How Calcium Absorption Works: Dairy, Vitamin D, and the Bone Density Evidence

Calcium is the most abundant mineral in the human body, with 99% of it stored in bones and teeth. But the number on a nutrition label tells you almost nothing useful. What matters is how much your intestine actually takes up, and that varies enormously depending on what you’re eating it with, which food it came from, and whether you have enough vitamin D.

Two Ways the Gut Absorbs Calcium

The intestine absorbs calcium through two separate pathways, and they don’t work equally.

The first is active transcellular transport. This is the efficient route. Calcium enters intestinal cells through TRPV6 channels on the cell’s surface, gets ferried across by a protein called calbindin-D9k, and pumps out the other side into the bloodstream (Peng et al., 1999, J Biol Chem). This pathway is saturable, meaning it can only process so much calcium at a time. It’s also tightly controlled by vitamin D.

The second pathway is passive paracellular diffusion. Calcium seeps between intestinal cells driven by concentration gradient. This requires no vitamin D. It’s slow, relatively inefficient, and becomes the dominant pathway when someone is vitamin D deficient.

Vitamin D’s job in all this is to upregulate the genes that produce TRPV6 and calbindin-D9k. When vitamin D is adequate, the active transport system runs at full capacity. When it’s not, you lose your most efficient absorption tool and depend almost entirely on passive diffusion. This is why vitamin D deficiency and calcium deficiency so often appear together.

Absorption Rates by Food Source

Not all calcium is created equal. Researchers measure fractional calcium absorption by feeding people a calcium source labeled with a stable isotope and measuring how much ends up in the blood or urine. The numbers are striking.

Dairy comes in around 30-32% absorption. That’s the reference point most people use, but it’s not the ceiling. Broccoli absorbs at roughly 50-61% (Weaver et al., 1999, Am J Clin Nutr). Kale and bok choy are in a similar range. These vegetables have low oxalate content, which is why their calcium is so accessible.

Spinach is the counterexample. It looks calcium-rich on paper, around 250mg per cooked cup. But it absorbs at only 5%. The reason is oxalate. Spinach contains massive amounts of it. Oxalate binds to calcium in the gut and forms calcium oxalate crystals that pass right through you. You absorb almost nothing.

The same logic applies to beet greens and Swiss chard: both are high in oxalate, both absorb poorly. Kale, bok choy, and broccoli are the smart choices if you’re relying on vegetables for calcium.

Fortified plant milks typically absorb comparably to dairy, around 30-32%, as long as you shake the carton (calcium can settle). The evidence here is solid (Heaney et al., 2000, Am J Clin Nutr).

Calcium supplements show different absorption profiles depending on the compound. Calcium carbonate absorbs best when taken with food because stomach acid helps dissolve it. Calcium citrate doesn’t require acid and absorbs better on an empty stomach. The difference matters most for people on proton pump inhibitors.

What Inhibits Calcium Absorption

Oxalate is the biggest inhibitor, as covered above. But it’s not the only one.

Phytate, found in whole grains and legumes, also binds calcium and reduces absorption. The effect is less dramatic than oxalate, but it adds up in plant-heavy diets. Soaking and fermenting grains reduces phytate content.

High sodium intake increases urinary calcium excretion. For every extra gram of sodium consumed, roughly 26mg of calcium is lost in urine. Over years, this matters for bone mass, though the clinical significance is debated.

Excess phosphorus isn’t an absorption inhibitor per se, but very high phosphorus intakes (from phosphate food additives in particular) may affect calcium balance in ways still being studied. The food additive phosphates in processed foods are a different chemical form than naturally occurring phosphorus and may behave differently in the body.

High caffeine intake has a modest calcium-depleting effect, though research suggests it’s only meaningful in people with already low calcium intakes.

The DASH Trial and Blood Pressure

Most people think of calcium in terms of bones, but the DASH trial (Appel et al., 1997, N Engl J Med) showed something important about calcium’s role in blood pressure. The DASH eating pattern, which was high in dairy foods, fruits, and vegetables, reduced systolic blood pressure by 11.1 mmHg in people with hypertension. The trial can’t isolate calcium as the cause, since the diet changed many things at once. But calcium’s vasodilatory role through smooth muscle relaxation is well established.

This is one reason calcium recommendations aren’t purely about bones.

The Bone Density Question

Here’s where the story gets more complicated. Dairy marketing has long pushed the message that milk builds strong bones and prevents fractures. The evidence is more mixed.

A 2007 meta-analysis by Bischoff-Ferrari and colleagues (Am J Clin Nutr) found that higher calcium intake didn’t consistently reduce hip fracture risk in men or women across cohort studies. The countries with the highest dairy intake also tend to have the highest fracture rates, though this ecological observation doesn’t prove causation and confounders abound.

What does appear to matter for bone density is total calcium adequacy (not necessarily from dairy), adequate vitamin D, load-bearing physical activity, and avoiding extreme deficiency. The evidence for calcium supplementation reducing fracture risk is mixed and has been questioned since large trials showed inconsistent results. Getting calcium from food sources remains better supported than supplementation.

The practical lesson: adequate calcium matters, but dairy isn’t the only way to get there, and the dairy-bones connection isn’t as strong as it once seemed.

This article is for educational purposes only. It’s not medical advice. Talk to your doctor or a registered dietitian before making significant changes to your diet.

Vitamin D metabolism and calcium absorption are inseparable, and you can read more about how your body activates and uses vitamin D at vitamin D metabolism. If you’re interested in how other minerals like iron behave similarly, iron absorption science covers the same active-versus-passive transport dynamics. The general principles behind why nutrients from different food sources absorb differently are covered in bioavailability.