Proper nutrition may help in preventing deaths or at least alleviating the symptoms of many chronic diseases. While the COVID-19 disease was still taking its toll, the world had to adjust to new life conditions, which could change nutritional habits. In this observational, cross-sectional study, we aimed to identify the potential correlations between sociodemographic factors and diet and the presence of common chronic diseases among Polish inhabitants. Furthermore, we tried to determine whether the COVID-19 pandemic led to changes in nutritional habits. Therefore, based on the online study (the National Test for Poles' Health (NTPH), we collected data from 376,102 and 200,000 respondents in two different time frames (before the COVID-19 pandemic: 2019-2020 and during the COVID-19 pandemic: 2021-2022, respectively). Despite the rapid global rise of the COVID-19 pandemic, among our study group, hypertension was still the most commonly occurring disease in both time frames (32.33% in 2019-2020 and 34.95% in 2021-2022, < 0.001). Furthermore, more chronic diseases were reported during the COVID-19 pandemic than in 2019-2020. Regarding sociodemographic factors, male respondents were more likely to develop hypertension and diabetes (OR = 1.35 CI 95% (1.28-1.43), < 0.001; and OR = 1.20 CI 95% (1.11-1.30), < 0.001). Vegetarian diet decreases the likelihood of hypertension, neurological disease, and diabetes (OR = 0.69, CI 95% (0.60-0.81), < 0.001; OR = 0.72, CI 95% (0.59-0.88), = 0.001; and OR = 0.73, CI 95% (0.55-0.96), = 0.026). In line with this, consuming meat meals increases the risk of hypertension (OR = 1.09, CI 95% (1.02-1.17), = 0.009). Interestingly, a reduced-sodium diet has an association with decreased morbidity of COVID-19 disease (OR = 0.72, CI 95% (0.63-0.82), < 0.001). This result brings new light to more research to be done to allow efficient prevention of this disease. In conclusion, our study shows the beneficial role of a balanced diet in reducing the incidence rate of common chronic diseases. Our findings may be educational for those who would like to change their nutritional habits and/or for public health professionals to suggest the implementation of proper diets to their patients.