Calcium: Absorption, Cofactors & the Paradox of Supplementation
Calcium is the most abundant mineral in the human body, with about 99% stored in bones and teeth. But the calcium story is far more nuanced than "drink milk for strong bones." How calcium is absorbed, where it ends up, and which cofactors determine its fate are all critical details that basic nutrition advice often misses.
Absorption Mechanisms
Calcium is absorbed through two distinct pathways: active transcellular transport (regulated by vitamin D and predominant at low intakes) and passive paracellular transport (driven by concentration gradients at higher intakes). Active absorption in the duodenum is the primary regulated pathway, while passive absorption along the entire intestine becomes more important at higher calcium intakes.
Only about 25-35% of dietary calcium is typically absorbed, though this varies based on vitamin D status, the food source, other dietary components, and individual factors like age and estrogen levels.
The Supplementation Paradox
Large calcium supplements taken in bolus doses raise serum calcium rapidly, which some research has associated with increased cardiovascular risk. This has led to the "calcium paradox" — populations with the highest dairy and supplement calcium intakes don't always have the lowest fracture rates. The issue may not be calcium itself but how it's delivered and whether the cofactors needed to route it properly are present.
Calcium Blockers
Oxalates in spinach, rhubarb, and beet greens bind calcium tightly, making the calcium in these foods largely unavailable. Phytic acid in grains and legumes also reduces calcium absorption, though less dramatically. The calcium-to-phosphorus ratio in the diet may also matter — very high phosphorus intake (common in processed food diets) may promote calcium excretion.
Food Sources
Dairy products, sardines (with bones), calcium-set tofu, bok choy, kale, broccoli, and fortified plant milks are reliable sources. Notably, the calcium in kale and broccoli is more bioavailable than the calcium in milk (absorption rate ~50-60% vs. ~30%) because these greens are low in oxalates.
External resources: NIH — Calcium Fact Sheet