In 2017, Longo was a co-author on the first human trial of whether fasting might reduce the risk factors for diseases like diabetes, cancer, and cardiovascular disease. The researchers randomized 100 people into one of two groups for three months: The first group ate anything they wanted, and the second fasted for five consecutive days each month. (By “fasted” here, I mean, they followed the fasting-mimicking diet.) After three months, the first group was crossed over into the fasting group, so the researchers could gather even more data on fasting.
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting. Some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting once 8–12 hours have elapsed since the last meal. Metabolic changes of the fasting state begin after absorption of a meal (typically 3–5 hours after eating).