America’s hyper-profitable obsession with its dogs and cats has reached a new peak. The year’s biggest private-equity buyout, announced Sunday for more than $8 billion, wasn’t for a big-idea Web startup or a profitable mega-firm, but for PetSmart, the strip-mall chain hawking kitty litter and puppy chow.

The pet-supply giant has built an empire on the country’s pet-food sales, which have nearly doubled since 2000, to $22 billion this year. And much of that money has come from a growing market of pricey “premium” dog and cat food, which Americans have spent a record-breaking $10 billion on this year.

Marketed as organic boosters of athletic performance and mental acuity, premium food now sells better than both low- and mid-priced foods combined, a lucrative symbol of how Americans see their pets worthy of gourmet chow. Consumer surveys by market researcher Mintel found 72 percent of U.S. pet owners said they considered their pets “part of the family,” and 79 percent said the quality of their pets’ food is as important as their own.

“It’s this big mature industry with a really devoted customer base that, you know, is willing to do pretty much just about anything for their pets,” said George Puro, a pet-market analyst with market researcher Packaged Facts. “With food, people think: What would they want to do for themselves, and should they be doing the same thing for their pets?”

PetSmart has struggled lately as sales of pet stuff has shifted to the bigger, savvier web storefronts of Target and Seeing a potentially undertapped goldmine, hedge funds have pressured the company for months to sell, and on Sunday the retailer agreed to sell itself to a team led by investment firm BC Partners, which will pay $83 a share, about 7 percent higher than the company’s Friday closing price.

Founded in the late ’80s, the pet chain now employs 54,000 workers across more than 1,300 stores in the U.S., Canada and Puerto Rico, most of which contain full-service veterinary centers. Its pet merchandise sales have boomed, led in part by product sponsorships with Martha Stewart and former Poison singer Bret Michaels, whose “Pets Rock” dogwear line includes a $49.99 spiked collar and a pink sequined coat.

But the company’s true strength has been in convincing pet parents to “trade up” from cheap bulk food to pricier organic, grain-free or premium blends, served fresh, frozen or raw.

“We see the continued humanization of pets, people treating their pets like family,” David K. Lenhardt, PetSmart’s president and chief executive, said in an earnings call last month. “We continue to feel very good about our ability to trade customers up.”

Pet-food companies have thrived by engineering reasons for pet owners to keep spending more on kibble. Purina Pro Plan offers 84 different dog foods, separated by “lifestage,” from puppy to senior, and specific interests, including weight management and other perks. One “Focus” food is marketed as helping “nourish brain and vision development.”

The company also sells nutritional supplement bars for dogs, called PRiME and ReFUEL, and recently rolled out a dog-training mobile app that tracks a dog’s “Greatness Points” and offers athletic and nutritional advice. (“Pro Tip: Real chicken is the #1 ingredient in the Pro Plan Sport product line.”)

About 55 percent of American households own a pet of some kind, including 45 million dog households and 30 million cat households, according to Packaged Facts, and most say their spending on pet stuff has grown. About half of all dog owners say they are now spending more on dog food and treats, while a third say they are spending more on pet supplies and professional dog-care services.

The pet business lends itself well to the upsell, largely because most pet owners can afford it. Half of all households with an income between $75,000 and $100,000 own a dog, compared to the 35 percent of households with incomes lower than $50,000.

Some of the trends of human diets have found their ways to the pet-store aisle. The Paleo Diet, a fad modeled after the eating habits of hunter-gatherer cave people, has led to “ancestral” pet foods, cooked with the kinds of proteins devoured by primitive dogs and cats.

Then, of course, there is what Euromonitor analyst Gina Westbrook called the “extreme pet humanizer,” who “sees their pet as a baby or even a fashion statement,” and who is often willing to spend lavishly on little pet luxuries “such as designer outfits and crystal encrusted drinking bowls.”

Pet parents increasingly expect the food they buy to offer “improved health, increased energy, enhanced performance and better resistance to diseases,” according to Roger Clemens, an adjunct professor in pharmacology at the University of Southern California who has written about the “humanization” of pet foods.

But the result, while energizing for the seller, can often mean little for the animal that eats it. One example, Clemens said: Despite the sale of gluten-free pet foods, gluten sensitivity is “not a health issue among most companion animals,” and rare cases have been resolved quickly without the need for special chow.

Selling pet food brings its own challenges, beyond even that its target customers can’t speak. Peter Ayton, an analyst with market researcher Mintel, wrote, “Competition comes from products which have no consumer value; i.e. the leftovers of humans.”