Oxalates

Oxalate is a plant compound that binds tightly to calcium in your gut, forming a complex that your body can't absorb. The classic example is spinach: it looks calcium-rich on a label, but only around 5% of that calcium actually gets absorbed, compared to roughly 30% from milk and about 50% from low-oxalate greens like kale, broccoli, and bok choy. So spinach, despite its reputation, isn't really a usable calcium source.

The key thing to understand is that this is a within-meal effect, not something that affects your whole body. Oxalate in a bite of spinach blocks the calcium in that same bite — it doesn't travel to your bones and pull mineral out, and it doesn't interfere with calcium you eat at other meals. Only a short list of foods are genuinely high in oxalate: spinach, Swiss chard, beetroot and beet greens, rhubarb, and to a lesser degree almonds and sweet potato.

There's no need to avoid these foods. Keep eating spinach if you enjoy it, just don't rely on it for calcium — get that from dairy or fortified alternatives, tinned fish with bones, calcium-set tofu, or low-oxalate greens. Cooking and discarding the water lowers oxalate content a lot. The main group who genuinely needs to manage oxalate is people with calcium-oxalate kidney stones, and even then the advice is usually to pair calcium with oxalate-containing foods at the same meal, not to cut calcium.