'Make America healthy again starts TOMORROW': Trump, Musk, RFK Jr feast on burgers

US President-elect Donald Trump's pick for health secretary, Robert F Kennedy Jr, has been pictured sharing a burger with his future boss, days after they promised to "make America healthy again."

Kennedy flew with Trump aboard his plane to an Ultimate Fighting Championship bout in New York on Saturday along with Tesla CEO Elon Musk and Donald Jr, the Republican's eldest son.

The four men are shown seated in front of trays of fast food in a photo posted online by Donald Jr on Sunday which he jokingly captioned: "Make America Healthy Again starts TOMORROW."

Trump and Kennedy, a scion of the famous Democratic political family, campaigned together ahead of the presidential election on November 5 promising to tackle processed food among other priorities.

Announcing Kennedy's nomination on Thursday, Trump said that "for too long, Americans have been crushed by the industrial food complex and drug companies."

Social media commentators noted that Kennedy appeared the least enthusiastic of the diners as he gingerly held an opened burger box, with fries and a bottle of cola in front of him.

The 70-year-old, a conspiracy theorist and anti-vaccine campaigner, has long argued that the United States needs to curb its obesity epidemic by reducing the high levels of sugar, fat and high-additive processed food in the national diet.

In a podcast posted last week, he was scathing about Trump's diet, saying, "The stuff that he eats is really bad."

"Campaign food is always bad, but the food that goes onto (Trump's) aeroplane is just poison," he told the Joe Polish podcast. "You don't have a choice, you're given [fast food]."

Trump has never made a secret of his love of fast food and diet cola, even briefly stopping to work at a fast food joint in Pennsylvania during his campaign in October.

"I love salt!" he told workers there.

Kennedy will need confirmation by the Senate to take up his job as health secretary, with some senior Republicans voicing concerns about his suitability for the position.