, fasting plasma glucose, total and low-density lipoprotein-cholesterol, triglycerides and uric acid but positively correlated with high-density lipoprotein-cholesterol.5 This suggests a positive role for adiponectin in the maintenance of cardiovascular health.Adiponectin appears to exert its effect in reducing insulin resistance by decreasing plasma fatty acid and triglyceride levels in muscle and liver cells.8-9
Chronic inflammation is also a hallmark of obesity, and inflammatory changes correlate with adverse health outcomes. As the adipocytes enlarge, cell metabolism is altered promoting the recruitment of macrophages and production of proinflammatory cytokines such as TNF-α. In addition to its positive contribution to markers of both diabetes type 2 and vascular health adiponectin has also been shown to down-regulate mediators of inflammation such as TNF-α and induce anti-inflammatory cytokines such as IL-10.10
While adiponectin can promote vascular health, reduce markers of inflammation and promote insulin sensitivity in humans, little is known about the mechanisms controlling production of this important adipokine.
Weight loss amongst obese and insulin-resistant individuals well as treatment with insulin-sensitizing drugs such as thiazolidinediones has been shown to increase adiponectin levels.6, 11-12 The control of adiponectin release by adipocytes is not currently well understood, but is likely to involve peroxisome proliferator-activated receptor gamma (PPAR-g) receptor activity, which is the master regulator of adipocyte differentiation and controls many adipocyte genes.
One study examined the role of caloric intake in adiponectin production and found that an intensive lifestyle modification program with caloric restriction and structured exercise amongst obese subjects with insulin resistance resulted in increased adiponectin levels.13 While studies on the effect of selected dietary components are essentially absent in humans, the effect of diet has been more extensively examined in rodents. Neschen et al,14 have reported that fish oils will increase adiponectin secretion from adipocytes through a PPARg-dependant mechanism. In mice fed a high-fat diet plasma adiponectin levels decreased significantly resulting in insulin resistance and hypertriglyceridemia (Yamauchi, 2001). Furthermore, adiponectin-deficient mice developed mild insulin sensitivity on a standard diet,15 but severe insulin resistance when fed a high-fat, high sucrose diet.16 No studies have examined the level of adiponectin in humans consuming a predominantly vegetarian diet
- To test and compare serum adiponectin levels in age, sex and BMI matched individuals consuming either a vegetable rich or vegetable poor diet.
- To correlate adiponectin levels to consumption of selected foods such as nuts, vegetables, fruits, whole grains, oils, fish, meat, dairy etc.
- To correlate adiponectin levels to saturated fat consumption.
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