The stored lipid is radioactive because it contains the radioactive components from the ingested 14C-labeled glucose.
Glycogenesis is the process by which the excess glucose present in the body is converted into a storage form called glycogen. This process helps to store glucose and is used to supply immediate glucose when required. When the glycogen storage cells are filled, then the glucose will be converted to fatty acids, fat, or lipids and stored in fat cells by a process called lipogenesis.
So when we subsequently supply 14C-labeled glucose, it will be converted first to glycogen. But once that is done, excess glucose will be converted to fat. This storage fat will acquire the 14C-labeled carbon within its structure and be radioactive.
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