A research design in which investigators deliberately change participants’ diets or lifestyles and compare outcomes against a control group. Unlike observational studies, intervention studies test cause and effect directly: researchers might assign one group a Mediterranean diet and another a standard diet, then track blood pressure, cholesterol, or disease events over months or years. Randomised controlled trials are the strongest form, reducing bias through random allocation. Such studies provide the evidence base for dietary guidelines and health claims, though they are expensive and demanding for participants. Careful design, compliance monitoring, and ethical approval are essential to produce reliable, meaningful results.