R.D

Calcium, copper, iron, magnesium, and zinc compete for absorption sites;

taking a single-mineral supplement may lessen the absorption of other

minerals. Thus, for example, calcium supplements should be taken between

meals by people who want to maximize their absorption of iron, magnesium,

and zinc from food.

• When a smaller quantity of calcium, iron, or zinc is consumed, the body

absorbs a higher percentage of the dose, compared to its absorption from a

large quantity. Thus, two 250 mg calcium supplements taken at different

times during the day deliver more calcium than a single 500 mg dose.

• Foods or beverages that accompany mineral intakes can affect absorption

negatively or positively. For example, the amount of iron absorbed can vary

sixfold or more, depending on whether factors that inhibit absorption (such

as cow’s milk, black tea, and wheat bran) or factors that enhance absorption

(such as vitamin C–rich foods) are part of a meal.

Refining whole foods—particularly grains—significantly impacts their mineral

content (for more information, see . Minerals are soluble in water; if cooking water is discarded, they can be lost. Steaming leads to greater nutrient

Vegans often soak, sprout, ferment, blend, or juice plant foods, which

significantly increases mineral availability. In nature, sprouting releases minerals

to support growing seedlings. When seeds, nuts, mung beans, lentils, and whole

grains are soaked or sprouted for food, the naturally present phytases (phytate-

splitting enzymes) become active, and the bound minerals are released. When

plant foods are juiced or blended, their cell walls are broken, releasing phytases

and increasing mineral bioavailability. 9

When plant foods are

fermented, as when soy is made into tempeh or miso, more than half the phytates

are broken down. Yeast has a similar effect on wheat; plus, with the acidity that

results from sourdough fermentation, phytate breakdown increases to as much as

97 percent. These common food preparation processes release bound calcium,

iron, magnesium, and zinc for absorption

OXALATES

Oxalates are tightly bound combinations of oxalic acid and minerals (calcium,

iron, magnesium, or zinc) that resist breakdown during food preparation and

digestion. Amaranth, beets, beet greens, cassava, chives, chocolate soy milk,

lamb’s-quarters, miso, parsley, purslane, sorrel, spinach, star fruit, Swiss chard,

and whole sesame seeds are among the most oxalate-dense plant foods. (For

specific amounts, search online for the US Department of Agriculture list,

“Oxalic Acid Content of Selected Vegetables.”

Of the minerals mentioned above, calcium is most tightly bound by oxalic

acid. For example, although spinach is calcium-rich, only about 5 percent of its

calcium may be absorbed by the body; the rest is transported out in the feces, bound by oxalic acid. Much of the iron in spinach can still be absorbed, because

the oxalate is primarily bound with calcium. Despite mineral binding, such

greens are still valuable dietary additions because they provide abundant folate,

vitamin K, beta-carotene, and numerous other protective nutrients and

phytochemicals unaffected by oxalates

The RDA is the average daily intake that meets a nutrient requirement of 97 to 98 percent of healthy individuals. Al is a value listed when there’s insufficient research to determine an RDA; it can be viewed as a sort of “best guess.” UL is the highest level of intake that won’t pose a risk of adverse effects for most healthy people when used indefinitely on a daily basis without medical supervision.

To see RDAs and Als for various age groups and for women during pregnancy and lactation, see

. For further information online, see Resources on38

RDA—Recommended Dietary Allowance

UL—Tolerable Upper Intake Level

AI—Adequate Intake

DV—Daily Value

When foods are soaked and the soaking water discarded, some oxalates are

disposed of along with bound minerals. Research has shown that boiling can

reduce the total oxalate content in spinach by 60 percent and in lentils by 16

The body achieves peak bone mass by about age 28.

Calcium also has crucial nonstructural functions, though these involve just 1

percent of the calcium in the body. These include roles in blood coagulation

(required after injury), muscle relaxation (without calcium, muscles remain tight

after contracting), nerve cell transmissions, and the regulation of cell metabolism

(such as the storage of energy as glycogen). Maintaining calcium intakes at

recommended levels also may help to prevent hypertension.

Calcium is vital for survival, and the body must keep the calcium levels in

blood and in interstitial fluid within a specific and narrow range.

Recommended Intake

The calcium RDA for adults is 1,000 mg per day, and 1,200 mg per day for

women over 50 and men over 70. The UL has been set at 2,500 mg per day to

age 50 and 2,000 mg for those older than 50. 20