Physiological Functions

Calcium is a primary structural constituent of the skeleton, but it is also widely distributed in soft tissue where it is involved in neuromuscular, enzymatic, hormonal, and other metabolic activity. The skeleton serves as a reservoir of calcium and other minerals. Labile reserves consist of calcium ions loosely held by electrostatic attraction to bone surface anions. These reserves are in dynamic equilibrium with ionized calcium in plasma. Labile reserves are continuously in flux, declining as tissue demand draws calcium from plasma, and subsequently replenished when calcium becomes available from intestinal absorption. Stable reserves consist of calcium in the mineral matrix of bone. Only when labile reserves are fully depleted will stable reserves be tapped to maintain plasma calcium levels. Release of calcium from stable reserves involves accelerated rates of bone resorption of bone in response to increased osteoclastic activity invoked by parathyroid hormone and vitamin D.

Just 1% of the total body pool of calcium is utilized to support nerve transmission, muscle contraction (including normal heart rhythm), blood clotting, and regulation of enzyme and hormone activities. Membrane calcium transport systems are involved in regulation of cellular osmolarity and peripheral vascular resistance. Additionally, this mineral assists in transport of nutrients and other substances across cell membranes and is required for binding of intrinsic factor to ileal receptors for absorption of vitamin B12.

Deficiency

Calcium deficiency is not associated with clinical symptoms until extensive bone mass is lost. When skeletal calcium reserves are depleted, minimal stress can promote bone fracture. Since calcium deficiency cannot be detected by clinical symptoms until it is advanced, assessment of calcium balance can provide information about the potential for calcium losses in the early stages when corrective action can be taken to prevent significant loss of skeletal mass. Calcium balance can be ascertained using dietary recall to estimate intake and identification of lifestyle and other factors that are known to influence urinary calcium losses. These factors are summarized below.

Increase Excretion:
High intakes of sodium and phoshates, excess alcohol consumption and cigarette smoking.

Decrease Excretion:
Adequate vitamin D and magnesium status.

Inadequate calcium intakes have also been associated with increased risk of pre-eclampsia, hypertension, and colon cancer.

Toxicity

In individuals who are prone to renal stone formation, excess calcium intake (usually in supplement form at doses > 2500 mg/day) may promote development of renal calculi. High calcium intakes may also cause constipation and interfere with absorption of iron and magnesium.

The upper limit of safety for calcium established by the Food and Nutrition Board of the Institute of Medicine is approximately 2,500 mg daily for adults. See table below for age- and gender specific guidelines.

		Calcium Tolerable
		Upper Intake Levels
		Life Stage	Calcium mg/day
		Infants
		     0-6 mo	N/A
		     7-12 mo	N/A
		Children
		     1-3 y	2500
		     4-8 y	2500
		Males, Females
		     9-13 y	2500
		     14-18 y	2500
		     19-70 y	2500
		     >70 y	2500
		Pregnancy
		     < 18 y	2500
		     19-50 y 	2500
		Lactation
		     < 18 y	2800
		     19-50 y	3000
		

Requirements

The Daily Reference Intakes (DRI) for calcium are shown in the table below.

		Calcium Requirements
		Daily Reference Intakes
		Life Stage	Calcium mg/day
		Infants
		     0-6 mo	210
		     7-12 mo	270
		Children
		     1-3 y	500
		     4-8 y	800
		Males
		     9-13 y	1300
		     14-18 y	1300
		     19-30 y	1000
		     31-50 y	1000
		     51-70 y	1200
		     >70 y	1200
		Females
		     9-13 y		1300
		     14-18 y 	1300
		     19-30 y	1000
		     31-50 y	1000
		     51-70 y	1200
		     >70 1200
		Pregnancy
		     <18 y 1300
		     19-30 y 1000
			31-50 y 1000
		Lactation
			<18 y 1300
			19-30 y	1000
			31-50 y 1000
		

Dietary Sources

Milk and other dairy products are the richest sources of calcium. Milk is an optimal calcium source because it is also fortified with vitamin D and also provides phosphate or sodium in amounts which favor calcium retention. Calcium-fortified orange juice, tofu and other soybean products, dried beans (pinto, navy, black, red, white), collard or mustard greens, and nuts are good non-dairy sources of dietary calcium. See table below for a detailed listing of dietary calcium sources.

Dietary Sources of Calcium
FOOD	Calcium (mg)
Yogurt, low fat, with fruit, 1 cup	448
Salmon, canned, with bones, 3.5 oz	380
Molasses, blackstrap, 2 Tbl	350
Milk, skim, 1 cup	300
Ice milk, soft-serve, 1 cup	274
Cheese, Swiss, 1 oz	272
Yogurt, frozen, 1 cup	240
Sardines, 3.5 oz with bones	240
Cheese, cheddar, 1 oz	204
Cheese, cheddar, 1 oz	204
Ice cream, vanilla, 1 cup	176
Rhubarb, cooked, 1/2 cup	174
Cheese, cottage, 2% fat, 1 cup	155
Spinach, frozen, cooked, 1/2 cup	140
Tofu, regular, 1/2 cup	130
Soynuts, 1/2 cup	119
Collard greens, 1/2 cup	120
Almonds, 1/2 cup	92
Ice cream, vanilla, regular, 1/2 cup	85
Navy or baked beans, 1/2 cup	64
Mustard greens, 1/2 cup	51
Orange, 1 medium	52
Okra, 1/2 cup	50
Halibut, baked, 3 oz	51
Kale, fresh, cooked, 1/2 cup	47
Broccoli, cooked from fresh, 1/2 cup	42