What is Calcium?

Nerve cells fire using calcium, muscle fibres contract using it, and blood will not clot without it. All of that draws on the small amount in the blood rather than on the skeleton.

Albumin binds a large share of the calcium in blood, so a change in blood protein moves the measured total while the calcium free to act stays where it was. Measuring albumin alongside calcium separates a change in the protein from a change in the calcium itself. Vitamin D governs how much calcium the gut absorbs from food and, with parathyroid hormone, which the body releases when blood calcium runs short, how much bone gives up.

Bone is calcium's reserve, giving it up when the blood runs short and taking it back when there is a surplus. That exchange is the largest influence on the circulating amount. Diet contributes only what the gut takes up, a fraction of what is eaten. At the other end, the kidney reclaims most of the calcium it filters rather than letting it go into urine.

Blood calcium barely moves from one day to the next. Against that steadiness, a difference still present a year later points to bone being rebuilt, gut absorption or the kidney's reclaiming, rather than to the previous day's food. This is Do Health's monitoring schedule, not a general recommendation for everyone.