Calcium
Most diets lack adequate calcium-rich foods.
There are several plant foods with bioavailable sources of calcium.
Required for muscle contraction
Needed for bones and teeth
Helps to regulate blood pressure and pH
Helps nerves to transmit signals
Involved in the release of some hormones
How it works
How much you need each day
The general U.S. population does not consume adequate calcium, and it is a “nutrient of public health concern; ” those at higher risk include 9 to 18-year-olds, [3,6] perimenopausal women, [3] and older adults (70+ years) [3,6]
Persons with diets that lack calcium-rich foods and/or supplements [1,2,5]
Persons with low vitamin D status/intake [2]
Alcoholics (due to low magnesium, which is required for PTH [5] and lowered calcium absorption [1,2])
Post-pubescent, amenorrheic women (e.g., athletes, anorexia) [1]
Persons with increased requirements (e.g., adolescents, older adults) [2]
Populations at risk for deficiency
Soybeans
Tofu made with calcium
Calcium-fortified plant milks and yogurts
Calcium-enriched 100% juice
Kale and other dark leafy greens
Mineral water with > 400 mg calcium per liter
Sources of nutrient
Vitamin D is needed for calcium to be absorbed into the body and to maintain calcium balance in the blood. [2] The higher the amount of calcium ingested at one time, the lower the amount absorbed. [1]
The highest calcium absorption potential occurs in the first few months of life. Breastfed infants absorb about 55 to 60% of calcium from breast milk. Although infant formula typically contains nearly double the amount of calcium than breast milk, calcium absorption averages about 40% in formula-fed babies. [2]
About 30% of calcium is absorbed from dairy and fortified foods, while nearly 60% of calcium is absorbed from certain green vegetables (e.g., broccoli, bok choy, kale). [2] For comparison of equal amounts of food:
Bioavailability
Calcium enters the body by active and passive transport. Active transport is concentrated in the duodenum (accounts for most of the calcium at low to moderate levels), whereas passive transport (most likely with higher doses from supplements) can occur throughout the small intestine. [2] However, in newborns, calcium is most absorbed passively and facilitated by lactose in breast milk. [2]
Calcium is challenging to get from the diet, and intake is often low. Needs increase in adolescents and older adults; thus, recommending a wide variety of calcium-rich or fortified foods or supplements may help meet calcium needs.
High dietary sodium intake is associated with calcium loss. High sodium intake has been associated with additional bone loss in menopausal women. [1,5]
High dietary phosphorus intake may increase calcium loss. With adequate calcium intake, this interaction may not affect bone health. However, those with large phosphorus intakes (e.g., soda) and inadequate calcium intake are at a higher risk for bone loss. [2,5]
High protein intake increases calcium urinary excretion but also increases calcium absorption; thus, there may be no net effect. [2,5] However, the higher acid load from high protein intake may increase the risk of bone loss [2] and kidney stones [5] in some individuals. Citrate, found in fruits and vegetables, may aid in pH balance and reduce the risk of calcium loss from bone. [1]
Calcium may interfere with the absorption of zinc, magnesium, and iron. [2] Thus, it may be best to take these supplements separately from foods/supplements rich in calcium. [5]
Breast milk calcium is taken from maternal skeletal stores. Although bone mineral density can decrease during exclusive breastfeeding between 2 and 6 months postpartum, it normally returns to baseline during 6 to 12 months post-weaning (including in adolescent mothers); a history of lactation does not increase the risk of osteoporosis or low bone mineral density. [2]
Breast milk calcium concentrations do not appear to be affected by dietary intake but have been shown to be associated with genetics. [2]
Notes
1. Calcium. Fact Sheet for Health Professionals. Office of Dietary Supplements. National Institutes of Health. Updated Mar 2020. Accessed Sept 2020. https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/
2. Institute of Medicine 2011. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press. https://doi.org/10.17226/13050.
3. U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 9th Edition. December 2020. Available at https://DietaryGuidelines.gov.
4. FoodData Central Database. United States Department of Agriculture. Accessed Oct 2021. https://fdc.nal.usda.gov.
5. Calcium. Linus Pauling Institute Micronutrient Information Center, Oregon State University. Updated Sept 2017. Accessed Sept 2020. https://lpi.oregonstate.edu/mic/minerals/calcium
6. Weaver C, Peacock M. Calcium. Adv Nutr .2019;10:546–548; doi: https://doi.org/10.1093/advances/nmy086.
7. WHO Recommendation: Calcium supplementation during pregnancy for prevention of pre-eclampsia and its complications. 2018. ISBN: 978 92 4 155045 1. https://apps.who.int/iris/bitstream/handle/10665/277235/9789241550451-eng.pdf
8. Bazzano AN, Green E, Madison, BMJ Open 2016;6:e009189. doi:10.1136/bmjopen-2015-009189
References
How much is too much
Increased risk of fractures, weakened bones, osteoporosis, and rickets (although more often from lack of vitamin D).
Note that hypocalcemia results primarily from health conditions (renal failure, medications) rather than calcium intake due to calcium homeostatic controls.
Deficiency signs and symptoms
Changes in dietary calcium typically do not affect blood levels because blood levels are tightly regulated. Thus, assessing dietary intake (e.g., diet recall) for calcium-rich foods and supplements is a common way to estimate adequacy. Additionally, bone mineral density, although not a direct measure of calcium, may be measured.
Measures of adequate status
Example Food Sources
* Not all products are listed; check labels for nutrient content. Values are general estimates and may be rounded. Values not provided by referenced sources are from products of multiple manufacturers.
High-dose supplement intake may increase the risk of kidney stones [1,6]
Pregnancy causes increased absorption of calcium, increasing the risk of hypercalciuric incidence and risk of kidney stones [2]
High intake of calcium supplements with large amounts of antacids, absorbable alkalis (e.g., baking soda), and milk may cause hypercalcemia (e.g., calcium-alkali syndrome) [5,6]
Increased risk of reduced zinc and iron absorption [6]
Although high intakes of calcium have the potential to cause hypercalcemia, it is most often due to primary hyperparathyroidism or malignancy. Hypercalcemia may cause renal insufficiency, [1] soft tissue and vascular calcification, [1,2,6] hypercalciuria, [1,6] and kidney stones. [1]
Toxicity signs and symptoms
When taken together, calcium may decrease the absorption of (take meds 2 hours before or 4-6 hours after calcium [5]):
Tetracycline and quinolone classes of antibiotics [1,5]
Bisphosphonates [1,5]
Sotalol (β-blocker) [5]
Levothyroxine [1,5]
Phenytoin [1]
Tiludronate disodium [1]
The following can decrease the absorption of calcium:
Mineral oil [1]
Stimulate laxatives [1]
H2 blockers (e.g., cimetidine) (with calcium carbonate and phosphates) [5]
Proton-pump inhibitors (e.g., omeprazole) (with calcium carbonate and phosphates) [5]
Glucocorticoids (e.g., prednisone) can cause calcium loss when used for months [1]
Aluminum and magnesium antacids increase calcium excretion in urine [1]
Calcium supplements may decrease blood levels of several antiretroviral medications (take separately 2 hours apart): [5]
dolutegravir (Tivicay)
elvitegravir (Vitekta)
raltegravir (lsentress)
Thiazide diuretics can interact with calcium supplements, increasing the risk of high blood calcium levels. [1,5]
Digoxin and high doses of calcium supplements may increase the risk of irregular heart rhythms [5]
Lithium may increase the risk of hypercalcemia [5]
Topical vitamin D analog cream, calcipotriene (with calcium carbonate and phosphate supplements), increases the risk of hypercalcemia [5]