Dietary calcium supports the health of your nerves, muscles, heart and bones. Several factors influence intestinal calcium absorption, including your vitamin D level; the activity of calcium regulatory hormones; the amount of calcium in your diet; and the chemical composition of the foods you eat. Grapefruit contains a chemical that limits the absorption of calcium from the fruit. However, grapefruit does not typically interfere with intestinal absorption of calcium from other foods.
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Oxalates and Calcium Absorption
Oxalates are chemicals commonly found in high concentrations in many plants. Oxalates form strong bonds with sodium, potassium and calcium, yielding oxalate salts. These salts resist breakdown in your digestive system, making the bound calcium, sodium or potassium unavailable for absorption. Grapefruit contains a moderate amount of oxalate, making it a relatively poor source of absorbable calcium. However, the amount of oxalate in grapefruit is unlikely to disrupt intestinal calcium absorption from other foods consumed with the fruit.
Potentially Interfering Foods
Although grapefruit does not contain enough oxalate to interfere with calcium absorption from a meal, other foods do. Spinach, rhubarb, Swiss chard, turnips, rutabagas, tofu, peanut butter, sweet potatoes, tangerines, blueberries and cocoa contain high concentrations of oxalates and may significantly reduce intestinal calcium absorption. Foods that contain high concentrations of another calcium-binding chemical, phytic acid, may also reduce dietary calcium absorption. Foods with large amounts of phytic acid include dry beans, tofu, nuts, seeds and products that contain wheat bran.
Calcium in Grapefruit
Grapefruit contains many healthful nutrients, but a relatively low concentration of calcium. A cup of fresh grapefruit contains less than 1 percent of the recommended daily allowance of calcium for adults. Similarly, grapefruit juice contains less than 1 percent of the RDA for adults. In general, eating grapefruit is unlikely to significantly contribute to your daily calcium intake.
Meeting Your Calcium Needs
The RDA for calcium takes into account the effects of various foods on your overall absorption of this nutrient. Therefore, if you are eating a healthful diet that includes the recommended servings from each of the food groups, you do not typically need to worry about food interactions. Examples of healthful, calcium-rich foods to incorporate into your diet include skim milk, nonfat cheese and yogurt, canned salmon and calcium-fortified cereals without added sugar. Because calcium absorption depends on an adequate level of vitamin D in your body, be sure to include vitamin D-rich foods in your diet. The RDA for calcium is 1,000 milligrams for adults through age 50. For women older than 50 and men older than age 70, the RDA for calcium is 1,200 milligrams.