Breakfast meals higher in protein promoted feelings of satiety, and those higher in fiber boosted beneficial gut microbiota, while both types led to weight loss and metabolic benefits, a small randomized trial showed.

Growing evidence supports early eating to control appetite and energy balance but few controlled studies assess the amount and/or type of breakfast needed to achieve optimal outcomes, according to Alexandra M. Johnstone, PhD, of the University of Aberdeen, Aberdeen, Scotland, and colleagues.

“The new paper follows on from our Cell Metabolism paper, which showed that morning-loaded calories were beneficial for appetite control,” Johnstone told Medscape Medical News. “So the next question was, what to eat for breakfast?”

The findings of the current study “support the concept that not one diet fits all people,” she said. “Both what and when we eat can influence health. A diet with high protein was good for appetite control and high fiber was good for gut health.”

The study was published online in the British Journal of Nutrition.

Breakfast Composition Key

Researchers conducted a randomized, within-participant diet intervention aimed at examining the effects of higher fiber (HF) and higher protein (HP) breakfasts in adults with overweight/obesity.

The primary outcome was energy balance, gauged by changes in body weight. Secondary outcomes were gut health (measured by changes in fecal microbiota composition and microbial metabolite concentrations) and subjective appetite, assessed with visual analogue scales.

Nineteen healthy adults with overweight or obesity (two women and 17 men; mean age, 57; mean BMI, 33.3) who ate breakfast regularly participated in two randomized 28-day weight-loss diets — HF or HP — with all food provided.

Participants were fed based on their measured resting energy requirements, according to the standard UK diet composition: 30% fat, 15% protein, and 55% carbohydrates.

The HF diet (35% fat, 15% protein, and 50% carbohydrates) contained at least 30 g/d of total dietary fiber for a 2000 kcal intake and was provided as mixed soluble and insoluble fiber sources to maintain palatability (eg, wheat bran, fava bean, lentil, buckwheat).

The HP diet (35% fat, 30% protein, and 35% carbohydrates) contained a mixed meat matrix to include poultry, fish, red meat, eggs, and dairy. Total dietary fiber was no more than 15 g/d for a 2000 kcal intake.

Participants ate only three meals a day, with 45% of daily calories consumed in the morning (big breakfast) and 20% of calories consumed at the evening meal. To allow for some behavioral (appetite) adaptation, the lunch (afternoon) meal could be consumed ad libitum up to 35% of daily calories, as provided.

Participants lost weight on both diets, with a mean loss of -4.87 kg on the HF diet and -3.87 kg on the HP diet. The HP diet was superior to the HF diet for suppressing subjective appetite. Relative to the HF meal, the area under the curve after the HP meal was significantly lower for appetite score and significantly higher for fullness.

These results indicate that despite there being a reduced energy intake from both weight-loss meals, the HP meal was better able to maintain the satiating capacity, while the HF meal resulted in lower satiety after eating.

Gut hormones (glucose-dependent insulinotropic polypeptide [GIP], GLP-1, Peptide YY, and ghrelin) were analyzed in plasma over time. The area under the curve for GIP was significantly lower on the HP diet than on the HF (28.4% lower) diet.

In contrast, the HP diet had a significantly higher GLP-1 area under the curve (14.8%) than the HF diet.

The fecal microbiota analysis showed beneficial groups of bacteria, including significant increases in proportional abundance of bifidobacteria and the butyrate-producers Faecalibacterium and Roseburia, on the HF diet.

Regarding the secondary outcomes, both diets showed cardiometabolic improvements resulting from weight loss, and beneficial effects on gastric emptying.

One limitation of the study was that it was conducted under free-living conditions, and this can increase the chances of participant noncompliance, the authors noted. In addition, a longer trial might yield different results.

Nevertheless, they concluded, “Breakfast composition has an important role in influencing subjective appetite with the higher-protein diet promoting greater feelings of satiety. The proportional abundance of putatively beneficial groups of gut microbiota was markedly higher on the fiber-enriched diet, which may be preferable for gut health.”

Longer Studies Needed

Commenting on the study for Medscape Medical News, American Gastroenterological Association spokesperson Sumant Inamdar, MD, University of Arkansas for Medical Sciences in Little Rock, Arkansas, said the study “reinforces the idea that breakfast composition is not just about calories. The macronutrient mix can meaningfully shape how people feel and how their metabolism and microbiome respond, even over a relatively short period.”

That said, Inamdar had several caveats. Not only was the cohort small but it was predominantly men, he noted, “which makes it harder to know how generalizable these results are, particularly to women, younger individuals, or patients with different metabolic or gastrointestinal conditions.”

The 4-week duration “is enough to observe short-term physiological changes but not long enough to tell us whether these effects translate into sustained weight loss, durable microbiome shifts, or meaningful clinical outcomes,” said Inamdar, who was not involved in the study.

“We know from nutrition research that early responses do not always predict long-term success, especially when it comes to adherence and real-world behavior,” he said. “In addition, the study mainly measured appetite, lab markers, and microbiome changes, which focus on short-term biological markers rather than real-world long-term outcomes like sustained weight loss or reduced disease risk.”

“In real life, people do not choose between protein and fiber,” he added. “They eat both, so studying balanced breakfasts that support satiety and gut health together would be more practical. We also need to connect microbiome changes to outcomes that matter, like symptoms, metabolic health, and quality of life, so these findings can truly guide everyday care.”

This work was supported by the Scottish Government as part of the Strategic Research Programme at the Rowett Institute (2017-2021).

AMJ reported holding a voluntary committee role with the British Nutrition Foundation, The Nutrition Society, and the Association for the Study of Obesity. Inamdar reported having no conflicts of interest.

Marilynn Larkin, MA, is an award-winning medical writer and editor whose work has appeared in numerous publications, including Medscape Medical News and its sister publication MDedge, The Lancet (where she was a contributing editor), and Reuters Health.