One of the body's highest priorities is maintaining blood calcium levels. The calcium stored in bone provides a bank of calcium for the blood. The blood can borrow and return calcium from bone as needed, so even with a dietary deficiency of calcium, blood levels of calcium remain normal while bone levels of calcium diminish, according to authors Eleanor Whitney and Sharon Rolfes in "Understanding Nutrition." You can have an inadequate calcium intake for years and only later in life realize the integrity of your bones has been compromised.
Calcium Balance
Three organ systems respond to calcium that rises too high or falls too low in the blood. The intestines, bones and kidneys along with vitamin D and two hormones, parathormone and calcitonin, return blood calcium to normal. Vitamin D's main function in the body is to maintain normal blood levels of calcium and phosphorus, according to MayoClinic.com. Calcium is found in some foods, added to other foods, supplements and medicines. It is the most abundant mineral in the body, found mostly in bone. The 1 percent of calcium in blood must be tightly regulated, according to the Office of Dietary Supplements.
Rising Calcium
If blood calcium rises above normal, calcium rigor occurs and causes muscles to contract and become unable to relax. Rising blood calcium signals the thyroid gland to secrete calcitonin. Calcitonin inhibits the activation of vitamin D and prevents calcium reabsorption in the kidneys. Calcitonin also limits calcium absorption in the intestines and inhibits osteoclast cells from breaking down bone, thus preventing the release of calcium into the blood. The result of these actions is lower calcium levels which inhibit calcitonin secretion to stop the cycle, according to "Understanding Nutrition."
Falling Calcium
Blood calcium below normal causes calcium tetany, characterized by uncontrolled muscle contraction. Falling calcium signals the parathyroid glands to secrete the hormone parathormone. Parathormone stimulates the activation of vitamin D. Both parathormone and vitamin D stimulate calcium reabsorption in the kidneys. Vitamin D enhances calcium absorption in the intestines. Vitamin D and parathormone stimulate osteoclast cells to break down bone and release calcium into the blood. The result of all these actions is higher blood calcium levels, which inhibits parathormone secretion to stop the cycle, according to "Understanding Nutrition."
Absorption
Factors that enhance calcium absorption include stomach acid, vitamin D, lactose and growth hormones. Growing children absorb 50 to 60 percent of the calcium they consume, compared to 30 percent absorbed by adults and 50 percent absorbed by pregnant women. Factors that inhibit calcium absorption include lack of stomach acid, deficiency of vitamin D, high phosphorus intake, high fiber diet, and phytates and oxalates that bind minerals such as calcium, according to "Understanding Nutrition."
Significance
Maintaining an appropriate amount of calcium in the blood is vital to life. Calcium helps regulate muscle contractions, blood clotting, nerve impulse transmission, hormone secretion and enzyme activation. Calcium in the blood also activates calmodulin, a protein that relays messages from the surface of the cell to the inside of the cell to help maintain normal blood pressure. Calcium may also protect against high blood cholesterol, diabetes, colon cancer and osteoporosis.
References
- "Understanding Nutrition, Ninth Edition"; Eleanor Noss Whitney and Sharon Rady Rolfes; (2002)
- MayoClinic.com: Vitamin D
- Office of Dietary Supplements: Dietary Supplement Fact Sheet: Calcium
- "Krause's Food, Nutrition, & Diet Therapy, 10th Edition"; Mahan and Escott-Stump; (2000)
