Is that Food REALLY a Good Source of Calcium?

I get frustrated when I see foods like spinach listed as good sources of calcium, even in nutrition textbooks. Here’s the confusion. One cup of cooked spinach has about 245 milligrams (mg) of calcium and one cup of milk has about 300 mg. So far, spinach sounds pretty good. The problem is that your body can only take up (absorb) about 5 percent of the calcium in spinach compared to over 30 percent of the calcium in milk.

Let’s do the math:

0.05 X 245 = 12 mg calcium absorbed per cup of spinach
0.32 X 300 = 96 mg calcium absorbed per cup of milk

The bottom line:

1) You absorb 8 times as much calcium from a cup of milk as you do from a cup of cooked spinach.
2) You must consume 8 cups of cooked spinach to get the same amount of calcium you get from one cup of milk.

Don’t get me wrong. Spinach is a great food for many reasons. But calcium is not one of them. Similar confusion exists for iron. Sorry Popeye, you can only absorb about one percent of the iron in spinach.

Bones Need To Get Enough Calcium

Providing your bones with enough calcium each day is one of the major components of a bone health regimen. For each day that you don’t absorb enough calcium to meet your needs, there is a net loss of calcium from your bones. If you fail to meet your needs day after day, there will be a steady decline in bone density and your risk of osteoporosis increases over time.

How Calcium Gets To the Bones

Getting calcium from your food to your bones requires a series of steps that involve digestion of food and freeing calcium from various food components that bind it. If the calcium remains bound up, it never gets into the body. The calcium just passes through the intestinal tract and your bones never see it.

When calcium can break free from other components in food, it has a good chance of being taken up (absorbed) by the cells along the intestinal tract. These intestinal cells can then release the calcium into the blood. Typically, about 30 percent of the calcium in food is taken up into the blood. However, the amount of calcium absorbed depends on the type of food consumed. The amount of calcium absorbed from a food can range from about 5 to 50 percent of the quantity in the food.

How To Get Enough Calcium To Your Skeleton

Don’t ask, “How much calcium is in that food?” Ask, “How much calcium can I absorb from that food?” In the U.S., the recommended daily calcium intake for adults (age19 to 50 years) is 1000 mg. This recommendation is based on the assumption that people can absorb about 300 mg of calcium from a diet that contains 1000 mg of calcium. This recommendation also assumes that the diet is composed of foods common to the U.S. diet, including milk products. If 300 mg of calcium is absorbed from the daily diet, it is likely enough calcium to replace all the calcium lost from the body that day. Consequently, there would be no net loss of calcium from the bones.

What Makes a Food a Good Source of Calcium?

For a food to be labeled a “good source” of calcium, U.S. food labeling law requires the legal serving size to contain at least 100 mg of calcium. That’s one tenth of the total daily recommended calcium intake (adults 19 to 50). However, there is a better way to think about this that takes into account how much calcium you can ABSORB from a food.

Using available information about the amount of calcium that is absorbed from various foods (fractional absorption), it is possible to determine “good sources” of calcium based on the ABSORBABLE calcium you need each day. So, if you should absorb about 300 mg of calcium each day, a “good source” should provide 10 percent of that or 30 mg of ABSORBABLE calcium in a customarily consumed amount of the food.

The table below lists some good food sources of absorbable calcium and shows what percentage of the calcium is typically absorbed from each food.

Some Good Food Sources of Absorbable Calcium

Type of Food & Amount	Calcium per Serving (mg)	Percent Absorbed	Amount Absorbed per Serving (mg)
Plain lowfat yogurt (1 cup)	448	32	144
Fruit flavor lofat yogurt (1 cup)	338	32	108
Milk, skim (1 cup)	316	32	101
Milk, 2% fat (1 cup)	285	32	91
Orange juice, calcium fortified (1 cup)	300	25	75
Cheese, mozzarella, part skim (1.5 ounces)	310	32	100
Soymilk, calcium fortified (1 cup)	300	24	72
Chinese Mustard Greens, cooked (0.5 cup)	212	40	85
Chinese Cabbage, cooked (0.5 cup)	79	54	43
Sardines, canned in oil (3 ounces)	324	27	88

Source

Titchenal, C. Alan and Joannie Dobbs. “A system to assess the quality of food sources of calcium.” Journal of Food Composition and Analysis (in press, corrected proof)) Available online 7 September 2006.