In the United States, 63% of adults self-reported consuming sugar-sweetened beverages (SSB) daily (Chevinsky et al., 2021). Chronic consumption of SSB is associated with an increased risk of hypertension (Kim and Je, 2016; Malik and Hu, 2019), type 2 diabetes (Imamura et al., 2015), and stroke (Bernstein et al., 2012; Narain et al., 2016; Pacheco et al., 2020). Moreover, many individuals consume artificially sweetened beverages (ASB) to lower caloric intake and for disease management (Fitch and Keim, 2012; Gardner et al., 2012). As such, approximately 20% of adults in the United States consume ASB daily (Fakhouri et al., 2012). However, daily consumption of ASB has been associated with an increased risk of stroke and dementia (Pase et al., 2017). Additionally, caffeine is common in SSB, ASB, and other beverages such as coffee and tea (Chou and Bell, 2007; Reyes and Cornelis, 2018). Regular consumption of caffeinated coffee is associated with a reduced risk of stroke (Larsson and Orsini, 2011). Thus, it is likely the ingredients that are unique to caffeinated coffee reduce stroke risk. Nevertheless, the consumption of SSB or ASB is associated with an increased risk of diseases associated with poor cerebral vascular function but very little is known about the acute cerebral vascular effects following the ingestion of these beverages.

The most common sweeteners in SSB are high-fructose corn syrup (HFCS; ~60% fructose, ~40% glucose) or sucrose (50% fructose, 50% glucose). HFCS consumption increases free fructose in the circulation which might contribute to the negative health outcomes associated with HFCS consumption (Le et al., 2012). The most common sweeteners in ASB are sucralose and aspartame. There is some evidence that these constituents of SSB and ASB can acutely affect hemodynamics and/or peripheral vascular function. The acute consumption of an HFCS beverage, but not a sucrose-sweetened beverage, elevated peak post-consumption systolic blood pressure, renal vascular resistance, and serum uric acid (Le et al., 2012; Chapman et al., 2020). The acute consumption of fructose alone results in elevated blood pressure and plasma levels of norepinephrine in humans (Jansen et al., 1987; Brown et al., 2008; Grasser et al., 2014). Additionally, there is a reduction in endothelial nitric oxide synthase expression in aortic endothelial cells exposed to fructose (Glushakova et al., 2008). Whereas, oral consumption, intraduodenal infusions, or cultured vascular cells exposed to artificial sweeteners, such as sucralose and aspartame, do not appear to acutely influence blood pressure or endothelial cell function (Memon et al., 2014; Pham et al., 2018; Schiano et al., 2020). However, there is evidence that 10 weeks of consuming artificial sweeteners (sucralose and acesulfame potassium) reduces acetylcholine-induced endothelium-dependent vasodilation in rodent aortas (Risdon et al., 2020). Caffeine is also a common ingredient in SSB and ASB that might influence cerebral vascular function. Caffeine is an adenosine receptor antagonist and inhibits vasodilation (Cameron et al., 1990; Coney and Marshall, 1998; Phillis and O’Regan, 2003). Specific to the cerebral vasculature, coffee ingestion reduces intracranial artery blood velocity and improves cerebral autoregulation (Sasaki et al., 2016). Although there is evidence for changes in peripheral vasculature with acute consumption, it is still unclear whether acute consumption of caffeinated beverages with different sweeteners (HFCS and artificial sweeteners) influences cerebral vascular function.

Cerebral vascular function, quantified by dynamic cerebral autoregulation and cerebral vascular reactivity, is vital to maintain cerebral blood flow during changes in arterial blood pressure, arterial blood gases, local metabolism, and alterations in autonomic nervous system activation (Willie et al., 2014). Arterial blood pressure fluctuations can occur during common activities of daily living such as moving from a seated to a standing position. Inadequate maintenance of cerebral blood flow during postural changes can result in light-headedness, dizziness, and in some cases syncope (e.g., orthostatic hypotension). Cerebral autoregulation and cerebral vascular reactivity to carbon dioxide are both modulated by local vascular factors (i.e., nitric oxide), myogenic responses, and autonomic innervation (Schmetterer et al., 1997; Zhang et al., 2002; Toda et al., 2009; Hamner et al., 2010). However, it is unknown if the acute consumption of SSB and/or ASB influence cerebral vascular function and the maintenance of cerebral blood flow during orthostatic challenges. Along these lines, cerebral vascular reactivity is the cerebral blood flow/velocity response to changes in the partial pressure of arterial blood gases, such as carbon dioxide (Kety and Schmidt, 1948; Hoiland et al., 2019) but it is currently not known how cerebral vascular reactivity is affected by the acute consumption of SSB and ASB.

Therefore, we investigated dynamic cerebral autoregulation and cerebral vascular reactivity to hypercapnia in healthy adults following the acute consumption of an SSB, an ASB, and water. Our first aim was to investigate the effects of commercially available caffeinated sugar- and artificially-sweetened soft drinks on cerebral vascular function. We hypothesized that the acute consumption of SSB and ASB would augment dynamic cerebral autoregulation and attenuate cerebral vascular reactivity to hypercapnia compared to water due to the effects of caffeine. Our second aim was to examine the effects of different sweeteners in commercially available caffeinated soft drinks on cerebral vascular function. Because our SSB contained HFCS, we hypothesized that the acute consumption of an SSB would attenuate dynamic cerebral autoregulation and cerebral vascular reactivity to hypercapnia compared to acute consumption of an ASB and water.

Twelve participants (age: 23 ± 2 years, height: 171 ± 10 cm, weight: 72 ± 11 kg; body mass index: 24 ± 3 kg/m²; four females) volunteered for the study. All participants self-reported to be free of cardiovascular, respiratory, metabolic, and neurological disorders and were regularly physically active. Female participants were tested within 10 days of the start of their self-reported menstrual cycle and were not pregnant (confirmed by a urine pregnancy test at the beginning of each study visit). All study visits were scheduled with at least 24 h between visits and started within the same hour of the day to account for any potential circadian variations. The study involving human participants was reviewed and approved by the Institutional Review Board of the University at Buffalo. The participants provided written informed consent to participate in the study. The study was performed in accordance with the latest revision of the Declaration of Helsinki with the omission of registration in a database.

There were no differences (P = 0.798) in pre-consumption USG among the water (1.009 ± 0.007), SSB (1.008 ± 0.006), and ASB (1.008 ± 0.006) study visits. At the post-consumption baseline (Table 1), heart rate was higher after SSB (P = 0.047) compared to water. MCAv (Figure 2A; both P < 0.002) and MCAc (Figure 2B; P < 0.041) were lower after SSB and ASB compared to water. MCAc was also lower after ASB compared to SSB (Figure 2B; P = 0.035). PCAc (Figure 2D) was lower after ASB compared to water (P = 0.007). The mixed effects model analyses revealed a significant effect for PCAv (Figure 2C; P = 0.031) and PETCO₂ (Table 1; P = 0.037), however, no multiple comparisons were significant (P > 0.071). There were no differences in mean arterial pressure, stroke volume, cardiac output, or total peripheral resistance between conditions at post-consumption baseline (Table 1).

We investigated the acute effects of consuming a caffeinated SSB and ASB on cerebral vascular function in healthy, young adults. The novel findings from our study indicate that compared to water: (1) resting MCAv, MCAc, and PCAc were lower after the caffeinated SSB and ASB; (2) dynamic cerebral autoregulation did not differ with the SSB compared to the ASB or between soft drinks and water during an orthostatic challenge; and (3) cerebral vascular reactivity to hypercapnia did not differ between the SSB and ASB but was attenuated after ASB compared to water.

MCAv was lower following consumption of both soft drink beverages compared to water. Cerebral vascular conductance, an indirect indicator of vasomotor tone (Claassen et al., 2007), was also lower in the MCA and PCA following both soft drinks compared to water. We also observed a significant effect of beverage consumption on PCAv, but the post hoc analysis was unable to determine differences among the beverages. Although unlikely, the lower MCAv, MCAc, and PCAc values could be a result of small changes in arterial CO₂ content as mean arterial pressure was not different between conditions. Cerebral artery blood velocity is sensitive to changes in arterial CO₂ content such that there is an estimated 2% decrease in velocity per mmHg decline in PETCO₂ (Ide et al., 2003; Hoiland et al., 2019). Although not significant, the changes in PETCO₂ compared to water were ~2 mmHg and ~1 mmHg lower following ASB and SSB, respectively. These numbers are similar to the non-significant ~2 mmHg difference in PETCO₂ after 300 mg of caffeine, however, our caffeine content (76 mg) was much lower than this (Blaha et al., 2007). Thus, it is unlikely that these small differences in arterial CO₂ content explain the entire magnitude of the lower MCAv following ASB (~18%) and SSB (~14%) compared to water. Both soft drinks listed identical amounts of caffeine (76 mg), which is an adenosine receptor antagonist that peaks in the plasma between 30 and 90 min after consumption (Mandel, 2002; Martins et al., 2020) and causes vasoconstriction of the cerebral the vasculature, thus reducing cerebral blood flow (Cameron et al., 1990; Coney and Marshall, 1998; Phillis and O’Regan, 2003). For example, 250 mg (Lunt et al., 2004) or 300 mg (Blaha et al., 2007) of caffeine ingestion reduced MCAv by 13% and 14%, respectively, in both adults who were healthy and post-stroke. It is currently not known if a dose-response relationship exists between caffeine ingestion and intracranial artery blood velocity. Nonetheless, the caffeine content in the ASB and SSB likely contributed to the lower resting intracranial artery blood velocities and indices of vasodilation (i.e., conductance) that we observed compared to water. Although we did not collect blood samples in this study, we tested participants within the window of peak plasma caffeine concentrations (Mandel, 2002; Martins et al., 2020).

In addition to caffeine, other constituents of the ASB and SSB may have modulated resting intracranial artery blood velocities and indices of vasodilation (i.e., conductance) that we observed compared to water. The ASB that we used contained the artificial sweeteners acesulfame potassium, aspartame, and sucralose. There is currently no evidence in humans to indicate that the acute consumption of these artificial sweeteners would attenuate resting cerebral blood flow. However, there is evidence that ingesting acesulfame potassium combined with sucralose for 10 weeks reduces endothelium-dependent relaxation to acetylcholine in rat aortas (Risdon et al., 2020). Therefore, chronic consumption of acesulfame potassium and sucralose may be needed before observing changes in resting intracranial cerebral artery blood velocity.

Acute consumption of an HFCS beverage increased renal vascular resistance (Chapman et al., 2020) and increased systolic blood pressure (Le et al., 2012) but did not affect arterial stiffness in young healthy adults (Freemas et al., 2021). Although we do not have pre-experimental beverage consumption blood pressure, there were no differences between the blood pressures at baseline between water and soft drink beverages but this could be due to a lower amount of caffeine than amounts known (>90 mg) to increase blood pressure (Nurminen et al., 1999). To our knowledge, there are no reports on the effects of HFCS on cerebral blood velocity and cerebral vascular function. However, there are reports of the isolated acute consumption of fructose. The acute ingestion of fructose mixed with water has been shown to reduce cerebral blood flow, assessed using magnetic resonance imaging arterial spin labeling, to various brain regions such as the thalamus, hippocampus, posterior cingulate cortex, fusiform gyrus, and visual cortex (Page et al., 2013). The acute consumption of fructose also reduced serum biomarkers of nitric oxide, increased serum uric acid, and impaired micro- and macrovascular endothelial function in healthy adults (Loader et al., 2017; Cai et al., 2018). Fructose-fed rats showed increased levels of oxidative stress after 7 days of consumption (Delbosc et al., 2005). There is also evidence that fructose ingestion modulates sympathetically-mediated increases in heart rate (Schwarz et al., 1992). Our findings likely support this as heart rate was higher at post-consumption baseline with the SSB compared to water and ASB, despite having the same caffeine amount as the ASB. Together, it is important to consider the route of administration, interaction with other ingredients in the drink (e.g., caffeine), the volume of fructose, the timing of measurements, etc. as these could influence modulators of cerebral vascular function similar to the peripheral vasculature.

We found that CVR_CO2 in the MCA was lower following the ingestion of ASB compared to water but observed no other differences between beverages or in the PCA. Using an animal model, relatively high doses of caffeine (10 mg/kg) have been shown to reduce the cerebral vascular response to hypercapnia by 40–44% (Estevez and Phillis, 1997). Using transcranial Doppler ultrasound, Blaha et al. (2007) also found that ingestion of 250 mg of caffeine reduced the MCAv response to hypercapnia by ~11%. Conversely, the intravenous infusion of 2.5 mg/kg body weight of caffeine did not reduce the cerebral blood flow response to hypercapnia assessed using blood-oxygenation-level-dependent magnetic resonance imaging (Chen and Parrish, 2009). Because we only observed a decrease in CVR_CO2 following ASB, even though both soft drinks contained identical caffeine content, we speculate that additional factors modulated this response. CVR_CO2 is used as an index of cerebral vasculature nitric oxide bioavailability as it facilitates increases in MCAv during hypercapnia (Iadecola and Zhang, 1994; Schmetterer et al., 1997). However, our study is the first to examine such a response following ASB. In this regard, flow-mediated dilation (a marker of peripheral vascular nitric oxide bioavailability; Joannides et al., 1995) was found to be not meaningfully different following the acute consumption of sucralose in healthy, young adults (Memon et al., 2014). Increases in cerebral blood flow during hypercapnia are also due to the hyperpolarization of endothelial cells via the activation of voltage-gated potassium channels resulting in vascular relaxation (Kitazono et al., 1995; Nelson and Quayle, 1995) which can be assessed with selective inhibitors, such as glibenclamide (Faraci and Heistad, 1998). Additional studies are warranted to replicate our results and determine which constituents of ASB contribute to the attenuated CVR_CO2.

Although we did not collect blood to quantify blood biomarkers such as uric acid, nitric oxide precursors, blood glucose and fructose, insulin, and lipids, we have previously shown that HFCS ingestion increases serum uric acid, serum glucose, and serum fructose following the identical SSB used in the current study (Chapman et al., 2020). Uric acid has been linked with reducing nitric oxide bioavailability (Khosla et al., 2005). Thus, we expected CVR_CO2 to be attenuated following the SSB. Our study is the first to report whether the consumption of SSB influences CVR_CO2, therefore it is unclear why CVR_CO2 was not affected by SSB ingestion. We speculate that the amount of circulating uric acid was not sufficient to lower nitric oxide bioavailability and influence CVR_CO2 in the intracranial arteries we examined. Furthermore, fructose consumption does cause small increases in circulating insulin (Le et al., 2012), which might have offset the vasoconstricting effects of uric acid and caffeine in the cerebral vasculature during hypercapnia (Katakam et al., 2009). Without blood samples, we are unable to identify blood biomarkers that could have helped elucidate specific mechanisms.

We found that dynamic cerebral autoregulation during an orthostatic stressor was not statistically different between the beverage conditions. Our study is the first to report how ASB and SSB influence dynamic cerebral autoregulation in humans. Moderate and large doses of caffeine (20 and 40 mg/kg body weight) administered to rats did not alter cerebral autoregulation during severe hypotension (Phillis and DeLong, 1986). Previous evidence from humans indicates that the rate of regulation, a marker of dynamic cerebral autoregulation, is greater (i.e., improved) 30 min following acute coffee ingestion that included 250 mg of caffeine among other ingredients that might contribute to augmented cerebrovascular function (Sasaki et al., 2016). Sasaki et al. (2016) posited that caffeine ingestion caused cerebral vasoconstriction, due to increased circulating catecholamines and antagonizing cerebral adenosine receptors, that lead to augmented dynamic cerebral autoregulation. We observed lower resting MCAc and PCAc following ASB compared to water indicating intracranial vasoconstriction. However, these changes in resting cerebral vasomotor tone did not translate to an augmented dynamic cerebral autoregulation. Thus, it is possible that caffeine content greater than our ASB and SSB or other drink constituents of the ASB and SSB that we tested is needed to influence cerebral vasoconstriction to the extent needed to influence dynamic cerebral autoregulation in humans during orthostatic stress.

Our study is the first investigation to report on the acute effects of caffeinated-soft drinks, both sugar- and artificially-sweetened, on cerebral vascular function. Our findings demonstrate: (1) lower blood velocity and conductance in the middle cerebral artery following an artificially sweetened beverage and a sugar-sweetened beverage and lower posterior cerebral artery conductance following an artificially sweetened beverage compared to water; and (2) lower cerebral vascular reactivity to hypercapnia in the middle cerebral artery following an artificially sweetened beverage compared to water. Contrary to our hypothesis, we did not observe any differences between water consumption and a sugar-sweetened beverage for cerebral vascular reactivity to hypercapnia or any differences between beverage conditions for dynamic cerebral autoregulation. We speculate that caffeine is the main ingredient that modulated resting intracranial artery blood velocity and cerebral vascular reactivity to hypercapnia following caffeinated beverages.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Institutional Review Board of the University at Buffalo. The patients/participants provided their written informed consent to participate in this study.

ER, MW, ZS, and BJ: study design. ER, MW, PK, and LP: collected data. ER, MW, PK, LP, and BJ: analyzed data. ER, MW, PK, and BJ: drafted manuscript. ER, MW, PK, LP, ZS, and BJ: edited and approved the final submission. All authors contributed to the article and approved the submitted version.