330 leaves of rocket:
When we were living in Bermuda one of the things I quickly learned was rocket was called “arugula“. If you are American this would seem entirely reasonable, but I would like to point out that Bermuda is a British Overseas territory – so I continue to use the British name.
The official Latin botanical name Eruca sativa doesn’t display transatlantic preferences, in fact Eruca in Latin means caterpillar! So no clues as to why or how we ended up with such different names for what is essentially a peppery lettuce leaf.
Why 330 leaves of rocket?
1 leaf contains about 1mg magnesium and the recommended daily intake of magnesium is 330mg. (300-360mg)
Magnesium is an important element is many physiological reactions.
Kidneys are fundamental to magnesium homeostasis (balance)
Is this something we need to be concerned about with PKD?
Low levels of magnesium have been associated with increased cardiovascular problems. One German study from 2011 demonstrated that for people with low serum magnesium the cardiovascular mortality was increased by around 40%. (Reffelmann et al, 2011) Because, as a group, people with PKD tend to also have cardiovascular problems this is probably important.
I learned that my own serum magnesium, at my last blood test, was 0.69 mmol/L. The result did not come with a normal range which prompted the research for this post.
In fact my own magnesium might be considered to be on the low side of normal. The serum magnesium in adults should be between 0.65 and 1.05 mol/L.
We need to go back to some basic chemistry:
Magnesium is a metal, silvery white in colour. If you did science in school you may recall standing well away from the teacher as they floated a small piece of magnesium on water and it ignited with bright white flames, or putting it in vinegar to produce hydrogen that burned with a popping sound.
Magnesium is found as mineral deposits in the earth’s crust, such as dolomite which is found in the mountain range of the same name, but it is also found in sea water where the concentration is around 55mMol/L. In some salt lakes the concentrations get much higher, reaching 198mMol/L in the Dead Sea.
The magnesium atom is in some ways similar to calcium. Both are important in the body and sometimes they will compete for the same binding sites. However, magnesium is almost always found in its hydrated state. This hydrated molecule of magnesium has two hydration shells – or two layers of water wrapped around it, while calcium only carries one hydration shell. This difference means that a hydrated magnesium molecule is huge compared with a hydrated calcium molecule, which makes it difficult for magnesium to pass through narrow channels in biological membranes. Magnesium needs special proteins to transport it inside the body that are capable of stripping off the hydration shells in order to deliver the bare magnesium ion to where it is used.
Most of the body’s magnesium, 99%, is located in bone, muscle and soft tissues. It is found in the surface layers of bones where it acts as a buffer to help keep the plasma magnesium steady. If plasma concentrations fall then there is a rapid release of the magnesium from bone. Because the amount of magnesium stored in bone falls with increasing age, the buffering capacity is reduced as we get older and we are less able to cope with big changes in our serum level of magnesium.
Inside the cell magnesium is an essential co-factor for many enzyme reactions – this means that some reactions will not happen or will be slow if the cellular magnesium concentration is low. It is important in ATP metabolism (energy production for biological reactions), muscle contraction, neurological function, vascular tone, clotting and cardiac rhythm. All of these are clearly essential, but of specific consideration in PKD patients, magnesium has a role in cell proliferation and transmembrane transport. We know that both of those processes are altered in PKD and so magnesium probably has an effect here – there is current research on this.
How does the body deal with magnesium?
We find high levels of dietary magnesium in green vegetables, nuts, seeds and unprocessed cereals. 10% of our magnesium intake actually comes from water. Then fruit, meat and fish will have intermediate levels. dairy has very low levels of magnesium.
We are back to our 330 leaves of rocket.
We need to take in around 300-360mg magnesium each day.
I leaf of rocket contains 1mg magnesium. There is slightly more in Swiss chard but slightly less in spinach and kale. Brazil nuts and sesame seeds contain around 100mg per ounce, with slightly less in walnuts.
For mackerel, 1g contains about 1mg magnesium, so a 3oz fillet (85g) has around 85mg magnesium.
Half a cup of beans or lentils will contain around 75mg magnesium and a serving (half a cup) of brown rice will contain around 40mg. 1 banana may have 30mg and dark chocolate has around 143mg for every 100g.
Even I could make a meal out of those ingredients to give me my daily recommended magnesium. Note however that there is some evidence that cooking improves the amount we can absorb and eating with protein is also suggested to increase absorption. One thing it is important to underline is that the proportion absorbed depends on the body’s magnesium status and not on the amount we eat. This has implications when trying to correct a deficit – it will take time to build up depleted levels, it cannot be done by just one large dose of magnesium.
So, assuming we have managed to ingest 360mg magnesium in one day. What happens to it?
Actually only 100mg of this will be absorbed into the blood stream, and 260mg will just carry on down through the gut.
As we said before, most of the magnesium is stored in bone and tissues so the levels in the blood are not accurate measures of the body magnesium content. Only 1% of total body magnesium is found in the serum.
Serum magnesium levels are affected by recent diet, exercise, and levels of haemolysis (breaking down of red blood cells) and bilirubin (from haemoglobin). Vegetarians will tend to a higher serum magnesium. After heavy physical exertion serum magnesium will rise, but after prolonged endurance type of exertion the levels will actually fall. Pregnancy also is associated with lower serum magnesium.
Lets go to the kidneys and consider how they deal with magnesium:
Magnesium is filtered by the glomerulus. about 2,400mg is filtered each day. In a healthy kidney then 2,300mg of this is reabsorbed.
Slightly unusually this reabsorption mostly takes place in the thick ascending Loop of Henle. (see previous posts for nephron anatomy). The amount reabsorbed, and therefore the amount excreted, is controlled by plasma magnesium levels, parathyroid hormone, ADH, glucagon, calcitonin and oestrogen. In a healthy adult with normal serum magnesium we can expect 100mg magnesium to be excreted in the urine over a day. Magnesium is excreted in a circadian rhythm – most of it is excreted overnight.
Measuring magnesium excretion through a 24 hour urine test actually gives a better indication of the body’s magnesium status, but it is a cumbersome and expensive test. It is however one used to identify renal wasting of magnesium (high loss in the urine) and occasionally to prove inadequacy of dietary intake (low loss in the urine). Alternatively a magnesium retention test can be undertaken where impaired absorption is suspected – high retention after a loading dose suggest a magnesium deficiency.
Because serum magnesium does not accurately reflect total body magnesium in humans, then assessing magnesium deficiency is not straightforward. (Of interest, in other mammals the serum levels will equilibrate with bone and tissue content over a period of 3-4 hours). You can have a subtle chronic magnesium deficiency and yet display normal levels in the serum.
There are, however, some situations where we know magnesium deficiency is common and likely. Hospitalised patients, especially those in intensive care, are likely to have low magnesium. The factors involved here are lower intake, stress, increased excretion, drugs and IV fluids. Particularly relevant for those of us with PKD are the drugs furosemide, cyclosporine, tacrolimus and omeprazole, all of which have been associated with low magnesium levels. Conditions such as diabetes, parathyroid impairment and alcohol excess may all contribute to lower magnesium levels.
Higher mortality has been linked to severe deficiencies of magnesium and chronic low levels have been linked to both hypertension and psychiatric illness.
Signs of low magnesium are tremor, irregular heart rhythm, fatigue and weakness, nausea, muscle cramps and skin tingling. But of course these are common symptoms in many situations and not at all diagnostic.
In advance chronic kidney disease the compensatory mechanisms may fail and we can actually end up with too much magnesium on board. This can be exacerbated by the use of laxatives and antacids which contain magnesium – both increasingly common over-the-counter medicines we might use as we get older.
It might also be advisable to take great care of you plan to swim in the Dead Sea – one case of near-drowning in the Dead Sea was associated with very high and nearly fatal serum magnesium.
Is there a bottom line to the magnesium story?
American studies have suggested that around 25% of the general adult population will consume less than half of the recommended daily allowance of magnesium. Refined foods tend to have low magnesium content. For example the process of changing brown rice to white rice loses about 80% of the magnesium content.
For people with ADPKD it is not simply a question of “more is good” because it depends so much on what kidney function you have, what medications you are taking, what other illnesses you may have, and lifestyle factors such as diet and exercise. It would be fairly safe to assume that in the early stages we can adjust our diet to give a magnesium intake within the 300-360mg per day. People on dialysis or after transplant or at the end stages of renal disease should make adjustments ONLY after discussing it with your personal doctor and dietician.
My lunch today: I shall be eating mackerel coated in sesame seeds with lentils and a large side salad of rocket – though maybe not 330 leaves. Needless to say this will be followed by chocolate!
Reffelmann et al. 2011 Low serum magnesium concentrations predict cardiovascular and all-cause mortality. Atehrosclerosis. 219:280-4
Wilhelm Jahnen-Dechent, Markus Ketteler; Magnesium basics. Clin Kidney J 2012; 5 (Suppl_1): i3-i14. doi: 10.1093/ndtplus/sfr163