Figure 1: Principles of the radiocarbon method.

(a) Lipid Δ¹⁴C values as a function of date of birth for samples collected after 2010. Above-ground nuclear bomb testing during the period of the cold war caused increased atmospheric ¹⁴C levels relative to stable isotope levels. These values decreased exponentially following implementation of a test ban treaty in 1963. Units are expressed in isotope ratio ¹⁴C/¹²C as Δ¹⁴C corrected for decay. Lipid age is determined by averaging lipid ¹⁴C incorporation into adipose tissue against the atmospheric ¹⁴C-curve (solid black line) over each individual’s lifetime to match sample ¹⁴C levels at the biopsy collection date (grey scatter). Low Δ¹⁴C values indicate that lipids have a high turnover rate (K_out), or equivalently that lipid residency times (lipid age) are short, with the relationship age=1/K_out. (b) Donor-paired sWAT lipid (triangles) and blood (filled circles) versus Δ¹⁴C levels as function of sample collection date. Each colour corresponds to a single donor. Lower blood Δ¹⁴C levels indicate a higher turnover rate. (c) Summary data showing the average age of blood and lipid. Values are mean±s.e.m. and compared by paired t-test. (d) Lipid age is used to assess triglyceride turnover as detailed in the Methods section. The main factors regulating triglyceride turnover in fat cells are the storage capacity (K_in) and removal capacity (K_out) of fatty acids. Such processes include fatty acid uptake, esterification, mobilization/re-esterification and oxidation. In an expanding adipose tissue, K_in>K_out, whereas the reverse is observed when adipose tissue mass is lost.