I almost paid to add a calcium blood test to my order this month.
I’m in the middle of a round of blood work and it’s not cheap. I kept going back and forth on whether to add the osteoporosis set of tests.
My mom is dealing with a lot of osteoporosis and calcium absorption trouble, and honestly, I’d love to avoid that in my future.
Before I checked the box, I took a look at what a calcium blood test could actually tell me in practice. What would I learn from it and what could I actually do about it?
I’m glad I looked. Here’s what a blood test can and can’t tell you about bone loss.
Why a normal blood test can’t detect bone loss
Bone loss can look like a normal calcium blood test. In fact, in ordinary osteoporosis, blood calcium is almost always normal.
Your body is designed to obsessively protect your blood calcium because calcium in your blood runs your heart, your nerves, and your muscles. So your body maintains it to this tiny range, no matter what you eat.
About 99% of your body’s calcium is in your bones. Only about 1% of it is in your blood. In your blood is where it's used. And your bones act as a reservoir, for when more is needed.
When calcium from food falls short, a hormone called PTH rises and pulls calcium out of bone to top up your blood. Your blood always stays normal, even when the calcium levels in your blood and bones are balanced. About 500 mg of calcium moves out of your bones and back in every day, just to maintain these levels.
And we see this directly in postmenopausal women who are actively losing bone: blood calcium stays normal on every diet. Essentially, the skeleton is sacrificed to protect calcium in the blood.
So is the problem even calcium?
Often, but not mainly.
For most women around menopause, the biggest force driving bone loss isn't too little calcium. It's losing estrogen.
Estrogen keeps bone breakdown in check. When it drops, bone is lost faster. And low calcium makes things worse. Older women eating little calcium or absorbing poorly are hit hardest.
You can’t just eat your way out of it
Eating more calcium also hits a ceiling. Your gut absorbs only a fraction of what you eat, roughly 30% from dairy and fortified foods.
And how much you absorb is driven again by PTH and active vitamin D: when calcium intake is low, your gut absorbs more, but when it’s high, it absorbs less.
So doubling your calcium intake doesn’t double how much you absorb. It’s about 45% at very low intakes and only about 15% at very high ones.
The good news
Calcium still helps. But you have to be low to begin with.
Paired with vitamin D, calcium cut total fractures by about 15% and hip fractures by about 30% across trials. In a trial of 36,000+ women, calcium plus vitamin D didn't lower hip fractures overall and slightly raised kidney stone risk.
🩺 If you’re worried about bone loss, ask for the bone test, not the blood test. A normal serum calcium says nothing about your bones. It stays normal even while bone is being lost. Ask for a DXA scan, and a vitamin D (25-OH-D) level if you take calcium.
🧮 Count your food calcium to the target once, then top up only the gap. Aim for 1,000 mg a day under 50, 1,200 over 50. Beyond that adds risk with no detectable benefits.
🟡 Take calcium with vitamin D. Calcium cut hip fractures only when paired with vitamin D.
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