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.
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"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.
