One Out, One In #5: calcium carbonate megadose → food calcium

One you drop. One you take instead.

3. A daily 1,000 mg carbonate swallow is a blunt tool: you absorb a smaller fraction, and you get more gas and constipation.

2. The big trial did not show a hip-fracture win in the intention-to-treat result. Kidney stones went up.

1. Count food first. Use D3 only if vitamin D is actually low. K2 is a research footnote, not the IN.

Calcium from food isn't the same thing as a bolus tablet. The NIH Office of Dietary Supplements (NIH ODS) says absorption from supplements is highest at ≤500 mg elemental per dose. About 36% of a 300 mg dose is absorbed, compared with about 28% of a 1,000 mg dose. The extra milligrams are partly wasted, then sit in the gut.

Carbonate is 40% elemental calcium. It wants a meal, especially if stomach acid is low. Citrate is 21% elemental calcium and less acid-dependent. Carbonate causes more gas, bloating and constipation. The label shows elemental calcium, not the weight of the salt.

The Australian products we checked:

The WHI study (Jackson 2006) followed 36,282 postmenopausal women taking 1,000 mg elemental carbonate + 400 IU D3 for about seven years. Hip bone mineral density (BMD) rose a little. Hip-fracture benefit was not significant in the intention-to-treat (ITT) result (HR 0.88). The adherent subgroup looked better, but that's a footnote, not the result. Kidney stones increased (HR 1.17).

The USPSTF's 2018 final recommendation is still current: daily doses of ≤1,000 mg calcium + ≤400 IU D don't prevent fractures in community-dwelling postmenopausal women (Grade D). A December 2024 draft is harsher. It's still a draft, so don't treat it as current.

Cochrane 2014 found a small hip-fracture win for calcium + D. Most of that number comes from the nursing-home story, about 1 fewer hip per 1,000 community older adults per year. That's not a blessing on Caltrate for the Chemist Warehouse shopper.

The heart-attack argument is contested: Bolland versus Lewis versus the National Osteoporosis Foundation. Food calcium is the default. Don't print “calcium pills cause heart attacks.”

If you bought the 1,000–1,200 mg once-a-day carbonate as a bone-health swallow, park it. Keep it only if a clinician has already prescribed calcium (osteoporosis protocol, hypoparathyroidism, PBS).

Count food first. In Australia, the target is about 1,000 mg a day for most adults, 1,300 mg for women over 50 and men over 70. The US says 1,200 mg for older women. If you're talking to an Australian GP, use the Australian figure.

Per serving (NIH ODS):

Spinach looks rich but absorbs badly (~5%). Three serves of dairy, or the tofu, sardine or fortified-soy versions, cover most of a 1,000 mg day.

If a GP later says your intake is still short, take split doses of ≤500 mg elemental calcium. Take calcium carbonate with a meal. Healthy Bones Australia’s consumer top-up is 500–600 mg a day, not 1,200 mg at 9pm. That belongs in a GP conversation. This letter isn't a product recommendation.

D3 footnote: Vitamin D is required for active gut absorption. Check 25-OH-D if your GP thinks it's needed. Don't make D3 the replacement bottle.

K2 footnote: MK-7 can carboxylate matrix Gla protein. AVADEC 2022 found no effect on aortic-valve calcium. A 2026 imaging trial (VitaK-CAC) slowed coronary calcium on a scan, not heart attacks or deaths. Hard no if you're on warfarin. Don't buy a cal-mag-K2 stack as the IN.

“I’m taking a large calcium carbonate tablet. Could we add up my food calcium first, check vitamin D if needed, and only supplement in split doses if I’m actually short?”

Stone history: “I have a history of calcium kidney stones. Can we keep dietary calcium and not add a big carbonate tablet without urology input?”

Levothyroxine, dolutegravir or a quinolone: “I’m on [medicine]. If I still need a calcium tablet, how should we time it?”

Warfarin plus a bone bottle with K2: “This calcium bottle has vitamin K2. I’m on warfarin. Should I avoid anything with MK-7?”