Various hormones are also affected. For example, production of insulin-like growth factor 1 (IGF-1) drops early and reaches very low levels by Day 3 or 4. It is similar in structure to insulin, which also becomes scarcer with fasting, and high levels of both have been linked to cancer.
As for treating cancer, Valter Longo, director of the Longevity Institute at the University of Southern California, thinks that short-term complete fasts maximize the benefits. He has found that a 48-hour total fast slowed the growth of five of eight types of cancer in mice, the effect tending to be more pronounced the more fasts the animals undertook.
Fasting is harder on cancer cells than on normal cells, he says. That's because the mutations that cause cancer lead to rapid growth under the physiological conditions in which they arose, but they can be at a disadvantage when conditions changes. This could also explain why fasting combined with conventional cancer treatment provides a double whammy. Mice with gliomas — very aggressive forms of cancer and the most commonly diagnosed brain tumor in people — were more than twice as likely to survive a 28-day study if they underwent a 48-hour fast accompanied by radiation therapy as were those that did not fast.
Could fasting prevent cancers from developing in the first place? Evidence is scant.
Longo says there are "very good reasons" why it should. He points out that high levels of IGF-1 and glucose in the blood and being overweight are risk factors for cancer, and they can all be improved by fasting.
Another risk factor is insulin, says Michelle Harvie at Britain's University of Manchester. Studying a group of women whose family history put them at high risk of developing breast cancer, she put half of them on a diet that cut calories by about 25 percent and half on a 5:2 fast. After six months, both groups showed a reduction in blood insulin levels, but the reduction was greater in the fasting group. Harvie's team is now analyzing breast biopsies to see whether this translates to fewer of the genetic changes associated with increased cancer risk.