This led to the revelation that the fat in the cells of the adipose tissue is in a continual state of flux. This was initially the work of a German biochemist, Rudolf Schoenheimer. In the early 1930s, while working at the University of Freiburg, Schoenheimer demonstrated that animals continually synthesize and degrade their own cholesterol, independent of the amount of cholesterol in the diet. After Hitler came to power in January 1933, Schoenheimer moved to New York, where he went to work at Columbia University. It was in New York that Schoenheimer collaborated on the development of a technique for measuring serum cholesterol and, by doing so, launched the medical profession’s obsession with cholesterol levels. Then, with David Rittenberg, he developed the technique to label or tag molecules with a heavy form of hydrogen known as deuterium*111 so that their movement through the metabolic processes of the body could be followed. Schoenheimer and Rittenberg put this technique to work studying the metabolism of fat, protein, and carbohydrates in the body.

Among their discoveries is that both dietary fat and a considerable portion of the carbohydrates we consume are stored as fat—or, technically, triglycerides—in the adipose tissue before being used for fuel by the cells. These triglycerides are then continuously broken down into their component fatty acids, released into the bloodstream, moved to and from organs and tissues, regenerated, and merged with fatty acids from the diet to reform a mixture of triglycerides in the fat cells that is, as Schoenheimer put it, “indistinguishable as to their origin.” Fat stored as triglycerides in the adipose tissue, and the fatty acids and triglycerides moving through the bloodstream are both part of the same perpetual cycle of fat metabolism. “Mobilization and deposition of fat go on continuously, without regard to the nutritional state of the animal,” as the Israeli biochemist Ernst Wertheimer explained in 1948, in a seminal review of this new science of fat metabolism.†112 “The ‘classical theory’ that fat is deposited in the adipose tissue only when given in excess of the caloric requirement has been finally disproved,” Wertheimer wrote. Fat accumulates in the adipose tissue when these forces of deposition exceed those of mobilization, he explained, and “the lowering of the fat content of the tissue during hunger is the result of mobilization exceeding deposition.”