Not long ago, "weight diversity speakers" advocating "fat acceptance" for the "fat pride" community would have been a gag on the US TV satire The Daily Show. Nowadays, they are a forceful voice in American politics.

Authors such as Marilyn Wann (Fat? So!) and Linda Bacon (Health at Every Size) challenge the assumption that fat is a problem. For the two-thirds of Americans now overweight or obese, their message is beguiling: being heavy does not equate with being unhealthy. What's really required, they argue, is a cultural attitude shift, whereby we stop sneering at the obese as over-indulgent slobs, accept the high-calorie habit as a defensible lifestyle choice and expand our aesthetic to embrace fat as beautiful.

Sure, artists from Rubens to Botero have celebrated the magnificence of mass, and western standards of beauty have grown literally too narrow. In a cheesier vein, Gok Wan, presenter of Channel 4's How to Look Good Naked, has laudably campaigned to restore the concept of the voluptuous and to ameliorate female self-loathing. Yet denying the link between obesity and poor health defies medical fact.

I have an older brother about whom I write with some reluctance, because I feel protective of him. He's topping 330lb: 24 stone. He was once 5ft 7in tall, but his vertebrae have compressed, and at 5ft 3in I now look him straight in the eye. I used to look up to him in every sense. I ended our last two visits in tears. My brother breaks my heart. He's obscenely smart, testing in adolescence (to the irritation of his siblings) as having a genius-level IQ. He's a well-read, intellectually adventurous man who can talk your ear off about why a meniscus is either concave or convex. But he's also a sadly good test case for the claim that one can be "healthy at every size".

My brother has diabetes, and has an average blood-sugar level — normal being about 5g – of 11.5g. His feet swell so that they don't fit in his boots. A bout of congestive heart failure nearly killed him. He can barely walk, and venturing out of his studio apartment is an ordeal. Obesity exacerbates his emphysema, and he drags a portable oxygen tank with him like a faithful dog. Not long ago, the tank's battery died at a bus stop. My brother went into respiratory arrest, and only a Good Samaritan who rushed off the bus got him to hospital in time to save his life. Every time I talk to my brother, I wonder if it's for the last time. Planning to see him during an author's tour in March, I'm counting the days, actively anxious that he won't still be with us three months from now.

My brother is also a good example of the kind of mitigating circumstances that sometimes attend being grossly overweight. Having been beaten up with a metal baseball bat in 1998 and broadsided by a careless driver while on his moped two years later, he boasts that his body clinks with "24 pieces of titanium" that set off alarms at airport security. The resultant chronic pain has made it impossible for him to exercise. The fact that my brother is fat is not, altogether, his fault.

However. He also eats too much.

I have buckets of sympathy for the obese, often subject to cruelty, ridicule, denunciation and contempt. Fatties are the one subgroup you can still make fun of on the BBC and not lose your job. Likewise, I sympathise with their recurrent sense of hopelessness. Dieting is odious and can require years of determination and sacrifice. I entirely understand the impulse to say screw it and have another piece of cake. But the "fat pride community" doesn't want my sympathy. They want acceptance. They want respect.

Respect, yes, for their feelings, for their humanity. But I won't simply accept that my brother is fat. And his only chance at a future is to refuse to accept himself that his weight gain is irrevocable. Nor can I quite put obesity on a par with being black, female or homosexual. While discrimination against heavy people should be illegal (save when fielding, say, job applications to lead tourists up Mount Kilimanjaro), to equate fat with race, gender and sexual orientation is to cast obesity as an unassailable state over which we have no control.

A once amply proportioned friend of mine was at last moved to go on a successful all-liquid diet when his doctor said starkly, "I don't have any old, fat patients." My brother is only 55, and without drastic intervention – gastric bypass surgery or a sudden resolve on his part that I fear is unlikely – I doubt he'll see 60. My brother is eating himself to death. I love him dearly, and I can't support any political movement that would have him believe he can be "healthy at any size".

An hour or so after this column was filed, Shriver's brother suffered a sudden respiratory crisis during a visit to their parents, and was admitted to hospital in New York. He died of cardiac arrest on 22 November.

• This article first appeared in Standpoint magazine.