Effect of 1,25-(OH)2D3 on jejunal absorption of magnesium in patients with chronic renal disease

Hypomagnesemia may be associated with symptomatic disease in patients with primary hyperparathyroidism

AIM

Magnesium (Mg) homeostasis is closely related to calcium (Ca) metabolism. Hypercalcemia inhibits the reabsorption of Mg from the kidneys, leading to hypomagnesemia. Therefore, patients with primary hyperparathyroidism (PHPT) are predisposed to hypomagnesemia. However, there are few studies on the clinical significance of hypomagnesemia in PHPT. The aim of this study was to retrospectively evaluate the association of hypomagnesemia with the clinical outcomes of PHPT.

MATERIALS AND METHODS

A retrospective evaluation was made of the data of 538 consecutive patients (478 females, 60 males) diagnosed with PHPT in our center.

RESULTS

The mean age of the study population was 56.5 ± 11.66 years. The mean serum Mg level was 2 ± 0.26 mg/dl. Asymptomatic disease was present in 241 (44%) patients. Symptomatic patients with osteoporosis, Ca level ≥11.2 mg/dl, and estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m2 had lower levels of Mg (p < 0.05). Hypomagnesemia was detected in 129 of 538 patients (23.9%). The patients with hypomagnesemia had a higher rate of symptomatic disease (80% vs. 48%, p < 0.0001). The serum parathormone (PTH) level was found to be higher in patients with hypomagnesemia and the lumbar and femur T-scores and serum vitamin D levels were lower (p < 0.05). Patients with hypomagnesemia had higher rates of kidney stones (34% vs. 21%, p = 0.003) and osteoporosis (74% vs. 32%, p < 0.001). Multivariate logistic regression analysis revealed that hypomagnesemia had a significant effect on the development of symptomatic disease (OR:6.88, CI 95%: 5.20-11.27, p < 0.001).

CONCLUSIONS

The current study results demonstrate that hypomagnesemia may be associated with a higher risk of osteoporosis and kidney stones in PHPT patients. Routine evaluation of serum Mg may predict the clinical outcomes of PHPT.

Regulation of magnesium balance: lessons learned from human genetic disease

Magnesium (Mg(2+)) is the fourth most abundant cation in the body. Thus, magnesium homeostasis needs to be tightly regulated, and this is facilitated by intestinal absorption and renal excretion. Magnesium absorption is dependent on two concomitant pathways found in both in the intestine and the kidneys: passive paracellular transport via claudins facilitates bulk magnesium absorption, whereas active transcellular pathways mediate the fine-tuning of magnesium absorption. The identification of genes responsible for diseases associated with hypomagnesaemia resulted in the discovery of several magnesiotropic proteins. Claudins 16 and 19 form the tight junction pore necessary for mass magnesium transport. However, most of the causes of genetic hypomagnesaemia can be tracked down to transcellular magnesium transport in the distal convoluted tubule. Within the distal convoluted tubule, magnesium reabsorption is a tightly regulated process that determines the final urine magnesium concentration. Therefore, insufficient magnesium transport in the distal convoluted tubule owing to mutated magnesiotropic proteins inevitably leads to magnesium loss, which cannot be compensated for in downstream tubule segments. Better understanding of the molecular mechanism regulating magnesium reabsorption will give new opportunities for better therapies, perhaps including therapies for patients with chronic renal failure.

Hair magnesium, but not serum magnesium, is associated with left ventricular wall thickness in hemodialysis patients

BACKGROUND

Extracellular magnesium (Mg) accounts for approximately 1% of the total body Mg. Clinically, serum Mg concentration is measured, but it does not necessarily reflect total body Mg status. Although relationships have been reported between reduced Mg and cardiovascular disease in non-dialysis patients, there have been few such studies in hemodialysis patients. It was hypothesized that reduced Mg, as represented by lower Mg concentration in the hair, would be associated with echocardiographic parameters in chronic hemodialysis patients.

METHODS AND RESULTS

Hair Mg concentration was measured in 79 male hemodialysis patients using inductively coupled plasma mass spectrometry, and the relationships between hair Mg concentration and echocardiographic parameters were investigated. There was no significant correlation between Mg concentration in the hair and in serum. Hair Mg concentration in the patients with high-left ventricular mass index (LVMI) was significantly lower than that in the low-LVMI patients. Hair Mg concentration correlated significantly and negatively with posterior left ventricular wall thickness, interventricular septum thickness, left ventricular wall thickness (LVWT), and relative wall thickness. Serum Mg concentration, however, did not correlate with any of these echocardiographic parameters.

CONCLUSIONS

In hemodialysis patients, hair Mg concentration is a biomarker, independent of serum Mg concentration. Hair Mg, but not serum Mg, was significantly and negatively associated with LVWT. Reduced tissue Mg concentration, as measured in the hair, may be associated with left ventricular hypertrophy in hemodialysis patients.

Studies of osmotic diarrhea induced in normal subjects by ingestion of polyethylene glycol and lactulose

The purpose of these studies was to gain insight into the pathophysiology of pure osmotic diarrhea and the osmotic diarrhea caused by carbohydrate malabsorption. Diarrhea was induced in normal volunteers by ingestion of polyethylene glycol (PEG), which is nonabsorbable, not metabolized by colonic bacteria, and carries no electrical charge. In PEG-induced diarrhea, (a) stool weight was directly correlated with the total mass of PEG ingested; (b) PEG contributed 40-60% of the osmolality of the fecal fluid, the remainder being contributed by other solutes either of dietary, endogenous, or bacterial origin; and (c) fecal sodium, potassium, and chloride were avidly conserved by the intestine, in spite of stool water losses exceeding 1,200 g/d. Diarrhea was also induced in normal subjects by ingestion of lactulose, a disaccharide that is not absorbed by the small intestine but is metabolized by colonic bacteria. In lactulose-induced diarrhea, (a) a maximum of approximate 80 g/d of lactulose was metabolized by colonic bacteria to noncarbohydrate moieties such as organic acids; (b) the organic acids were partially absorbed in the colon; (c) unabsorbed organic acids obligated the accumulation of inorganic cations (Na greater than Ca greater than K greater than Mg) in the diarrheal fluid; (d) diarrhea associated with low doses of lactulose was mainly due to unabsorbed organic acids and associated cations, whereas with larger doses of lactulose unmetabolized carbohydrates also played a major role; and (e) the net effect of bacterial metabolism of lactulose and partial absorption of organic acids on stool water output was done dependent. With low or moderate doses of lactulose, stool water losses were reduced by as much as 600 g/d (compared with equimolar osmotic loads of PEG); with large dose, the increment in osmotically active solutes within the lumen exceeded the increment of the ingested osmotic load, and the severity of diarrhea was augmented.

Jejunal and ileal adaptation to alterations in dietary calcium: changes in calcium and magnesium absorption and pathogenetic role of parathyroid hormone and 1,25-dihydroxyvitamin D

Previous balance studies have shown that fractional calcium absorption is increased by a low and reduced by a high calcium diet. The present studies were done to determine which segment of the small intestine is most sensitive to alterations in dietary calcium, and to see if dietary calcium intake has an effect on the intestinal absorption of another divalent cation, magnesium. Absorption was measured during constant perfusion of 30-cm segments of jejunum and ileum of normal subjects after 4 or 8 wk of a high (1,900 mg/d) or a low (20 mg/d) calcium diet. We found that calcium absorption rate was higher when subjects had been on a low than when they had been on a high calcium diet; the ileum responded more rapidly and more completely than the jejunum. Similar results were obtained with magnesium, but only the difference in the ileum was statistically significant. Sodium and xylose absorption were not influenced by dietary calcium intake. The serum concentrations of parathyroid hormone and 1,25-dihydroxyvitamin D were higher on the low than on the high calcium diet. We conclude that the ileum is more sensitive than the jejunum to changes in dietary calcium intake, and that ileal adaptation probably plays a major role in protecting the body against a deficiency or excess of body calcium that otherwise would occur when dietary calcium is abnormally low or high. Calcium intake influences ileal magnesium absorption in a similar fashion; it is not known whether or not this serves a protective function. Our data are compatible with the concept that adaptation to dietary calcium intake is mediated by changes in the serum concentrations of parathyroid hormone and 1,25-dihydroxyvitamin D.