Inclusion Criteria:  Newly diagnosed recurrent /metastatic head and neck squamous cell cancer that is not amenable to local therapy with curative intent (surgery or radiation therapy with or without chemotherapy) and initiating standard of care immune checkpoint blocker- (Nivolumab, pembrolizumab, Atezolizumab, Avelumab or Durvalumab with or without chemotherapy), for the first time. Patients must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded) as outlined in RECIST version 1.1. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1.  Exclusion Criteria:  BMI< 18.5. Diabetes mellitus, hyperthyroidism, pregnancy, any eating disorder including anorexia nervosa or bulimia, metabolic disorders that may affect gluconeogenesis or adaptation to short fasting periods Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, autoimmune disease requiring systemic steroids, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements. Patients who have known leptomeningeal metastases or untreated or symptomatic brain metastases. Treated, asymptomatic brain metastasis can be included.