The patterns of cancer incidence, for many cancers, are similar to those of heart disease, diabetes, and obesity, which alone suggests an association between these diseases that is more than coincidental. This was the basis of Cleave’s speculation, of Dennis Burkitt’s, and of those cancer epidemiologists who argued that dietary fat caused breast cancer. But if dietary fat, red meat, man-made chemicals, or even the absence of fiber cannot explain the “strikingly similar” patterns of disease distribution, as the Harvard epidemiologist Edward Giovannucci remarked about colon cancer and Type 2 diabetes in 2001, then something else most likely does.

Those cancers apparently caused by diet or lifestyle and not related to tobacco use are either cancers of the gastrointestinal tract, including colon and rectal cancer, or cancers of what are technically known as endocrine-dependent organs—breast, uterus, ovaries, and prostate—the functions of which are regulated by hormones. This connection between these diet-and life-style-related cancers and hormones has been reinforced by the number of hormone-dependent factors linked to cancers of the breast and the endometrium (the lining of the uterus). All suggest that estrogen plays an important role. All these cancers, with the possible exception of pancreatic and prostate cancer, appear to increase in incidence with weight gain. These associations together imply both a metabolic and a hormonal connection between diet and cancer. This in turn led breast-cancer researchers to focus their attention on the likely possibility that obesity increases the incidence of breast cancer by increasing estrogen production.

The most direct evidence linking overweight or overnutrition to cancer comes from animal experiments. These date back to the eve of World War I, when Peyton Rous, who would later win a Nobel Prize, demonstrated that tumors grow remarkably slowly in semi-starved animals. This line of research lapsed until 1935, when the Cornell University nutritionist Clive McCay reported that feeding rats just barely enough to avoid starvation ultimately extended their lifespan by as much as 50 percent. Seven years later, Albert Tannenbaum, a Chicago pathologist, launched a cottage research industry after demonstrating that underfeeding mice on very low-calorie diets, as McCay had, resulted in a dramatic inhibition of “many types of tumors of divergent tissue origin.” In one experiment, twenty-six of fifty well-fed mice developed mammary tumors by a hundred weeks of age—the typical lifespan of lab mice—compared with none of fifty that were allowed only minimal calories. Tannenbaum’s semi-starved animals not only lived longer, but were more active, he reported, and had fewer “pathologic changes in the heart, kidneys, liver, and other organs.”*63

To explain this inhibitory effect, Tannenbaum considered an idea that had originated in the 1920s with Otto Warburg, a German biochemist and later Nobel Prize winner. Warburg had demonstrated that tumor cells quickly develop the ability to survive without oxygen and to generate energy by a process of fermentation rather than respiration. Fermentation is considerably less efficient, and so tumors will burn perhaps thirty times as much blood sugar as normal cells. Incipient tumors in these calorie-restricted lab animals, it was thought, cannot obtain the huge amounts of blood sugar they need to fuel mitosis—division of the nucleus—and continue proliferating.