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What is the Best Calcium to Take?


  • Your ability to absorb calcium is based on the dose and form of the nutrient.
  • Calcium carbonate is the most common form of calcium in supplements, but this mineral is poorly absorbed and can cause constipation.
  • Of all the forms of calcium on the market, calcium citrate is one of the most bioavailable forms.


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By Dr. John Neustadt

When most people think about important nutrients for bone health, the two that most often come to mind are calcium and vitamin D. The FDA allows supplement companies to claim that “Adequate calcium and vitamin D throughout life, as part of a well-balanced diet, may reduce the risk of osteoporosis.” 

However, there are many types of calcium on the market. There’s calcium carbonate, calcium citrate, coral calcium, calcium citrate-malate, algae calcium, and more. Which one is best? For the reasons outlined below, I prefer calcium citrate.


When marketing their products, some companies claim that their calcium has superior absorption. Your ability to absorb calcium is based on the dose and form of the nutrient. 

Regardless of the type of calcium, your body can only absorb about 500 mg at a time.1 Therefore, a calcium supplement should not give you more than 500 mg in each serving. 

In addition to the dose, the form of calcium is important. Minerals, like calcium, are always attached to a carrier molecule, such as carbonate, coral calcium, citrate, citrate-malate, or algae calcium. Your body does not absorb all forms equally.

Calcium Carbonate

Calcium carbonate is the most common form of calcium in supplements, but this mineral is poorly absorbed and can cause constipation.2 The calcium used in most clinical trials is calcium carbonate, which accounts for those side effects reported in the studies. 

Absorbing calcium carbonate requires stomach acid to separate the calcium from the carbonate. As we age, however, low stomach acid becomes more common. An estimated 10–21% of people 60-69 years old, 31% of those 70-79 years old, and 37% of those 80 years and older have low or no stomach acid.3 Stomach acid production also decreases if you take acid-blocking medications, have an untreated H. pylori infection, have an autoimmune condition, or are chronically stressed. This decreases the absorption and increases the risk of side effects like abdominal cramping, nausea, and constipation. 

Coral Calcium

As the name implies, coral calcium comes from coral reefs. It is simply calcium carbonate with some magnesium and trace minerals.4 According to a 2020 review article, coral calcium puts a “burden” on the stomach and intestines since the main component is calcium carbonate, which is difficult to absorb.5 Therefore, people taking coral calcium can get the same side effects as those taking calcium carbonate. 

Calcium Citrate

Of all the forms of calcium on the market, calcium citrate is one of the most bioavailable forms. A study of 21 volunteers (4 men and 17 women, ages 22-60) found that a mean of 40.2% of calcium as calcium citrate was absorbed.6 The lowest amount of calcium that was absorbed was 33.5% and the most was 46.9%. The absorption of calcium citrate is approximately 24% higher than calcium carbonate.1 

Even with low stomach acid, calcium citrate is still absorbed better. When stomach acid is low, people absorb 200% more calcium from calcium citrate than calcium carbonate.7 In fact, calcium citrate is the preferred calcium supplement when someone has low stomach acid or is on an acid-blocking medication.1 A clinical trial with 63 postmenopausal women showed that taking calcium citrate for two years preserved bone mineral density in the lumbar spine and leg compared to placebo.8

Additionally, if you struggle with calcium oxalate, cystine, or uric acid kidney stones, calcium citrate is the preferred form since citrate prevents the formation of these kidney stones.9, 10 

Calcium Citrate-Malate

The absorption of calcium citrate-malate is comparable to calcium citrate. In a study with twelve postmenopausal women ages 45-60, calcium citrate-malate has the same absorption as calcium carbonate.11 In a clinical trial with 301 postmenopausal women, calcium citrate-malate also preserved bone density over two years compared to placebo and calcium carbonate.12

Algae Calcium

Calcium from plant sources, such as algae, is also available in dietary supplements. There is not much data on the absorption of calcium from algae. A small study published in 2010 tested the absorption of algal calcium in ten volunteers. The results showed a mean absorption of 23.1%. The amount of calcium absorbed varied widely among volunteers, with as little as 16.7% and up to 29.5% being absorbed.13 

Bone Strength

As I discuss in my article on MK4 vs MK7, just because you absorb something doesn’t mean it’s working. It’s crucial to understand that an absorption study doesn’t give us any information about whether or not a substance is active in the body, if it has any health benefits, or if one form of the nutrient is better than another. For that, we need to look at clinical trial results. 

While most people, including doctors, equate bone mineral density with bone strength, they’re not the same. Research in the 1990s concluded that a bone density test predicts less than half of people who will break a bone. More precisely, in 2008, a study determined that a bone mineral density (BMD) test predicts that only 44% of women with osteoporosis and 21% of men will break a bone.14 

Instead, in clinical trials, the most accurate indicator of bone strength is whether or not someone breaks a bone. In this regard, despite the differences in the absorption of types of calcium, there’s no indication that one type is better than another. 

A review of 26 clinical trials, most of which gave calcium carbonate, concluded that calcium supplements decreased total fractures by 11% and vertebral fractures by 14%. Importantly, the volunteers in these studies were not deficient in calcium and vitamin D when they started taking the supplements.15 

When someone has low calcium and vitamin D, taking these nutrients might be even more helpful. In a multicenter, randomized, placebo-controlled clinical trial with 3270 elderly women (mean age 84) living in nursing homes, researchers evaluated the ability of calcium and vitamin D to promote bone strength. Notably, these women had elevated parathyroid hormone (PTH) and low vitamin D at the start of the study. Low vitamin D causes elevated PTH, so it’s not surprising that supplementing with vitamin D decreased the women’s PTH. 

After 18 months, taking the dietary supplements reduced hip fractures by 43% and all non-vertebral fractures by 32% compared to women taking the placebo.16 After three years, the benefits were not as great, but still excellent, with a 29% reduction in hip fractures and 24% fewer non-vertebral fractures.17 This study was repeated later in a different group of women and showed essentially the same results.18

Calcium Safety

While calcium carbonate and coral calcium cause more side effects than other types of calcium, the safety of calcium supplements depends on how much you’re getting, not which type you’re taking. 

Clinical guidelines from the Bone Health and Osteoporosis Foundation (BHOF) and the American Society for Preventive Cardiology recommend consuming less than 2,000-2,5000 mg of total calcium daily. This is from all sources—diet and dietary supplements—and matches the National Academy of Medicine recommendation.19 The European Food Safety Authority came to a similar conclusion. Its position agrees that people should not consume more than 2,500 mg of calcium per day.20

Importantly, while getting less than 2,000 to 2,500 mg per day is important for avoiding calcium toxicity, there is no evidence that getting more than 1,200 mg daily gives you greater bone health benefits.

How much should you take?

For most people, how much calcium they should get from supplements depends on their diet. American women get about 800 mg of calcium daily from diet, and men get about 1,000 mg daily. For many women, therefore, taking an extra 400 mg as a calcium dietary supplement is enough to reach the US RDA for calcium. However, even if women take 1,000 mg of calcium as a dietary supplement, most will still be below the upper limit of 2,000-2,500 mg set by the National Academy of Medicine. Others, like those who don’t eat dairy, may need more. But you don’t have to eat dairy to get enough calcium. Plants are a great source. 

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1 Straub DA. 2007;22(3):286-96. 

2 Nicar MJ, Pak CY. 1985;61(2):391-3.

3 Hurwitz A, Ruhl C. 1997;278(20):1659-1660. 

4 Marcason W. 2003;103(10):1319. 

5 Xu Y, Ye J, Zhou D, Su L. 2020;10(1):18425.

6 Harvey JA, Zobitz MM, Pak CY. 1988;3(3):253-8. 

7 van der Velde RY, Brouwers JR, Geusens PP, et al. 2014;58

8 Ruml LA, Sakhaee K, Peterson R, et al. 1999;6(6):303-11. 

9 Frassetto L, Kohlstadt I. 2011;84(11):1234-42. 

10 Sakhaee K, Poindexter JR, Griffith CS, 2004;172(3):958-61. 

11 Rzymski P, Pischel I, Conrad F, et al. 2016;242(1):45-50.

12 Dawson-Hughes B, Dallal GE, Krall EA, et al. 1990;323(13):878-83. 

13 Uenishi K, Fujita T, Ishida H, et al. 2010;2(7):752-61. 

14 Liu H, Paige NM, Goldzweig CL. 2008;148(9):685-701. 

15 Bolland MJ, Leung W, Tai V, et al. 2015;351:h4580.

16 Chapuy MC, Arlot ME, Duboeuf F, et al. 1992;327(23):1637-42. 

17 Chapuy MC, Arlot ME, Delmas PD, et al. 1994;308(6936):1081-2. 

18 Chapuy MC, Pamphile R, Paris E, et al. 2002;13(3):257-64. 

19 Kopecky SL, Bauer DC, Gulati M, et al. 2016;165(12):867-868.

20 EFSA Panel on Dietetic Products N, Allergies. Scientific Opinion on the Tolerable Upper Intake Level of calcium. EFSA Journal. 2012;10(7):2814.

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