Magnesium

Magnesium is the quiet helper of your metabolism. More than three hundred enzymes need it, from the enzymes that make energy from food to the enzymes that copy your DNA and let your muscles relax after a contraction. It keeps the potassium channels in your cells closed, lets your parathyroid glands release PTH and lets your heart beat in a regular rhythm. Of all the magnesium in your body about sixty percent sits in your bones, almost all the rest in your cells, and only one percent in your blood. That one percent is what the test measures, and that is the limitation of this value. Your body keeps blood magnesium up by taking it from the bones and the cells, so the blood value stays normal for a long time while the store in the tissue is already shrinking. A low blood value therefore means the deficiency has existed for a while, and a normal value does not rule out a deficiency. The kidneys regulate removal: they hold on to magnesium when there is little and excrete it when there is plenty, and anything that disturbs that regulation shifts the value. Your report shows magnesium in mmol/L. Normal is about 0.73 to 0.98 mmol/L for a man and 0.77 to 0.98 mmol/L for a woman, where the labs overlap. Below 0.4 mmol/L and above 3.0 mmol/L the requester is phoned the same day, because the heart rhythm can then be at risk.

Magnesium is the value you read when potassium or calcium stay low while you top them up. Without magnesium the kidneys leak potassium away and the parathyroid glands release no PTH, so a low potassium and a low calcium only recover once magnesium is topped up. Anyone who takes diuretics, acid-reducing medicines or a lot of alcohol loses magnesium through the kidneys or does not absorb it, and that is the most common reason a low potassium or calcium will not correct. For anyone who is healthy and does sport, magnesium is more interesting than the blood value shows. Muscle cramps, twitching eyelids, poor sleep and a heart that skips are the symptoms of a deficiency in the tissue, and those can be there with a normal blood value. A diet with few nuts, seeds, legumes, whole grains and green vegetables and a lot of processed food delivers less magnesium than the body needs, and sweat carries it away. A generous intake from food goes with a lower blood pressure, a better blood sugar and less cardiovascular disease, and that is an effect of the store in the cells, not of the number in your blood. The upper end is almost always the kidneys. Healthy kidneys excrete every surplus, so a high magnesium means they can no longer do that, or that more comes in through medicines such as magnesium-containing laxatives or antacids than they can handle. A magnesium supplement hardly lifts the blood value with healthy kidneys.

A low magnesium almost always means loss or poor absorption, and rarely too little food alone. Usually it is the first and rarely the last: acid-reducing medicines, which with long-term use block absorption in the gut; diuretics, which make the kidneys release magnesium; and alcohol, which makes the kidneys excrete magnesium and often makes the diet one-sided. After that come long-lasting diarrhoea or a bowel disease, and a poorly controlled diabetes, in which the kidneys lose magnesium along with the sugar. The symptoms are cramps, twitching, tiredness and a heart that skips, and a low potassium or calcium that will not recover is often the first clue. A high magnesium means the kidneys cannot get rid of it. With kidneys that filter less, magnesium builds up, certainly if magnesium-containing laxatives, antacids or supplements are added. With healthy kidneys a high value is rare, and then usually a matter of dehydration or a tube in which the red cells broke. Above 3.0 mmol/L the muscles go weak and the heart slows, and that should be assessed the same day. Magnesium belongs beside potassium and calcium, because those do not recover while magnesium is low, beside creatinine and the eGFR, which say whether the kidneys can get rid of it, and beside your symptoms and your diet, because a normal blood value does not rule out a deficiency in the tissue.