One recent morning, curiosity led me to a video promoting Mr. Kennedy’s Make America Healthy Again movement. With the hospital’s cacophony of ventilator and heart rate monitor sounds in the background, I watched Mr. Kennedy stand in a grassy field and promise to find and target the root causes of chronic illness — with few actual specifics.

Critics of seed oils, like RFK Jr., say they are toxic and fueling high rates of chronic diseases. Such claims baffle nutrition scientists.