Breakfast that accelerates weight loss. What researchers observed in people who chose fiber over protein

Scientists are reevaluating an older idea about breakfast: when the main meal of the day is moved to the morning, the effects on weight and metabolic health may be better. But the type of food chosen in the first part of the day is decisive.

Researchers at the University of Aberdeen, Scotland, compared two low-calorie diets in overweight or obese adults. One of them was high in fiber, the other in protein, and the daily menu emphasized a hearty breakfast. In both cases, good results were recorded: weight loss and improvements in metabolic parameters, but the mechanisms were different. The protein diet reduced hunger, while the high-fiber version led to slightly greater weight loss and a more balanced gut microbiome.

“This study continues our research published in Cell Metabolism, which showed that a higher caloric intake in the morning is beneficial for appetite control. So the next question was: what exactly should we eat for breakfast,” Alexandra Johnstone, lead author of the study, told Medscape Medical News.

The biological clock influences metabolism

The study, published in British Journal of Nutrition in February 2026, it is part of research on chrononutrition, a field that analyzes the connection between meal times and the way the body processes nutrients. Numerous studies have revealed that when the main meal of the day is served in the morning and dinner remains lighter, the values ​​of some metabolic indicators tend to be better.

The explanation lies in the circadian rhythm, the 24-hour biological clock that regulates the sleep-wake cycle, hormonal fluctuations and body temperature. The body processes food more efficiently in the early part of the day, while in the evening, the metabolism slows down, as the body prepares for rest.

According to research, glucose tolerance is better in the morning and gradually decreases throughout the day. Blood sugar control is influenced by melatonin, the hormone secreted in the evening that signals the brain that it is time to rest. Melatonin causes the pancreas to produce less insulin and simultaneously reduces the ability of fat tissue to absorb glucose from the blood.

“Melatonin sends the signal to the body that it needs to rest and that it should not eat,” Marta Garaulet, a professor of physiology and nutrition at the University of Murcia in Spain, who was not involved in the study, told The Washington Post.

Basically, a meal at 8:00 in the morning has completely different metabolic effects compared to the same meal at 8:00 in the evening.

Hearty breakfast versus hearty dinner

An earlier randomized clinical trial, led by Daniela Jakubowicz of Tel Aviv University, Israel, followed overweight women with metabolic syndrome. The participants followed a hypocaloric diet of approximately 1,400 kcal per day for 12 weeks. The difference was the distribution of calories throughout the day.

One group consumed about 700 kcal at breakfast and only 200 kcal at dinner. The other group did the opposite. Women who ate more in the morning lost an average of almost 8 kilograms, while participants with their main meal in the evening lost about 3.3 kilograms. Greater decreases in waist circumference, blood glucose, insulin and triglycerides were also seen in the same group.

Another research supporting the importance of breakfast, published in Obesity Reviews, pooled the results of nine clinical trials that included a total of 485 adults. Participants who ate most of their calories in the early part of the day lost more weight than those who ate predominantly in the evening and had better cholesterol, blood sugar and insulin sensitivity.

Protein or fiber: what should breakfast contain?

The study led by Johnstone's team also looked at the composition of breakfast. The researchers wanted to see if metabolic outcomes differed when the main meal of the day was protein-based or fiber-based.

The 19 participants (17 men and two women, with an average age of 57.4 years and an average body mass index of 33.3) followed two separate low-calorie diets for 28 days, with all meals provided by the researchers. Both diets had the same caloric distribution: 45% of daily calories at breakfast, 35% at lunch (in a free regime, within the limit of the allocation) and only 20% at dinner.

The difference was the composition of the food. The first diet was high in fiber (50% carbohydrates, 15% protein, and 35% fat), and the menu included lentils, beans, buckwheat, wheat bran, and whole grains. The second diet had a higher protein intake (30% protein, 35% carbohydrate and 35% fat) mainly from fish, poultry, eggs, red meat and dairy.

The fiber diet ensures the equivalent of 30 grams of fiber per day (for an intake of about 2,000 kilocalories), according to European recommendations. The protein diet provided only half of this amount.

For the protein option, breakfast included scrambled eggs, scrambled eggs, boiled eggs, sausage, turkey bacon, toast, fruit smoothies and cereal. In the high-fiber one, participants ate whole grains, oatmeal with seeds and raisins, muesli, bean frittata (Italian omelette), zucchini muffins, beans on toast, and fruit smoothies in the morning.

Who lost more weight

Both diets improved some cardiometabolic indicators. Blood pressure, blood sugar, and insulin decreased, and fat mass, waist circumference, and hip circumference decreased. Blood lipid values ​​also underwent favorable changes.

But the people who followed the high-fiber diet lost an average of 4.87 kilograms, compared to 3.87 kilograms in the group that followed the high-protein diet. Participants in the protein variant group reported less hunger and greater satiety throughout the day. The effect is explained by the fact that the proteins stimulate the intestinal hormones GLP-1 and PYY, which transmit the satiety signal to the brain.

The researchers also analyzed the thermic effect of food, that is, the energy consumed by the body for digestion and metabolism. This was lower in the high-fiber diet than in the high-protein diet.

Blood glucose decreased in both groups, but more so in the case of the fiber diet: 10.2% on an empty stomach and 10% after a meal. In the high-protein diet, the decreases were 8.4% and 6.9%, compared to the maintenance diet.

Fiber keeps the colon healthy

One of the biggest differences between the two diets occurred at the level of the gut microbiome. Analysis of stool samples showed that the high-fiber diet was associated with greater microbial diversity, an indicator correlated in numerous studies with better metabolic function. The authors of the study describe the composition of the microbiota in this group as more favorable.

Specifically, the high-fiber diet favored the proliferation of butyrate-producing bacteria such as Anaerostipes hadrus, Roseburia faecis, and Faecalibacterium prausnitzii. At the genus level, Bifidobacterium, Faecalibacterium, and Roseburia were associated with the high-fiber diet, while Streptococcus was associated with the high-protein diet.

Butyrate and other short-chain fatty acids (SCFAs), such as acetate and propionate, are compounds produced by gut bacteria when they ferment dietary fiber. Previous research has linked them to reduced inflammation, cardiovascular protection and a lower risk of colon cancer. In the present study, total SCFA was higher in the high-fiber diet.

Johnstone noted that he specifically wanted to look at the effects of a high-fiber breakfast because most people don't meet the recommendation of about 30 grams of fiber a day.

There is no universal recipe

The study's author emphasized that in choosing between a high-protein and high-fiber diet, personal goals should be considered.

“The results show that there is no one-size-fits-all diet. Both what we eat and when we eat can influence health,” added the researcher.

In other words, for people who experience acute hunger throughout the day, a protein-rich breakfast might be more helpful. Conversely, a higher fiber intake seems more favorable for the gut microbiome and for some metabolic indicators.

Thomas M. Holland, a physician and researcher at Rush University who was not involved in the study, said that some of the observed improvements are explained by the calorie restriction itself. However, the differences between protein and fiber influenced hunger, energy consumption during digestion, and metabolites produced by gut bacteria. Holland believes longer studies with larger and more diverse groups are needed and suggests testing regimens that combine high protein and fiber intakes in a focused eating pattern early in the day.

What a healthy breakfast looks like, concretely

Those who do not have an appetite in the morning can opt for a light snack upon waking, followed by a hearty lunch and a moderate dinner. The last meal of the day should be eaten at least three hours before bedtime so that digestion can take place before melatonin levels rise. Marta Garaulet also recommends a nocturnal break of about 12 hours between the last meal and breakfast the next day.

To increase fiber intake at breakfast, foods such as oatmeal, muesli, nuts, seeds, beans, fruits, vegetables and whole grains can be added. A concrete example would be a bowl of plain Greek yogurt with berries, nuts, chia seeds and oatmeal, along with a slice of toasted wholemeal bread with almond butter. Another option is psyllium, a type of fiber that can be consumed as a supplement (capsule or powder dissolved in water).

Those who prefer protein in the morning can choose cottage cheese, poached eggs, turkey bacon or protein smoothies.

Limitations of the study

The study also has some limitations. The number of participants was small, only 19 people, and most of them were men. Only two participants were women, and the average age of the group was approximately 57 years. For this reason, the results cannot be easily extrapolated to younger populations or to groups with a different distribution between women and men.

Participants went about their normal lives during the study, not in a strictly controlled clinical environment. In addition, each diet was followed for 28 days. The interval is sufficient to observe changes in the gut microbiome, which can occur within a few days, but does not allow for the assessment of long-term effects.

However, the study used a crossover design, where each participant followed both diets and served as their own point of comparison. This approach allows for clearer evaluation of the effects of the intervention, even on a small sample. In addition, the results are in agreement with other research on chrononutrition and the distribution of calories in the first part of the day.