The most powerful part of the speech (which you can watch in full below) is perhaps when Hawking says: "For what it's worth, how being sedentary has been a major health problem is beyond my understanding."
The professor relates his work as a cosmologist, in which he sees the world "as a whole," to what he describes as "the most serious public health problems of the 21st century."
See Stephen Hawking throughout his life:
"Today too many people die from complications related to overweight and obesity. We eat too much and move too little," Hawking says in the video ad.
When talking of the solution, he says: "It's not rocket science," and recommends simply that people eat less and take up more physical activity.
After playing the video of Hawking delivering his speech, the low-budget ad ends with three written statements: "Physical inactivity is now the world's fourth leading cause of death," "Required physical activity per day: Adults 30 minutes," and "Required physical activity per day: Children 60 minutes."
Watch the ad in full here: