Is Spinach Bad for Your Kidneys?

Spinach is unusual in being high in both potassium and oxalate at the same time. A 90 g serving of cooked spinach (a typical side portion) delivers around 460 mg of potassium and roughly 750 mg of oxalate. By comparison, a portion of broccoli has about 220 mg potassium and 5 mg oxalate.

This combination is the reason renal dietitians and stone clinics consistently single spinach out. For someone with CKD it is a potassium issue; for someone with calcium-oxalate stones it is an oxalate issue; for many patients, it is both.

About 80% of UK kidney stones are calcium-oxalate. The oxalate in stones comes partly from your liver's metabolism and partly from food. High dietary oxalate increases urinary oxalate, which combines with calcium in the kidney tubules to form crystals.

Spinach is the single biggest food contributor to dietary oxalate in the typical UK diet. Other high-oxalate foods include rhubarb, beetroot and beetroot leaves, Swiss chard, almonds, peanuts, dark chocolate, and strong tea drunk in large volumes. Stone clinics usually ask calcium-oxalate stone formers to limit or avoid spinach completely.

One practical mitigation: dietary calcium taken at the same meal binds oxalate in the gut and reduces absorption. A small amount of spinach eaten with a dairy-containing meal causes a smaller urinary oxalate rise than the same amount eaten alone.

Many patients assume cooked spinach is 'safer' because the volume shrinks. From a kidney point of view, the opposite is closer to the truth. Cooking removes water but does not meaningfully remove potassium or oxalate, so the cooked product is more concentrated per gram.

• 30 g raw spinach (handful for a salad): ~ 170 mg potassium, ~ 290 mg oxalate.
• 90 g cooked spinach (typical side, from ~ 250 g raw): ~ 460 mg potassium, ~ 750 mg oxalate.

So a small bowl of cooked spinach can deliver more potassium and oxalate than several large salads.

If you have been asked to limit spinach, useful lower-potassium leafy greens include:

• Lettuce (cos, iceberg, gem): 100–150 mg potassium / 60 g
• Watercress: 100 mg potassium / 30 g
• Rocket: 75 mg potassium / 30 g
• Cabbage (boiled): 150 mg potassium / 80 g
• Pak choi (boiled): 200 mg potassium / 80 g
• Chinese leaf (raw): 150 mg potassium / 80 g
• Kale (boiled, small portion): 200–300 mg potassium / 80 g — moderate, watch portion size

These also tend to be much lower in oxalate. Boiling and discarding the cooking water further reduces potassium in any leafy green.

Spinach is a common ingredient in green smoothies, juices and 'detox' drinks. Even modest amounts can deliver a large potassium and oxalate load because the spinach is uncooked, often combined with other high-potassium ingredients (banana, kale, coconut water, dates), and consumed in liquid form that makes the volume easy to underestimate. There have been published case reports of acute oxalate nephropathy (acute kidney injury caused by oxalate) from heavy daily smoothie use.

If you have CKD, avoid daily spinach-based smoothies. If you make smoothies, base them on lower-potassium fruit (berries, apple, pineapple) and watercress or rocket instead of spinach and kale.

• CKD Stages 1–3, normal blood potassium, no stone history: occasional small portion of cooked spinach is fine.
• CKD Stage 4–5 / dialysis: limit or replace with lower-potassium greens; agree daily totals with your renal dietitian.
• Calcium-oxalate stone formers: avoid spinach, rhubarb, beetroot leaves, Swiss chard. Drink plenty of plain water. Take dietary calcium with meals.
• Anyone on a daily green smoothie: re-think the recipe. Frequency and volume matter more than 'organic' or 'fresh' status.
• Pregnancy + CKD: discuss with your renal team; usual stage-specific advice applies.