In this study we are reporting for the first time that eggs modulate the response of certain inflammatory cytokines during a weight loss intervention using a CRD. Subjects consuming the eggs presented a better response to adiponectin and CRP, two major markers of inflammation and of CHD risk. This effect could be due to the high concentration of lutein, a potent antioxidant present in the egg yolk.

Adiponectin

There was a significant increase in adiponectin in both the EGG and SUB groups, with a greater increase in adiponectin observed in the EGG group. An anti-atherogenic adipokine, adiponectin decreases adhesion molecule expression that occurs after inflammation [24] and also decreases TNF-α production by macrophages [25]. Reductions in weight and fat mass in obese individuals is often accompanied by an increase in serum adiponectin [26], accordingly, we observed a significant reduction in trunk fat in this study following the intervention. Inflammatory cytokines suppress adiponectin expression [27], so the increase in adiponectin may also be directly related to reduction in these inflammatory markers and because of a selective decrease of CRP only by eggs, a higher increase of adiponectin was observed for this group. Paraoxonase is an antioxidant carried on HDL that protects against LDL oxidation [28], and with inflammation, there is a reduction in paroxonase levels [29]. The increased HDL-C and additional adiponectin observed in the EGG group may have resulted in elevations of paraoxonase, which may explain improvements in the inflammatory profile. Direct administration of adiponectin stimulates fatty acid oxidation in hepatic and muscle tissues, which leads to decreased triglycerides [30]. Increased adiponectin levels are associated with increased insulin sensitivity, decreased triglycerides, and increased HDL-C [31]. Accordingly, we found that the EGG group presented a significant increase in plasma HDL-C in addition to higher levels of adiponectin. Egg intake is associated with significant increases in HDL-C and concentration of anti-atherogenic large HDL particles[16, 17]. Large HDL particles are correlated with decreased CHD risk [17, 32] because they can more efficiently remove excess cholesterol by returning it to the liver for excretion. It is postulated that adiponectin may influence reverse cholesterol transport (RCT) by increasing HDL production in the liver by promoting apoA-1 synthesis and the ABCA1 pathway [33]. Additionally, accelerated RCT results in decreased secretion of ApoB-100 containing lipoproteins [34], which is evidenced by the reduction in triglycerides. Lipoprotein lipase (LPL) catalyzes the hydrolysis of triglycerides from ApoB-100 containing particles. Several studies show that adiponectin is highly correlated with LPL activity [35, 36]. In addition, there were significant reductions in insulin for both groups. Carbohydrate restriction improves insulin resistance by lowering insulin levels and the disinhibition of hormone sensitive lipase, promoting triacylglycerol hydrolysis [13]. As a result, there is an increase in cellular fatty acid uptake and oxidation [13]. The increase in fatty acid oxidation decreases hepatic triglyceride production and the synthesis and secretion of triglyceride-rich VLDL particles. Taken together, these data suggest that interventions which increase HDL may also be directly involved in improving the inflammatory response and increasing insulin sensitivity as demonstrated by the increased levels of plasma adiponectin.

CRP

At week 12, CRP was reduced only in the EGG group. Serum CRP is an important marker of vascular inflammation and can be used to predict atherosclerosis [7]. Reduced CRP levels are found after weight loss interventions and are inversely correlated to adiponectin [37]. The reduction in adipose tissue due to weight loss reduces the number of pro-inflammatory cytokines (TNF-α, IL-8.) secreted by monocytes, and the number of macrophages and endothelial cells present in the adipocytes [7]. Macrophages and endothelial cells produce CRP, so the reductions in adipose tissue as well as a reduction of carbohydrates work in parallel to mediate the reduction in CRP. Previously, Tannock et al. [38], found that the addition of 4 whole eggs/day for 4 weeks was associated with increased levels of CRP in lean, insulin sensitive individuals, however, there was no effect on CRP in obese or insulin resistant individuals. The subjects in the current study were obese individuals undergoing weight loss and we found that additional dietary cholesterol lowered CRP levels. In previous studies, diets high in monounsaturated fatty acids and polyunsaturated fatty acids have been associated with decreased serum CRP [39]. Additionally, eggs contain the potent antioxidants lutein and xeazanthin, which might play a role in the reduction of inflammation observed in the EGG group. Both groups had significant increases in dietary intake of lutein, but only the EGG group experienced an increase in plasma. These findings correspond with results from another study that found increases in the size and number of HDL particles is associated with increased plasma lutein [18]. We speculate that the reduction of CRP in the EGG group may be the result of increased HDL-C preventing CRP induced upregulation of inflammatory adhesion molecules, possibly through enhanced paraoxonase activity from antioxidants and fatty acids contained within the egg yolk, which can break down oxidized lipids to counteract proinflammatory effects.

TNF-α, IL-8, and MCP-1

TNF-α is an inflammatory cytokine that is secreted 7.5 times higher from the abdominal adipose tissue of obese subjects than their lean counterparts [40]. A few experimental studies show that weight loss through short-term caloric restriction can reduce circulating levels of TNF-α [14, 41], however a reduction of fat mass does not always signal a reduction in TNF-α [42, 43]. We did not observe a change in fasting TNF-α in either experimental group. These discrepancies are likely the result of the duration and method of weight loss. Further studies are needed to understand the relationship between TNF-α and central obesity. Additionally, IL-8 did not change after the intervention either. Stimulated by oxidized low-density lipoprotein, IL-8's released from macrophages plays a key role in the development of atherosclerosis [44]. IL-8 allows adhesion of monocytes to the endothelium [45], which promotes the atherosclerotic process. TNF-α stimulates IL-8 synthesis in the adipocytes [46], so the equivocal results found for IL-8 production likely relates to the failure of TNF-α levels to change after the intervention.

MCP-1 levels were significantly reduced for the SUB group at week 12, while no change was observed for the EGG group. MCP-1 is an inflammatory chemokine mostly produced by endothelial cells and macrophages. In visceral adipose tissue, an elevation in MCP-1 is associated with an increased ability to attract macrophages, which reinforces the inflammatory cycle [25]. In endothelial cells, leptin secreted from adipose tissue enhances MCP-1 synthesis [47]. At this point, it is not clear why the SUB group had a greater reduction in MCP-1 compared to the EGG group. However, the reduction of MCP-1 observed in both groups, although not significant in the EGG group, may be the result of decreased circulating leptin due to weight loss in the current study. We have also observed that subjects in both groups reduced levels of leptin following the CRD intervention (Ratliff and Fernandez, unpublished observations).

VCAM-1 and ICAM-1

VCAM-1 and ICAM-1 levels did not change throughout the intervention for either group. Circulating adhesion molecules play an active role in the atherogenic process by adhering circulating leukocytes into vessel walls. Proteolysis of membrane bound molecules releases soluble forms of VCAM-1 and ICAM-1 into the bloodstream, which can serve as markers of endothelial inflammation [48, 49]. Endothelial VCAM-1 and ICAM-1 are upregulated in response to TNF-α [50]. In the current study, there was no change in TNF-α observed in either group, which may result in the unchanged serum VCAM-1 and ICAM-1 values. A previous study found that caloric restriction in conjunction with exercise for one year lowered plasma VCAM-1 and ICAM-1 in overweight women [51]. The brief duration of the current study, gender, or the lack of structured exercise might also explain why there was not a reduction in cellular adhesion molecules for either group.