NSAIDs interact with the endocannabinoid system and its endocannabinoids, as COX2 have been shown to utilize endocannabinoids as substrates, and may have a key role in both the therapeutic and adverse effects of NSAIDs, as well as in NSAIDs-induced placebo responses.
Definition: An independent variable is the variable that is changed or controlled in a scientific experiment. Independent variables are the variables that the experimenter changes to test their dependent variable.
To reiterate, the independent variable is the thing over which the researcher has control and is manipulating. In this experiment, the researcher is controlling the food intake of the participant. The dependent variable is believed to be dependent on the independent variable.