Bone density in perimenopause: when the loss starts

Most women file bone density under things to deal with in their sixties. The steepest bone loss of your life is likely to happen a decade or two before that, inside a window most people don't know is open until it has closed.

Why the timing catches people off guard

Estrogen is one of the main brakes on the cells that break down old bone. As it becomes erratic and then declines, that brake loosens, and resorption starts outpacing rebuilding.

The SWAN cohort tracked bone density against each woman's final menstrual period rather than her age, and the picture is sharper than "after menopause." Loss began about one year before the final period and slowed about two years after it. Across that roughly three-year window, spine density fell 2.46% per year and femoral neck 1.76% per year — several times the rate on either side of it.

The clinically awkward part: in the year before the final period, when loss had already started, 68% of those women were still classified as early perimenopausal by their cycle patterns. Menstrual staging doesn't tell you the window has opened.

What actually moves the needle

Calcium: the RDA is 1,000 mg/day through age 50 and 1,200 mg from 51. Split it across meals — absorption from supplements is highest at doses of 500 mg or less. Food first, supplements filling the gap.

Vitamin D: the RDA is 600 IU, and the target blood level is genuinely contested. NIH treats 20 ng/mL as adequate for bone health across the population — but that's a floor, not an optimum, and the 2024 Endocrine Society revision declined to name an optimal number at all. A 2025 summary of dose-response meta-analyses found that most disease-specific recommendations land between 30 and 50 ng/mL. That's the range worth aiming for. The number that keeps a population out of deficiency and the number that's good for you are not the same question.

Resistance training: bone is living tissue that responds to mechanical load, and this is one of the few levers that acts on bone directly rather than through what you swallow.

What to actually do

If you're in your forties with risk factors — family history, early perimenopause, a history of disordered eating, long-term steroid use — ask about a baseline DEXA. A baseline doesn't diagnose anything. It gives you a number to compare against on the far side of the window, which is the whole point of knowing the window exists.