Breakfast has long been hailed as the most important meal of the day, and over time, its purported benefits have expanded to include weight control. But it turns out eating breakfast to stave off hunger and avoid overeating later may be questionable advice.

Whether we eat or skip breakfast “has minimal discernible effect on body weight,” says David Allison, dean of the Indiana University School of Public Health at Bloomington, who has conducted research on the subject. “The results at this point seem quite clear.”

So how did it become conventional wisdom that eating breakfast is good for our waistlines, and why does this notion continue to hold such sway? Much of the credit goes to makers of the quintessential breakfast food — cereal.

Beginning in the early 20th century, American cereal maker and food manufacturer C.W. Post elevated breakfast’s status with advertising that hawked the ability of Grape-Nuts to do everything from helping children grow to boosting adults’ brain power.

“Because of his innovative promotional techniques, Post’s influence on the morning meal . . . was profound,” writes food historian Abigail Carroll in her book “Three Squares: The Invention of the American Meal.” Post’s advertising shaped not only what people ate but also what they expected breakfast to do for their health.

By the 1950s, ads for Grape-Nuts were touting its alleged impact on weight. One ad, for example, showed a smiling young woman holding a sexy dress against her thin body as a heavier woman glared jealously at her. Another proclaimed that the “trimmest weight watchers just happen to eat Post Grape-Nuts.”

The theme continued in a 1960s Grape-Nuts ad campaign featuring a mom named Caroline Burke and her look-alike teenage daughter, Dale. The iconic TV commercial showed a young man mistakenly grabbing Caroline in the swimming pool and then exclaiming, “Oh no, Mrs. Burke. I thought you were Dale!” Her secret to staying so slim? Exercise and Grape-Nuts for breakfast.

In the decades that followed, Kellogg’s reinforced the message about breakfast and weight through ads for Special K. People old enough may remember the “pinch an inch” commercials. Said one: “If you can pinch an inch” on your waist — and who can’t?—“the Kellogg’s Special K breakfast may help you lose weight.”

Likewise, the Special K Challenge ad campaign promised that consuming the cereal for breakfast and one other meal every day could lead to the loss of up to six pounds or a jeans size in two weeks. Kellogg’s could point to published research supporting the claim — which the company helped to fund.

Financial support for breakfast-related studies by cereal makers has also probably helped shape our perceptions of the morning meal. In most cases, such research has found that breakfast eaters tend to weigh less than breakfast skippers. Some studies without industry funding have yielded similar results. The problem is that many of these studies tend to show only associations, not cause and effect. It could be that breakfast eaters are thinner because of other lifestyle habits or traits that research didn’t account for.

Sorting this out requires randomized trials, the kind of studies that can show cause and effect. And in such research, which randomly assigns participants to either eat or skip the morning meal, breakfast tends to come up short.

Pooling results from seven trials — a type of study known as a meta-analysis — in 2019, Australian researchers found that participants assigned to eat breakfast did not lose more weight. Nor did they consume fewer calories. Breakfast eaters on average took in 260 more calories per day than breakfast skippers.

A separate meta-analysis in 2020 by Harvard researchers concluded that people who skipped breakfast lost slightly more weight than those who ate breakfast.

“Still, the evidence is not strong enough to recommend eating or skipping breakfast,” says Marta Guasch-Ferré, senior author of the Harvard study.

One limitation of all this research is that it focuses just on whether people ate breakfast — not what or how much they consumed. Starting your day with, say, a giant stack of chocolate-chip pancakes is not the same as eating steel-cut oats with berries.

“It is likely not a good strategy for overall health and weight management to consume a breakfast full of sugars, refined carbohydrates, and processed meat, and may be better to skip breakfast than to eat low quality foods,” Guasch-Ferré says. But she adds that a healthful breakfast is “totally fine.”

Registered dietitian and author Carolyn O’Neil advises people not to force themselves to eat in the morning if they aren’t hungry.

But if you do eat breakfast, she says it should consist of “a combination of foods that will help nourish you and also give you staying power.”

That means including protein, fiber and fat to help fill you up. One of O’Neil’s favorites is sliced tomatoes and cheese on a whole-grain English muffin. Foods such as whole-wheat toast with peanut butter or scrambled eggs with lots of vegetables can also fit the bill.

The morning meal doesn’t have to consist of conventional breakfast foods, O’Neil says. It can include leftovers from dinner.

If you opt for sweet breakfast treats such as doughnuts or pastries, she recommends limiting portions and combining them with foods high in fiber and protein to prevent a spike in blood sugar that will be followed by a crash.

When it comes to cereal, look for brands that are relatively high in fiber and low in sugar. If your favorite cereal isn’t so healthful, try mixing it with one that is. “Think of it as a tossed salad,” O’Neil says.

As for coffee with your breakfast, research shows that it can be part of a healthy breakfast (and protective against many diseases) as long as you don’t overdo it or load it up with cream and sugar.

Whatever you eat, O’Neil suggests avoiding a large breakfast, which can make you feel sluggish. Instead, keep a healthful snack such as plain yogurt and nuts on hand to tide you over until lunch if necessary.

Allison says it’s possible that eating breakfast may have other benefits such as increasing alertness, boosting mood and improving metabolic health, although like weight control, these effects are “not truly well proven.”

In the end, whether to eat breakfast is an individual decision based on your preferences and goals.

“Try different things and see what works for you,” Allison says.