Answer to Question 1
Calcium appears almost exclusively in three classes of foods: milk and milk products; green vegetables such as broccoli, kale, bok choy, collards, and turnip greens; and a few fish and shellfish. Milk and milk products typically contain the most calcium per serving. Many greens are also good choices, but a complication enters inabsorption. It is not clear to what extent calcium is absorbed from certain green vegetables (notably, spinach and Swiss chard), whereas calcium is known to be very well absorbed from milk. Milk contains both vitamin D and lactose, both of which enhance calcium absorption and promote bone health. Milk and milk products also normally supply about 40 percent of people's intake of riboflavin.
Some food components interfere with calcium absorption. Phytate is one example; it occurs in some fruits, vegetables, and whole grains, as well as in nonherbal tea. Oxalate is found in beet greens, rhubarb, and spinach, among other foods. These binders seem to depress absorption of the calcium present in the same food as the binder but do not depress the absorption of calcium from other foods consumed at the same time. Because fiber in general seems to hinder calcium absorption, the higher your diet is in fiber, the higher it should also be in calcium. This fact doesn't detract from the overall value of high-fiber foods, which are important and nutritious for many reasons.
Answer to Question 2
A
Cambrian trilobite, with a shell made of calcium carbonate.
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