Magnesium Flashcards
Magnesium distribution
- 55-60% located in bone
- 20-25% located in soft tissues
- 1% located in extracllular fluids
Magnesium sources
- Nuts and seeds
- Legumes and whole grains, especially oats, barley, and brown rice
- Most vegetables, especially green leafies, corn, and carrots
- Seafood and dairy products
- Coffee and tea, cocoa and chocolate
- Blackstrap molasses
Magnesium absorption
• In the small intestine
o Primarily in the distal jejunum and ileum
o May also be absorbed in the colon especially if disease has interfered with absorption in the small intestine
• 2 routes
o Saturable, carrier-mediated active transporter
• When intake is low
• More efficient
• Stimulated by vitamin D
o Simple diffusion with higher intakes
• Factors that enhance absorption:
o Vitamin D
o Simple sugars such as lactose and fructose
• Factors that inhibit absorption
o Phytates
o Fiber
o Excessive unabsorbed fatty acids
o Other minerals - especially calcium and phosphorus
Magnesium transport
• In the blood Mg is found:
o 50%-55% free
o 33% bound to protein (mostly albumin)
o 13% complexed with ions
Magnesium homeostasis regulation
• Regulatory mechanism is unclear • However, as with calcium PTH increases plasma concentrations of magnesium by: o Increasing intestinal absorption o Decreasing renal excretion o Enhancing bone resorption
Magnesium functions
• Bone
o 70% associated with Ca and phosphorus as part of the bone crystal matrix
o 30% found on the surface which is most likely an exchangeable pool to maintain blood levels
• Required as a cofactor for over 300 enzymatic reactions
o Participates in all aspects of energy metabolism including glycolysis, beta-oxidation and the TCA cycle
• Also involved in a wide range of physiological processes including:
o Neuronal activity
o Cardiac excitability
• Still other functions of magnesium:
o Inhibits platelet aggregation
o Promotes dilation of blood vessels
o Antispasmodic effect on skeletal smooth muscle
Magnesium excretion
- Urine – most (85-95%) of the filtered Mg is reabsorbed
- Feces – mostly unabsorbed Mg but some endogenous as well
- Perspiration
Magnesium mild deficiency symptoms
o Anxiety, panic attacks, depression, fatigue, insomnia
o Muscle cramps/twitches, tremors
o Chest tightness, dyspnea
o Faintness, headaches, decreased concentration, memory loss, confusion
o Palpitations, cardiac arrhythmias
Magnesium severe deficiency symptoms
o Muscle weakness, abnormal gait
o Vertigo, seizures, hallucinations, stupor, coma
o Electrolyte disturbances: hypokalemia, hypocalcemia
o Anorexia, nausea, vomiting
o Muscle fasciculations, myoclonal jerks
o Physical exam: +ve Chvostek and Trousseau signs, hyperreflexia, nystagmus
Magnesium increased risk of deficiency
o Chronic stress
o Excessive alcohol intake
o Chronic diarrhea or prolonged vomiting
o Diabetes mellitus
Magnesium clinical indications
- Cardiovascular: arrhythmias, congestive heart failure, hypertension
- Gastrointestinal: constipation, esophageal spasm
- Musculoskeletal: fibromyalgia, muscle cramps, osteoporosis
- Neurological: Headaches, restless leg syndrome
- Gynecological: dysmenorrhea
- Psychiatric: anxiety, depression
- Pulmonary: asthma
Magnesium preparations
• Many forms to choose from:
o Oxide, citrate, glycinate, aspartate, lactate, hydroxide, sulfate, amino acid chelate
• Mg oxide
o Low cost
o High proportion of elemental Mg
o One study showed it was not as well absorbed as Mg citrate but another study showed that Mg oxide, citrate, and amino acid chelates all have similar absorption
• Mg (bis)glycinate
o Similar absorption to oxide but may cause less diarrhea
• Mg aspartate
o Aspartate may increase Mg absorption
o However, aspartate is an excitatory neurotransmitter and may be neurotoxic in amounts >8g
• Mg sulfate
o Acts as a non-reabsorbable anion in the kidney which may interfere with an associated potassium deficiency
o Also most likely to cause diarrhea – sometimes used as a laxative
o Useful as a bath slat to relax sore, stiff muscles
• Avoid enteric-coated Mg chloride (Slow-Mag) as it was found to have lower absorption
Magnesium toxicity
• Tolerable upper intake level (UL): 350 mg of non-food sources • Diarrhea is a relatively common dose-related side effect of Mg supplementation • Symptoms of hypermagnesemia: o Nausea o Flushing o Double vision o Slurred speech o Weakness, muscular paralysis
Magnesium contraindications
o End-stage renal disease
o Myasthenia gravis
o Hypermagnesemia
o Urinary tract infection with elevated urinary phosphates
• Use with caution in patients with hyperparathyroidism
Magnesium nutrient interactions
• Calcium
o May decrease tissue levels of Mg and therefore may accentuate signs of deficiency
o Mg is needed for PTH secretion but high levels inhibit its release
o Mg is also required for the effects of PTH on bone, kidney, and the GI tract
o Hydroxylation of vitamin D in the liver requires Mg
• Phosphorus – Mg inhibits phosphorus absorption
• Potassium – Mg is essential for uptake of potassium from the serum into cells
o Therefore in patients that are deficient in Mg and potassium, supplementation with potassium may not correct the deficiency unless Mg is also given and supplementing with Mg alone may increase cellular uptake of potassium and worsen the hypokalemia
• Vitamin B6 – Increases intracellular uptake of Mg and vice versa
• Thiamine – large doses of thiamine may increase the need for Mg and may exacerbate a Mg deficiency
• Zinc – high doses of zinc decreases Mg absorption
