Most of the worries about keto are vaguer than people think, but kidney stones are an exception: this is a real, documented risk with a clear mechanism, and it deserves a straight treatment rather than either dismissal or alarm. The good news is that the same understanding that explains the risk also tells you how to avoid it, and for the average person drinking enough water the danger is modest.
What the evidence shows
The clearest data comes from children treated with the strict therapeutic ketogenic diet for epilepsy, where the diet is followed deeply and for a long time under medical supervision. In a Johns Hopkins cohort of children on the diet, around 6.7 per cent developed kidney stones. That is well above the background rate, and it established kidney stones as a recognised side effect of strict, sustained ketosis. It is worth holding onto the context, though: these were children on a far more rigorous version of keto than most adults follow, often with other risk factors, so the figure is the high end of the range rather than what a typical low-carb dieter should expect.
Why ketosis raises the risk
The mechanism is well understood and hinges on urine chemistry. Producing ketones and burning a lot of fat raises the acid load the body has to deal with. To buffer that acid, the body draws on citrate, and citrate happens to be one of the most important natural inhibitors of kidney stone formation. So as acid rises, urinary citrate falls, and in the Johns Hopkins data it fell dramatically, from around 252 milligrams a day before the diet to about 52 on it. At the same time, ketogenic diets tend to make the urine more acidic and can increase the amount of calcium passed in the urine. Combine low citrate, acidic urine, more urinary calcium and, often, low fluid intake, and you have close to ideal conditions for both calcium and uric acid stones to form.
Who is most at risk
The risk is not evenly spread. It is highest for people on strict, prolonged ketosis, people who do not drink enough, people eating very high amounts of protein (which adds to the acid and uric acid load), and anyone who has formed kidney stones before, since a past stone is the strongest predictor of a future one. A moderate low-carb dieter who drinks plenty of water and eats plenty of vegetables sits at the low-risk end. If you have ever had a kidney stone, or have any kidney condition, this moves from a general caution to a specific reason to talk to your doctor before starting.
How to prevent it
Here is the encouraging part: prevention is mostly straightforward, and it maps directly onto the mechanism. Drink plenty of water, consistently, because dilute urine is the single biggest protection and low fluid intake is the common thread in stone formation. Replace the lost citrate: in the epilepsy data, giving potassium citrate roughly halved the stone rate, from about 10 per cent down to around 3 per cent, and for everyday purposes citrate-rich fluids like water with lemon juice help, with prescribed potassium citrate reserved for higher-risk people under medical care. Eat plenty of low-carb vegetables, which are alkalising and counter the acid load while also providing fibre. And keep protein sensible rather than sky-high, since very high protein intakes add to both the acid and the uric acid burden. None of this is onerous, and together it removes most of the risk.
Keeping it in proportion
It is worth stepping back. Kidney stones are a genuine, mechanism-backed risk of ketosis, best documented in the strict therapeutic setting, and they are unpleasant enough to take seriously. But they are also among the most preventable of keto’s downsides, addressed largely by drinking water, eating vegetables and not overdoing protein, all of which you should be doing anyway. For most people following a sensible, well-hydrated, plant-rich low-carb diet, the risk is low and manageable rather than a reason to avoid keto altogether. For anyone with a stone history or kidney disease, it is a doctor’s conversation first.
The bottom line
Strict ketosis raises kidney stone risk by lowering urinary citrate, acidifying the urine and increasing urinary calcium, a pattern documented most clearly in children on therapeutic keto, where stones affected nearly seven per cent. The defences are simple and effective: drink plenty of water, get citrate from lemon water or, where needed, prescribed potassium citrate, eat plenty of vegetables, and keep protein moderate. Respect the mechanism, take the easy precautions, and for most people kidney stones need not be part of the keto story, but if you have had one before, see your doctor first.
This is general information about the ketogenic diet, not medical advice. If you have had kidney stones or have any kidney condition, speak to a doctor before starting keto. Seek medical care for severe flank pain, blood in the urine, or other signs of a kidney stone.
Source: Sampath A, Kossoff EH, Furth SL, Pyzik PL, Vining EPG. Kidney stones and the ketogenic diet: risk factors and prevention. Journal of Child Neurology. 2007;22(4):375-378. Read it here.