The actor, who is known to have changed his size for several roles, most recently gained weight for his starring turn in 2016's War Dogs in which he played arms dealer Efraim Diveroli.
Jonah is thought to have been at his heaviest weight ever during filming in 2015.
After filming wrapped, the actor called on his close friend and former co-star, Channing Tatum, for help to drop the weight.
"I gained weight for this movie War Dogs," the actor told Jimmy Fallon in late 2016. "And then I wanted to get in better shape, so I called Channing Tatum, and said, 'Hey, if I ate less and go to a trainer, will I get in better shape?" And he said, 'Yes, you dumb motherf**ker, of course you will, it's the simplest thing in the entire world'."
The actor has also been using a nutirionist to keep an eye on his diet, though this hasn't always gone to plan.
In the same interview with Fallon, Jonah revealed he accidentally sent his daily food diary to the rapper Drake, instead of his Doctor. Makes sense, to be honest:
Keep on doing you, Jonah!