Forget about a black man becoming president. America has passed a new milestone this week, and one few expected to see. The most famous dieter in a weight-obsessed country has announced that she is chucking out the calorie counter: ladies and gentlemen, Oprah Winfrey has stepped off the scales.

Oprah has announced on the cover of her magazine, O, that she is giving up her lifelong, highly emotional, sporadically successful, often not, diet. Yet judging from the accompanying photo of her, shrugging unhappily in a tracksuit, this proclamation is less triumphantly liberating than sadly defeatist.

It is hard to overstate just how earth-shattering an event this is, for both dieters and Oprah fans alike (admittedly, there is quite a crossover in that Venn diagram). Never mind her poverty-torn upbringing, the sexual abuse she suffered as a child, the racism she has overcome - the only part of her life that Oprah has complained about repeatedly has been her weight.

And, oh, we Oprah fans rode on that rollercoaster with her. We were there during her liquid-diet phase (it was the 80s, obviously). We were there when she dragged out a wheelbarrow filled with 60lb of animal fat to show how much weight she had lost (before promptly putting it back on). We were there when she was at her lightest (160lb) and her heaviest (237lb), numbers that this otherwise private woman (she may encourage everyone else to let it all hang out, but she herself almost never gives interviews) has shared with the world.

No matter what, the weight always went back on. She writes touchingly in O about the self-loathing it has engendered, such as the time she hated being on stage with Tina Turner and Cher because "I felt like a fat cow". That sentence - combining A-list fabulousness with empathetic normality - sums up Oprah's success.

Oprah's dieting has not just proved to a fame-obsessed world that fame can still mean fallibility, but also that thin isn't always desirable. Oprah just looks wrong thin. It is simply not her build, which explains why she could never stay that way. But more importantly, whatever her weight, Oprah rocks. If there was a choice between being a 7st actress moping around doing nothing, and a whatever-weight Oprah being Oprah, surely the only person who would struggle to make the obvious choice is, alas, Oprah herself.