It isn't 1984. The "big brother" of this novel's title is just that — the big brother of the woman who tells the story, and a really big guy. This kind of bigness — i.e., fat — comes as a surprise to his little sister Pandora, now a middle-aged woman living in Iowa, running a wildly successful enterprise of her own invention and raising the two children of her somewhat prissy, self-employed, furniture-making, fitness-mad husband, Fletcher.

Lionel Shriver, author of "We Need to Talk About Kevin" and "So Much for That," among other mostly wonderful novels, has a lot to say about a lot of things. And I mean that not at all facetiously, because Shriver's willingness to wade into the controversial waters of contemporary issues, and her ability to do so with consummate intelligence and knowledge without compromising narrative integrity (without, that is, making her characters mouthpieces or her stories object lessons) are what make her work distinctive and consistently interesting.

Here, the subject is obesity — or ostensibly obesity, because the shocking fatness of Pandora's brother, once a rakishly good looking and thoroughly cool jazz musician, prompts all manner of reflections on eating, food, sustenance, body images and prejudices, appearance, family, fame, the true coordinates of the American heartland. Pandora, narrating, is the source — or at least the conduit — of all these observations and opinions; and until an unexpected and somewhat dubious turn near the end of the novel, she seems reliable — smart and self-aware, unsentimental and even occasionally remote, but fully engaged in the project that her brother Edison becomes when he arrives for an extended visit, carrying as baggage the considerable handicap of weighing nearly 400 pounds.

Entering a family home already well divided between the indulgent and the ascetic, and between the financially successful and the failing, Edison also acts as something of a free radical, catalyzing a whole series of disturbing developments, not least of these a standoff between Pandora and her threatened, less-than-tolerant husband. Already standing between husband and wife is Pandora's shared past with Edison, as the children of the star of a huge long-running hit sitcom whose television family, and whose onetime fame, still echoes through his children's lives. So when Edison shows up, and Pandora takes time off to see her brother through a radical yearlong diet, she seems to have chosen one family (the old) over another (Fletcher's), and questions about the nature of a marriage enter the mix.

The diet — the story of a heroically undertaken significant change — is pretty nearly irresistible. But what really powers this story, an outsize look at the most basic of human activities, eating, is a search for the definition, and appreciation, of "ordinary life." "I'd always resisted this expression," Pandora says. "Nothing was ordinary about the seemingly-small-but-secretly-ample delights to which it alluded." It is against the broad backdrop of Edison that Shriver brings such delights clearly into view.

Ellen Akins is a writer in Cornucopia, Wis., and the author of "World Like a Knife."

Lionel Shriver.