What is trans fat, exactly? Essentially, it’s the type of fat that’s created when hydrogen is added to the chemical structure of a fat, typically vegetable oil. The addition of hydrogen helps to make the fat (and therefore the food it’s used in) more shelf-stable, preventing it from easily spoiling when left out unrefrigerated. Partial hydrogenation (the process of adding hydrogen to fat) also creates a semi-solid fat, which is necessary in order to prevent foods from melting at room temperature.