A high-sugar breakfast may trigger a “rest and digest” state that dampens cognitive focus

Starting the day with a sugary pastry might feel like a treat, but new research suggests it could sabotage your workday before it begins. A study published in the journal Food and Humanity indicates that a high-fat, high-sugar morning meal may dampen cognitive planning abilities and increase sleepiness in young women. The findings imply that nutritional choices at breakfast play a larger role in regulating morning physiological arousal and mental focus than previously realized.

Dietary habits vary widely across populations, yet breakfast is often touted as the foundation for daily energy. Despite this reputation, statistical data indicates that a sizable portion of adult women frequently consume confectionaries or sweet snacks as their first meal of the day. Researchers identify this trend as a potential public health concern, particularly regarding productivity and mental well-being in the workplace.

The autonomic nervous system regulates involuntary body processes, including heart rate and digestion. It functions through two main branches: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic branch prepares the body for action, often described as the “fight or flight” response.

Conversely, the parasympathetic branch promotes a “rest and digest” state, calming the body and conserving energy. Professional work performance typically requires a certain level of alertness and physiological arousal. Fumiaki Hanzawa and colleagues at the University of Hyogo in Japan sought to understand how different breakfast compositions influence this delicate neural balance.

Hanzawa and his team hypothesized that the nutrient density of a meal directly impacts how the nervous system regulates alertness and cognitive processing shortly after eating. To test this, they designed a randomized crossover trial involving 13 healthy female university students. This specific study design ensured that each participant acted as her own control, minimizing the impact of individual biological variations.

On two separate mornings, the women arrived at the laboratory after fasting overnight. They consumed one of two test meals that contained an identical amount of food energy, totaling 497 kilocalories. The researchers allowed for a washout period of at least one week between the two sessions to prevent any lingering effects from the first test.

One meal option was a balanced breakfast modeled after a traditional Japanese meal, known as Washoku. This included boiled rice, salted salmon, an omelet, spinach with sesame sauce, miso soup, and a banana. The nutrient breakdown of this meal favored carbohydrates and protein, with a moderate amount of fat.

The alternative was a high-fat, high-sugar meal designed to mimic a common convenient breakfast of poor nutritional quality. This consisted of sweet doughnut holes and a commercially available strawberry milk drink. This meal derived more than half its total energy from fat and contained very little protein compared to the balanced option.

The researchers monitored several physiological markers for two hours following the meal. They measured body temperature inside the ear to track diet-induced thermogenesis, which is the production of heat in the body caused by metabolizing food. They also recorded heart rate variability to assess the activity of the autonomic nervous system.

At specific intervals, the participants completed computerized cognitive tests. These tasks were designed to measure attention and executive function. Specifically, the researchers looked at “task switching,” which assesses the brain’s ability to shift attention between different rule sets.

The participants also rated their subjective feelings on a sliding scale. They reported their current levels of fatigue, vitality, and sleepiness at multiple time points. This allowed the researchers to compare the women’s internal psychological states with their objective physiological data.

The physiological responses showed distinct patterns depending on the food consumed. The balanced breakfast prompted a measurable rise in body temperature and heart rate shortly after eating. This physiological shift suggests an activation of the sympathetic nervous system, preparing the body for the day’s activities.

In contrast, the doughnut and sweetened milk meal failed to raise body temperature to the same degree. Instead, the data revealed a dominant response from the parasympathetic nervous system immediately after consumption. This suggests the sugary meal induced a state of relaxation and digestion rather than physiological readiness.

Subjective reports from the participants mirrored these physical changes. The women reported feeling higher levels of vitality after consuming the balanced meal containing rice and fish. This feeling of energy persisted during the post-meal monitoring period.

Conversely, when the same women ate the high-fat, high-sugar breakfast, they reported increased sleepiness. This sensation of lethargy aligns with the parasympathetic dominance observed in the heart rate data. The anticipated energy boost from the sugar did not translate into a feeling of vitality.

The cognitive testing revealed that the sugary meal led to a decline in planning function. Specifically, the participants struggled more with task switching after the high-fat, high-sugar breakfast compared to the balanced meal. This function is vital for organizing steps to achieve a goal and adapting to changing work requirements.

Unexpectedly, the high-fat, high-sugar group performed slightly better on a specific visual attention task. The authors suggest this could be due to a temporary dopamine release triggered by the sweet taste. However, this isolated improvement did not extend to the more complex executive functions required for planning.

The researchers propose that the difference in carbohydrate types may explain some of the results. The balanced meal contained rice, which is rich in polysaccharides like amylose and amylopectin. These complex carbohydrates digest differently than the sucrose found in the doughnuts and sweetened milk.

Protein content also likely played a role in the thermal effects observed. The balanced meal contained significantly more protein, which is known to require more energy to metabolize than fat or sugar. This thermogenic effect contributes to the rise in body temperature and the associated feeling of alertness.

The study implies that work performance is not just about caloric intake but the quality of those calories. A breakfast that triggers a “rest and digest” response may be counterproductive for someone attempting to start a workday. The mental fog and sleepiness associated with the high-fat, high-sugar meal could hinder productivity.

While the results provide insight into diet and physiology, the study has limitations that affect broader applications. The sample size was small, involving only 13 participants from a specific age group and gender. This limits the ability to generalize the results to men or older adults with different metabolic profiles.

The study also focused exclusively on young students rather than full-time workers. Actual workplace stress and physical demands might interact with diet in ways this laboratory setting could not replicate. Additionally, the study only examined immediate, short-term effects following a single meal.

It remains unclear how long-term habitual consumption of high-fat, high-sugar breakfasts might alter these responses over months or years. Chronic exposure to such a diet could potentially lead to different adaptations or more severe deficits. The researchers note that habitual poor diet is already linked to cognitive decline in other epidemiological studies.

Hanzawa and the research team suggest that future investigations should expand the demographic pool. Including male participants and older workers would help clarify if these physiological responses are universal. They also recommend examining how these physiological changes translate into actual performance metrics in a real-world office environment.

The study, “High-fat, high-sugar breakfast worsen morning mood, cognitive performance, and cardiac sympathetic nervous system activity in young women,” was authored by Fumiaki Hanzawa, Manaka Hashimoto, Mana Gonda, Miyoko Okuzono, Yumi Takayama, Yukina Yumen, and Narumi Nagai.