The 5:2 intermittent fasting (FAST) intervention consisted of two nonconsecutive days of fasting per week that could be chosen freely according to the patient's weekly schedule. The minimum amount of time to count as a fasting day was 24 hours, but patients were advised to aim for a complete fasting day including the nights before and after that day. On a fasting day, only water and unsweetened tea or coffee was allowed. However, given that fat is the macronutrient interfering the least with the metabolic adaptions to fasting, while carbohydrates disturb the most \[26\], patients were allowed to consume small amounts of bone broth/meat broth, coconut oil, butter/ghee or heavy cream as an "emergency plan", i.e., in case that they felt they needed some energy-containing foods to complete an initiated fasting day.
The primary study outcome was the feasibility of the FAST intervention and its effects on longitudinal body composition changes from baseline until the final week of radiotherapy (body mass, fat mass, fat-free mass, muscle mass, extracellular and total body water). Secondary endpoints were absolute changes in metabolic parameters, hormones, and overall quality of life scores in the FAST group. As a surrogate marker for insulin resistance, the triglyceride-glucose index (TyG) was calculated according to TyG=ln(fasting triglycerides \[mg/dl\]×fasting glucose \[mg/dl\]/ 2).
