Calcium

What Is Calcium?

Calcium is the most plentiful mineral in the body. About 99% of it sits in bones and teeth as a hard crystal that gives them their strength. The remaining 1% circulates in blood and fluid around cells, and that small share does most of the day-to-day work.

What Calcium Does Besides Building Bone

Calcium is the signal that makes a muscle fiber contract, including the heart muscle. Nerve cells need it to pass a message to the next cell, blood vessels need it to tighten and relax, and glands need it to release hormones.

Because those jobs cannot pause, the body holds blood calcium in a tight range no matter what you eat. When a meal brings too little, hormones release calcium from bone to cover the gap. A blood test therefore says almost nothing about calcium intake: the level stays normal while the skeleton pays the bill.

Calcium and Bone Through Life

Bone is broken down and rebuilt all the time. In children and teenagers building wins, which is why the US RDA is highest, 1,300 mg, from age 9 to 18. In early and middle adulthood the two roughly balance. Later, and especially after menopause, breakdown runs ahead, and a long shortfall can end in osteoporosis.

How Well Calcium Is Absorbed From Food

The gut needs vitamin D to take calcium in, and it absorbs a smaller share as the amount in one sitting rises. Two plant compounds also get in the way: oxalate, found in spinach and rhubarb, and phytate, found in whole grains, beans, seeds and nuts. Both bind calcium in the gut so less of it is absorbed.

Spinach is the clearest example. Per 100 g it has about half the calcium of milk, but the body absorbs only about 5% of spinach calcium, against about 30% from milk. Kale, broccoli and bok choy have little oxalate, and their calcium is absorbed about as well as milk's.

How Much Calcium One Day of Food Gives

One day built from foods in the Caltide library, in usual portions, with no supplements and no fortified foods.

The milk, yogurt and cheese alone give 77% of the total. Leave dairy out and the same day needs several other sources to catch up.

Who Is Most Likely to Be Low in Calcium

In US surveys, teenage girls and older adults are among the groups most likely to get less than their RDA. People who avoid dairy because of lactose intolerance, a milk allergy or a vegan diet need other sources such as calcium-set tofu, fortified plant drinks and low-oxalate greens. Women after menopause absorb less calcium and lose more from bone as estrogen falls.

Common Calcium Mistakes

Taking 1,000 mg in one tablet. Absorption is best at 500 mg or less at a time. Two smaller doses, morning and evening, deliver more calcium to the body than one large one.

Counting the compound as calcium. A 1,250 mg calcium carbonate tablet holds about 500 mg of calcium. The calcium figure on the label, not the weight of the salt, is the one to add up.

Forgetting that food counts toward the limit. The upper limit, 2,500 mg a day or 2,000 mg over 50, covers food and supplements together. Someone who eats plenty of dairy and also takes a high-dose tablet can pass it without noticing.