Where the calcium from a supplement goes

The gut absorbs only part of a calcium tablet.^[1] The blood carries that part, and bone and the kidneys take calcium from the blood.^[2] With a meal, healthy women absorbed 28.6% of a 500 mg dose.^[1] Bone holds 98% of the calcium in the body.^[3] Joint cartilage has no blood vessels.^[4] In trials of calcium supplements in people over 50, bone density rose by 0.7 to 1.8%. The gain at 2 years and later was about the same as at 1 year.^[5]

Where does the rest of the dose go?

Most of a 500 mg dose never reaches the blood.^[1] In the model, by 24 hours about 350 mg has left with stool. Of the 150 mg absorbed, about 97 mg extra leaves in urine and about 50 mg stays in bone. The model puts 0 mg in joint cartilage. These are model values.^[2] The calcium stored in the body depends on the calcium in food and on how much of it the gut absorbs. It also depends on how much the kidneys pass into urine and on how bone is rebuilt.^[2]

Does the size of one dose matter?

Yes. The share absorbed falls as the dose grows. The body absorbs about 36% of a 300 mg dose and 28% of a 1000 mg dose. Absorption from supplements is highest with doses of 500 mg or less.^[3] The same holds over a whole day. Middle-aged women absorbed about 45% of their calcium at intakes near 200 mg a day, and about 15% at intakes above 2000 mg a day.^[11]

In the model, the gut absorbs 38% of 250 mg and 23.5% of 1000 mg. For 1000 mg, this is less than the 28% that the NIH gives.^[3]

Calcium supplements are absorbed better with food. Calcium carbonate is 40% calcium by weight and calcium citrate 21%, and a US supplement label lists the amount of calcium itself. Calcium supplements can cause gas, bloating or constipation in some people. Smaller doses more often, or doses with meals, can ease these effects.^[3]

What does one dose do to blood calcium?

Very little. The body holds blood calcium in a narrow range, so a blood test of calcium does not show whether a person gets enough calcium.^[3] After a 1 g dose, ionized calcium rose more in women who absorbed calcium normally than in women who absorbed it poorly.^[9]

In the model, 500 mg raises ionized calcium from 1.220 to 1.247 mmol/L, a rise of about 2%. PTH falls to 58% of its usual level at 3.7 hours and is almost back, at 96%, by 24 hours. These are model values.^[9]

The gut needs vitamin D for active transport of calcium, and low vitamin D status lowers absorption.^[3] In the model, low vitamin D cuts the share absorbed from 500 mg from 30% to 19.5%, or 97 mg. A 1-year trial gave vitamin D or a placebo to 163 women after menopause with low vitamin D. Across blood levels of 25-hydroxyvitamin D from 50 to 165 nmol/L, the share absorbed from a 100 mg calcium dose rose only from 52% to 58%. The authors wrote that their results question the value of adding vitamin D for absorption, except when this level is very low, under 25 nmol/L.^[12]

What does a daily dose do to bone density?

It raises it a little, once. In the meta-analysis, the gain was similar with 1000 mg a day or more and with less, and with or without vitamin D. It was also similar in people who got less or more than 800 mg of calcium a day from food. More calcium from food raised bone density about as much as supplements did.^[5] The authors concluded that such small gains are unlikely to lower the risk of fractures by a clinically significant amount.^[5]

In a trial, 208 African-American women after menopause got more than 1200 mg of calcium a day, with or without vitamin D. Their bone density rose during the first year and then fell. The authors estimated one cycle of bone rebuilding at about 9 months.^[13]

In the model, the gain is larger when food gives little calcium: +1.24% at 12 months with 400 mg a day from food, and +0.84% with 1000 mg. The trials found no such difference between people who got less or more than 800 mg a day from food.^[5] Both lines in the model also fall by 0.5% a year, a loss with age that goes on. After menopause, women lose about 1% of their bone density a year on average.^[3]

Does calcium help the knee joint?

The body stores its calcium in bone: 98% of it is in the bones.^[3] In one open-label trial, 38 women around menopause with low bone density took 1200 mg of calcium carbonate a day for 6 months as the comparison group. Their back and knee pain changed little, and the change was not significant. In the other group, which took 1660 mg a day of a calcium product made from bone (ossein-hydroxyapatite complex), pain fell significantly.^[14]

Among the benefits of calcium, Joint comfort is listed with 14 studies. Its other top studies did not test plain calcium supplements:

• monthly knee injections of sodium bicarbonate with calcium gluconate, in 74 people with knee osteoarthritis^[15]
• an aragonite-based implant placed in cartilage and bone defects of the knee, in 251 people^[16]
• calcium fructoborate, 110 mg twice a day for 14 days, in 60 people with knee discomfort^[17]
• needling with shock-wave therapy, against shock-wave therapy alone, for calcium deposits in shoulder tendons, in 80 people^[18]

Who should be careful with calcium supplements?

The US recommended intake for adults is 1000 to 1200 mg a day, depending on age and sex. The upper limit for adults is 2500 mg a day up to age 50 and 2000 mg a day from age 51.^[3] More calcium from supplements might raise the risk of kidney stones, although two later reviews found no such link.^[3]

Calcium supplements can lower the absorption of some medicines:^[3]

• levothyroxine, a thyroid hormone: take it at least 4 hours apart from calcium carbonate^[3]
• quinolone antibiotics such as ciprofloxacin: take them 2 hours before or 2 hours after calcium^[3]
• dolutegravir, an HIV medicine: take it 2 hours before or 6 hours after calcium^[3]

Lithium together with calcium supplements may raise the risk of too much calcium in the blood. People who take these or other medicines every day should talk with a doctor or pharmacist about calcium.^[3]

In short

• With a meal, healthy women absorbed 28.6% of a 500 mg calcium dose, and a larger share of smaller doses.^[1]
• Bone holds 98% of the calcium in the body.^[3]
• Joint cartilage has no blood vessels.^[4]
• After a 1 g dose, ionized calcium in the blood rose and PTH fell, and both levelled off at 3 to 4 hours.^[9]
• In people over 50, calcium supplements raised bone density by 0.7 to 1.8%, and the gain at 2 years and later was about the same as at 1 year.^[5]
• In one trial, 1200 mg of calcium carbonate a day for 6 months did not significantly change back and knee pain.^[14]
• The upper limit for adults is 2000 to 2500 mg of calcium a day, depending on age.^[3]

What this figure leaves out

The knee comes from the Human Reference Atlas, version 1.3: the left knee of a woman, shared under the CC BY 4.0 licence. The parts used are the femur, tibia, fibula, kneecap, meniscus and joint cartilage.^[19] The atlas has no ligaments, no joint capsule and no cartilage on the tibia. The cartilage in the figure is built from the surface of the atlas with a thickness of 2.3 mm. Adult joint cartilage is 2 to 4 mm thick; the 2.3 mm was not measured on this knee.^[8]

The motion and the numbers come from a small balance model of one dose over 24 hours and of a daily dose over 24 months. Its share absorbed is set to about 30% of 500 mg with a meal.^[1] Its blood calcium and PTH change most at 3 to 4 hours, as after a 1 g dose in women.^[9] Its step in bone density lies within the 0.7 to 1.8% of the trials.^[5]

Each orange dot stands for 10 mg of calcium. The dots are a teaching aid, not single atoms. The model leaves out differences between people, such as age, menopause and the form of calcium.^[3] For education; not for diagnosis.