The aim of this study was to quantitatively summarize the association of dietary magnesium (Mg) intake with serum C-reactive protein (CRP) levels in the general population.
Observational and experimental studies through February 2013 were reviewed in PubMed and EMBASE. Additional information was retrieved through Google or hand search of related reference lists. The main outcome is either adjusted geometric mean of CRP or odds ratio (OR) of having serum CRP 3 mg/l. Meta-regression was used to determine the linear association of dietary Mg intake and adjusted geometric means of CRP levels. A fixed-effects model was used to pool ORs of interest, comparing those in the lowest with those in the highest group of dietary Mg intake.
A data set derived from seven cross-sectional studies including 32 918 participants was quantitatively assessed. A weighted inverse association between Mg intake and serum CRP levels was observed (β-coefficient: −0.0028; 95% confidence interval (CI), −0.0043 to −0.0013; Ptrend=0.001) from four cross-sectional studies. The pooled OR (95% CI) of having CRP 3 mg/l was 1.49 (1.18–1.89) on comparing the lowest to the highest group of Mg intake from three studies with the data available. Qualitative assessment among five intervention studies also showed a potential beneficial effect of Mg intake on serum CRP levels.
This meta-analysis and systematic review indicates that dietary Mg intake is significantly and inversely associated with serum CRP levels. The potential beneficial effect of Mg intake on chronic diseases may be, at least in part, explained by inhibiting inflammation.
Magnesium (Mg), an essential mineral, is found abundantly in whole grains, green leafy vegetables, legumes and nuts, and it is required by hundreds of body physiologies involving over 350 enzymes.1 However, according to the National Health and Nutrition Examination Survey (NHANES), 1999–2000, about 60% of US population consumed inadequate dietary Mg. Dietary Mg intake has been related to several health outcomes including those related to metabolic and inflammatory processes such as hypertension, metabolic syndrome,2, 3, 4 type 2 diabetes,5 cardiovascular diseases,1, 6, 7, 8 osteoporosis and some cancers (for example, colon, breast).9 One suggested mechanism for the beneficial effect of Mg intake is that Mg may reduce the levels of C-reactive protein (CRP)—a well-documented indicator of a low-grade or chronic inflammation.
Laboratory studies have linked Mg deficiency to acute inflammatory response mediated by calcium, N-methyl-D-aspartic acid or N-methyl-D-aspartate, interleukin-6 (IL-6) and tumor necrosis factor-alpha.9 So far, findings from epidemiological studies on the association of Mg and CRP levels are not consistent. Several cross-sectional studies reported an inverse association between serum Mg concentrations and CRP levels in children,10 women4 and obese patients.11 Also, in a study of samples from NHANES 1999–2002, a national representative population in the United States, people with total daily Mg intake below the recommended daily allowance (RDA) were 40% more likely to have elevated CRP levels.12 In addition, a cross-sectional study also found an inverse association between dietary Mg intake and serum CRP levels.13 However, another recent cross-sectional study found no association between dietary Mg intake and CRP levels.14 Therefore, we summarized the literature quantitatively to estimate the overall association of Mg and CRP levels by conducting a meta-analysis as well as systematic review.
Materials and methods
Data source and study selection
We searched the databases including PubMed, EMBASE and Google up to February 2013, along with the references obtained from the identified studies and reviews using the terms ‘magnesium’, ‘Mg’, ‘dietary micronutrient’ combined with ‘C-reactive protein’, ‘high sensitivity C-reactive protein’, ‘CRP’, ‘hs-CRP’ and ‘biomarkers of inflammation’.
Observational studies, reporting either mean or odds ratio (OR), are included in the quantitative meta-analysis. Other studies that included one cohort study, two cross-sectional studies reporting outcomes (correlation coefficient (r) or ORs) on continuous scale and five intervention studies that reported correlation coefficients, geometric mean, or median level of CRP or changes in CRP levels between baseline and the end of study are included for a systematic review.
Data were carefully extracted from the original studies independently by two authors (DD and PX), and any disagreements were resolved by consensus. The data that we collected included the first author’s name, year of publication, study name, number of participants, age, percent of male participants, exposure assessment and category, outcome assessment, adjusted covariates and adjusted average levels of CRP or ORs of having serum CRP 3 mg/l with 95% confidence intervals (CIs) for corresponding categories and/or continuous exposure.
Data synthesis and analysis
The included studies were categorized into two groups: cross-sectional studies with adjusted geometric mean as the outcome and cross-sectional studies with OR as the outcome. We extracted adjusted geometric means of CRP levels with 95% CIs as well as median Mg levels for each quintile of Mg intake in cross-sectional studies.4, 13, 14, 15 In all the studies, data from fully adjusted models were used.
Data transformed to their natural logarithms (ln) were used to compute the corresponding standard errors and inverse variance. A random-effects meta-regression16 was used to assess the overall linear relation between Mg intake and geometric mean levels of CRP with inverse variance as weights for each study. We also extracted ORs with 95% CIs in three cross-sectional studies.10, 17, 18 Natural logarithms-transformed ORs and 95% CIs were used to compute the corresponding standard errors. Fixed-effects models were used to combine ORs of having CRP 3 mg/l by comparing participants in the lowest with those in the highest group of Mg intake because there was no significant heterogeneity found among studies. Cochran’s chi-square test was used to examine heterogeneity among included studies and I2 was computed to determine the degree of inconsistency across studies.19, 20 Publication bias was assessed by Egger’s test21 and Begg’s test.22 All analyses were conducted using STATA statistical software (version 12.1, Stata Corp., College Station, TX, USA). All statistical tests were two-sided and P-value 0.05 was considered statistically significant.
Initially, 60 articles were identified, but 53 articles were excluded because they did not meet the prespecified inclusion criteria. So, seven cross-sectional analyses were included in the meta-analysis. The study selection process for the meta-analysis is presented in Figure 1. Two more cross-sectional studies and one cohort study were also identified and included in the systematic review because the results could not be pooled with other studies (Table 1). In addition, five intervention studies were identified for systematic review (Table 2).
The final data set for this meta-analysis is comprised of 32 918 participants from seven cross-sectional studies. Six studies were conducted in the USA and one study in Italy (Table 1). Five studies included both men and women, and two studies consisted women only. Five studies used semiquantitative food frequency questionnaires to collect dietary data, whereas other two studies used secondary dietary data from NHANES (1999–2000 and 1999–2002). In the original studies, dietary Mg intake was either categorized into three/four groups on the basis of RDA levels10, 18 or divided into tertiles or quintiles.4, 13, 14, 15, 17 Serum CRP was measured by using high sensitivity assay techniques (Table 1).
In the three cross-sectional studies10, 17, 18 that calculated ORs as the effect sizes, the median Mg intake ranged from 205 to 397.9 mg/day. In the four cross-sectional studies that calculated geometric means as the outcomes, median dietary Mg intake ranged from 225 to 422 mg/day. In almost all studies, age, body mass index (BMI), smoking, physical activity, alcohol intake and dietary calorie intake were considered as potential confounders.
As shown in Figure 2, for the four cross-sectional studies that measured geometric mean CRP, the pooled estimate indicated that Mg intake (mg/day) was inversely associated with serum CRP levels (mg/l), with the meta-regression model:
Statistically significant heterogeneity was found among these studies (I2=94.5%).
The combined association between adjusted OR of having serum CRP 3 mg/l with each unit (mg/day) increment in Mg intake is presented in Figure 3. A statistically significant inverse association was observed (pooled OR: 1.49; 95% CI: 1.18–1.89). Non-significant heterogeneity among the studies was found (I2=38.8%, P=0.195). In addition, no evidence of publication bias existed (Egger’s test: P=0.308; Begg’s test: P=0.602).
Two cross-sectional studies11, 23 and one prospective cohort study24 were not included in the meta-analyses because the exposure measurement, effect size or the study design was different. The two cross-sectional studies found that serum Mg level was significantly inversely associated with serum CRP levels. In a study23 conducted in Mexican children aged 10–13 years old, a continuous decrease in mean CRP levels from the lowest tertiles (2.45 mg/l, s.d.=10.6) to the highest tertile (0.8 mg/l, s.d.=0.5) of serum Mg level was observed. In a similar study11 conducted in adults aged from 23 to 52 years old, it was found that serum Mg level was significantly inversely correlated to serum CRP levels (r=−0.39, P<0.002). In a prospective cohort study24 conducted among young adults aged 18–30 years, the researchers found a significant inverse association between dietary Mg intake and serum CRP levels, β-coefficient (95% CI) in the highest quintile of Mg intake was −0.160 (−0.262 to −0.058, Ptrend<0.01).
In addition, five Mg intervention studies (Table 2) reported significant inverse association between Mg supplementation and serum CRP levels. Two of the studies were conducted in the USA and the rest were conducted in Mexico, Israel and Iran, respectively. The age of participants ranged from 30 to 85 years. The duration of the studies ranged from 4 weeks to 4 months. One of the intervention studies25 measured CRP at baseline and at the end of study, and reported it in natural logarithmic scale. The ln(CRP) decreased significantly after the intervention. Another study26 showed that CRP levels decreased by 1.6 mg/l in the Mg intervention group, whereas it increased by 1.5 mg/l in the placebo group (P<0.002). Two other studies did not find a significant association between Mg supplementation and CRP levels. Of note, one study27 found that CRP levels increased after 500 mg elemental Mg supplementation in the form of Mg citrate for 4 weeks, whereas it decreased in the placebo group. The difference in changes between groups was not significant (P=0.50).
Evidence from this meta-analysis and systematic review indicates that dietary Mg intake is inversely associated with serum CRP levels. Our findings are robust as the meta-analysis is based on both continuous and binary outcomes, and the results are supportive of each other. In addition, the study participants are comprised of male and female, adults and children with a wide age range, which improve the generalizability of the findings. In addition, the summarization of findings both from observational and intervention studies either in the meta-analyses or in the systematic review makes the available information on the association of Mg and serum CRP levels more aggregated.
Some limitations should also be considered when interpreting the results from this meta-analysis. First, most of the studies had just a single measure of exposure/effect sizes, for example, mean level of CRP, ORs or correlation coefficients. The different measures of exposure and effect size made it difficult or even impossible to pool the results and estimate the overall association. For example, we had to exclude two studies11, 23 from the meta-analysis because they reported mean CRP (s.d.), and/or OR (95% CI) on the basis of a continuous scale of Mg intake. Second, none of the primary studies considered the health impact of high Mg intake (hypermagnesemia, for example, serum Mg >1.9 mEq/l28). Anyway, no sufficient evidence to date indicates any substantial adverse effect of dietary Mg overdose, although two case studies reported that Mg intake above 6 mEq/l, a rare phenomenon in a general population, caused parathyroid gland dysfunction, respiratory, cardiac and CNS dysfunction.29, 30 Of note, an estimated 75% of Americans have daily Mg intakes less than the RDA.31
In addition, the possibility of residual confounding from primary studies cannot be completely excluded, although various potential confounders including lifestyle and demographic variables were well adjusted in the primary studies.
Five Mg intervention studies25, 26, 27, 32, 33 used different doses of Mg supplementation (50–450 mg/day) for relatively short durations (4 weeks to 4 months). These studies cannot be pooled because of the different measures of outcome across studies. Nevertheless, all these intervention studies reported a generally inverse association between Mg supplementation and serum CRP levels, which is consistent with the pooled estimate from observational studies. In addition, a study conducted in patients with cardiac surgery found a moderate inverse correlation between serum Mg concentration and preoperative CRP levels.34
Findings from this meta-analysis are biologically plausible. Studies indicate that Mg deficiency may increase CRP production mediated through interlinked chains of the following events. Inadequate dietary Mg intake depletes extracellular Mg ion and consequently leads to the activation of macrophages and influx of calcium ions into cells (adipocytes, neuronal and peritoneal cells). The increased calcium level in the cells causes enhanced Mg need to block the influx of calcium ion, which further leads to the increased stimulation of N-methyl-D-aspartic acid or N-methyl-D-aspartate receptors. The increased stimulation of N-methyl-D-aspartic acid or N-methyl-D-aspartate results in the opening of channels nonselective to cations, thus increasing calcium ions in neuronal cells.9 This leads to the release of neurotromediators (for example, substance P) and inflammatory cytokines. Major pro-inflammatory cytokines including IL-6 and tumor necrosis factor-alpha are released into the bloodstream and act as signaling molecules to promote the release of CRP from the liver as a part of the acute phase response, which further prolongs the inflammatory response in the body.1 CRP production by liver is regulated by tumor necrosis factor -alpha and IL-6.9 A well-documented link between Mg deficiency and acute inflammatory response in animal models characterized by leukocyte and macrophage activation, release of inflammatory cytokines and acute phase proteins, and excessive production of free radicals has also been reported in experimental settings.35, 36 Studies based on infrared spectrometry techniques have also demonstrated substantial alteration in secondary structures of human CRP in the presence of Mg ions.37
In summary, findings from this meta-analysis and systematic review indicate that dietary Mg intake is inversely associated with serum CRP levels. Our results suggest that the potential beneficial effect of Mg intake on the risk of chronic diseases may be, at least in part, explained by inhibiting inflammation. Because inflammation is a risk factor of various chronic diseases, increasing Mg intake is certainly of great public health significance.
This study was partially supported by NIH grants (R01HL081572 and R01ES021735).
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European Journal of Clinical Nutrition (2015)