In 1863, Dr. James Caleb Jackson, a staunch nutrition advocate, created the world’s first recipe for what would become granola. The dish (he called it granula) consisted of dense, unsweetened bran nuggets, soaked in milk.

But if Dr. Jackson were alive today, it’s unlikely he would recognize the modern incarnation of his creation. Despite its reputation as a breakfast health food, granola has become nothing less than a sweet dessert.

Commercial varieties of granola are often loaded with enough added sugar to rival a slice of chocolate cake. Homemade granola recipes are often no better, calling for ample amounts of refined sugar, maple syrup, honey and other sweeteners.

Granola bars and cereals are widely marketed as wholesome and natural, or made with whole grains, which helps to give the products a health halo. But experts say they are junk foods in disguise. Even the federal government’s dietary guidelines label granola as a “grain-based dessert,” placing it in the same category as cookies, doughnuts and cake.

A survey by the New York Times Upshot this summer found that a majority of Americans described granola and granola bars as “healthy,” even though most dietitians know better.

“When I think of granola, I think of piles of sugar,” said Cassie Bjork, a registered dietitian who runs a popular health blog. “It’s advertised as a healthy choice. But the reality is that it’s usually not.”

Sales data suggest that the popularity of granola is at its peak, with Americans spending nearly $2 billion annually on granola products. In 2014, the market for private-label granola and related cereals in the United States exceeded $750 million, according to Statista, a market research firm. Sales of granola bars and similar snack bars topped $1 billion, led by the General Mills brand Nature Valley.

Serving sizes on granola labels can vary widely, from a quarter cup to one-half cup. But the average person eating granola for breakfast is probably consuming closer to a full cup, said Andy Bellatti, a Las Vegas-based dietitian.

A cup of Nature Valley Oats ’n Honey Protein Granola has 24 grams of sugar and actually contains more sugar than protein by weight. Nature Valley also has a Peanut Butter ’n Dark Chocolate Protein Granola that has 30 grams of sugar per cup. Bear Naked Honey Almond Granola has 20 grams of sugar per cup.

By comparison, a slice of chocolate cake with frosting has 26 grams of sugar. A cup of ice cream has 28 grams of sugar. A regular Krispy Kreme glazed doughnut has 10 grams of sugar, according to the Krispy Kreme website.

“At its most basic level, granola is just super sweet, crunchy oatmeal,” Mr. Bellatti said. “Apart from some specialty brands, you can’t find unsweetened granola.”

That wasn’t always the case. Dr. Jackson’s original granula consisted of little more than graham flour, which was baked into brittle sheets, broken into little pieces and then baked again. It was more like an unsweetened early version of Grape Nuts than the granola of today.

Dr. Jackson wanted an alternative to the more common breakfasts of that time, which were typically some combination of meat, bread and cheese, and griddlecakes fried in grease and doused with syrup, butter or jam, said Abigail Carroll, a cultural historian and the author of “Three Squares: The Invention of the American Meal.”

Dr. Jackson was a health reformer who ran a sanitarium in Dansville, N.Y. — about 80 miles east of Buffalo — where he advocated vegetarianism and a diet of simple foods. His granula was probably not very tasty by today’s standards. But giving up ham and griddlecakes for simple grains often relieved indigestion and made a lot of people feel better, Ms. Carroll said.

Eventually, Dr. John Harvey Kellogg, another health reformer who owned a rival sanitarium in Michigan, stole Dr. Jackson’s recipe. When Dr. Jackson sued, Dr. Kellogg changed the name of his cereal to granola, which he and his brother, Will Keith Kellogg, experimented with, leading to the creation of cornflakes.

Dr. Kellogg wanted to keep his cereals sugar-free — he was a Seventh-day Adventist who famously advocated against alcohol, meat, sugar, tobacco and sexual activity. But his brother, Will, insisted on sweetening them, and the two parted ways. Will went on to launch the Kellogg Company, and its enormously popular sweet cereals, which eventually included Froot Loops and Cocoa Krispies.

While sugary cereals soared in popularity over the following decades, granola remained a mainstay in the Seventh-day Adventist community, which is known to shun foods high in sugar, salt and other additives. Packaged granola is believed to have first appeared in the 1960s when one Adventist and granola promoter, Layton Gentry — nicknamed Johnny Granola-Seed by Time magazine — sold his recipe of rolled oats, wheat germ and sesame seeds to two food companies. As granola crossed over from the Seventh-day Adventist community to the mainstream, it became increasingly laden with sugar.

By the 1970s, one popular granola recipe by the celebrity nutritionist Adelle Davis, whose cookbooks and health books sold millions of copies, called for five cups of rolled oats, one cup of wheat germ and one cup of honey, among other things. At around the same time, the Quaker Oats Company, Kellogg’s and General Mills also began introducing their own sweetened, mainstream versions of granola.

There are signs that some consumers are rethinking their penchant for sweetened cereals. A study last year by the market research firm Packaged Facts found that two out of three Americans said they were seeking out grocery foods with fewer and simpler ingredients. The NPD Group recently found that sugar is the No. 1 ingredient Americans are looking to avoid.

As a result, some lightly sweetened brands of granola are starting to emerge. Mr. Bellatti, the dietitian, said he often recommends Mamma Chia’s vanilla almond granola, which has about 10 grams of sugar per cup.

Homemade granola is also an option, but many recipes include lots of sugar, which gives granola its clumpy texture. Amy Roskelley, owner of the Super Healthy Kids website, makes a sugar-free version using egg whites for clumping, and cinnamon and fresh fruit like blueberries for flavor.

Ms. Bjork, the nutrition blogger, says granola is best avoided altogether, and she labeled granola bars one of “Five Foods to Skip This Year” on her website.

“People are always surprised when I tell them it’s not as healthy as they think it is,” she said.