Final answer:
Calcium absorption from nuts and whole grains is inhibited by phytates and oxalates, which bind to calcium and form insoluble salts. Excessive dietary fiber also affects absorption, and vitamin D is essential for efficient calcium uptake.
Step-by-step explanation:
Dietary calcium is not well absorbed from nuts and whole grains due to interactions with phytate (found in cereal grains) and oxalate. These compounds bind to calcium to form insoluble calcium salts, which cannot be absorbed through the intestines. Additionally, excessive dietary fiber can interfere with the absorption of calcium.
Calcium absorption is facilitated by activated vitamin D, and a deficiency of this vitamin can lead to reduced calcium absorption. Foods high in calcium, such as dairy products, green leafy vegetables, and certain fish with bones, are beneficial for maintaining adequate calcium levels in the body.