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Journal of the American College of Nutrition, Vol. 25, No. 6, 509-513 (2006)
Published by the American College of Nutrition

Serum Magnesium and Type-2 Diabetes in African Americans and Hispanics: A New York Cohort

Earle C. Chambers, PhD, Stanley Heshka, PhD, Dympna Gallagher, EdD, Jack Wang, MS, F. Xavier Pi-Sunyer, MD and Richard N. Pierson, Jr, MD

New York Obesity Research Center, St. Luke’s-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York, NY

Address reprint requests to: Dr. Earle C. Chambers, Obesity Research Center, 1090 Amsterdam Ave., Ste. 14A, New York, NY 10025. E-mail: ecc2106{at}columbia.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 CONCLUSIONS
 ACKNOWLEDGMENTS
 REFERENCES
 
Objectives: The aim of this study was to examine the relationship of serum Mg with stage of diabetes measured by fasting serum glucose in a cohort of 485 African American and Hispanic adults.

Methods: The cross sectional Rosetta study was designed to assess body composition in a multi-racial cohort of healthy adults living in New York City. The data utilized for the current analyses were collected during the years 1990 to 2000. Serum Mg and glucose were measured after a 10–12 hour fast. Dual-energy x-ray absorptiometry was used to measure fat mass (FM) and fat free mass (FFM).

Results: The mean age of the cohort was 53 ± 16 years. Hispanics had significantly lower (p < 0.05) mean serum Mg levels (0.82 ± 0.07 mmol/L vs. 0.85 ± 0.07 mmol/L) and FFM (48.8 ± 10.9 kg vs. 50.9 ± 10.3 kg) compared to African Americans. In both race/ethnic groups, individuals classified as having diabetes had significantly (p < 0.001) lower serum concentrations of Mg (0.80 ± 0.07 mmol/L) compared to the normal group (0.84 ± 0.07 mmol/L).

Conclusions: These results show that in African American and Hispanic adults, those with diabetes have lower serum Mg levels compared to those classified as pre-diabetic or those with normal fasting glucose levels.

Key words: serum magnesium, body composition, adults, type-2 diabetes, minority health


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 CONCLUSIONS
 ACKNOWLEDGMENTS
 REFERENCES
 
The disproportionate risk of diabetes among racial/ethnic minorities in the U.S. is well documented [1,2]. Epidemiological studies have shown that deficiency of certain major minerals may increase the risk for type-2 diabetes [3,4] and cardiovascular disease (CVD) [5–7] particularly among populations of African descent [8,9]. Research has shown that low serum magnesium (Mg) levels cluster with the components of the metabolic syndrome [10–12] increasing the risk for type-2 diabetes. It is unclear why those with diabetes have reduced serum Mg. Clinical research studies have shown that Mg has favorable effects on glucose homeostasis and may play a role in reducing the risk of type-2 diabetes in adults [11,13].

It has been estimated that 11.1% of overweight U.S. adults have pre-diabetes [14] using a fasting glucose sample between 6.10 and 7.00 mmol/L (110 and 125 mg/dL). According to a report using data from the National Health and Nutrition Examination Survey (NHANES), the prevalence of pre-diabetes is significantly higher in Hispanics than African Americans and clusters with CVD risk factors consistent with the metabolic syndrome [14]. An inverse relationship between serum or dietary Mg and diabetes risk has not been shown consistently across racial/ethnic groups [11]. Whether low Mg levels are significantly associated with diabetes risk in minority groups that consume less dietary Mg and have lower serum Mg [15,16] would have significant public health implications. Therefore, it would be of clinical importance to determine whether lower levels of Mg can be detected among those that fall into the pre-diabetes categorization and at high risk for diabetes.

The specific aim of the following study was to examine the relationship of serum Mg levels with normal, pre-diabetes and diabetes categorizations of fasting glucose adjusting for body size in African American and Hispanic adults in New York City.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 CONCLUSIONS
 ACKNOWLEDGMENTS
 REFERENCES
 
The Rosetta study was a cross sectional study designed to assess body composition in a multi-racial cohort of healthy adults living in New York City [17]. The data utilized for the current analyses were collected during the years 1990 to 2000. Since the specific aims of the current analysis centered on African Americans and Hispanics, these race/ethnic groups were extracted from the larger Rosetta database. The number of individuals with diabetes in Whites (n = 8) and Asians (n = 2) were quite small in this cohort. The current analysis consisted of 485 participants for whom blood and body composition data were available. Demographic information was collected by questionnaire during a clinic visit. The inclusion criterion for the study has been described elsewhere [17]. Briefly, each participant underwent a physical exam, including a brief medical history. Those with a history of cancer, renal failure, chronic use of steroid medication, liver disease, drug abuse, or alcoholism were excluded from the study. A fasting serum sample was collected on a portion of the entire Rosetta sample. Those with incomplete data on serum Mg or glucose were excluded from the analysis. Informed consent was given in writing by all study participants prior to enrollment in the study. All procedures and consent forms were approved by the Institutional Review Board of the St. Luke’s-Roosevelt Hospital Center, New York, NY.

Body Composition
Dual-energy x-ray absorptiometry was used to measure fat mass (FM) and fat free mass (FFM). The scan was completed with a whole-body pencil beam DPX system (GE Lunar Radiation Corp., Madison, WI; version 3.1 software). Detailed methods for image acquisition have been described elsewhere [18]. All scans were read at the Body Composition Unit of the New York Obesity Research Center and the intraclass correlation coefficient for %fat estimation by DXA is 0.99 [18]. Body mass index (BMI, kg/m2) and waist circumference (WC) were measured using standard procedures [17,19].

Blood Analysis
Serum Mg and glucose were collected after a 10–12 hour overnight fast. Blood analysis was performed by a commercial laboratory (Quest Diagnostics Incorporated, Teterboro, NJ). Diabetes status was determined according to the American Diabetes Association (ADA) criteria (Normal (<6.10 mmol/L); Pre-Diabetes (≥6.10 mmol/L – 6.99 mmol/L); Diabetes (≥7.00 mmol/L)) [20]. All individuals categorized as having provisional type-2 diabetes according to ADA criteria were newly diagnosed as such.

Statistical Analysis
Spearman correlation analysis was performed to determine the linear relationship between serum Mg and glucose and the body composition parameters. Stratified analysis by race was done based on the specific aim of the study and a significant one-way interaction term of race and Mg in multivariate regression analysis. Chi-square and one-way analysis of variance with F-tests were used to compare the demographic, biological and body composition parameters in African Americans and Hispanics. General linear modeling was used and the mean values are displayed for Mg by diabetes status adjusting for age, gender and BMI. Bonferroni corrections were used to adjust for multiple comparisons. A p-value less than 0.05 and a p-value greater than 0.05 but less than 0.10 was considered statistically significant and marginally significant, respectively. The interaction term of race and diabetes status was marginally significant at the p < 0.10 level (p = 0.07) in the model utilizing the entire sample.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
CONCLUSIONS
 ACKNOWLEDGMENTS
 REFERENCES
 
Hispanics were significantly younger (49 ± 15 years; range 20–110 years) than African Americans (58 ± 16 years; range 20–107 years) in the study cohort (p < 0.001). There was a prevalence of diabetes of approximately 12% in both African Americans and Hispanics. Approximately 30% of the cohort was classified as having pre-diabetes according to ADA criteria (Table 1). Women (n = 271) had marginally higher levels of serum Mg than men (n = 214) (0.84 ± 0.07 mmol/L vs. 0.82 ± 0.07 mmol/L; p = 0.07). Table 1 shows that Hispanics had significantly lower mean levels of Mg and FFM and marginally higher WC compared to African Americans. Spearman correlation analyses showed that Mg was significantly correlated with age and marginally correlated with glucose and % body fat (Table 2). None of the other body composition parameters were associated with Mg.


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  		Table 1. Prevalence of Diabetes and Mean Levels (Standard Deviation) of Serum Magnesium and Glucose and Body Composition Parameters by Race

 

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  		Table 2. Spearman Correlations between Serum Magnesium, Glucose and Body Composition Variables

 
Mean levels of Mg were compared by diabetes status to determine if significantly lower Mg levels could be observed among those classified as having pre-diabetes adjusting for age, gender and BMI. Fig. 1 shows no significant difference in mean levels of Mg in the pre-diabetes group compared to the normal group in African Americans or Hispanics. Only individuals in the diabetes group had significantly lower levels of Mg compared to the normal group. The interaction term of gender and diabetes status was included in the race/ethnicity stratified model, however, the term was not significant in either race/ethnic group.


Figure 1
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  		Fig. 1. Mean levels of serum magnesium with standard error bars stratified by race/ethnicity and diabetes status adjusting for age, gender and BMI. Fasting glucose: Normal (<6.10 mmol/L); Pre-Diabetes (≥6.10 mmol/L – 6.99 mmol/L); Diabetes (≥7.00 mmol/L). * p < 0.05 (Normal as reference).

 

   DISCUSSION
 
Dietary sources of Mg include foods such as whole grains, legumes, nuts and dairy products. In a dietary recall assessment of the NHANES 1999–2000 data, dietary intake of magnesium was significantly lower in African Americans compared to Caucasians and Hispanics in the U.S. [15]. In the current study, the serum levels of Mg were lower in the Hispanic group compared to the African American group. It should be noted that the NHANES samples largely Mexican Americans where the majority of the Hispanic population in the Rosetta study are Puerto Rican. Studies have shown differences in dietary intake within the Hispanic population [21]. It is not clear, from the data available, whether the differences observed here are due to dietary intake or differences in the metabolism of Mg by race/ethnic group.

As a metabolic cofactor, Mg is important in energy metabolism and glucose homeostasis [22]. Low levels of Mg can impede phosphorus bond dependent reactions of the many enzymes related to glucose metabolism. Moreover, low levels of Mg may impair insulin receptor function through increased microviscosity of the plasma membrane and thus decrease insulin sensitivity [23]. As such, the lower Mg levels among those classified as diabetic in the current study may result from increased insulin resistance in this group. This is consistent with many other reports that have shown Mg to be associated with insulin resistance and diabetes risk [4,8,11,13,24,25].

In the current study, the participants were healthy (defined by self-report) at the time of their clinical visit. Therefore, those that were classified as diabetic by fasting serum sample would have been undiagnosed. Nevertheless, it is possible that the lower levels of Mg among those with diabetes may result from increased urinary excretion [22,26,27] of Mg and thus show reduced levels of Mg in the serum [28]. Increased urinary excretion of Mg signifies a decline in renal function among diabetics [29,30] and is often a sign of uncontrolled diabetes. The limited ability of the kidneys to retain Mg may be due, in part, to states of hyperglycemia among those with diabetes [28].

A few limitations of the current study should be noted. First, no information was collected on dietary intake including dietary supplements. Dietary supplementation of Mg is known to vary by ethnic group [15]. Since, there are no dietary data available the degree to which supplemental Mg affects the differences observed in the current study cannot be determined from the data available. Secondly, there was only one serum measurement of Mg taken per participant so issues of the reliability of Mg measurements should be considered. It is unlikely that this will change the interpretation of the results observed in this study since there is no reason to suspect a difference in the blood collection procedures by race/ethnic group which would bias the interpretation of the mean levels reported here. Furthermore, the results from the current study are consistent with other cross-sectional [6] and large prospective studies [11] using a single measurement of serum Mg. Lastly, Mg only exists in very small amounts in the blood [31] and is not always a precise indication of intracellular Mg content [32]. As such, extrapolations to Mg deficiency should be viewed with caution.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSIONS
ACKNOWLEDGMENTS
 REFERENCES
 
These data confirm the results shown in other studies of significantly lower levels of serum Mg among those with fasting glucose levels equivalent to the ADA criteria for diabetes among racial/ethnic minorities. However, low levels of Mg are not clearly observed among those with pre-diabetes in this cohort.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSIONS
 ACKNOWLEDGMENTS
REFERENCES
 
The Rosetta Study was supported in part by a grant (DK37352) from the National Institute Diabetes and Digestive and Kidney Disorders (NIDDK). This analysis of the Rosetta study data was supported in part by the New York Obesity Research Center grant also from NIDDK (DK26687-25 and DK07759).

Received July 12, 2005. Accepted June 1, 2006.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSIONS
 ACKNOWLEDGMENTS
 REFERENCES
 

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