The writer, who is worth more than 560 million dollars, denied that she "does pilates, yoga, jogs, has botox injections and has cut out saturated fats".
Rumours that the writer was following a Hollywood-style health and beauty regime started circulating after she appeared noticeably slimmer and more polished in her recent public appearances.
She wrote on her website: "In the interests of accuracy I must point out that, far from losing weight, I've gained a good bit ... So J K Rowling's top tip for today is: eat more."
The mother of three joked: "Perhaps my next project could be a revolutionary diet book?"
Rowling also slammed size zero models and Paris Hilton-type celebrities "whose only function in the world appears to be supporting the trade in overpriced handbags and rat-sized dogs".
The 42-year-old described them as "empty headed, self-obsessed, emaciated clones," reports the Daily Mail.
The author was recently featured in the top 20 best-dressed women by Harpers Bazaar beating supermodel Kate Moss in the style stakes.