This study is the first to show that IFCR with liquid meals can produce potent decreases in CHD risk, and that these effects are mediated in part by improvements in adipokines. More specifically, we show here that IFCR-L is an effective diet therapy to modulate lipid indicators of CHD risk, i.e. reduce LDL cholesterol, triglycerides, and the proportion of small LDL particles, while increasing LDL peak particle size. Our findings also demonstrate that these favorable changes in lipids were related to reduced waist circumference (visceral fat mass) and reductions in pro-atherogenic adipokines, such as leptin and TNF-alpha.
Although both of the interventions produced favorable changes in lipids, superior modulations were shown in the IFCR-L group when compared to the IFCR-F group. The reductions in plasma lipids by IFCR-L (LDL-cholesterol: 19%, triglycerides: 20%) are similar to what has been reported in previous trials of IF[2, 3]. For instance, in a trial by Williams et al., obese subjects consumed a very-low calorie diet (VLCD; <500 kcal/d) 1 day per week, and ate ad libitum every other day of the week. After 20 weeks of treatment, LDL cholesterol and triglyceride concentrations decreased by 10% and 52%, respectively. In the trial by Harvie et al., obese women underwent 2 days of VLCD (600 kcal/d) and ate ad libitum on every other day of the week, for 24 weeks. Post-treatment LDL cholesterol and triglyceride levels were reduced by 10% and 17% from baseline. The mechanism by which IFCR modulates circulating lipid concentrations is not clear. Nonetheless, recent evidence from CR studies indicate that the oxidation of circulating free fatty acids (FFA) is increased during periods of weight loss, while FFA synthesis is decreased. Lower availability of precursor FFA results in a reduction in hepatic synthesis and secretion of very-low density lipoprotein (VLDL) into plasma. Lower secretion of VLDL contributes to reduced plasma concentrations of LDL, since VLDL is quickly converted to LDL in the circulation. Although this mechanism has only been demonstrated for CR, it possible that IFCR may alter circulating lipids in a similar fashion.
The greater improvement in lipid profile by IFCR-L is most likely due to the more pronounced reductions in body weight and visceral fat mass observed. After 8 weeks of treatment, body weight and waist circumference, an indirect indicator of visceral fat, decreased to a greater extent in the IFCR-L (4 kg and 6 cm, respectively) when compared to the IFCR-F group (3 kg and 4 cm, respectively). The greater weight loss by the IFCR-L group is not surprising as these individuals had a larger daily calorie deficit relative to the IFCR-F group (453 kcal/d, 250 kcal/d, respectively). These decreases in waist circumference in the IFCR-L group were related to reductions in LDL cholesterol concentrations. An accumulation of adipose tissue in visceral depots may contribute to the development of dyslipidemia in several ways. For instance, lipolysis of fat tissue in visceral adipocytes is higher than that of subcutaneous adipocytes. This can lead to an augmented efflux of FFA from visceral depots. These FFAs released from visceral fat are then collected by the portal vein and reach the liver at much higher concentrations than they do the systemic circulation. In the liver, these FFA trigger the augmented production of triglycerides, and the elevated secretion of VLDL. The increased secretion of VLDL then results in higher plasma levels of LDL. In view of this, it is conceivable that the decrease in visceral fat by IFCR played a role in the reduced LDL cholesterol concentrations shown here.
Decreases in pro-atherogenic adipokines, such as leptin (26%), IL-6 (19%), TNF-alpha (25%), and IGF-1 (10%) were observed by the IFCR-L group. No changes in adipokines were noted in the IFCR-F group, however, which is most likely due to insufficient weight loss to achieve changes in these parameters. The decreases in leptin noted here are similar to those observed by Harvie et al.. After 24 weeks of IF, obese women experienced potent reductions in leptin of 40%. The greater reductions in leptin in this previous trial are most likely due to the greater weight loss achieved with the longer trial duration. In the IFCR-L group, lower plasma leptin was related to decreased triglyceride levels. In vitro studies demonstrate that leptin is a potent stimulator of lipolysis and fatty acid oxidation in adipocytes and other cell types. Accordingly, leptin is also a regulator of circulating triacylglycerol concentrations. Reduced leptin levels were also related to lower LDL cholesterol levels post-treatment, though the mechanism by which leptin may be involved in reducing LDL cholesterol concentrations is not clear. We also observed a positive association between visceral fat mass and leptin levels. Thus, it is possible that the decrease in visceral fat by the IFCR-L intervention stimulated a reduction in leptin, which in turn contributed to the lipid profile improvements demonstrated here. Reductions in TNF-alpha were also correlated to decreases in plasma triglycerides. Evidence in rodent models indicates that TNF-alpha induces hyperlipidemia by increasing hepatic triglyceride production. Thus, a reduction in circulating TNF-alpha by IFCR-L may be related to the reductions in triglycerides observed.
This study is limited in that it did not carefully control for food intake by providing food-based meals to the intervention groups, i.e. dinner meal for the IFCR-L group, and 3 meals/d for the IFCR-F group. Providing all the meals during the study would allow for a more precise assessment of dietary adherence. Another disadvantage is that only female subjects were employed, and as such, the applicability of these findings to males remains uncertain.
Taken together, our results suggest that IFCR with liquid meals is an effective diet therapy to reduce body weight, visceral fat mass, and lipid indicators of CHD risk. Our findings also demonstrate that the beneficial modulations in vascular disease risk by IFCR may be mediated, in part, by reductions in visceral fat mass and pro-atherogenic adipokines. This study is an important first step to understanding the underlying mechanisms that mediate the cardio-protective effects of this novel diet regimen.