"I swear I used to be taller." Women in my practice say some version of that after a routine physical, half-joking, half-unsettled.

They expected the same height they had carried for twenty years and got a different number. Nobody explained why.

Your spine is not collapsing. It is responding to a shift in how bone rebuilds itself after 45.

Bone is living tissue that constantly remodels through two types of cells: osteoblasts lay down new bone, and osteoclasts break old bone apart. Estrogen keeps the two in balance by restraining osteoclasts and supporting osteoblasts.

When estrogen declines during the menopausal transition, osteoclast activity increases and outpaces osteoblast activity. More bone is taken apart than replaced.

The spine takes the largest hit because vertebrae are packed with trabecular bone, the spongy interior that turns over faster than the dense outer shell in the limbs. As that interior loses density, the vertebrae compress slightly under ordinary body weight.

That compression is where the lost centimeter comes from. It adds up over years without pain to mark it, so the first clue is usually a number at the doctor's office.

The SWAN study, published in the Journal of Bone and Mineral Research in 2012, followed 862 women through the menopausal transition and measured this directly. Women lost roughly 10 percent of their spine bone density over about ten years, with the steepest decline concentrated in the two to three years around the final menstrual period.

Ever notice you keep rounding up to the height you had at 40? A 15-year study of 1,421 women, published in Osteoporosis International in 2007, found that more than 70 percent had lost over a centimeter of height, and the rate of height loss tracked with bone loss elsewhere in the body.

This is not a sign that your skeleton is giving out. The loss is well documented, and the bone you still have responds to what you do with it.

Loading the skeleton matters more than most people expect. Two or three sessions a week of stair climbing, bodyweight squats, or resistance bands stimulate bone-building cells in a way that a flat walk does not.

Bone responds to mechanical stress the way muscle does: when force travels through the skeleton during a squat or a stair climb, osteoblasts read that load as a reason to lay down new tissue.

How much can a cup of yogurt really do for bone? More than you might think.

The body absorbs calcium most efficiently in smaller amounts spread across meals, not in one large dose. A cup of plain yogurt carries about 415 milligrams, a cup of milk about 300, and a serving of cooked kale about 180, which together cover most of the 1,200 milligrams the NIH recommends daily for women over 50.

If you have lost more than about an inch and a half from your tallest adult height, fractured a bone from a minor fall, or have sudden sharp back pain, bring it up with your doctor. A bone density scan can measure exactly where you stand.

Your skeleton after 45 is working with less estrogen and more time. That is biology, not a verdict.

You will not hit 1,200 milligrams of calcium most days, and your bones care about the long arc, not any single meal.