Is oxalic acid in spinach really dangerous and causes kidney stones?

THE’oxalic acidalso known as oxalate, is a substance that does not always have a “good reputation” in food science: it is said that it can promote the formation of kidney stones and for this reason many people tend to limit or even avoid foods that are rich in them, such as spinach. But are we sure that the danger applies to everyone? The answer, in reality, is nuanced: oxalate can only be a problem in particular conditions. Generally, for most healthy people, moderate consumption of spinach is not dangerous, but the risk of forming kidney stones increases if there are predispositions individualas for those suffering from kidney failure. In this case, it is better to avoid certain foods such as spinach, or, if we really cannot do without them like Popeye, there are small tricks to reduce the negative effect of oxalates, such as boiling them or eating them with foods rich in calcium, such as milk, and obviously avoiding spinach “binges”.

Spinach and oxalates: attention only in particular conditions

Some studies have shown that spinach represents a significant source of oxalate, the same which can then crystallize in the kidneys in the form of stones. However, saying that “spinach causes stones” is, perhaps, one oversimplification: it is not always easy to establish how much of the risk is due to the individual food, because they also come into play other factorssuch as individual, genetic and metabolic ones.

Some studies highlight that the relationship between the consumption of oxalates (therefore our beloved spinach) and the risk of kidney stones is real, but weakand is more relevant only in those who already have particular predispositionssuch as kidney failure or previous episodes of stones. In extreme cases it can lead to a condition called oxalate-nephropathywhere large amounts of oxalate directly damage the kidneys, but it is of rare events not linked to the moderate consumption of vegetables in healthy subjects. In summary, spinach is not a “universal enemy”: its oxalates become a problem only in an already predisposed and inherently fragile clinical context.

How do kidney stones form if we take too much oxalic acid?

To understand why kidney stones form if we take excess oxalates, it is important to understand what it means chemical solubility of substances, a value that tells us how well a compound can dissolve in a certain solvent, such as water. Table salt and sugar are foods with good solubility in water: in fact, just put a little in a glass and, with the help of a shaking spoon, they dissolve immediately. Right away… in certain quantities! This is exactly the key concept: if we put too sugar or salt in the glass, we can stir with the spoon as much as we want but if we exceed a certain quantity, they will never melt again, remaining undissolved as a background body.

Solubility tells us exactly this: for a given volume of solvent (water) only a certain amount of substance dissolves; if we exceed it, the excess substance becomes like magic insoluble. The same goes for oxalic acid in spinach: initially it dissolves without problems and is well absorbed by the body, but has reduced solubility. It means that the maximum quantity of substance that can dissolve is very low and as soon as we exceed a certain intake limit nothing dissolves anymore.

So what happens to our kidneys? Passed the solubility limitthe much-discussed kidney stones are formed: solid crystals of calcium oxalatethe most common form of kidney stone! Real pebbles, which aren’t exactly good for our kidneys.

How to reduce your risk without giving up spinach

The good news is that although oxalates cannot be completely eliminated from foods, they exist practical measures to limit their effects. After all, better safe than sorry, right? Correct cooking already does a lot: the boiling it significantly reduces the oxalate content and discarding the cooking water would seem to amplify the effect. Food combinations are also important: consume spinach together with foods rich in calciumsuch as cheese or yogurt, allows calcium to bind to oxalate already in the intestine, preventing it from being absorbed and reaching the kidneys.

At the same time, hydrate well is fundamental: the more water we drink, the more diluted the urine remains, and the lower the possibility that crystals will form. Finally, it is useful to avoid “excesses” in a single meal, as well as too high doses of vitamin C which can be metabolised by transforming itself into oxalate.

In practice, spinach must not disappear from the diet: with some caveats, more so for “predisposed” subjectscan remain part of a healthy diet, without unnecessarily increasing the risks to the kidneys.