American Journal of Clinical Nutrition, Vol. 88, No. 3, 797-800,
September 2008
© 2008 American Society for Nutrition
ORIGINAL RESEARCH COMMUNICATION |
Association of the melanocortin-4 receptor V103I polymorphism with dietary intake in severely obese persons1,2,3
Michaela Pichler,
Barbara Kollerits,
Iris M Heid,
Steven C Hunt,
Ted D Adams,
Paul N Hopkins and
Florian Kronenberg
1 From the Division of Genetic Epidemiology; Department of Medical Genetics, Molecular and Clinical Pharmacology; Innsbruck Medical University, Innsbruck, Austria (MP, BK, and FK); the Institute of Epidemiology, Helmholtz Center Munich, German Research Center for Environmental Health, Neuherberg, Germany (IMH); the Institute of Information Management, Biometry and Epidemiology; Ludwig-Maximilians University of Munich, Munich, Germany (IMH); and the Cardiovascular Genetics Division, University of Utah School of Medicine, Salt Lake City, UT (SCH, TDA, and PNH)
2 Supported by the Genomics of Lipid-associated Disorders—GOLD of the Austrian Genome Research Programme GEN-AU (to FK), the German National Genome Research Net and a subcontract of the 1 R01 DK 075787-01A1 from the NIH/NIDDK (to the Helmholtz Center Munich–Institute of Epidemiology), the Munich Center of Health Sciences (MC Health) as part of LMUinnovativ, and grant no. DK-55006 from the National Institute of Diabetes and Digestive and Kidney Diseases (to SCH).
3 Reprints not available. Address correspondence to F Kronenberg, Division of Genetic Epidemiology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, Schöpfstrasse 41, A-6020 Innsbruck, Austria. E-mail: florian.kronenberg{at}i-med.ac.at.
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ABSTRACT
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Background: Several studies have reported that carriers of the 103I allele of the melanocortin-4 receptor (MC4R) gene had lower body weight than did persons with the wild-type genotype. A recent study found an association of the MC4R 103I variant with carbohydrate intake, which may mediate some of the association of this variant with leanness.
Objective: The purpose of the study was to investigate the association between the MC4R V103I polymorphism and the dietary intake of persons with severe obesity, which was derived by using the Willett food-frequency questionnaire.
Design: The MC4R V103I polymorphism was genotyped in a group of 1029 severely obese white subjects with an average body mass index (BMI; in kg/m2) of 46.0 (range: 33–92).
Results: Carriers of the 103I allele had significantly higher daily energy (364 kcal/d or 19%; P = 0.03) and carbohydrate (57 g/d or 27%; P = 0.01) intakes than did noncarriers, but there was no relation with BMI. No notable association of this polymorphism with lipid and glucose variables of the metabolic syndrome was observed.
Conclusions: The higher dietary intake of carbohydrates in severely obese persons with the MC4R 103I variant is in line with previous findings. It may indicate a differential effect on body size measures in extremely obese subjects as compared with the general population.
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INTRODUCTION
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The melanocortin-4 receptor gene (MC4R) is highly expressed in the hypothalamus, where the central control of feeding and energy balance is located (1). Whereas loss-of-function mutations of this gene cause monogenic forms of obesity, several mutations that do not disrupt the gene have an uncertain influence on weight regulation and obesity. One of the most common and widely investigated MC4R polymorphisms is the V103I missense variant, which has a relatively low population frequency of 2–4%. Several studies, including meta-analyses, have investigated the influence of this variant on body mass index (BMI) and showed that heterozygote carriers of V103I may have an 18–30% lower risk for obesity (2-6). It is interesting that a recent study found a borderline significant association of the MC4R 103I variant with carbohydrate intake, which may mediate some of the association of this variant with leanness (7).
In the present study, we investigated, with the use of high-quality information on energy and carbohydrate intakes, whether the MC4R 103I variant shows an association with weight by influencing the amount and quality of food intake in severely obese subjects.
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SUBJECTS AND METHODS
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Subjects
The study included 1029 white persons from the same geographical region of Utah who were recruited for severe obesity [BMI (in kg/m2): 33–92] and who had been described in detail elsewhere as part of studies in the past few years (8, 9). Briefly, the investigated persons either were seeking gastric bypass surgery or were randomly chosen from a population-based sample of severely obese persons not seeking gastric bypass surgery. The examination of patients undergoing gastric bypass surgery was done before the intervention.
Twenty-one percent of the patients belonged to obesity class II (BMI: 35 to <40) and 78% belonged to obesity class III (BMI: 
40). The daily amount of energy intake (kcal/d), including the total amount of fat, carbohydrates, and proteins, was derived from the Willett food-frequency questionnaire (10), which was administered to 926 severely obese subjects. Bioelectrical impedance equipment (RJL Systems Analyzer; Quantum II, Clinton, MI) was used to determine the percentage body fat.
Written informed consent was obtained from all subjects. The study was approved by the Institutional Review Board of the University of Utah.
Laboratory methods
Blood samples were collected after an overnight fasting period. Lipid and lipoprotein concentrations were measured as previously described in detail (11). Genotyping of the MC4R V103I polymorphism (rs2229616) was done in the Genotyping Unit of the Gene Discovery Core Facility (Innsbruck Medical University, Innsbruck, Austria) by using a 5' nuclease allelic discrimination Taqman assay (Applied Biosystems, Foster City, CA) in all subjects with sufficient amount and quality of DNA. All carriers of the 103I allele and a similar number of carriers of the wild-type genotype were verified by sequencing. Genotypes were available in 1029 persons from the Utah study group, and the genotyping success rate was 99.2%.
Statistical analysis
An age- and sex-adjusted general linear regression model was used to estimate the association of the genotypes of the MC4R V103I polymorphism with quantitative phenotypes as dependent variables. To control for outliers, we excluded those subjects with values above or below 4 SDs of the respective continuous variables. We had 80% power at a 5% type 1 error rate to detect a difference of 
500 kcal in the total daily amount of energy consumed and a difference of 69 g/d in daily carbohydrate intake between carriers and noncarriers of the variant (corresponding to 25% and 32%, respectively, of the daily intake). Statistical analyses were performed with SPSS software (version 15.0; SPSS Inc, Chicago, IL) and SAS software (version 9.1; SAS Inst, Cary, NC).
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RESULTS
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Clinical characteristics and laboratory data of the study participants are reported in Table 1
. The primary analysis investigated the daily age- and sex-adjusted dietary intake and observed that carriers of the 103I variant had a mean 19% higher total energy intake (364 kcal/d; P = 0.03) than did homozygote wild-types (Table 2). A major part of this higher energy intake was due to a 27% higher carbohydrate intake (57 g/d; P = 0.01) and a slightly (15%) higher fat intake (10.7 g/d; P = 0.13) (Table 2). The fact that the association between the MC4R V103I polymorphism and total energy intake was no longer significant (P = 0.83) after adjustment for carbohydrate intake indicates that the entire association of total energy intake is explained by the carbohydrate intake. The association between the 103I variant and carbohydrate intake remained significant (P = 0.01), and estimates persisted when we additionally adjusted for BMI.
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TABLE 2 The association of the melanocortin-4 receptor (MC4R) V103I polymorphism with age- and sex-adjusted measures of dietary intake (primary analysis) and with measures of body composition and lipoprotein and glucose metabolism (secondary analysis) in a group of 1029 severely obese persons1
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Because 81.4% of the subjects were women, we performed a sensitivity analysis by excluding all men from the analysis. We observed even stronger associations for total calorie intake (409 kcal/d; P = 0.01) and especially for carbohydrate intake (64 g/d; P = 0.005) in the 103I variant carriers.
In the secondary analysis, we did not detect a significant association (P = 0.17) between the MC4R V103I polymorphism and BMI; estimates were 1.89 higher for the heterozygote genotype carriers than for the wild-type carriers (Table 2
). When we adjusted this analysis for carbohydrate intake or daily energy intake, the BMI estimates became even weaker (increases of 1.10; P = 0.44). When we extended the analysis to other measures of body size and lipid and glucose metabolism, we observed that waist and percentage body fat in carriers of the 103I variant trended in the same direction as BMI but were not significant. We did not observe any significant association of this polymorphism with lipid or glucose metabolism.
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DISCUSSION
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A recent population-based study described a borderline significantly higher carbohydrate intake for heterozygote MC4R V103I genotype carriers (7). Because a carbohydrate-rich diet is associated with leaner body composition (12), it was speculated that this polymorphism plays a role in appetite regulation in humans by reducing the quantity or modifying the quality of food intake. These effects may modulate central obesity, as well as lipid and glucose metabolism. In line with those findings (7), we observed a significantly higher carbohydrate intake in carriers of rare alleles in the present study of severely obese subjects. However, these genotype carriers had also a general increase in energy intake, which may explain why we did not find a lower BMI, such as has been reported in the literature. Previous studies in population-based subjects or spanning the entire range of BMI showed a lower BMI in carriers of rare alleles (2-6). This association may be disturbed in severely obese subjects, as seen in the present study and in the only other large studies investigating persons of obesity class II and III (5, 13).
It can be speculated that the rare allele of the MC4R V103I may modulate appetite toward higher carbohydrate intake, which may have strongly differential effects on different population groups. For the general population, there may be a tendency toward an association of a leaner phenotype with greater carbohydrate intake, whereas, for a subgroup already in severe obesity, this appetite modulation is connected with higher total energy intake, which counteracts a leaner phenotype. According to mediator analysis proposed by Prentice (14), the partly mediating role of carbohydrate intake is supported by the analysis of BMI after adjustment for carbohydrate intake. This analysis yielded lower association estimates for BMI, which supports the idea that the influence of this polymorphism on body size measures is mediated by nutritional behavior.
Even though the present study included 1029 patients, most of whom were severely obese, one may assume that our findings could be limited by the small number of carriers of the rather rare 103I genotype. Whereas we had 70% power for the observed higher intake of carbohydrates, our power to detect a BMI difference of 1.5 was only 20%. Furthermore, the nutritional variables are very difficult to assess, and they usually involve substantial uncertainty (15), which aggravates the power issue. However, statistical theory teaches that such random uncertainty in the regression outcome variable would not impose a bias on the estimate, because the uncertainty can be considered to be nondifferential between variant carriers and noncarriers. Therefore, the association estimate with the true underlying carbohydrate intake could be expected to be the same as that observed, but with smaller CIs and therefore better P values.
Underreporting of dietary intake in obese persons is also a well-known problem. However, because we analyzed a "case-only" group, systematic underreporting could be accounted for by subtracting a constant from the outcome variable values. This step would result in neither biased estimates nor a loss of precision, because the underreporting can be considered to be independent of the genotype status (15).
Finally, it is highly unlikely that our finding is a technical artifact of genotyping, because we confirmed each 103I genotype carrier and a similar number of wild-type genotype carriers by sequencing. We propose that future large-scale studies may particularly analyze the association of MC4R polymorphisms stratified by severity of obesity. If our preliminary finding can be confirmed, we propose a differential influence of this polymorphism in severely obese persons compared with the general population. It is conceivable that several interconnected mediators in appetite regulation are heavily disturbed in severely obese persons, which could change the influence of this polymorphism on the spectrum of nutritional items ingested.
In summary, the higher dietary intake of carbohydrates in severely obese persons with the MC4R 103I variant is in line with previous findings and may indicate a differential consequence for body size measures in extremely obese subjects as compared with the general population. This possibility, however, requires confirmation in independent studies or meta-analyses.
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ACKNOWLEDGMENTS
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We appreciate the technical assistance of Anke Gehringer and Markus Haak from the Division of Genetic Epidemiology, Innsbruck Medical University.
The authors' responsibilities were as follows—IMH and FK: developed the hypothesis for this study; MP, IMH and FK: reviewed the relevant literature; SCH, TDA and PNH: performed the data collection; MP, BK, and FK: performed the data analysis; MP and FK: wrote the draft of the manuscript; and all authors: contributed to the interpretation of data and the final version of the manuscript. None of the authors had a personal or financial conflict of interest.
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Received for publication February 21, 2008.
Accepted for publication June 6, 2008.
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ABSTRACT
INTRODUCTION
