David Cameron is engaged in a "great patriotic struggle" to shed the pounds and has given up bread as part of his new regime.
The Prime Minister, who said his downfall was having a snack in the middle of the day, vowed to "cut the carbs and go for a run".
With the general election looming Mr Cameron insisted he was "relatively fit and healthy" at 48 and "I don't think I've gone mad yet".
David Cameron has given up bread, this is his face on bread
In an interview with BBC Sussex the 48-year-old Prime Minister said he intended to serve the full five-year term if returned to Downing Street after the May 7 poll.
The Prime Minister said: "I'm passionate about this job. It's a huge honour to do it. I'm putting myself forward for a full term and the British people are my boss and if they want me to do that they will vote for me, if they don't they will chuck me out and get somebody else to do the job.
"But it's an honour to do it, I'm relatively young at 48, I try to stay relatively fit and healthy."
He joked: "I don't think I've gone mad yet, although I'm sure that will come."
The Prime Minister has previously acknowledged he had put on weight over the festive period.
Setting out his own personal cuts programme he said: "I'm trying hard. It is a great patriotic struggle. It's difficult when you are on the road a lot - this sounds like a feeble excuse.
"I'm trying my hardest, I'm trying to go for a run every other day. I'm trying to cut the intake."
He added: "My downfall is just having, when you get hungry in the middle of the day, having a snack.
"That's the thing you have got to cut out, isn't it? I'm giving up bread, bread's off at the moment.
"Cut the carbs and go for a run. It worked before, maybe it will work again, who knows."
Mr Cameron acknowledged the public would be able to tell whether he was making progress: "I can't get away with it because people will judge pretty clearly whether the pounds have come off or gone on."