In an extract from the band's autobiography Unsaid Things... Our Story published in The Mirror, the singer confessed that this led to him developing unhealthy diet obsessions.
"I was obsessing about being the fat one in the band. I wanted to lose weight, but rather than going about it the sensible way, I pretty much stopped eating," he said.
"I'd go to Starbucks and order a frappuccino and blueberry muffin and that was me for the day. Not eating apart from my daily Starbucks became my routine.
"Not only would I beat myself up internally if I consumed anything else, I'd be in a foul mood if we were working somewhere there was no Starbucks.
"One day my car was in the garage so I got up extra early and walked several miles to our local town to get one. It was an obsession - and a deeply unhealthy one."
Fletcher explained how he was diagnosed with bipolar disorder last year and prescribed anti-depressants and given hypnotherapy tapes.
McFly bandmate Dougie Poynter previously told how his battle with depression over drink and drug abuse led to a suicide attempt.