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Journal of the American College of Nutrition, Vol. 23, No. 5, 510S-513S (2004)
Published by the American College of Nutrition

Magnesium, Insulin Resistance and Body Composition in Healthy Postmenopausal Women

Maria J. Laires, PhD, Helena Moreira, PhD, Cristina P. Monteiro, Luis Sardinha, PhD, F. Limão, Luisa Veiga, Alice Gonçalves, A. Ferreira and Manuel Bicho, MD

Biochemistry Laboratory (M.J.L., C.P.M., F.L., A.F.), PORTUGAL
Exercise and Health Laboratory (L.S., A.G.), PORTUGAL
Faculty of Human Movement, U.T.L., Cruz Quebrada, Sports Department, University of Trás-os-Montes e Alto Douro, Vila Real (H.M.), PORTUGAL
U.T.L., Lisbon’s School of Health Technologies (L.V.), PORTUGAL
Genetics Laboratory, Faculty of Medicine, U.L., Lisboa (M.B.), PORTUGAL

Address reprint requests to: Prof. Dra. Maria José Laires, Laboratório de Bioquímica, Faculdade de Motricidade Humana, Estrada de Costa 1495-688 Cruz Quebrada, PORTUGAL. E-mail: mjlaires{at}fmh.utl.pt


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Objective: This study was conducted to determine the association between magnesium (Mg), body composition and insulin resistance in 136 sedentary postmenopausal women, 50 to 77 years of age.

Methods: Diabetics, hypertensives and women on hormonal replacement therapy were excluded and the remaining 74 were divided according to BMI≥25 (obese: OG) and BMI<25 kg/m2 (non-obese: NOG). Nutritional data disclosed that intakes were high for protein and saturated fat, low for carbohydrates, polyunsaturated fat and Mg and normal for the other nutrients, according to recommended dietary allowances (RDA). Mg values in red blood cells (RBC-Mg) and plasma (P-Mg), were determined, as were fasting glucose, and insulin levels, Homeostasis Model Assessment (HOMA), body mass index (BMI), body fat percent (BF %), abdominal fat (AF) and free fat mass (FFM).

Results: RBC-Mg values were low in both groups when compared with normal values. There were significant differences in body composition parameters, HOMA and insulin levels, with higher basal insulin levels in OG. RBC-Mg was directly correlated with insulin, HOMA and FFM in both groups, according to Pearson correlations. HOMA in OG was also directly correlated with BMI, FFM and AF. In NOG, HOMA was only correlated with FFM. The low RBC-Mg levels observed were probably due to low Mg intake and to deregulation of factors that control Mg homeostasis during menopause.

Conclusions: Both Mg deficit and obesity may independently lead to a higher risk for insulin resistance and cardiovascular disease.

Key words: rbc Mg, serum Mg, Mg deficiency, insulin resistance, body composition, postmenopause, fat-distribution, obesity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Menopause transition is associated with a pattern of obesity in which a high proportion of body fat is deposited in trunk and abdomen. Obesity, in particular visceral fat, is associated with high risk for insulin resistance and cardiovascular disease [1,2]. Magnesium (Mg) deficiency or depletion has often been associated with both pathologies, and changes in Mg concentration occur in menopause [3–5]. This study was conducted to determine the association of body composition parameters with Mg levels and insulin resistance.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects
The study sample included 136 Caucasian women aged between 50 and 77 years old recruited from an exercise intervention program. Inclusion factors were determined from a detailed history and physical examination. All women were sedentary (they performed less then 30 min of exercise/week), non-smokers, and postmenopausal (amenorrhoea of ≥12 months). When diabetics (5.9%), hypertensives (16.2%) and women on hormonal replacement therapy (28.72%) were excluded, 74 subjects remained.

Using as criteria for obesity BMI≥25 Kg/m2 [6] this group was divided into obese (OG; n = 55) and non obese (NOG; n = 19). The subjects were informed about the protocol design and signed a consent form. The study was approved by the Faculty of Human Movement.

Anthropometric Measurements and Body Composition
All measurements were made after a 12 hours overnight fast. Body weight was measured with an electronic scale to the nearest 0.1 Kg (Seca, Hamburg, Germany). Height was measured with a wall-mounted stadiometer to the nearest 0.5 cm. Body mass index (BMI) (Kg/m2) was derived from weight (Kg) and height (m). Dual-energy X-ray absorptiometry was used to assess body fat percentage (BF %), abdominal fat (AF) and Free Fatty Mass (FFM) (QDR-1500, Hologic, Waltman, USA, pencil beam mode, software version 5.67 enhanced whole body analysis).

Biochemical Analysis
Blood was collected by venipuncture after a 12 hours overnight fast. Red blood cell Mg (RBC-Mg) and plasma Mg (P-Mg) were determined by atomic absorption spectrophotometry. Fasting glucose (FG) was assessed by spectrophotometry, insulin (FI) by RIA, which values were used to determine glucose intolerance by the formula for Homeostasis Model Assessment (HOMA) [7]: HOMA = FI (µUI/mL) x FG (mmol/L)/22.5 that was used as an indirect measure of insulin resistance.

Nutritional Analysis
Nutritional information was collected using a three-day food record and the analysis was made with Food Processor (Nutrition analysis software, version 7.4, by ESHA Research, Salem, Oregon, 1999).

Statistical Analysis
One sample t test was used to compare nutritional intake with RDA values. Student’s t test for unpaired data was used to determine significant differences in body composition variables, biochemical variables and age, between OG and NOG. Associations between continuous variables were quantified by using Pearson’s product-moment correlation coefficients. The level of significance was set at p < 0.05. The statistical analysis was performed using SPSS (version 10.0).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Nutritional analysis is given in Table 1. According to the RDA, these values were significantly high for protein and saturated fat and significantly low for carbohydrates, polyunsaturated fat and Mg. The other nutrients intake was normal [8].


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  		Table 1. Nutritional Evaluation

 
The body composition and biochemical characteristics of the two groups of women are given in Table 2. P-Mg values were normal and RBC-Mg values were low in the two groups when compared with the normal values [9]. Concerning insulin and glucose, the values were consistent with the literature [10] in both groups but, although all the diabetics were excluded, the HOMA levels observed in OG were considerably high [7].


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  		Table 2. Body Composition and Biochemical Characteristics of the Subjects

 
When comparing OG with NOG, the mean values of BMI, FFM, BF %, AF, insulin and HOMA were significantly higher in OG. In OG glucose levels present a greater dispersion when compared with NOG. There were significant direct correlations between RBC-Mg and insulin (NOG: r = 0.812, p = 0.000; OG: r = 0.346, p = 0.012); RBC-Mg and HOMA (NOG: r = 0.792, p = 0.000; OG: r = 0.273, p = 0.05) and RBC-Mg and FFM (NOG: r = 0.293, p = 0.035; OG: r = 0.587, p = 0.010) in both groups, according to Pearson’s correlations. In OG, HOMA was also directly correlated with BMI (r = 0.361, p = 0.007), FFM (r = 0.346, p = 0.023) and AF (r = 0.372, p = 0.006). In NOG, HOMA was correlated only with FFM (r = 0.527, p = 0.025).


   DISCUSSION
 
In this study we excluded diabetic, hypertensive and women on hormonal replacement therapy because these factors can influence the studied parameters [3,11]. Nutritional analysis revealed poor food habits that can be important risk factors for obesity and other pathologies. RBC-Mg values were low in the two groups, probably as a result of menopause-induced alterations in Mg metabolism [3]. Accumulation of visceral adipose tissue with age is accelerated after menopause. Central distribution and accumulation of adipose tissue is often associated with insulin resistance [1]. HOMA and insulin mean values were significantly higher in OG. This is in accordance with the work of Amatruda & Welle [12]. Although no significant differences were found for RBC-Mg between groups, a higher mean value was observed in OG. These results are in accordance with those of De Leeuw and co-workers [13] that showed higher basal insulin levels and increased RBC-Mg concentration in obese women when compared with classical gynecoid fat distribution. The mean value of FFM was higher in OG, probably due to increased estrogen synthesis by adipose tissue [14]. The direct correlation between AF and HOMA may be explained by the highly lipolytic activity of the adipose cells that are poorly inhibited by insulin, as compared with adipocytes from other fat depots [15,16].

It has been proposed by Bjorntrop [17] that adipose tissue located within the abdominal cavity releases high levels of fatty acids, which are drained directly from the portal circulation to the liver, with resultant impairment of hepatic intermediary metabolism, leading to metabolic complications of obesity. High free fatty acid levels have also been associated with reduced hepatic clearance of insulin [18,19]. The correlation observed in OG between HOMA and AF suggests greater cardiovascular risk in this group. In recent years, central distribution and accumulation of adipose tissue and the concomitant insulin resistant dyslipidemic state have emerged as important components of a cluster of metabolic abnormalities that are strongly related to coronary heart disease [20,21].

The direct correlations observed in both groups between RBC-Mg and insulin is consistent with the work of Paolisso & Barbagallo [22], who found an increase of intracellular Mg upon insulin modulation. Insulin may be responsible for shift of Mg from the extracellular to intracellular space [23,24].

We also found direct correlations between RBC-Mg and HOMA, but these correlations are lower in OG, which can be explained by the significantly higher number of such women with levels of glucose above 100 mg/dl. Insulin is involved in erythrocyte Mg transport, especially in the presence of normal glucose levels [25]. The higher value of RBC-Mg in the obese can be related to higher insulin levels in normoglycemic subjects [26–28].


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSION
ACKNOWLEDGMENTS
 REFERENCES
 
Both Mg deficit and obesity may independently lead to a higher risk for insulin resistance and cardiovascular disease.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSION
 ACKNOWLEDGMENTS
REFERENCES
 
The authors would like to thank Mrs. Fátima Raposo for the proficient technical assistance.

Received August 5, 2004.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES
 

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