In an old house, beams don't break all at once. First, voids appear inside them, like in a sponge, and only later does one of the beams give way. Bones age just as quietly: the spongy part, resembling a weave of thin struts, loses its supports long before a person feels pain. Scientists decided to test whether substances that slow aging can halt this quiet dismantling of load-bearing structures.
For the experiment, they took mice of a special breed known as four-way cross mice: each individual is genetically unique, like a human. Some received acarbose with food, others 17α-estradiol, and still others rapamycin. All three substances are known to extend the lifespan of rodents. The animals' bones were scanned on a micro-CT scanner, which takes X-ray images with a resolution of thousandths of a millimeter.
The result was unexpected. In females, all three drugs noticeably increased trabecular bone volume: rapamycin by 34%, 17α-estradiol by 47%, and acarbose by 15% compared with untreated mice of the same age. In males, the effect was weak and unconvincing. Importantly, the bone was not regrown: the struts did not become thicker, there were simply more of them. The drugs prevented the builders from dismantling old beams, but they did not build new floors.
The dense outer layer of bone, similar to tree bark, responded very little. In females at 22 months, two drugs slightly increased its fraction in cross-section, but the thickness of the cortex itself did not change. This means the geometry changed, not that new tissue appeared.
Previously, it was unknown how these three substances affect bone during long-term use, especially in different sexes. Now it is clear: in females they preserve spongy bone, in males almost not at all, and the dense layer remains indifferent. For future medicine, this is a warning: effects cannot be transferred from one sex to another or from one tissue to another. The quiet repair of load-bearing beams follows its own rules, and they do not always coincide with the general laws of longevity.
UM-HET3 mice are "four-breed mutts": they are produced by crossing four inbred strains so that each individual is genetically unique, just like a person. That is exactly why they are used in the geroprotector testing program of the US National Institute on Aging.
In Kim Stanley Robinson's novel Aurora, colonists have to grapple with bone aging under low gravity, and geroprotectors are mentioned there as part of a combination therapy. Although the book is about space rather than earthly old age, the idea of combining drugs that slow the aging of different tissues echoes the conclusions of this work.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.