The purpose of this study was to report observations about changes in emerging risk factors following weight loss induced by a CRD with or without soluble fiber supplementation in overweight and slightly obese men. Though comparisons of these changes cannot be made to other forms of diet therapy, these data contribute to the small amount of evidence currently available about the effects of CRD on markers of CVD risk not traditionally measured. The main findings of the study were that a CRD resulted in a spontaneous reduction in caloric intake in 93% of subjects and favorable effects on a number of non traditional biomarkers including a significant reduction in plasma Lp(a) in 76% of subjects and a significant reduction in the inflammatory markers CRP in 69% and TNF-α in 76% of subjects. There were no effects of adding additional soluble fiber to a CRD on these responses. Though dietary fiber has been shown to reduce CVD risk, it appears that the significant weight loss in our subjects has overshadowed any potential benefits from the fiber supplementation.
Consistent with other reports [
20‐
22], simply instructing subjects to restrict carbohydrates resulted in a spontaneous reduction in energy intake (mean 718 kcal/day), suggesting that a CRD may be easier to adhere to than diet therapies that specifically target energy restriction. The reduction in caloric intake was predominantly due to the 74% reduction in carbohydrate. There is a common belief that CRD are high in fat, but in this study the absolute amount of total fat and saturated fat intake was unchanged and protein intake only modestly increased.
Homocysteine
Weight loss can lead to increases in homocysteine concentrations [
23,
24], and alter the dose-response relationship between both serum folate and vitamin B-12 levels (i.e., higher serum folate and B-12 levels are required to maintain the same homocysteine level in those losing weight) [
25]. The carbohydrate restricted diet supplemented with folic acid in this study did not affect homocysteine concentrations. This is consistent with other work [
26] and a prior study we published showing that weight loss induced by a low-fat diet and a 400 μg folic acid supplement resulted in no effect on homocysteine [
27]. Therefore, vitamin supplementation or emphasis on folic acid-rich foods may be an important component of a weight loss program to prevent increases in homocysteine.
Homocysteine may be elevated following the ingestion of a high-protein meal even in individuals accustomed to higher protein intake. However, homocysteine levels tend to be normalized following fasting for an extended period of time (as in preparation for a blood draw) [
28]. Postprandial measurements were not taken, though any potential increases in homocysteine appear to have been transient given the fasting plasma levels.
Lp(a)
Elevated plasma Lp(a) is a risk factor for cardiovascular disease because of both atherogenic and thrombotic properties [
4]. The 12% decrease in plasma Lp(a) is a novel finding considering that plasma Lp(a) levels are reported to have a strong genetic influence [
29,
30] and that diet usually has little positive influence on Lp(a) levels [
5,
31]. Important to note is that most diet interventions that have examined Lp(a) response emphasized fat restriction with moderate to high carbohydrate intake, which sharply contrasts the macronutrient contribution in the current intervention. Reducing total and saturated fat intake has been shown to increase Lp(a) during weight maintenance [
32] and have no effect on Lp(a) during weight loss [
33]. The significant decrease in Lp(a) in this study suggests that carbohydrate restriction, as opposed to fat restriction, may play a greater role in modulating Lp(a) levels during weight loss. Diets relatively high in trans fatty acids in comparison to saturated or unsaturated fatty acids can significantly increase plasma Lp(a) [
34]. Thus, the 38% reduction in trans fatty acid consumption could also have also influenced this parameter.
Reports about weight loss and change in plasma Lp(a) are varied. Significant weight loss achieved via a very-low calorie diet in obese individuals has been shown to both reduce [
35] and not reduce plasma Lp(a) [
36]. However, a very-low calorie diet and weight loss may only reduce plasma Lp(a) in subjects with elevated baseline values (> 20 mg/dL). Our results are in agreement with previous reports that reductions in plasma Lp(a) correlate strongly with baseline Lp(a), but not with weight loss [
36,
37].
CRP, IL-6, and TNF- α
Both the acute-phase reactant CRP and the inflammatory cytokine TNF-α were significantly reduced over time. These results extend our previous findings that weight loss achieved through carbohydrate restriction can improve plasma levels of inflammatory biomarkers [
38] indicating that these results persist through 12 wk. Previous reports [
38,
39] suggest that weight loss – rather than the macronutrient composition of the diet used to achieve weight loss – is key to reducing plasma markers of inflammation. In the present study, a 7.9% weight loss was accompanied by an 18.5% reduction in CRP, which is consistent with other reports where low fat [
40] and CRD [
41] were used to achieve weight loss. We did not find a correlation between weight or fat loss and changes in any inflammatory marker. Change in CRP was significantly correlated with baseline levels, which is similar to a previous report utilizing a CRD [
41]. Another positive alteration in inflammatory status was the reduction in TNF-α. Obese individuals release greater quantities of TNF-α, which is at least partially derived from adipose tissue [
42]. The significant reductions in adipose tissue likely played a role in reducing plasma TNF-α. As such, it appears that carbohydrate restriction could be considered an option for overweight men to improve cardiovascular risk.
We also examined the individual data for CRP and TNF-α. The range for subjects with relative reductions in CRP (n = 21) was -2.9 to – 71.9%, and the range for subjects with relative increases in CRP (n = 8) was 1.5 to 43.0%. Fourteen of 21 subjects who reduced CRP had relative reductions greater than 20%. Five of eight subjects who had increases in CRP had elevations greater than 20%. Though the mean reductions and individual data indicate more individuals reduced CRP, some individuals had increases, indicating that weight loss from a CRD may not be universally appropriate for reducing CRP. However, CRP concentrations in individuals who experienced elevations after 12 wk was 1.36 mg/L (range = 0.14 to 4.17 mg/L), which is within the normal range. Similarly, 22 subjects had relative reductions in TNF-α. Our observations indicate that a CRD is generally effective at reducing both CRP and TNF-α, though, as with any dietary therapy, individual cases should be evaluated to determine appropriate treatment.
An unexpected result was that IL-6 was unchanged from baseline to week 12. IL-6 has often been reported to be reduced following weight loss [
43,
44] but not always [
40,
45]. IL-6 is thought to be important in the inflammatory cascade associated with atherogenesis because of its role in both stimulating several cellular adhesion molecules and smooth muscle cell proliferation and migration [
42]. IL-6 also acts as a messenger cytokine [
46] and that derived from adipose tissue is thought to provide the principle stimulus for hepatic CRP production [
16,
42]. Though a considerable amount of plasma IL-6 in healthy individuals is thought to be derived from adipose tissue (approximately 30%), a large proportion is derived from other tissue [
47]. Thus, though weight loss – particularly fat loss – would be expected to reduce plasma IL-6, other influencing factors likely play an important role.
One potential tissue that may be implicated is skeletal muscle. It has been reported that consuming a CRD leads to an approximate 50% reduction in skeletal muscle glycogen [
48], which could potentially affect plasma IL-6 levels. Steensberg et al. [
49] hypothesized that contracting skeletal muscle releases IL-6, which acts in a hormone-like manner to stimulate hepatic glucose output. Subsequent work indicated that IL-6 release from skeletal muscle is increased in response to reduced glycogen levels [
50]. It is important to note that reduction of glycogen was accomplished through exercise rather that diet. However, the response of IL-6 to reduced glycogen, taken in conjunction with observations that infusion of recombinant IL-6 in humans increases liver glucose output [
50,
51], suggest a potential explanation for the unchanged IL-6 levels (despite weight loss) in the present study. Future research is needed to confirm the role of IL-6 in glycogen depletion resulting from a CRD.
CRD have been consistently shown to reduce triglycerides and increase HDL-C, yielding a beneficial effect on CVD risk, particularly for individuals with metabolic syndrome [
52]. For the current study we correlated triglyceride and HDL-C responses with emerging risk factors to determine if plasma lipid responses were related to changes in other risk factors for CVD. We found no relationship between response of traditional markers for CVD (HDL-C and triglyceride response) and relative changes in TNF-α, IL-6, CRP, Lp(a), and homocysteine. As such, we believe that individuals who have a less favorable triglyceride and HDL-C response to a CRD do not necessarily have an inferior response in terms of CVD risk markers not traditionally measured.