Is there any meal more confusing than breakfast?

For years we all heard that breakfast is the most important meal—a jump-start for your body and brain and a tool for managing weight . Then the message on breakfast got muddied.

Several big studies appeared to knock down earlier research suggesting that eating breakfast aids weight loss. Concerns about ultraprocessed foods made people wary of morning staples like cereal and flavored yogurt. And the rise of intermittent fasting , which restricts eating to particular time windows, made skipping breakfast popular—and let people feel good about it.

Younger adults, in particular, have joined the breakfast backlash. Nearly a quarter of people between the ages of 20 and 39 skip it, while overall 15% of U.S. adults do, according to federal data.

The latest thinking among nutrition researchers, though, boils down to this: A healthy breakfast is a good idea for most people, and skipping it to shed pounds probably won’t work.

As ever, the details matter. What people eat—and how much—has a big impact over time. A regular breakfast of sugary pastries, for example, could nudge the numbers on the scale upward, whereas a serving of Greek yogurt with berries could make them fall. Perhaps surprisingly, some of the studies casting doubt on breakfast’s weight-loss benefits didn’t take details like those into account.

Breakfast may also keep you from eating late at night , which research shows raises the risk of obesity.

One of the biggest reasons to eat breakfast: It is when people typically get a big share of key nutrients. While the meal only contributes about 21% of a U.S. adult’s daily calories, the average breakfast provides 42% of vitamin D and 30% of vitamin A, iron and folate, according to an analysis of federal data by a committee of scientists advising the U.S. government on its next round of dietary guidelines.

“The foods we’re eating are more nutrient-dense and more healthful at breakfast, compared to some of the other times we eat,” says Heather Eicher-Miller , a professor of nutrition science at Purdue University who was on the federal dietary-guidelines committee.

Cereals are often fortified with vitamins and minerals, she notes, and milk and yogurt first thing in the morning may be the only time many adults have calcium and vitamin D-rich dairy products. Our breakfasts also tend to contain less of what we don’t need, like excess saturated fat and sodium, at least compared with what we eat later in the day.

Breakfast and weight

The original prescription to eat breakfast to lose weight was spurred by several studies years ago that found people who ate it had a lower body-mass index and a reduced risk of obesity than breakfast skippers.

Researchers speculated this was because skipping breakfast would cause people to be hungrier, eat more and make poorer food choices later in the day, says Christopher Gardner , a nutrition scientist and professor of medicine at Stanford Medicine.

That hypothesis lost considerable ground when scientists tested the approach in randomized controlled trials. One 2019 analysis of 13 studies found people assigned to eat breakfast ate more calories daily. Another study published in the journal Obesity in 2020 found people directed to skip breakfast lost weight after about two months.

But since then, some leading researchers have looked at those results with a more skeptical eye. Those trials were relatively short, and meaningful weight loss generally happens over time. And the studies often didn’t look at what people actually ate.

“The quality of the breakfast makes a big difference,” says Cristina Palacios , chair and professor in the department of dietetics and nutrition at Florida International University, who was also on the federal dietary-guidelines committee. The idea that skipping breakfast can help people lose weight was refuted by a report released by that committee in January.

For the die-hards

What about people who can’t stomach the idea of eating in the morning?

Don’t force yourself, says Flavia Cicuttini , a professor at Monash University in Australia who has studied breakfast consumption.

She’s part of a group of scientists who say there is no one-size-fits-all approach to breakfast. So is Stanford’s Gardner, who says your response to the meal is highly individual and likely influenced by your personal body clock and metabolism.

“A lot of people function just fine without breakfast, and I know I’m personally a wreck without breakfast,” he says.

One way breakfast can help with weight is curbing late-night eating, says Marie-Pierre St-Onge , associate professor of nutritional medicine at Columbia University Irving Medical Center. When calories are eaten closer to bedtime, you burn less fat, she adds.

For a study published in the journal Obesity, St-Onge and colleagues had people spend two weeks eating all their meals and snacks between 9 a.m. and 7 p.m. (Dinner was at 6 p.m., with a snack at 7 p.m.) For two more weeks, participants were directed to eat between 1 p.m. and 11 p.m. (Dinner was at 10 p.m.; a bedtime snack was at 11 p.m.) The diets remained the same. People burned the same calories in both windows. But when it came to fat, they burned less of it around the later dinner and snack.

What does this mean for intermittent fasters? If you do skip breakfast, try to stop eating by 7 p.m., St-Onge says. And you can up your nutrient intake by adding healthy breakfast foods to your lunch , snack or dinner menu.

What makes a good breakfast

Pick a meal combining protein, carbohydrates, healthy fats and fiber, says Maya Vadiveloo , associate professor of nutrition at the University of Rhode Island. Use breakfast to get in the fruits, vegetables and whole grains that most Americans don’t eat enough of.

Vadiveloo’s recommendations:

These breakfasts of about 300-400 calories should stave off hunger for three to four hours, she says.

What to keep to a limit? Many yogurts, cereals and breakfast bars are loaded with added sugars. Savory alternatives like bacon-egg-and-cheese sandwiches are high in saturated fat and sodium. With cereals, Vadiveloo suggests sticking to those with less than eight grams of sugar per serving.

Write to Andrea Petersen at andrea.petersen@wsj.com