Spinach and rhubarb are healthy foods, but they contain oxalates, which can interfere with the absorption of calcium in the body. Oxalates are natural occurring compounds found in v plants that can bind to calcium in the digestive tract, forming insoluble calcium oxalate. It reduces the amount of calcium that is available for absorption into the bloodstream, most of the calcium will not be utilized in the body. Those who rely on these two vegtables may not get that amount of calcium.

It is also a risk for kidney stones. When calcium combines with oxalates to form calcium oxalate, it can cause the formation of stones in susceptible individuals. It's a good idea to look for calcium in other areas such as dairy products to be on the safe side.