Breakfast plays an important role in supplying energy and nutrients [1
], yet breakfast skipping and selection of foods of low nutritional quality are common, especially in urban students [2
]. In addition to nutritional benefits, there is growing evidence that breakfast may enhance cognitive and academic performance [3
]. To obtain effective nutritional strategy, it is important to clarify the contributions of different macronutrients in breakfast on cognitive performance. Glucose is the main fuel for brain function. Intake of carbohydrates has a direct effect on blood glucose levels and, in turn, on cognitive function [5
]. Both long- [7
] and short-term [9
] effects of consumption of specific kinds of fats on cognitive function have also been demonstrated. As to protein, prior works have demonstrated both long- [10
] and short-term [12
] effects of consumption of high-protein meal on cognitive performance compared to an isoenergetic high-carbohydrate meal. However, in the studies investigating the acute effects of protein on cognitive performance, the protein content consumed was much higher than that indicated in the Recommended Dietary Allowance (RDA). Even though the RDA recommends 0.8 g protein per kg body weight per day in the United States [14
] and 60 g per day for males older than 12 years in Japan [15
], the high-protein meals used in these studies contained 80–100 g per meal. The effect of adequate amounts of protein within a single meal on cognitive performance needs to be clarified.
Dairy products are good source of protein. Frequent dairy product consumption is found to be associated with better cognitive performance [16
], although the underlying mechanism is still to be determined. An acute beneficial effect of dairy milk on cognition is also reported in persons with higher fasting glucose [18
]. It is well recognized that the protein fraction in bovine milk contains a balanced profile of essential amino acids and is highly digestible, and its amino acids are highly absorbable [19
]. However, absorption of the components in bovine milk is slowed by gastric acid—induced coagulation of caseins, which constitute up to 80% of the milk protein fraction [21
]. To suppress this aggregation, an acidification procedure for milk has been developed using stabilizers as well as a dispersant [22
]. Amino acids from the acidified milk appeared to be absorbed more quickly compared with untreated milk, as was shown by a greater increase in the plasma amino acid level after ingestion in rats.
A mental arithmetic task is one of the tasks to assess cognitive performance. Higuchi demonstrated that nutritionally balanced breakfast improved the task performance of the paper-based mental arithmetic task compared to a high-carbohydrate breakfast [23
]. Certain physiological indices may be associated with cognitive performance [24
]. One such index is body temperature. Within an optimal thermal range, increased body temperature has been associated with improved cognitive performance and alertness in humans, as shown by studies in which the circadian phase was controlled [24
]. Another index that may be associated with cognitive performance is heart rate variability (HRV). HRV is the fluctuation in the beat-to-beat interval in the heart rate, resulting from the interaction between the sympathetic and parasympathetic arms of the autonomic nervous system [27
]. It has been suggested that HRV reflects the capacity of individuals to respond to changing environmental demands, and it has often been considered to be a biomarker of the ability to regulate cognitive, emotional and behavioral processes [26
We hypothesized that a special milk protein drink may have an acute beneficial effect on cognition and modify associated physiological indices. To test this, we performed a crossover study to investigate the acute effects of an acidified milk protein drink on cognitive performance as well as subjective feeling, body temperature, and HRV compared to an isoenergetic placebo drink.
The present study demonstrated that subjects improved in cognitive performance when they consumed an acidified milk protein drink containing 16 g of milk proteins compared with an isoenergetic placebo drink. In addition, we confirmed that subjects, when they consumed the milk protein drink, also showed an increased HRV, which has been recently suggested to correlate with cognitive function. To the best of our knowledge, this is the first study to show that a single administration of a protein supplement is associated with improved cognitive performance and increased HRV in comparison with isoenergetic-fed controls.
Recent reviews have suggested that breakfast consumption may acutely improve memory, attention, motor and executive function, although the contribution of macronutrients on cognitive function is inconclusive [32
]. Our result shows a significant increase in the number of correct answers in the UKT when subjects consumed an acidified milk protein drink. The UKT consists of simple mental arithmetic and handwriting and is used to measure cognitive task performance [33
]. Mental arithmetic requires the involvement of several cognitive domains including short-term memory and sustained attention [37
]. Handwriting is also a complex perceptual–motor skill [38
]. On the other hand, the subjects’ scores on the Stroop test did not improve after they drank the protein drink. This might be partly because the Stroop test results were measured inappropriately. In this study, we used the paper-based version of the Stroop test repeatedly throughout the test sessions, such that subjects answered the same questions repeatedly within the same day; the subjects may have memorized the questions as a result. The Stroop test measures executive function, which would involve higher and more complex cognitive processes [39
]. Therefore, it is also possible that consumption of milk protein drinks may not have an acute effect on executive function. In addition, we found no differences in the changes in subjective feeling between drink conditions. Thus, we concluded that the ingestion of the acidified milk protein drink must have improved cognitive performance without changing the subjects’ feelings.
The body temperature was similar between the drink conditions. While protein feeding has been observed to have a higher thermic effect compared with other macronutrients [40
], it is unlikely that the 16 g of protein in the test drink was enough to increase the peripheral body temperature of the subjects in this study.
The HF and RMSSD of HRV during the UKT were higher when the subjects ingested the acidified milk protein drink compared with the control drink. It is assumed that both RMSSD and HF, which are the indices of the time domain analysis and frequency domain analysis of HRV, respectively, indicate the vagal modulation of the cardiac function. Thus, the increases in RMSSD and HF should indicate that ingestion of the milk protein drink was associated with stronger vagal activity. However, HRV may be more than just an indicator of simple cardiac and autonomic function [26
]. Growing evidence suggests that a higher vagally mediated HRV is associated with greater cognitive performance [43
]. To account for this relationship, a neurovisceral integration model has been proposed [49
]. The neurovisceral integration model explains how multilevel neural control can adaptively coordinate cognitive and autonomic response depending on one’s current goals and context (e.g., a particular cognitive task, the state of the body, and the external world). The model explains, in addition, that the higher HRV might be the index for greater prefrontal level control, which is sensitive to goals and context and required to successfully perform demanding cognitive/attentional tasks. The increase in HRV after intake of the acidified milk protein drink may have correlated with greater prefrontal level control, which in turn would have led to the improved UKT scores.
The mechanism for the relationship of milk protein drinks with improved cognitive performance, compared with the placebo, remains to be elucidated. One hypothesis involves the distinct dynamics of neurotransmitter precursors after ingestion of the different nutrients, which would influence brain function. Ingestion of either proteins or carbohydrates modifies the uptake of tryptophan and tyrosine into the brain and the conversion to serotonin and catecholamines, respectively [51
]. In addition, a recent review described how the manipulation of tryptophan could affect the gut–brain axis, which could in turn affect cognitive function [53
]. Previous animal study has shown that ingestion of acidified milk may lead to greater post-exercise muscle protein synthesis through a greater increase of essential amino acids in plasma compared with bovine skim milk [22
]. Thus, the ingestion of milk protein drinks may have affected cognitive performance through changes in neurotransmitter precursor concentrations, although changes in amino acids levels after consumption of the acidified milk should be investigated in humans. Another hypothesis is that the peptides generated from acidified milk protein might have affected cognitive function. Nakamura et al. reported that ingestion of 10 g of casein hydrolysates was associated with improved cognitive performance in healthy young men, with increased HRV also observed [54
]. In the acidification process, stabilizers and dispersants are added to prevent aggregation of milk proteins [22
], apparently leading to quick digestion of the milk protein, as shown in the greater increase in amino acids in the blood after the acidified milk protein consumption. Digestion of the acidified milk protein test drink in our study may have generated peptides similar to those in the casein hydrolysates. Further study comparing the acidified milk protein with bovine milk protein is necessary to understand the relationship of milk protein acidification with cognitive performance and HRV.
However, there are limitations in determining the detailed mechanism for the changes in cognitive function observed in this study. The difference in the amount of glucose, rather than the proteins, between the 2 test drinks could have affected cognitive performance. An ‘inverted U’-shaped dose-response curve was observed for the relationship of plasma glucose level with cognition [56
]. Acute glucose fluctuations may impair cognitive performance [57
] and a stable metabolic condition should, therefore, improve cognitive performance [58
]. In addition, the 2 test drinks differ in other components as well as proteins and carbohydrate constituents: namely, the stabilizers and acidifier for the acidification procedure, and the sweeteners and flavors, which were added so the test drinks would have a similar taste and appearance. These components might have affected cognitive performance and autonomic activity in this study. Some factors involved in cognition were not considered in this study. Individual differences in fasting glucose appear to be relevant for cognition [59
] and baseline glucoregulation has been demonstrated to moderate postprandial cognition [18
] even in healthy young adults. Links between obesity and cognitive function have also been reported in young adults [60
]. Further studies are necessary to clarify the mechanism by which the acidified milk protein drink improved the subjects’ cognitive performance and also increased their HRV.