Calcium
Most diets lack adequate calcium-rich foods.
There are several plant foods with bioavailable sources of calcium.
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Required for muscle contraction
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Needed for bones and teeth
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Helps to regulate blood pressure and pH
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Helps nerves to transmit signals
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Involved in the release of some hormones
How it works
Recommended daily amount
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Soybeans
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Tofu made with calcium
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Calcium-fortified plant milks and yogurts
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Calcium-enriched 100% juice
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Kale and other dark leafy greens
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Mineral water with > 400 mg calcium per liter
Sources of nutrient
Daily upper limits
Bioavailability
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:
Calcium absorption may lower with foods rich in oxalates (e.g., spinach) or phytates (e.g., nuts, grains), but not always. [1] The effect of these compounds on calcium is highly variable, and food properties and combinations (i.e., food matrix, digestion, and effect of the total meal) influence the amount absorbed. [2]
For example, spinach can reduce calcium absorption from milk, but wheat (except wheat germ) does not. [2] The availability of calcium in amaranth is about 5%, while in sesame seeds, it is 20%. [2] Furthermore, yeast increases calcium availability due to an enzyme that breaks down phytate in grains. [5]
Overall, for people who eat a variety of food, these oxalate and phytate interactions “likely have little nutritional consequence,” and the recommended daily intake values have taken these differences of a mixed diet into account. [1]
Calcium supplements
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Absorption is highest when the dose is 500 mg or less. [1]
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Calcium carbonate is best taken with food since it needs stomach acid, whereas calcium citrate may be taken with or without food. [1]
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Calcium citrate has been shown in some, but not all studies to be more bioavailable than calcium carbonate supplements. [2]
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Calcium may interfere with zinc, iron, and magnesium; it is best to take supplements separately. [2]
Examples of nutrient-rich foods
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
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
Deficiency signs and symptoms
Increased risk of fractures, weakened bones, osteoporosis, and rickets (although more often from lack of vitamin D).
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
The content provided is for informational purposes only and may not be an exhaustive list of potential interactions.
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]
Potential drug-nutrient interactions
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There is a higher risk for pre-eclampsia in persons with low calcium intake. [7]
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The WHO recommends reducing the risk of pre-eclampsia for pregnant women with low dietary calcium intake with 1.5 – 2 grams of oral elemental calcium divided into 3 doses, preferably taken at mealtimes. [7]
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The American College of Obstetrics and Gynecologists does not make specific recommendations for calcium for hypertensive disorders during pregnancy but notes that calcium may be useful in some populations. [8]
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The Society of Obstetricians and Gynaecologists of Canada and the Society of Obstetric Medicine of Australia and New Zealand recommend at least 1 gram of calcium per day in women at risk for pre-eclampsia. [8]
Calcium and pre-eclampsia
Heading 3
Notes
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