What you eat for breakfast, not just how much, can significantly impact appetite control, weight loss, and the composition of gut microbiota, according to a new study published February 11, 2026, in the British Journal of Nutrition.
Researchers at the University of Aberdeen conducted a randomized, crossover trial involving 19 overweight or obese adults, finding distinct effects from high-fiber and high-protein breakfasts within a calorie-restricted diet. The study investigated the impact of different macronutrient compositions on appetite, energy balance, and gut health markers.
The trial involved participants following a maintenance diet, followed by either a high-fiber weight loss (HFWL) diet or a high-protein weight loss (HPWL) diet, with a washout period between each phase. Both weight loss diets were designed around a “big breakfast” concept, with 45% of daily calories consumed in the morning. The HFWL diet emphasized sources like lentils, fava beans, buckwheat, and wheat bran, while the HPWL diet included fish, poultry, eggs, red meat, and dairy.
Participants on the HFWL diet experienced an average weight loss of 4.87 kg, compared to 3.87 kg on the HPWL diet. Both diets led to reductions in fat mass and fat-free mass, but the reduction in fat-free mass was more pronounced after the HFWL diet. Interestingly, the HPWL diet was found to promote greater feelings of fullness, or satiety, while the HFWL diet had a more positive impact on gut microbiota composition.
Analysis of fecal samples revealed significant differences in gut microbiota between the two diets. The HFWL diet was associated with increased levels of butyrate-producing bacteria, including Anaerostipes hadrus, Roseburia faecis, and Faecalibacterium prausnitzii. The HPWL diet, conversely, showed an association with the genus Streptococcus. Levels of Bifidobacterium, Faecalibacterium, and Roseburia were all higher with the HFWL diet.
Further analysis showed that total short-chain fatty acids (SCFAs), including acetate, butyrate, and propionate, were significantly lower in participants following the HPWL diet compared to those on the HFWL diet. SCFAs are known to play a crucial role in gut health and overall metabolic function. A related study highlighted in PubMed demonstrated that whole-grain wheat consumption exerts a prebiotic effect on the human gut microbiota, increasing levels of beneficial bacteria like bifidobacteria and lactobacilli.
The study also measured metabolic markers. Both diets resulted in significant reductions in lipid levels and improvements in insulin sensitivity. However, the HPWL diet led to a greater decrease in HOMA-β, a measure of pancreatic beta-cell function, while the HFWL diet showed a more favorable impact on glucose levels. Resting metabolic rate (RMR) decreased after both weight loss diets compared to the maintenance diet, and the thermic effect of food (TEF) was lower with the HFWL diet.
Researchers noted that the study cohort was predominantly male, with an average age of 57.4 years and a body mass index of 33.3 kg/m2, which may limit the generalizability of the findings. The study, as published in news-medical.net, focused on short-term effects and further research is needed to determine the long-term sustainability and health implications of these dietary approaches.
The findings suggest that tailoring breakfast composition within a calorie-restricted diet could be a valuable strategy for weight management and improving metabolic health. The choice between a high-protein or high-fiber breakfast may depend on individual goals, with the HPWL diet potentially offering greater satiety for long-term adherence and the HFWL diet promoting a more beneficial gut microbiota profile.