A worm one millimeter long eats food containing special atoms, and they get built into its body: into muscles, skin, gut. Then the worm is switched to ordinary food and researchers track how quickly the tagged atoms leave. This reveals which parts of the body renew faster and which more slowly.
When food becomes scarce, but not so scarce that the worm is starving, the body starts operating differently. It doesn't slow everything down; it rearranges things: some tissues renew faster, others stay as they were. For instance, muscles and skin begin replacing their proteins and fats more often, while the gut keeps its former rhythm. It's like renovating a house on a tight budget: you fix what matters most first.
Previously, such details were invisible: conventional methods blended all cells into one mush and showed only an average. Now we can see how diet redistributes resources in the body, and this helps explain why calorie restriction extends life. The method can also be applied to other animals to study aging and disease.
The worm's cuticle, composed mainly of collagen, after labeling showed a ¹⁵N/¹⁴N ratio of about 15,000, which is thousands of times higher than the natural level and demonstrates almost complete saturation with isotopes.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.