Hypophosphatemia (serum phosphate < 2.5 mg/dL) is a major concern when initiating nutritional support. We evaluated which factors contribute to hypophosphatemia development in critically ill patients, as well as the association between hypophosphatemia and mortality. A retrospective cohort study of patients who were ventilated for at least 2 days in a 16-bed mixed ICU. Data collected includes demographics, Acute Physiology & Chronic Health Evaluation 2 (APACHE2) admission score, Sequential Organ Failure Assessment score at 24 h (SOFA24), hourly energy delivery, plasma phosphate levels during the first 2 weeks of admission, ICU length of stay (LOS), length of ventilation (LOV), and mortality (ICU and 90 days). For the hypophosphatemia development model, we considered mortality as a competing risk. For mortality analysis, we used the Cox proportional hazards model considering hypophosphatemia development as a time-varying covariate. 462 patients were used in the analysis. 59.52% of the patients developed hypophosphatemia. Several factors were associated with a decreased risk of hypophosphatemia: age, BMI, pre-admission diabetes diagnosis, APACHE2, SOFA24, first kidney SOFA score, hospital admission time before ICU admission, and admission after liver transplantation. Admission due to trauma was associated with an increased risk of hypophosphatemia. Survival analysis with hypophosphatemia as a time-varying covariate showed a protective effect of hypophosphatemia from mortality (HR 0.447, 95% CI 0.281, 0.712). Age, APACHE2, and SOFA24 score were found to be significantly associated with ICU mortality. Fasting duration in the ICU before nutritional support initiation was not found to be significantly associated with hypophosphatemia. We examined several fasting intervals (12 h, 24 h, 36 h, 48 h, 60 h, 72 h). In each fast interval, we compared the prevalence of hypophosphatemia among patients who fasted the specified length of time, with those who did not fast for the same length of time. In each fasting interval, hypophosphatemia prevalence was lower in the fasting group compared to the non-fasting group. However, this difference was insignificant. BMI, APACHE2, and hospital LOS before ICU admission were inversely associated with hypophosphatemia development. Fasting for up to 72 h in the ICU before starting nutritional support did not affect hypophosphatemia occurrence. Hypophosphatemia was associated with lower mortality.