"Tumors are very hungry," said Joseph Sun, an immunologist and the study's co-corresponding author. "They take up essential nutrients, creating a hostile environment often rich in lipids that are detrimental to most immune cells. What we show here is that fasting reprograms these natural killer cells to better survive in this suppressive environment." Natural killer (NK) cells are white blood cells that live up to their name, destroying other cells infected with viruses and cancer cells. Although their primary job is to kill invaders, NK cells also communicate with other immune cells, releasing proteins called cytokines to signal them to attack harmful cells. The more NK cells there are invading a cancer tumor, the better the patient's prognosis.