The calcium in dairy products is not absorbed into the body as easily as that in many dark green leafy vegetables.
For example, calcium is much more easily absorbed from kale than cow’s milk. However, while spinach contains a lot of calcium, it is bound to a substance called oxalate which inhibits calcium absorption, so it is important to obtain calcium from low-oxalate green vegetables (eg broccoli, cabbage, bok choy, watercress). Grains, nuts and seeds contain a substance called phytic acid which until recently was also considered to hinder calcium absorption; however phytic acid is now believed to have only a minor influence. Caffeine and smoking has been shown to reduce calcium absorption.