Unit 9 – Vitamins and Minerals Part 2

9.3 Calcium: Critical for Bones and Throughout the Body

As discussed on the previous page, bones are comprised of two components:  minerals and protein.  Other minerals, including magnesium, fluoride, sodium, and potassium, play supporting roles. This page focuses on calcium, and we’ll cover potassium, magnesium, and fluoride on the following page.

Calcium Functions and Regulation

Calcium is the most abundant mineral in the body. Most of the body’s calcium—more than 99%—is stored in bone, which is important for bone strength and structure. The remaining 1% of the body’s calcium is found in the blood and soft tissues, but it is here that calcium performs its most critical functions. For example, calcium is required to transmit every nerve impulse and electrical signal from nerve cells. It’s also required for every cycle of muscle contraction and relaxation.  Calcium also plays vital roles in blood pressure regulation, blood clotting, enzyme activation, hormone secretion, and signaling between cells.1

The body rigorously controls blood calcium levels in a very tight range. The many roles of calcium around the body are critical to daily survival, so maintaining homeostasis, or a steady state, of blood calcium levels, is a high priority in the body. If blood calcium drops, your body initiates several mechanisms to restore homeostasis, including drawing calcium from the bone. While the calcium stored in bone is essential for bone’s long-term strength and structure, it also serves as a calcium reserve that can be drawn upon to support the vital functions of calcium in the body if blood calcium drops too low. Long-term consequences of inadequate dietary calcium mean you continuously draw down the calcium stores in your bones, resulting in declining bone mineral density and increased risk of fracture.

Calcium requirements are highest for children and adolescents, who are growing and building bones, and for older adults, who are losing bone density. The RDA for calcium for children 9 to 13 years old and teens 14 to 18 is 1,300 milligrams per day. The RDA for adults is 1,000 mg per day but increases to 1,200 mg per day for women ages 51 and up and for men ages 71 and older.

Dietary Sources of Calcium

In the typical American diet, calcium is obtained mostly from dairy products.  One cup of milk contains approximately 300 milligrams of calcium, which is about a third of the RDA for calcium for most adults.  Foods fortified with calcium, such as cereals, plant-based beverages, and orange juice, also provide one-third or greater of the calcium RDA. Smaller amounts of calcium are naturally present in plant-based foods such as legumes, leafy greens, and nuts. With careful planning, adequate calcium can be obtained from non-dairy sources.

Bar graph showing dietary sources of calcium compared with the RDA adults of 1,000 mg. Top sources include cheese, tofu, yogurt, fortified orange juice and soymilk, milk, canned salmon with bones, and fortified breakfast cereals. Food sources pictured include cheese, milk, soymilk, yogurt with almonds, soybeans, and chia seeds.

Figure 9.4. Dietary sources of calcium. Examples of good sources pictured include cheese, milk, fortified soymilk, yogurt with berries, edamame (immature soybeans), and chia seeds. Source: NIH Office of Dietary Supplements and Dietary Guidelines for Americans, 2015-2020.

Calcium bioavailability, or the amount of dietary calcium absorbed from the intestine into the bloodstream, can vary significantly. In general, calcium absorption is highest in infants and young children—who need relatively high amounts of calcium for building bone—and declines with age. With higher calcium intake, especially from supplements, bioavailability decreases to prevent excessive calcium absorption. Some chemical components of plant foods, including phytic acid (found in whole grains, beans, seeds, soy, and nuts) and oxalic acid (found in spinach, collard greens, sweet potatoes, rhubarb, and beans), bind to calcium and reduce bioavailability. Despite reduced absorption, these foods can still provide a significant amount of calcium.

DID YOU KNOW?  It takes 3 cups of milk to make  1 cup of Greek yogurt.  (Type a nutrient in the box to compare the  amounts in the dairy products.)

Comparison of Nutrients in Skim Milk and Greek Yogurt

Nutrient 1 Cup Skim Milk 3 Cups Skim Milk 1 Cup Greek Yogurt
(From Skim Milk)
Energy (kcal) 84 252 133
Protein (g) 8.26 24.78 23.1
Carbohydrate (g) 12.47 37.41 8.2
Calcium (mg) 299 897 250
Magnesium (mg) 27 81 25
Potassium (mg) 382 1146 320
Riboflavin (mg) 0.45 1.34 0.63
Vitamin D IU 115 445 0
Vitamin B-12 (mcg) 1.23 3.69 1.70
Sodium (mg) 103 309 82

It takes 3 cups of milk to make 1 cup of Greek yogurt. Use the table to compare the differences. (Data from USDA's Food Data Central as of February 22, 2022.)


Calcium Deficiency and Toxicity

A long-term calcium deficiency may result from a continuous draw of calcium stores from the bone. Osteoporosis significantly increases a person’s risk of fractures. Nutrition surveys in the United States show that groups at greatest risk of dietary calcium inadequacy include adolescents and older adults, especially female teens and older women.1

Too much calcium can also cause problems, although this is rarely caused by excessive intake of calcium from foods. High calcium intake from supplements has also been associated with an increased risk of kidney stones and, in some studies, an increased risk of cardiovascular disease. It can also cause constipation.1





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Introduction to Nutrition and Wellness Copyright © 2022 by Janet Colson; Sandra Poirier; and Yvonne Dadson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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