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© 2007 American Society for Nutrition J. Nutr. 137:1757-1762, July 2007

Nutrient Physiology, Metabolism, and Nutrient-Nutrient Interactions

Plasma Vitamin C Is Inversely Related to Body Mass Index and Waist Circumference but Not to Plasma Adiponectin in Nonsmoking Adults1,2

Carol S. Johnston3,*, Bonnie L. Beezhold3, Bo Mostow3 and Pamela D. Swan4

3 Department of Nutrition and 4 Department of Exercise and Wellness, Arizona State University, Mesa AZ 85212

* To whom correspondence should be addressed. E-mail: carol.johnston{at}asu.edu.


   ABSTRACT
TOP
 ABSTRACT
Introduction
 Materials and Methods
 Results
 Discussion
 LITERATURE CITED
 
We examined the relationships between plasma vitamin C, adiposity, and the collagen-like adipokine, adiponectin. Of 118 sedentary, nonsmoking adults participating in the cross-sectional trial (35 men and 83 women aged 38.7 ± 1.0 y with BMI of 30.4 ± 0.6 kg/m2, plasma vitamin C concentrations of 43.5 ± 1.3 µmol/L, and plasma adiponectin concentrations of 8.9 ± 0.3 mg/L), 54% were obese and 24% were overweight. Plasma vitamin C was inversely related to BMI, percentage of body fat, and waist circumference in both women and men (r = –0.383 to –0.497, P < 0.025). In women but not men, these associations remained significant after controlling for body mass. Plasma vitamin C was directly related to plasma adiponectin in the women after controlling for age and vitamin C supplement use (r = 0.222, P = 0.049) but not after controlling for body mass. Twenty obese men and women participated in an intervention trial and consumed an energy-restricted diet low in vitamin C ({approx}38 mg/d) for 8 wk. Subjects were stratified by age, gender, and BMI and randomly assigned to receive placebo or vitamin C (500 mg) capsules daily. At baseline, plasma adiponectin was directly related to plasma vitamin C (r = 0.609, P = 0.021) and inversely related to body mass (r = –0.785, P = 0.001). Body mass decreased significantly during the 8 wk study in both the vitamin C (n = 6, –5.9 ± 0.9 kg) and placebo groups (n = 8, –6.5 ± 0.7 kg). Plasma adiponectin increased 13% from baseline by wk 8 in both groups (P < 0.05). In summary, plasma vitamin C was inversely related to markers of adiposity, particularly in women, but vitamin C supplementation did not influence the circulating concentration of adiponectin.


   Introduction
TOP
 ABSTRACT
 Introduction
Materials and Methods
 Results
 Discussion
 LITERATURE CITED
 
Adipose tissue is an important endocrine organ that secretes a range of bioactive regulatory proteins called adipokines, the most abundant of which is adiponectin. (1,2). Unlike the other adipokines, such as tumor necrosis factor alpha (TNF{alpha}), leptin, and interleukin-6 (IL-6),5 adiponectin concentrations in plasma are lower in obese subjects than in nonobese subjects. The paradoxical downregulation of adiponectin during obesity is a result of altered gene expression induced by oxidative stress (3–5). Weight loss reduces inflammation and restores homeostatic control of adiponectin secretion (6–8). Adiponectin protects against insulin resistance and vascular reactivity by altering metabolic processes in liver, muscle, and cardiac tissue (9–11). Many of these effects have been attributed to the adiponectin-dependent activation of signaling molecules such as adenosine monophosphate-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor-{alpha} (PPAR{alpha}) (10–13). Moreover, hypoadiponectinemia has been identified as an independent risk factor for type 2 diabetes, coronary artery disease, and the metabolic syndrome (14–16).

Adiponectin shares sequence homology with collagen X, VIII, and with complement protein C1q (1,2). Furthermore, there are 5 conserved lysine residues (Lys 28, 60, 63, 72, and 96) within the collagenous domain of adiponectin that are hydroxylated (17). Vitamin C is a cofactor in hydroxylation reactions and functions in the biosynthesis of collagen and complement component C1q (18,19). In vitamin C deficiency, collagen in cartilage and tendons is reduced 50% (20) and C1q concentrations in blood are reduced 25–50% (21). It is not known whether plasma vitamin C concentrations are related to adiponectin concentrations in vivo. We examined the relationships between plasma vitamin C, adiponectin, body mass, and fasting insulin concentrations in a sample of healthy adults. Additionally, we conducted a weight loss trial in obese adults to examine whether vitamin C depletion during active weight loss attenuates the increase in adiponectin induced by reductions in body fat.


   Materials and Methods
TOP
 ABSTRACT
 Introduction
 Materials and Methods
Results
 Discussion
 LITERATURE CITED
 
    Subjects. Nonsmoking, generally sedentary men and women, aged 20–60 y, were recruited from a campus community through verbal and electronic advertisement. Overweight (BMI between 25 and 29.9 kg/m2) and obese (BMI >30 kg/m2) individuals were encouraged to participate as well as individuals with normal body weights. Participants were healthy by self-report with no history of chronic disease, including diabetes, heart disease, or cancer. Participants recorded whether they currently took vitamin C supplements. Blood sampling and anthropometric measurements were completed following an overnight fast ({approx}12 h). Participants with BMI ≥30 kg/m2 were invited to participate in an 8-wk weight loss trial. All participants provided informed consent, and the study was approved by the Institutional Review Board at Arizona State University.

    Measurements. Body stature was measured with a standard stadiometer. Waist circumferences were recorded in duplicate to the nearest 0.10 cm with a Gulick tape measure to ensure constant tension during measurement. Body fat mass was estimated using a bioelectrical impedance scale (Tanita) or, for the weight loss trial, by air displacement plethysmography (Bod Pod, Life Measurement).

    Weight loss trial. Twenty obese men and women were stratified by age, gender, and BMI and randomly assigned to the control (CON) or vitamin C (VC) group. A double-blind, parallel intervention design was used. Both dietary groups consumed a high carbohydrate (60% of energy), low fat (<30% of energy) diet based on the U.S. Dietary Guidelines (22; Table 1). The diet provided ≥67% of the recommended dietary intakes for the micronutrients with the exception of vitamin C ({approx}38 mg/d; 40–50% of the recommended dietary intake). All foods and beverages were provided to participants Monday through Fridays during the 8-wk trial. On weekends, subjects were instructed to follow a similar diet plan but were able to select and prepare foods independently. Nutrient intakes were calculated from diet records maintained on weekend days; energy and vitamin C intakes were similar on weekdays and weekends (Table 1). Daily energy intakes were individually adjusted to provide {approx}70% of that needed for weight maintenance. The placebo (white flour) and vitamin C (500 mg vitamin C; Twin Laboratories) capsules were identical in appearance and distributed weekly to participants. CON and VC subjects were instructed to consume 1 capsule (placebo or vitamin C, respectively) daily with food.


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  		TABLE 1 Energy and nutrient contents of the diets consumed by subjects participating in the intervention trial on weekdays (experimentally prepared diet) and on weekends (self-selected diet) for 8 wk1

 
    Analyses. Blood was processed immediately and plasma frozen (–45°C) until analyses. For the vitamin C analysis, fresh plasma was treated with an equal volume of 10% trichloroacetic acid, and the supernatant frozen. Vitamin C was measured spectrophotometrically (23). Plasma adiponectin and insulin concentrations were measured using RIA methodology (cat. no. HADP-61HK, Linco Research; ICN Pharmaceuticals).

    Statistical analysis. Data are reported as means ± SE, and all statistical analyses were performed using SPSS for Windows, version 14. Data were assessed for homogeneity of variance by Levene's test, and data not normally distributed were log transformed prior to analyses. A univariate ANOVA with the Least Significant Difference post hoc test, controlling for gender and age, was used to assess differences between means. The independent t test was used to assess differences between 2 means. Pearson correlation coefficients and partial correlation coefficients controlling for body mass or for age and use of vitamin C supplements were computed to assess relationships between variables. A multivariate general linear model for repeated measures was used to determine significant time and time x group interactions for the intervention trial. Nominal data were compared using the chi-square statistic. Differences were considered significant at P < 0.05.


   Results
TOP
 ABSTRACT
 Introduction
 Materials and Methods
 Results
Discussion
 LITERATURE CITED
 
    Cross-sectional trial. The BMI of participants (35 men, 83 women, aged 38.7 ± 1.0 y) was 30.4 ± 0.6 kg/m2 (range: 18.7 to 47.9 kg/m2); 78% of the sample was classified as overweight or obese. The plasma vitamin C and adiponectin concentrations were 43.5 ± 1.3 µmol/L and 8.9 ± 0.3 mg/L, respectively. About 44% of the sample reported current use of vitamin C supplements, but the use of supplements did not vary by BMI category. Subject characteristics are displayed in Table 2.


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  		TABLE 2 Characteristics of adults in the cross-sectional trial grouped by BMI12

 
The plasma vitamin C concentration was inversely correlated with BMI, percentage of body fat, and waist circumference in both women and men, was indirectly related to plasma insulin in women, and tended to be positively correlated with plasma adiponectin in women (P = 0.055; Table 3). Correlations with BMI, percentage of body fat, and waist circumference remained significant after controlling for age and the use of vitamin C supplements, and correlations with plasma adiponectin in women became significant. Controlling for body mass eliminated all of the significant associations in the men. In women, significant associations between plasma vitamin C and BMI, body fat percentage, and waist circumference remained after controlling for body mass, but plasma vitamin C and adiponectin were not significantly related after controlling for body mass.


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  		TABLE 3 Pearson and partial correlations of plasma vitamin C with plasma adiponectin, BMI, body fat percentage, waist circumference, and plasma insulin for adults in the cross-sectional trial

 
    Weight loss trial. Six participants in the weight loss trial (4 VC; 2 CON) were unable to comply with the diet protocol due to travel and/or job constraints and did not complete the 8-wk study. Age, body mass, BMI, and plasma analytes, with the exception of plasma insulin, did not differ at baseline between VC (n = 6) and CON (n = 8) groups (Table 4). Two VC participants had extreme fasting insulin concentrations at baseline, and the insulin data reported herein represent 4 VC and 8 CON participants.


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  		TABLE 4 Baseline characteristics of adults participating in the intervention trial1

 
At baseline, plasma adiponectin was directly related to plasma vitamin C (r = 0.609, P = 0.021) and inversely related to body mass (r = –0.785, P = 0.001). At trial wk 4 and 8, plasma vitamin C concentrations varied significantly by group (Fig. 1A). Body mass decreased significantly in both groups after the 8-wk intervention (–5.9 ± 0.9 for VC and –6.5 ± 0.7 kg for CON groups; Fig. 1B). Plasma adiponectin concentrations increased 13% in both diet groups (P < 0.05; wk 8 vs. baseline; Fig. 1C), and fasting insulin concentrations were reduced 25–28% (P < 0.05) (Fig. 1D).


Figure 1
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  		FIGURE 1  Plasma vitamin C (A), body mass (B), plasma adiponectin (C), and plasma insulin (D) at 0, 4, and 8 wk in adults in the VC and CON groups. Values are means ± SE; VC, n = 6 (except in D, n = 4); CON, n = 8. Time and time x group interactions were determined by multivariate general linear model for repeated measures.

 

   Discussion
TOP
 ABSTRACT
 Introduction
 Materials and Methods
 Results
 Discussion
LITERATURE CITED
 
Our data show that in women, plasma vitamin C was inversely related to BMI after controlling for age, vitamin C supplement use, and body mass. The inverse relationship between plasma vitamin C and adiposity has been documented in several reports. In 1989, Schectman et al. (24) noted a significant inverse relationship between BMI and plasma vitamin C concentrations among 11,592 participants of the NHANES II. More recently, abdominal obesity, as measured by waist-hip circumferences, was inversely related to plasma vitamin C among 19,000 adults participating in the European Prospective Investigation into Cancer and Nutrition Norfolk cohort study (25). This relationship remained significant after adjusting for age, BMI, vitamin supplement use, cigarette smoking, and social class. Other studies have found reduced serum concentrations of antioxidant vitamins in obese people (26,27), but it is not known whether adiposity reduces vitamin concentrations, perhaps a result of increased oxidative stress or dietary differences, or whether vitamins influence lipid accumulation in the adipocyte.

Previously, we reported that fat oxidation during moderate exercise was significantly reduced in individuals with marginal plasma vitamin C compared to individuals with adequate plasma vitamin C (28). Moreover, fat oxidation during exercise was enhanced in the former group by normalizing plasma vitamin C concentrations. Vitamin C is a cofactor required for the biosynthesis of carnitine, a metabolite required for the transport of long chain fatty acids across the mitochondrial membrane for subsequent fat degradation and oxidation (29). Carnitine deficiency is associated with reduced fat oxidation and lipid accumulation in muscle (30); moreover, carnitine supplementation (3 g/d for 10 d) has been demonstrated to increase fat oxidation by 20% in slightly overweight subjects (31). Muscle carnitine is reduced substantially in vitamin C depletion (32), and reduced muscle carnitine may hinder fat oxidation contributing to obesity in some individuals (31–35).

Adiponectin also promotes fatty acid uptake and oxidation in skeletal muscle via the activation of AMPK and PPAR{alpha}, which function as sensors in the control of energy metabolism (36). Although the collagen-like structure of adiponectin is well characterized (1,2), it is not known whether plasma vitamin C is related to adiponectin concentrations in vivo as demonstrated for collagen and for the collagen-like complement component C1q (20,21). In our study, plasma vitamin C was significantly related to plasma adiponectin in a cross-sectional sample of normal weight, overweight, and obese women controlling for age and vitamin C supplement use (r = 0.222, P = 0.049). However, controlling for body mass eliminated this association (P = 0.7) suggesting that body mass strongly influences adiponectin concentrations and/or vitamin C concentrations.

An inverse association between adiponectin and body mass has been well documented (37–40). As adipose tissue expands, macrophages infiltrate adipocytes and increase production of the inflammatory cytokines IL-6 and TNF{alpha} (41,42), and in vitro investigations indicate that these cytokines directly inhibit adipose tissue adiponectin mRNA levels (39,40). In 6 obese subjects who reduced their body weight by ~20 kg over 20 wk, investigators observed an inverse correlation between the weight loss–induced increment in adiponectin and the decrement in IL-6 (40).

These data imply that the synthesis of adiponectin does not rely on vitamin C; yet, it should be noted that only 4% of women (n = 3) had below-adequate plasma vitamin C concentrations [vitamin C <23 µmol/L; see (43)] and none of the women were vitamin C deficient (vitamin C <11.4 µmol/L). Vincent et al. (44) reported that a daily antioxidant treatment (800 IU vitamin E, 500 mg vitamin C, 10 mg ß-carotene for 8 wk) increased plasma adiponectin concentrations in overweight young adults, a response attributed to reductions in oxidative stress and the inflammatory cytokine, IL-6.

Adiponectin concentrations were lower in men (n = 35, 7.1 ± 0.5 mg/L) than in women (n = 83, 9.6 ± 0.4 mg/L, P = 0.001). Other investigators reported that plasma adiponectin concentrations are gender-specific (45,46). Nishizawa et al. (45) found that plasma adiponectin concentrations were significantly higher in castrated mice than in sham-operated control mice and that testosterone treatment reduced plasma adiponectin concentrations in both castrated and sham-operated mice. Hence, androgens decreased plasma adiponectin. In men, plasma vitamin C was not directly or indirectly related to plasma adiponectin. Plasma vitamin C concentrations were similar in the men (41.5 ± 1.7 µmol/L) and the women (44.9 ± 1.7 µmol/L), and none of the men had below-adequate vitamin C. To determine conclusively whether plasma vitamin C is associated with adiponectin concentrations, a sufficient number of vitamin C–adequate and vitamin C–below adequate men and women will need to be identified and tested.

Weight loss was associated with a 13% rise in adiponectin in well-nourished men and women consuming either low vitamin C ({approx}40 mg vitamin C/d) or vitamin C supplemented ({approx}540 mg vitamin C/d) energy-reduced diets for 8 wk. Both diet groups lost ~6.5% total body mass (Fig. 1B) and 15% fat mass during the trial (data not shown). These data indicate that when dietary energy is tightly controlled, vitamin C supplementation does not promote the loss of body fat; moreover, dietary vitamin C restriction does not hinder the weight loss–induced increase in adiponectin secretion. In a larger sample, Naylor et al. (47) was able to demonstrate that vitamin C supplementation (3 g/d, a dosage level {approx}6 times that used in the present trial) was associated with greater weight loss (2.5 kg) than placebo (1.0 kg, P < 0.05) after 6 wk. These 38 obese participants followed dietary advice for weight loss, but they were not placed on controlled, energy-reduced diets. Hence, the outcome of our trial may have been limited by a small sample and the level of supplementation. Long-term investigations relating vitamin C intake to weight maintenance and/or the propensity to gain weight in free-living populations would help elucidate the subtle effects of vitamin C on body mass.

In conclusion, plasma vitamin C was inversely related to adiposity, particularly in women, and this association was independent of body mass and age. Because one-third of Americans have marginal plasma vitamin C concentrations (48), this is an important observation worthy of further investigation. Plasma vitamin C, however, was not associated with plasma adiponectin independent of body mass. If vitamin C is required for the synthesis of collagen-like adiponectin, only severe vitamin C deficiency is likely to impact this antiatherogenic and antiinflammatory adipokine.


   ACKNOWLEDGMENTS
 
We thank Alison L. Eldridge, PhD, RD from General Mills, Bell Institute of Health and Nutrition for advice regarding study design. We also thank Justine Condron for diet preparation and diet analyses and Michael Stroup for phlebotomy and technical assistance.


   FOOTNOTES
 
1 Supported by a grant from the General Mills, Bell Institute of Health and Nutrition. Back

2 Author disclosures: C. S. Johnston, B. L. Beezhold, B. Mostow, and P. D. Swan, no conflicts of interest. Back

5 Abbreviations used: CON, control; IL-6, interleukin-6; VC, vitamin C. Back

Manuscript received 11 January 2007. Initial review completed 4 February 2007. Revision accepted 13 May 2007.


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 Discussion
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