Spinach components have been shown to inhibit growth and proliferation of cervical cancer cells in the laboratory and carcinogen-induced colon cancer in mice. Dietary intake of spinach has been found to be associated with lower risks of head and neck, lung, gallbladder, stomach, liver, bladder, prostate and ovarian cancer in population studies.

Carotenoids and glycolipids isolated from spinach have been demonstrated to cause dose-dependent growth inhibition in breast cancer cells. Several population studies have found that spinach consumption is associated with lower risk of breast cancer. Spinach consumption may help counteract the cancer-promoting effects of the heme iron in red meat.

Baby spinach has higher flavonoid concentration than mature spinach. Red spinach (Amaranthus gangeticus) is a plant used in South Asian cooking that is closely related to common spinach. Based on the few studies that have been performed, red spinach appears to have anti-cancer activities similar to that of common spinach.

Spinach contains oxalic acid and purines; intake of spinach has been shown to increase the risk of kidney stone and gallstone formation, as well as gout. Individuals prone to stones or gout may want to limit or avoid spinach. Oxalates interfere with calcium absorption, and therefore spinach should not be eaten at the same time as calcium-rich foods by breast cancer patients and others to whom calcium levels are important. Boiling reduces the oxalic acid content of spinach and increases iron availability.