This study aims to identify changes in diet, physical activity, sleep duration, and lifestyle behaviors, which are the main determinants of health, during the COVID-19 pandemic. Changes in nutrition, physical activity, and sleep patterns of 3294 adults (18-65 years old) living in Turkey were examined through an online questionnaire. Evaluation of the periods before and during the pandemic revealed a significant increase in body weight and BMI values in both genders (70.5±15.12 kg, 71.5±15.58 kg, p<0.001, 24.0±4.24 kg/m2, 24.3±4.36 kg/m2 p<0.001, respectively). The mean Mediterranean Diet Adherence Score of the participants was 7.0±2.37, with 25.0% low, 60.5% moderate, 14.5% high-level Mediterranean diet pattern. While it was found that there was a difference across genders, the physical activity duration was less than 30 minutes or 30-45 minutes in the majority of women (80.8%), and 30-45 minutes or more than 45 minutes in the majority of men (68.8%) (p<0.001). The frequency of consumption of all food groups increased significantly compared to the pre-quarantine period (p<0.001). Compared to the pre-quarantine period, the participants' water consumption (46.7%) and sleep duration (44.3%) increased, while their physical activity levels (57.5%) decreased. The results of this study confirm that nutrition and lifestyle behaviors have changed drastically during the pandemic. Based on the relationship between the immune system and nutrition, it is crucial to raise awareness in society about healthy, adequate, and balanced nutrition and a physically active life.