Effects of Varying Caffeine Dosages and Consumption Timings on Cerebral Vascular and Cognitive Functions: A Diagnostic Ultrasound Study

Abstract

Caffeine is consumed owing to its stimulatory effects; however, its excessive intake triggers adverse effects. Herein, we analyzed changes in physiological cerebrovascular and cognitive functions following the consumption of 100 and 200 mg of caffeine in healthy adults after 0/30/60 min to ascertain appropriate caffeine consumption habits. The peak systolic velocity (PSV), pulse wave velocity (PWV), and common carotid artery (CCA) diameter were measured using diagnostic ultrasound. Cognitive function was evaluated using the accuracy rate and response time on the two-back task. Percutaneous oxygen saturation (SpO₂) and heart rate (HR) were assessed using patient monitoring systems. After consuming 100 mg of caffeine, systolic blood pressure (SBP) increased (p > 0.05) and SpO₂ and accuracy rate improved by 30 min (p = 0.018 and p = 0.356) but declined by 60 min (p = 0.924 and p = 0.055). HR and response time continuously decreased (p = 0.209 and p = 0.061, respectively), while PWV showed no change (p > 0.05). After consuming 200 mg of caffeine, SBP (p < 0.05), diastolic blood pressure (p = 0.004 and p = 0.820), and SpO₂ (p = 0.002 and p = 0.666) increased significantly, while the accuracy rate (p = 0.634 and p = 0.055, respectively) and response time (p < 0.05) decreased. PWV remained unchanged (p > 0.05). The results revealed distinct dose-dependent patterns on physiological and cognitive changes, with SBP and SpO₂ exhibiting greater changes when a higher dose was consumed in a short duration. Although moderate caffeine intake may support vascular health and cognitive function, excessive intake reduces blood flow, alters vascular elasticity, and impairs cognitive activation. These findings highlight the need for guidelines to ensure safe and effective caffeine consumption.

1. Introduction

Caffeine is consumed worldwide in various forms such as coffee, chocolate, beverages, and pharmaceuticals [1]. The caffeine content varies by form, with approximately 17 mg/oz in coffee, 0.8 mg/oz in cocoa and hot chocolate, and 30–100 mg in medications [2]. Most caffeine intake occurs through coffee, which is consumed at various times throughout the day, such as after waking up or during meals [1,3]. Globally, since 2002, the coffee consumption per capita has increased by 37% over the last 20 years [4], resulting in a paralleled increase in caffeine intake. Currently, the U.S. FDA recommends a maximum daily caffeine intake of 400 mg for healthy adults. Similarly, Health Canada recommends maximum daily doses of 45–85 mg/day for children aged <12, 2.5 mg/kg/day for adolescents aged >12 years, 400 mg/day for adults, and 300 mg/day for pregnant women [5]. The European Food Safety Authority (EFSA) recommends daily caffeine intake limits of 0.2–2.0 mg/kg body weight (bw)/day for children (3–10 years), 0.4–1.4 mg/kg bw/day for adolescents (10–18 years), 400 mg/day for adults, and 200 mg/day for pregnant women. The Ministry of Food and Drug Safety of South Korea recommends maximum daily intakes of 2.5 mg/kg bw/day for children and adolescents, 400 mg/day for adults, and 300 mg/day for pregnant women [6,7]. Despite these guidelines, public awareness remains low, resulting in excessive caffeine consumption without consideration of the recommended limits. The primary driver of this consumption is the stimulatory effect of caffeine.

Caffeine exerts its stimulatory effects through multiple mechanisms. Its structural similarity to adenosine allows it to act as an antagonist, with stimulatory effects being a key outcome [8]. Adenosine typically binds to the A1 and A2A receptors in the brain, thereby reducing the release of glutamate and dopamine and inducing drowsiness and sleepiness [9]. However, caffeine binds to these receptors non-selectively, blocking adenosine-mediated signaling and enhancing neurotransmitter release. This results in reduced drowsiness, improved brain function, and sympathetic nervous system activation, which increases catecholamine release and alertness [8,10,11]. Additionally, caffeine induces vasoconstriction, elevates blood pressure (BP) [8,12,13], increases renal blood flow, and inhibits ion and fluid reabsorption, leading to diuretic effects [8]. After exerting these effects, caffeine is metabolized by the liver and excreted by the kidneys through the urine [14,15]. Its half-life in the body averages 3–5 h and varies according to physiological and environmental factors [8,16,17].

The positive effects of caffeine extend beyond its stimulatory effects. It activates lipase, promotes fat breakdown [14,15,18], enhances muscle strength by affecting muscle contraction [8,15,19], and aids in digestion by stimulating gastrin release and gastric acid secretion in the stomach and duodenum [20,21]. However, excessive caffeine intake can cause dehydration due to excessive diuresis, gastritis, or ulcers due to the overproduction of gastric acid [8,21], in addition to psychological side effects such as anxiety, tension, and insomnia [22]. High accessibility to caffeine also affects adolescent consumption, potentially causing electrolyte imbalances that impair growth and development, with potentially adverse effects on mental health during critical periods of physical and emotional change [23,24]. In pregnant women, excessive caffeine intake may result in caffeine crossing the placental barrier owing to its lipid solubility, potentially causing miscarriages or other complications [25].

The effects of caffeine, both in terms of dosage and duration, are well documented. While small doses of caffeine may improve cognitive function and vascular health, excessive amounts can lead to reduced blood flow, an increased heart rate (HR), and impaired cognitive performance [18,19,20]. Previous studies have reported cardiovascular responses, such as increased BP and HRs within 30–60 min after caffeine consumption, as well as improvements in cerebral oxygen saturation and working memory [21,22]. However, most studies have focused on specific doses (>300 mg) and have not systematically analyzed dose-dependent differences or combined vascular and cognitive changes over time.

To address the above knowledge gaps, the present study aimed to analyze the physiological and cognitive responses to varying doses of caffeine intake in a short duration using diagnostic ultrasonography and cognitive function task.

2. Materials and Methods

2.1. Participants

This study was approved by the Institutional Review Board (IRB No. 1044396-202404-HR-067-01). The participants in this study were recruited through bulletin boards of recruiting target institutions or public announcements posted on social networks, and 30 participants (male: 15; female: 15) voluntarily participated (

Table 1. Participant information (mean ± SD).

	Age (Years)	Height (cm)	Weight (kg)	BMI (kg/m2)
Males (n = 15)	22.93 ± 1.33	174.93 ± 6.25	74.73 ± 11.27	24.35 ± 2.94
Females (n = 15)	22.53 ± 1.36	159.72 ± 3.22	53.38 ± 5.62	20.89 ± 1.75
Total (n = 30)	22.73 ± 1.33	167.33 ± 9.15	64.06 ± 13.94	22.63 ± 2.96

Abbreviations: BMI, body mass index; SD, standard deviation.

). Prior to the experiment, participants were informed about this study’s purpose, procedures, and equipment safety. All participants provided their written informed consent to participate in this study.

The criteria for inclusion were healthy adults over the age of 20 without any disease, and the exclusion criteria are those who have a history of diseases possibly affecting cardiovascular function, take cardiovascular drugs, are physically weak or mentally ill, and are students affiliated with the same department as the researchers.

2.2. Experimental Protocols and Data Acquisition

The participants completed two experimental sessions held at the same time, one week apart. To minimize external factors influencing the results, the participants refrained from smoking and eating for 6 h before each session, avoided caffeine consumption for 12 h, and abstained from alcohol consumption for 24 h. The 12 h caffeine restriction was set according to its half-life: approximately 3–5 h.

During the measurements, all participants were seated on a fixed chair in the same position, with the back of their heads touching the wall. Velcro tape and a polyester band were placed around their foreheads to secure their heads in place and prevent any movement away from the wall.

A diagnostic ultrasound system (E-CUBE i7, ALPINION MEDICAL SYSTEMS Co., Ltd., Seoul, Republic of Korea) and a linear transducer array (L3-12T, ALPINION MEDICAL SYSTEMS Co., Ltd., Seoul, Republic of Korea) were used to locate and mark the right CCA for consistent measurements. The ultrasound measurements were performed by a radiologic technologist with experience in multiple ultrasound-related studies. BP was monitored using a smartwatch (SM-R850; Samsung Electronics, Suwon, Republic of Korea) worn on the left wrist and calibrated individually according to the manufacturer’s instructions. The HR and peripheral oxygen saturation (SpO₂) were measured using a patient monitor (Bionics, Chuncheon, Republic of Korea), with sensors attached to the left index finger.

To assess cognitive performance, the participants practiced the N-back test once before the experiment to familiarize themselves with the task. The two-back (2B) test was then conducted using the DMDX software (DMDX version 6, University of Arizona Psychology Department, Tucson, AZ, USA). In this test, numbers ranging from 0 to 9 appeared randomly on the screen 40 times at 3 s intervals. Participants were required to indicate whether the current number matched the one presented two trials earlier by selecting “positive” or “negative.”

Subjective measures were evaluated using the visual analog scale (VAS) for comfort, Karolinska Sleepiness Scale (KSS) for drowsiness, and the Chalder Fatigue Scale (CFS) for fatigue. The VAS ranges from the most comfortable state (0) to the most uncomfortable state (10), whereas the KSS ranges from highest alertness (1) to greatest sleepiness (9) [26,27]. The CFS uses a 14-item Likert scale (eight items for physical fatigue and six for mental fatigue), with responses scored as better than usual (0), same as usual (1), worse than usual (2), and much worse than usual (3), yielding a total score of 0–42, with higher scores indicating greater fatigue [28].

Physiological and biosignal measurements were taken at three time points: before caffeine ingestion (0 min), 30 min after ingestion (30 min), and 60 min after ingestion (60 min), based on caffeine’s approximate absorption time of 45 min in the bloodstream [29]. Brightness mode (B-mode) and Doppler-mode ultrasound images of the right CCA were recorded for 5 s, capturing eight cardiac cycles (Figure 1). Blood pressure (BP) was measured concurrently using a smartwatch.

Participants consumed caffeine immediately following the 0 min measurement period. Caffeine doses were prepared using stick coffee (Kanu Mini Mild-Roasted Coffee Sticks; Dongsuh Foods Corporation, Incheon, Republic of Korea), containing 33 mg of caffeine per stick. For each trial, three (approximately 100 mg) or six sticks (approximately 200 mg) were dissolved in 200 mL of water. In the first experiment, the participants were randomly assigned to consume either 100 mg or 200 mg of caffeine, and the alternate dose was consumed in the second experiment. Therefore, a single-blind method was used, in which the participants were unaware of the intake amount.

Ultrasound images of the CCA were saved in DICOM format and analyzed using DICOM viewer software (Radiant DICOM Viewer Version 2021.2.2 (64-bit), Medixant, Poznan, Poland). The maximum diameter (MaxD) and minimum diameter (MinD) were measured from the B-mode images, while the average peak systolic velocity (PSV) over 5 s was calculated from the Doppler-mode images. HR and SpO₂ values were averaged over 30 s of measurement, and systolic blood pressure (SBP) and diastolic blood pressure (DBP) were recorded using the smartwatch application.

The pulse wave velocity (PWV), a measure of arterial stiffness, was calculated using SBP, DBP, MaxD, and MinD according to Equations (1) and (2). Cognitive performance data, including accuracy rate and response time from the 2B test, were extracted using the DMDX software.

$$\mathrm{Stiffness} \mathrm{Index} (\mathrm{S}\mathrm{I})=\mathrm{l}\mathrm{n}\left({\displaystyle \frac{\mathrm{S}\mathrm{B}\mathrm{P}}{\mathrm{D}\mathrm{B}\mathrm{P}}}\right)\times \left({\displaystyle \frac{\mathrm{M}\mathrm{i}\mathrm{n}\mathrm{D}}{∆\mathrm{D}}}\right)$$

(1)

where DD = MaxD − MinD

$$\mathrm{Pulse} \mathrm{Wave} \mathrm{Velocity} (\mathrm{P}\mathrm{W}\mathrm{V})=\sqrt{{\displaystyle \frac{\mathrm{S}\mathrm{I}\times \mathrm{D}\mathrm{B}\mathrm{P}}{2\times \mathrm{B}\mathrm{D}}}}$$

(2)

where blood density (BD) = 1.050 g/cm³

The raw data for BP, diameter, PWV, PSV, HR, SpO₂, accuracy rate, and response time were normalized using Equation (3) to calculate the change rates for statistical analysis.

$$\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e} \mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e} (\mathrm{C}\mathrm{R})={\displaystyle \frac{\mathrm{V}alue at 0,30,\mathrm{o}r 60 \mathrm{m}\mathrm{i}\mathrm{n}}{\mathrm{V}alue at 0 \mathrm{m}\mathrm{i}\mathrm{n}}}$$

(3)

2.3. Statistical Analysis

Statistical analyses were conducted using Jamovi software (version 2.2.5, free software, https://www.jamovi.org, accessed on 15 October 2024). Time-dependent changes (0, 30, and 60 min) were analyzed using repeated-measures analysis of variance (RM ANOVA). Sphericity was tested, and Greenhouse–Geisser corrections were applied where necessary. Tukey’s post hoc test was performed for significant effects (p < 0.05). Changes in caffeine dose (100 mg vs. 200 mg) were analyzed using paired t-tests. For subjective measures, the means and standard deviations were calculated to observe trends according to caffein doses and consumption times. Statistical significance was set at p = 0.05.

3. Results

All measurements were expressed as relative change rates, with the baseline at 0 min set to 1. Overall, SBP showed no significant differences at 30 (1.003 ± 0.037) or 60 min (1.009 ± 0.030) following the intake of 100 mg of caffeine (p > 0.05). However, following a 200 mg intake, SBP significantly increased at 30 (1.014 ± 0.029) and 60 min (1.029 ± 0.044) (p < 0.05) (

Table 2. Comparison of blood pressure parameters using RM ANOVA.

Variation	Caffeine Dose	Time	Mean ± SD	F	p	Factor 1	Factor 2	Mean Difference ± SE (Factor 1–Factor 2)	t	pTukey
SBP	100 mg	0 min	1	1.4	0.254	0 min	30 min	−0.00301 ± 0.00669	−0.450	0.895
		30 min	1.003 ± 0.03666				60 min	−0.00921 ± 0.00552	−1.667	0.235
		60 min	1.009 ± 0.03026			30 min	60 min	−0.00620 ± 0.00436	−1.420	0.344
	200 mg	0 min	1	10.3	<0.001 *	0 min	30 min	−0.01400 ± 0.00536	−2.610	0.037 *
		30 min	1.014 ± 0.02936				60 min	−0.02930 ± 0.00804	−3.640	0.003 *
		60 min	1.029 ± 0.04403			30 min	60 min	−0.01530 ± 0.00563	−2.720	0.029 *
DBP	100 mg	0 min	1	0.179	0.836	0 min	30 min	−0.00221 ± 0.00866	−0.255	0.965
		30 min	1.002 ± 0.04777				60 min	0.00262 ± 0.00842	0.311	0.948
		60 min	0.997 ± 0.04639			30 min	60 min	0.00483 ± 0.00706	0.685	0.774
	200 mg	0 min	1	7.85	0.001 *	0 min	30 min	−0.02122 ± 0.00595	−3.564	0.004 *
		30 min	1.021 ± 0.03207				60 min	−0.02551 ± 0.00752	−3.390	0.006 *
		60 min	1.022 ± 0.03649			30 min	60 min	−0.00429 ± 0.00711	−0.603	0.820

Abbreviations: DBP, diastolic blood pressure; F, F-statistic; p, p-value; SBP, systolic blood pressure; SE, standard Error; t, t-statistic. * p < 0.05

).

DBP showed no significant differences at 30 (1.002 ± 0.048) or 60 min (0.997 ± 0.046) following 100 mg of caffeine intake (p > 0.05), whereas for the 200 mg intake, significant increases were observed from 0 to 30 min and 0 to 60 min (p = 0.004 and p = 0.006, respectively); however, no significant differences were noted between 30 and 60 min (p = 0.820) (Table 2 and Figure 2).

Significant differences were observed between 0 and 30 min (p = 0.016), but no significant differences were observed between 0 and 60 or 30 and 60 min (p = 0.504 and p = 0.138, respectively) following 100 mg of caffeine intake. For the 200 mg intake, no significant differences were observed at 30 min (1.001 ± 0.028) or 60 min (0.999 ± 0.030) (p > 0.05) (

Table 3. Comparison of diameter using RM ANOVA.

Variation	Caffeine Dose	Time	Mean ± SD	F	p	Factor 1	Factor 2	Mean Difference ± SE (Factor 1–Factor 2)	t	pTukey
MaxD	100 mg	0 min	1	4.610	0.014 *	0 min	30 min	−0.01777 ± 0.00594	−2.990	0.016 *
		30 min	1.020 ± 0.02951				60 min	−0.0071 ± 0.00628	−1.130	0.504
		60 min	1.008 ± 0.03317			30 min	60 min	0.01067 ± 0.00541	1.970	0.138
	200 mg	0 min	1	0.130	0.878	0 min	30 min	−0.00070 ± 0.00504	−0.136	0.990
		30 min	1.001 ± 0.02800				60 min	0.00214 ± 0.00549	0.389	0.920
		60 min	0.999 ± 0.02998			30 min	60 min	0.00282 ± 0.00665	0.424	0.906
MinD	100 mg	0 min	1	4.840	0.011 *	0 min	30 min	−0.01902 ± 0.00707	−2.689	0.031 *
		30 min	1.020 ± 0.03820				60 min	−0.01298 ± 0.00520	−2.497	0.048 *
		60 min	1.013 ± 0.02849			30 min	60 min	0.00604 ± 0.00632	0.956	0.610
	200 mg	0 min	1	0.427	0.655	0 min	30 min	0.00229 ± 0.00674	0.340	0.938
		30 min	0.999 ± 0.03647				60 min	0.00545 ± 0.00571	0.953	0.612
		60 min	0.996 ± 0.03033			30 min	60 min	0.00315 ± 0.00520	0.606	0.818

Abbreviations: F, F-statistic; MaxD, maximum diameter; MinD, minimum diameter; p, p-value; SE, standard error; t, t-statistic. * p < 0.05

).

MinD showed significant differences from 0 to 30 min and 0 to 60 min (p = 0.031 and p = 0.048, respectively), but no significant differences were found between 30 and 60 min (p = 0.610) after 100 mg of caffeine intake. For the 200 mg intake, no significant differences were observed from 30 min (0.999 ± 0.036) and 60 min (0.996 ± 0.030) (p > 0.05) (Table 3 and Figure 3).

The PWV showed no significant differences at 30 (1.005 ± 0.136) or 60 min (1.053 ± 0.112) following 100 mg of caffeine intake (p > 0.05). For the 200 mg intake, no significant differences were observed at 30 (1.014 ± 0.162) or 60 min (1.001 ± 0.128) (p > 0.05) (

Table 4. Comparison of PWV using RM ANOVA.

Variation	Caffeine Dose	Time	Mean ± SD	F	p	Factor 1	Factor 2	Mean Difference ± SE (Factor 1–Factor 2)	t	pTukey
PWV	100 mg	0 min	1	2.860	0.066	0 min	30 min	−0.00245 ± 0.0254	−0.0966	0.995
		30 min	1.005 ± 0.13648				60 min	−0.05081 ± 0.0208	−2.4374	0.055
		60 min	1.053 ± 0.11166			30 min	60 min	−0.04836 ± 0.0253	−1.9090	0.156
	200 mg	0 min	1	0.695	0.503	0 min	30 min	−0.02844 ± 0.0313	−0.9090	0.639
		30 min	1.014 ± 0.16218				60 min	−0.00319 ± 0.0229	−0.1390	0.989
		60 min	1.001 ± 0.12751			30 min	60 min	0.02525 ± 0.0243	1.0390	0.559

Abbreviations: F, F-statistic; p, p-value; PWV, pulse wave velocity; SE, standard error; t, t-statistic.

and Figure 4).

The PSV showed significant differences between 0 and 30 min (p < 0.001); however, no significant differences were observed between 0 and 60 min and 30 and 60 min (p = 0.056 and p = 0.407, respectively) following 100 mg of caffeine intake. For the 200 mg intake, no significant differences were observed from 30 min (0.960 ± 0.069) or 60 min (0.961 ± 0.099) (p > 0.05) (

Table 5. Comparison of the PSV, HR, and SpO₂ using RM ANOVA.

Variation	Caffeine Dose	Time	Mean ± SD	F	p	Factor 1	Factor 2	Mean Difference ± SE (Factor 1–Factor 2)	t	pTukey
PSV	100 mg	0 min	1	7.98	<0.001 *	0 min	30 min	0.0768 ± 0.0185	4.140	<0.001 *
		30 min	0.927 ± 0.10060				60 min	0.0526 ± 0.0217	2.420	0.056
		60 min	0.956 ± 0.11457			30 min	60 min	−0.0242 ± 0.0186	−1.300	0.407
	200 mg	0 min	1	2.11	0.131	0 min	30 min	0.02918 ± 0.0150	1.951	0.144
		30 min	0.960 ± 0.06945				60 min	0.03403 ± 0.0189	1.797	0.190
		60 min	0.961 ± 0.09885			30 min	60 min	0.00484 ± 0.0195	0.248	0.967
HR	100 mg	0 min	1	8.04	0.001 *	0 min	30 min	0.0112 ± 0.0064	1.760	0.209
		30 min	0.988 ± 0.03054				60 min	0.0310 ± 0.00875	3.540	0.005 *
		60 min	0.973 ± 0.03853			30 min	60 min	0.0198 ± 0.00814	2.430	0.061
	200 mg	0 min	1	6.09	0.008 *	0 min	30 min	0.0187 ± 0.00830	2.250	0.080
		30 min	0.980 ± 0.04016				60 min	0.0298 ± 0.01049	2.840	0.022 *
		60 min	0.974 ± 0.05511			30 min	60 min	0.0111 ± 0.00662	1.670	0.235
SpO2	100 mg	0 min	1	5.28	0.008 *	0 min	30 min	−0.0043 ± 0.00148	−2.914	0.018 *
		30 min	1.004 ± 0.00808				60 min	−0.0038 ± 0.00153	−2.482	0.049 *
		60 min	1.003 ± 0.00808			30 min	60 min	0.0005 ± 0.00133	0.379	0.924
	200 mg	0 min	1	11.5	<0.001 *	0 min	30 min	−0.00693 ± 0.00181	−3.821	0.002 *
		30 min	1.006 ± 0.00913				60 min	−0.00834 ± 0.00211	−3.947	0.001 *
		60 min	1.008 ± 0.01113			30 min	60 min	−0.00142 ± 0.00164	−0.866	0.666

Abbreviations: F, F-statistic; HR, heart rate; PSV, peak systolic velocity; p, p-value; SE, standard error; SpO₂, peripheral oxygen saturation; t, t-statistic. * p < 0.05

and Figure 5).

HR showed significant differences between 0 and 60 min (p = 0.005) but no significant differences between 0 and 30 min and 30 and 60 min (p = 0.209 and p = 0.061, respectively) after 100 mg of caffeine intake. For the 200 mg intake group, significant differences were only observed from 0 to 60 min (p = 0.022), with insignificant changes observed between 0 and 30 and 30 and 60 min (p = 0.080, p = 0.235, respectively) (Table 5 and Figure 6).

SpO₂ showed significant differences between 0 and 30 min and 0 and 60 min (p = 0.018 and p = 0.049, respectively) but no significant differences between 30 and 60 min (p = 0.924) after 100 mg of caffeine intake. For the 200 mg intake, significant differences were observed from 0 to 30 min and 0 to 60 min (p = 0.002 and p = 0.001, respectively) but not between 30 and 60 min (p = 0.666) (Table 5 and Figure 7).

The accuracy rate significantly differed between 30 and 60 min (p = 0.009); however, no significant differences were observed between 0 and 30 min and 0 and 60 min (p = 0.356 and p = 0.613, respectively) following 100 mg of caffeine intake. For the 200 mg intake, significant differences were observed between 0 and 60 min (p = 0.020) but not observed from 0 to 30 and 30 to 60 min (p = 0.634 and p = 0.055, respectively) (

Table 6. Comparison of results of the two-back test using RM ANOVA.

Variation	Caffeine Dose	Time	Mean ± SD	F	p	Factor 1	Factor 2	Mean Difference ± SE (Factor 1–Factor 2)	t	pTukey
Accuracy rate	100 mg	0 min	1	3.31	0.044 *	0 min	30 min	−0.01305 ± 0.00932	−1.400	0.356
		30 min	1.013 ± 0.04842				60 min	0.00903 ± 0.00947	0.953	0.613
		60 min	0.991 ± 0.04923			30 min	60 min	0.02207 ± 0.00684	3.226	0.009 *
	200 mg	0 min	1	5.44	0.007 *	0 min	30 min	0.00537 ± 0.00586	0.917	0.634
		30 min	0.995 ± 0.03218				60 min	0.02034 ± 0.00708	2.874	0.020 *
		60 min	0.979 ± 0.04009			30 min	60 min	0.01497 ± 0.00618	2.423	0.055
Response time	100 mg	0 min	1	30.4	<0.001 *	0 min	30 min	0.0445 ± 0.0094	4.740	<0.001 *
		30 min	0.963 ± 0.04238				60 min	0.0701 ± 0.01048	6.690	<0.001 *
		60 min	0.934 ± 0.04590			30 min	60 min	0.0256 ± 0.00706	3.620	0.003 *
	200 mg	0 min	1	32.6	<0.001 *	0 min	30 min	0.0232 ± 0.00660	3.520	0.005 *
		30 min	0.976 ± 0.03408				60 min	0.0637 ± 0.00928	6.860	<0.001 *
		60 min	0.938 ± 0.04713			30 min	60 min	0.0404 ± 0.00784	5.160	<0.001 *

Abbreviations: F, F-statistic; p, p-value; SE, standard error; t, t-statistic. * p < 0.05

and Figure 8).

The response time showed significant differences over time following both the 100 mg and 200 mg caffeine intake, with reductions observed between 30 min (0.963 ± 0.042, 0.976 ± 0.034) and 60 min (0.934 ± 0.046, 0.938 ± 0.047) (p < 0.05) (Table 6 and Figure 9).

Caffeine dosage significantly affected SBP at 60 min (p = 0.016) but not at 30 min (p = 0.145) (

Table 7. Comparison of the change rate by caffeine dose using paired t-test.

Variation	Time	Mean Difference ± SE (100 mg–200 mg)	t	p
SBP	30 min	−0.01097 ± 0.00732	−1.4985	0.145
	60 min	−0.02008 ± 0.00786	−2.5530	0.016 *
DBP	30 min	−0.01852 ± 0.00939	−1.9720	0.059
	60 min	−0.02498 ± 0.01097	−2.2760	0.031 *
MaxD	30 min	0.01953 ± 0.00783	2.495	0.019 *
	60 min	0.00837 ± 0.00638	1.311	0.201
MinD	30 min	0.02147 ± 0.00930	2.307	0.029 *
	60 min	0.01718 ± 0.00844	2.035	0.052
PWV	30 min	−0.00904 ± 0.04381	−0.0206	0.838
	60 min	0.05229 ± 0.03464	1.5100	0.143
PSV	30 min	−0.03298 ± 0.02186	−1.509	0.144
	60 min	−0.00489 ± 0.03181	−0.154	0.879
HR	30 min	0.00818 ± 0.00897	0.912	0.373
	60 min	−0.00185 ± 0.01311	−0.141	0.889
SpO2	30 min	−0.002 ± 0.00205	−0.976	0.338
	60 min	−0.00437 ± 0.00213	−2.051	0.050 *
Accuracy rate	30 min	0.01782 ± 0.01168	1.526	0.139
	60 min	0.01242 ± 0.01464	0.849	0.404
Response time	30 min	−0.01296 ± 0.01064	−1.218	0.235
	60 min	−0.00403 ± 0.0117	−0.357	0.724

Abbreviations: DBP, diastolic blood pressure; HR, heart rate; MaxD, maximum diameter; MinD, minimum diameter; PSV, peak systolic velocity; PWV, pulse wave velocity; p, p-value; SBP, systolic blood pressure; SE, standard Error; SpO₂, peripheral oxygen saturation; t, t-statistic. * p < 0.05

). Similarly, DBP significantly differed at 60 min (p = 0.031) but not at 30 min (p = 0.059) (Table 7). Furthermore, MaxD and MinD exhibited significant differences at 30 min (p = 0.019 and p = 0.029, respectively) but not at 60 min (p = 0.052) (Table 7). PWV, PSV, and HR showed no significant dose-dependent effects over time (p > 0.05), whereas the SpO₂ differences were significant at 60 min (p < 0.05) but not at 30 min (p = 0.338) (Table 7). The accuracy rate and response time were not significantly affected (p > 0.05) (Table 7).

At a caffeine intake of 100 mg, the VAS score was 3.13 ± 2.19 at 0 min, 3.07 ± 2.03 at 30 min, and 2.80 ± 2.04 at 60 min. This value decreased by 0.06 at 30 min compared to that at 0 min and by 0.27 at 60 min compared to that at 30 min. For a caffeine intake of 200 mg, the VAS was 2.87 ± 2.16 at 0 min, 2.83 ± 2.09 at 30 min, and 2.57 ± 2.05 at 60 min. This value decreased by 0.03 at 30 min compared to that at 0 min and by 0.26 at 60 min compared to that at 30 min.

For a caffeine intake of 100 mg, the KSS was 4.87 ± 1.59 at 0 min, 4.00 ± 1.46 at 30 min, and 4.13 ± 1.41 at 60 min. This value decreased by 0.46 at 30 min compared to 0 min and by 0.28 at 60 min compared to 30 min. When the caffeine intake was 200 mg, the KSS was 5.00 ± 1.55 at 0 min, 3.87 ± 1.22 at 30 min, and 3.87 ± 1.28 at 60 min. The 200 mg intake decreased by 1.13 at 30 min compared to that at 0 min and showed no further decrease from 30 min to 60 min (0.00 change).

For a caffeine intake of 100 mg, the CFS was 16.30 ± 5.53 at 0 min, 14.40 ± 5.62 at 30 min, and 16.00 ± 5.64 at 60 min. This value decreased by 1.90 at 30 min compared to that at 0 min and increased by 2.10 at 60 min compared to that at 30 min. For a caffeine intake of 200 mg, the CFS was 14.80 ± 5.58 at 0 min, 13.20 ± 5.64 at 30 min, and 12.80 ± 5.31 at 60 min. This value decreased by 1.60 at 30 min compared to that at 0 min and decreased further by 0.40 at 60 min compared to that at 30 min (

Table 8. Results of subjective evaluation.

Variation	Caffeine Dose	0 min	Δ	30 min	Δ	60 min
VAS	100 mg	3.13 ± 2.19	−0.06	3.07 ± 2.03	−0.27	2.8 ± 2.04
	200 mg	2.87 ± 2.16	−0.03	2.83 ± 2.09	−0.26	2.57 ± 2.05
KSS	100 mg	4.87 ± 1.59	−0.46	4.00 ± 1.46	−0.28	4.13 ± 1.41
	200 mg	5.00 ± 1.55	−1.13	3.87 ± 1.22	0.00	3.87 ± 1.28
CFS	100 mg	16.30 ± 5.53	−1.90	14.40 ± 5.62	2.10	16.00 ± 5.64
	200 mg	14.80 ± 5.58	−1.60	13.20 ± 5.64	−0.40	12.80 ± 5.31

Abbreviations: CFS, Chalder Fatigue Scale; KSS, Karolinska Sleepiness Scale; VAS, visual analog scale.

).

In short, the VAS scores decreased slightly at 60 min compared to those at 0 min for both doses, although this difference was insignificant. The KSS decreased at 30 min and showed minor increases at 60 min, indicating reduced sleepiness following caffeine intake. The CFS scores showed a transient reduction at 30 min with a slight rebound at 60 min for 100 mg and continued reduction at 200 mg (Table 8).

4. Discussion

In this study, the effects of caffeine intake on BP, HR, SpO₂, blood vessel diameter, and cognitive function were analyzed. The BP change was not significant when 100 mg was ingested in a short duration, but BP increased significantly after 200 mg of intake. HR tended to decrease at both doses, and SpO₂ increased further when 200 mg was ingested. In the cognitive function evaluation, the accuracy improved until 30 min after 100 mg of intake but decreased thereafter, and the accuracy continuously decreased when 200 mg was ingested. On the other hand, the reaction time was shortened under all conditions, suggesting the possibility that caffeine may have a positive effect on rapid cognitive response.

SBP demonstrated a general upward trend over time across all levels of caffeine intake, with a significant increase observed only at 200 mg. This increase could be attributed to the stimulation of the sympathetic nervous system, leading to catecholamine release, which induces vasoconstriction and subsequent BP elevation [8,10,13,20]. The most pronounced increase occurred approximately 45 min post-intake, aligning with the peak absorption of caffeine [29,30]. DBP showed a modest increase for up to 30 min following 100 mg of intake, followed by a decrease at 60 min. Conversely, a 200 mg intake resulted in a gradual increase in DBP up to 60 min. These observations are consistent with the mechanisms underlying SBP changes, indicating that the limited increase in DBP after 30 min of consuming 200 mg of caffein may reflect a physiological threshold influenced by blood flow. The changes in SBP and DBP were more pronounced with 200 mg than with 100 mg, with significant differences observed only at 60 min, indicating that higher caffeine doses exert greater effects on BP.

The MaxD and MinD increased at 30 min compared to the baseline with a 100 mg intake but decreased by 60 min. Following an intake of 200 mg, MaxD slightly increased at 30 min and subsequently slightly decreased at 60 min, while MinD progressively declined. These findings suggest that high doses of caffeine have a strong effect on vasoconstriction. Changes in MaxD and MinDs were more pronounced at 100 mg than at 200 mg, with significant differences observed only at 30 min, indicating a shorter recovery period for vessel diameter compared to BP [9,13].

PWV remained relatively stable at 30 min for all doses. At 100 mg, it increased slightly at 60 min, likely due to the smooth muscle contraction triggered by elevated intracellular calcium levels, resulting in vascular stiffening and increased wave propagation speed [31,32]. In contrast, the PWV at 200 mg remained constant for up to 60 min. Neither time nor dosage resulted in significant differences in the PWV. Previous studies have reported either the maintenance of or an increase in PWV following acute caffeine intake [32,33]. Differences in measurement methods, such as foot-to-foot versus the one-point approach, may account for these discrepancies. The latter provides more localized measurements, thereby reducing variability.

Overall, PSV exhibited a decreasing trend 30 min after intake across all doses. Recovery was observed after 60 min for 100 mg, whereas a slight decrease was observed for 200 mg. Although the reduction observed at 30 min was more substantial at a 100 mg caffein dose, both doses showed similar levels at 60 min, without significant differences. This aligns with the effects of caffeine on the carotid and peripheral arteries, where vasoconstriction impedes smooth blood flow owing to reduced adenosine activity [8,34,35]. HR decreased significantly over time for all doses, with greater reductions at 200 mg than at 100 mg at 30 min. After 60 min, both of the doses resulted in similar reductions. This trend is likely a baroreflex response to elevated BP, aimed at reflexively lowering HR [36,37,38]. SpO₂ increased across all doses up to 30 min, followed by a slight decline at 100 mg, and continued to increase at 200 mg up to 60 min. These results are consistent with previous findings indicating that a low dose of caffeine reduces tissue oxygen consumption, leading to increased SpO₂ levels [39,40].

In cognitive function tests, comprising the 2_B test assessment, the accuracy rate increased at 30 min for 100 mg but returned to baseline by 60 min, whereas it showed a continuous decline until 60 min after a 200 mg dose. The transient improvement at 30 min with the lower dose reflected the peak absorption and immediate effects of caffeine [29,30]. However, the subsequent decline highlights the short-lived nature of the cognitive effects of caffeine. Increased sympathetic activity due to higher caffeine doses likely contributed to the reduced accuracy. The participants’ stationary posture during the experiment may have activated the default mode network, which is associated with reduced focus and concentration [41,42,43,44,45,46]. The response time consistently improved over time for all doses, indicating enhanced cognitive arousal with caffeine intake, regardless of the dose.

Subjective measures, such as the VAS, KSS, and CFS, demonstrated increased comfort and reduced sleepiness and fatigue at 30 and 60 min for both doses. VAS and KSS showed greater changes at 100 mg, whereas CFS showed greater changes at 200 mg. These findings indicate the arousal effects of caffeine, which are mediated by dopaminergic modulation in the prefrontal cortex, which enhances mood and relaxation [47,48,49]. The general expectation of the effects of caffeine may have influenced the subjective evaluations regardless of the actual dose [50].

Overall, our findings indicate that caffeine exerts both dose- and time-dependent effects on physiological and cognitive parameters. Moderate caffeine intake appears to be beneficial for maintaining vascular health and cognitive activation, while excessive intake may lead to adverse vascular changes and cognitive impairment.

The limitations of this study include that the participants of the experiment were limited to their 20 s, so it was not possible to compare the results according to age, and the effect of caffeine on the patients with any disease could not be confirmed. In addition, the change after 60 min could not be confirmed because the measurement was made within a limited time up to 60 min after caffeine intake. The number of study participants is still small. Therefore, in a future study, it is necessary to supplement these limitations and increase the number of samples to conduct studies on various age groups and patients with diseases and additional studies on changes after 60 min.

Unlike some previous studies that were conducted in the lying down position and/or with a single dose [32,51], this study was conducted in a sitting position and with two different doses. In addition, this study examined the vascular physiology of caffeine, cognitive function, and subjective changes in comparison to the previous study [51].

5. Conclusions

Caffeine, which induces physiological and cognitive changes, is widely consumed in various forms, such as in coffee, chocolate, and medications. This study quantitatively analyzed the effects of caffeine intake (100 mg and 200 mg) and elapsed time (0, 30, and 60 min) on vascular physiology, cognitive function, and other subjective measures. The results revealed no significant changes in blood pressure or cognitive function with a 100 mg dose. However, the 200 mg dose significantly increased systolic blood pressure and diastolic blood pressure and impaired cognitive accuracy. Subjective measurements indicated reduced sleepiness and fatigue across all doses, reflecting the arousal effects of caffeine. This study highlighted the importance of moderately consuming caffeine. Low-dose caffeine intake over a short period is relatively safe and supports vascular and cognitive health, whereas high-dose caffeine intake over a short period causes vascular constriction and cognitive decline.

The practical implications of this study are as follows. Consuming a moderate amount of caffeine can promote vascular health and cognitive function. In addition, individualized guidelines should account for varying physiological responses to caffeine. Lastly, strategic use of the positive effects of caffeine, coupled with measures to prevent excessive consumption, is crucial.

Future research should explore the effects of caffeine in diverse age groups, health conditions, and the long-term effects of caffeine consumption to establish comprehensive guidelines.