The Take That frontman explained that after he was dropped as a solo artist in 1999, he began to eat more and withdrew from public appearances.
"I felt horrible. I felt s**t," he told Radio Times "For someone so big, I felt incredibly small. I'd decided, 'OK, nobody wants me, but I don't want to do it anyway, and to make sure I don't do it again, this is how I'm going to look.' I realised afterwards it was a form of depression."
Barlow added that his wife Dawn convinced him to visit his doctor about his weight gain after it began to affect his health.
"The fat was a protective coating. As I got bigger, I found I didn't need a hat or sunglasses. I had my disguise wrapped around me. Then one day, in December 2001, Dawn said, 'Babe, I think you should go and see the doctor.'
"I knew what she meant. I was wheezy and out of breath with even the slightest exertion. It would take me three rolls to get off the bed and I was reduced to wearing XXXL tracksuits. I hadn't dared stand on the scales, but I forced myself - 16st 8lbs. I booked an appointment."
The X Factor judge also revealed that his bandmate Robbie Williams "hurt" him before they reconciled.
> Robbie Williams not quitting Take That, says band rep
Watch the video for Gary Barlow's 1997 single 'Love Won't Wait' below: