Magnesium intake and serum C-reactive protein levels in children

ARTICLE

Auteur(s) : Dana E King, Arch G Mainous III, Mark E Geesey, Tina Ellis

Department of Family Medicine, Medical University of South Carolina, 295 Calhoun Street, Charleston, South Carolina 29425, USA

Materials and methods

High sensitivity C-reactive protein (CRP) was measured as part of the NHANES 99-02 physical and laboratory examination. Standard phlebotomy techniques were used to obtain specimens (see website). The threshold for elevated CRP was defined by American Heart Association (AHA) guidelines that designate CRP levels ≥ 3.0 mg/L as associated with high cardiovascular risk in adults [4]; no such levels have been established for children.Dietary intake in the NHANES 99-02 is based on recollection of foods eaten the previous day by the respondent coupled with known nutritional content of each of these foods (24 hour recall). The US Office of Dietary Supplements of the National Institutes of Health (http://ods.od.nih.gov/index.aspx) and the Institute of Medicine (IOM) have established Recommended Daily Allowances (RDA) of magnesium intake based on gender and age (table 1). For each child in the study population we calculated the percentage of the RDA that they had consumed based on gender and age. Three groups were established based on magnesium consumption: less than 75% of the RDA, 75-99% of the RDA, and 100% or more of the RDA.

Demographic variables (age, race, sex) were included as control variables because of their known impacts on CRP [11]. Additional control variables were included that might be linked to eating behavior or also influence CRP level. We controlled for body weight (BMI > 85%ile) because of its link to diet and its known association with CRP [4, 6]. We also controlled for income level (above or below the poverty level), exercise, fiber intake, and calorie intake [9, 12]. Because the quantity of magnesium and fiber consumed may be linked to the total amount of food consumed, total caloric intake was incorporated as another control variable [5, 13].

Because of this complex sampling design, appropriate weighting factors (based on statistical stratification and population estimates) must be taken into account when calculating population-based frequency estimates. We used SUDAAN (Research Triangle Institute, Research Triangle, NC), a specialized statistical program that accounts for the complex weighting of the NHANES 99-02 sample [14]. Using SUDAAN allowed us to correct for unequal probabilities of selection and different response rates, ensuring that the results can be generalized to the non-institutionalized civilian population of the U.S. Thus the percentages and odds ratios in this study represent weighted values. SUDAAN also adjusts the standard errors to account for the weighting, stratification, and clustering of the complex sampling design to ensure that expressed p values are valid [15].

Descriptive statistics for the sample were performed to illustrate the demographic characteristics and dietary magnesium intake. For each of the demographic variables (age, race, gender, etc.), the association with the dietary magnesium quartile group was examined using χ² analyses.

Magnesium intake then was examined in adjusted multivariate logistic regression analyses predicting elevated serum CRP (≥ 3.0 mg/L). The covariates age, race, gender, BMI, income, exercise, fiber intake, and total caloric intake were included in the models to control for their effects. Standardized betas, p-values, odds ratios, and 95% confidence intervals were obtained from the logistic regression output. Statistical significance was defined as ≤ 0.05 without correction for multiple-comparisons, since there was only one mineral examined, and the specific analyses were planned in advance.
Table 1 US RDA for magnesium based on age and gender (US Office of Dietary Supplements of the National Institutes of Health).

Results

In logistic regression analyses, controlling for age, race, sex, income level, exercise, fiber intake, BMI, and total calories, children with a consumption of < 75% of RDA were 58% more likely to have elevated CRP (OR 1.58, 95% CI 1.07-∞, table 3). Overweight children with Mg intake below the RDA were not more likely to have an elevated CRP than children with Mg intake above the RDA (table 3).
Table 2 Demographics of children ages 6-17 in the US, according to the presence or absence of elevated serum CRP. Sample size of 5007 individuals representing a US population estimate of 41,089,564.

Discussion

The implications of these findings are that Mg intake may be an important determinant of levels of inflammation in children and adolescents as it is in adults. Mg deficiency may play a role in putting individuals at increased cardiovascular risk at a young age. The findings also have some implications as to whether magnesium plays a direct and important role in regulating inflammation. Magnesium functions in many important metabolic pathways and is a coenzyme for almost 300 reactions including many involving muscle contraction. Magnesium has also been found to play a key role in endothelial metabolism and function [16]. Because children with elevated CRP frequently also have other accompanying cardiovascular risk factors, maintaining sufficient magnesium for important physiologic processes is especially important [6]. Healthier diet habits are needed to assure adequate magnesium intake in children for these vital metabolic pathways.

Elevated CRP levels are also more common in children with symptoms of the metabolic syndrome [17]. Children with the metabolic syndrome were approximately three times more likely to have an elevated CRP level than children without the metabolic syndrome [17]. It is unknown whether children with an elevated CRP level are more likely to experience cardiovascular complications in the future. However, it will be important to assess the various cardiovascular risk factors into adulthood.

Previous studies have demonstrated an association between diet and elevation of serum CRP. Most notably, high dietary fiber intake has been associated with lower CRP [9, 18, 19]. In those studies, other nutrients were not found to be associated with CRP levels, including carbohydrates and saturated fats [9, 18]. However, in some studies dietary fiber intake was found to be highly correlated with magnesium intake [9]. The question arises as to whether the lower levels of CRP are due to the intake of fiber, magnesium, or some other nutrient in foods that are a common source of both (e.g., bran). By controlling for fiber intake in the current study, we were able to test for and confirm an independent association with magnesium.

The study has some important limitations that should be taken into account. First, dietary information was obtained from 24-hour dietary recall, which may not accurately represent an individual’s intake over the last several weeks or months. Second, participants may have exaggerated or overestimated their intake of healthy foods such as vegetables and legumes which are relatively high in magnesium. However, if true, then population estimates of magnesium intake are likely to be too low, and the real associations between Mg and CRP may be stronger than we have illustrated. Third, the strength of the association between magnesium supplement intake and CRP is modest, and may be due to other unmeasured factors. Further, we can make no definitive statement regarding cause and effect in a cross-sectional analysis.

Conclusion

References

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