Why are dialysis patients with End-Stage Renal Disease (ESRD) given Magnesium (Mg) oxalate, which has a low absorption rate, instead of Magnesium (Mg) glycinate, which has a better absorption profile?

Magnesium Supplementation in Dialysis Patients: Why Absorption Doesn't Matter

In dialysis patients with ESRD, magnesium balance is determined almost entirely by dialysate magnesium concentration, not by oral absorption, making the choice between magnesium oxalate and magnesium glycinate clinically irrelevant. 1, 2, 3

The Fundamental Physiology in ESRD

Renal Excretion is the Primary Route

• In healthy individuals, the kidney is the major route of magnesium elimination from the body, and renal excretion maintains magnesium homeostasis 2
• Once patients reach end-stage renal disease on dialysis, they are largely dependent on the dialysate magnesium concentration for maintaining serum magnesium and magnesium homeostasis 1
• The limited ability of the failing kidney to excrete magnesium loads means that dialysate composition becomes the dominant determinant of magnesium balance 2, 3

Intestinal Absorption is Impaired in CKD

• Chronic renal failure is characterized by impaired intestinal magnesium absorption, likely related to deficient synthesis of active vitamin D metabolites by the non-functioning kidney 2
• This impairment means that even highly bioavailable oral magnesium formulations will have reduced absorption in ESRD patients 2

Why Oral Magnesium Formulation Choice is Irrelevant

Dialysate Overwhelms Oral Intake

• Changes in dialysate magnesium concentration are the primary tool used to manipulate magnesium balance in dialysis patients, not oral supplementation 2
• The dialysate magnesium concentration can be adjusted to reduce renal osteodystrophy, alleviate uremic pruritus, or retard arterial calcification 2
• During hemodialysis sessions, magnesium flux across the dialysis membrane is bidirectional and depends on the concentration gradient between blood and dialysate 1, 3

Magnesium Balance in Dialysis is Tightly Controlled

• In continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis, the major determinant of magnesium balance is the concentration of magnesium in the dialysate, not dietary or supplemental intake 2
• Patients with ESRD on dialysis can develop either hypomagnesemia or hypermagnesemia depending on dialysate composition, regardless of oral intake 1, 3

Clinical Implications

When Magnesium Supplementation Might Be Considered

• Hypomagnesemia in dialysis patients should first be addressed by adjusting dialysate magnesium concentration rather than oral supplementation 4, 1
• Dialysis solutions containing magnesium should be used to prevent electrolyte disorders during kidney replacement therapy 4
• The use of dialysis and replacement fluids with increased magnesium concentration is particularly indicated when regional citrate anticoagulation is used, as magnesium is lost in the effluent as magnesium-citrate complexes 4

The Rare Case for Oral Supplementation

• If oral magnesium supplementation is attempted in dialysis patients (which is uncommon), the 4% absorption of magnesium oxalate versus higher absorption of magnesium glycinate becomes meaningless because:
  • Intestinal absorption is already severely impaired in ESRD 2
  • Any absorbed magnesium will be immediately subject to removal during the next dialysis session 1, 3
  • The dialysate concentration will equilibrate serum levels regardless of oral intake 2

Common Pitfalls to Avoid

• Do not attempt to manage magnesium balance in dialysis patients primarily through oral supplementation - this approach ignores the fundamental physiology of ESRD 1, 2
• Avoid magnesium-containing antacids and laxatives in dialysis patients, as these can lead to toxic magnesium concentrations when renal excretion is absent 2
• Do not assume that higher oral bioavailability translates to clinical benefit in patients whose magnesium balance is controlled by dialysate composition 1, 3

The Bottom Line

The question itself reflects a misunderstanding of magnesium homeostasis in ESRD. Dialysis patients don't "get" magnesium oxalate as a therapeutic choice based on absorption characteristics - if they receive any oral magnesium at all, the formulation is irrelevant because dialysate magnesium concentration is the overwhelming determinant of their magnesium status 1, 2, 3. The appropriate clinical approach is to adjust dialysate magnesium concentration to achieve target serum levels, not to select oral magnesium formulations based on absorption profiles 4, 2.