For 156 days, a guinea pig named Khryun drank water laced with a heavier version of hydrogen. Over that period, the isotope worked its way into the animal's fats, proteins, and pigments, converting an ordinary rodent into something researchers could read like a timestamped record of its own chemistry.
The results, published in the International Journal of Molecular Sciences, answer a question that sounds simple and has been surprisingly hard to pin down: how long does it actually take the body to replace the molecules it is made of?
