Energy Drink Effects on Hemodynamics and Endothelial Function in Young Adults

Energy products, especially energy drinks, are being consumed more and more worldwide, especially by young adults ranging in ages between 20 and 39 years [1]. Additionally concerning is the fact that younger children and adolescents are often targeted in the marketing campaigns of the energy drink manufacturers [2‒4]. Energy drinks are flavored beverages that contain artificial caffeine, guarana (a natural source of caffeine), sugar, amino acids, herbs, and vitamins [5].

They are promoted as a way to boost physical energy and mental performance, and more recently as a coffee substitute [5]. Emerging evidence has linked energy drink consumption with negative health consequences including risk-seeking behaviors, poor mental health, adverse cardiovascular effects, and metabolic, renal, or dental conditions [4, 6].

In the cardiovascular system, energy drink consumption by young healthy individuals has been associated with elevations in heart rate, blood pressure, and reduced endothelial function and hypercoagulability [7‒11]. In addition, several groups have noted prolongation in the QTc interval associated with energy drink consumption [7, 8, 12, 13]. The goal of this study was to evaluate changes in endothelial function, hemodynamic, and electrocardiogram parameters before and after consumption of a popular energy drink in young healthy subjects.

The study was undertaken at the McGovern Medical School at the University of Texas-Houston after Institutional Review Board approval, IRB reference number: HSC-MS-13-0043. The study group was formed after presenting to various meetings of the medical students, emailing class lists, and posting on their intranet sites. The volunteers were assessed to make sure they met the inclusion and exclusion criteria and were then asked to read and sign the consent form.

1. A normal baseline electrocardiogram.

2. A normal resting blood pressure.

3. The ability to abstain from tea, coffee, cola, alcohol, chocolate, or any other source of caffeine 24 h prior to testing.

4. The ability to provide informed consent.

1. History of cardiac or pulmonary disease.

2. History of diabetes mellitus.

3. History of hypertension.

4. History of cigarette smoking.

5. Currently taking any of the following medications and unable to stop for 1 week prior to testing: caffeine tablets, theophylline, and medications that contain caffeine.

6. Currently drinking an energy beverage on a regular basis (at least twice a week) and unable to stop for 1 week prior to testing.

7. Currently pregnant.

The following baseline demographics were obtained: age, gender, weight, height, race, and BMI. After fasting for at least 8 h prior to testing, the subjects received a baseline electrocardiogram and underwent testing of endothelial function using the technique of flow-mediated dilatation (FMD) by a single registered vascular ultrasonographer, who was certified by the University of Wisconsin Atherosclerosis Imaging Research Program Core Laboratory [14]. After resting supine for 10 min in a temperature-controlled room, a blood pressure cuff was placed on the widest part of the proximal right forearm approximately 1 cm distal to the antecubital fossa. Using a 10-MHz resolution linear array vascular ultrasound transducer with a Philips iE33 ultrasound machine, the brachial artery was located above the elbow and scanned in longitudinal sections. After recording baseline B-mode digital images of the brachial artery and spectral Doppler images of flow, the forearm cuff was inflated to 250 mm Hg for 5 min to induce reactive hyperemia. Immediately after deflation, spectral Doppler images were obtained to verify hyperemia. FMD of the brachial artery was measured approximately 90 s after cuff deflation. The relative FMD (%) was calculated as the ratio between the largest post-cuff release and the baseline diameter. Each image was checked for quality control, and each artery diameter was measured from the media to media using the automated software, Vascular Tools 6, an FDA approved software to aid analysis of FMD of brachial arteries (Medical Imaging Applications, LLC, Coralville, IA, USA). We ran the software 3 separate times on the images at 3 different QRS complexes with slightly different readings obtained, and then averaged them.

The subjects then drank an energy drink in approximately 1 min. This was a 24-oz can of Monster Energy, whose contents include 54 g sucrose, glucose, sucralose, maltodextrin, sodium 360 mg, sodium citrate, sodium Chloride, caffeine 240 mg, taurine 2,000 mg, niacin 40 mg 200% RDA, niacinamide, pyridoxine 4 mg 200% RDA, cyanocobalamin (B12) 12 mcg 200% RDA, riboflavin (B2) 3.4 mg 200% RDA, ginseng extract 400 mg, glucuronolactone, inositol (B8), guarana extract, L-carnitine, all listed as a part of a 5,000 mg “Energy Blend,” and sodium benzoate.

During the 90-min period between energy drink consumption and repeat FMD, the subjects sat on the measuring chair and relaxed. Specifically, they were not allowed to eat, drink, or move about. The subjects had FMD repeated at 90 min after consumption. Hemodynamic measurements were also monitored at baseline as well as 90 min post-energy drink consumption. The subjects were in the supine position for all measurements. The time of 90 min was chosen as pilot data suggested maximal effect on FMD at this time. Specifically, during the pilot study, FMD measurements were made at 60 and 90 min following consumption of the energy drink, and the maximal FMD occurred at 90 min.

Microsoft Excel was utilized for the statistical analysis. The discrete variables were analyzed using a Student’s t test for unpaired samples. A two-sided p < 0.05 was considered indicative of statistical significance. ANOVA testing was performed for electrocardiogram variables to compare statistical significance at baseline and 90 min post-consumption.

Forty-four volunteers met inclusion criteria. Their demographics are present in Table 1. The average age was 24.7 with 34 (77%) being male. The cohort comprised of mostly Caucasians (52%) and Asians (27%). The average BMI was 23.4 kg/m².

Hemodynamics data were normal at baseline (Table 2). Fifteen minutes following energy drink consumption, systolic and diastolic blood pressure both increased by 16 mm Hg on average, while heart rate increased on average by 17 bpm. Ninety minutes following energy drink consumption, systolic blood pressure was no longer significantly different from baseline; however, diastolic blood pressure (67.6 ± 6 vs. 69.2 ± 6.5; p = 0.02) as well as heart rate (62.3 ± 9.1 vs. 67.1 ± 8.7; p = 0.00005) was still significantly increased. There was no change in electrocardiogram parameters at baseline or after energy drink consumption (Table 3).

FMD analysis was statistically notable for a significant attenuation in endothelial function measured via brachial artery diameter 90 min post-energy drink consumption. Pre-energy drink %FMD was 5.1 ± 4.1%, while post-energy drink %FMD was 2.8 ± 3.8% (p = 0.004) (Fig. 1).

Endothelial cells constitute the inner lining of blood vessels and have metabolic and synthetic functions. When endothelial cells function abnormally or there is “endothelial dysfunction,” it is associated with poor vascular reactivity, pro-thrombosis, pro-adhesion, proinflammation, and growth promotion.

Mechanistically, endothelial dysfunction, where the endothelium’s ability in regulating vascular resistance is impaired, may reduce blood flow. Exposure to cold, mental arithmetic, anger, exercise, cigarette smoking, cocaine, excess food, or alcohol may reduce endothelial function [10]. Endothelial cell function is closely related to cardiovascular risk, with impairment being involved in the pathogenesis of atherosclerosis and coronary artery disease.

Impairment of endothelial cell function is also related to a decrease in the bioavailability of nitric oxide, a vasodilator, and inhibitor of platelet aggregation, which also has anti-inflammatory and anti-proliferative properties [11]. Endothelial cell function is commonly measured indirectly by FMD in the brachial artery, which is well validated, and serves as a strong predictor of cardiovascular events [11].

Studies suggest that a brachial artery FMD response to post-occlusion hyperemia in young healthy adults is approximately 7.0%, range 5.0–9.5% [15]. In our study, our average FMD at baseline was 5.1%, within the normal range. Ninety minutes following energy drink consumption, the FMD was reduced to 2.8%, which represented a significant reduction (p = 0.004). Lower FMD values like this have been described in patients with coronary artery disease and peripheral arterial disease; also, reduced FMD has been associated with cardiovascular risk factors including hypertension, obesity, hyperlipidemia, smoking, and diabetes mellitus [15]. Whether this acute reduction we noted in FMD acutely after energy drink consumption is clinically significant is unknown. Further research to evaluate FMD in those with more chronic exposures to energy drinks, such as those consuming them daily, is recommended.

Several recent reviews on cardiovascular complications associated with energy drink consumption suggest that the impact on endothelial function could be a factor in subsequent cardiac events [8]. Some of their ingredients individually or in combination may be associated with reduced endothelial function [11].

When healthy young adults ingested energy beverages, we found significant acute increases in heart rate as well as systolic and diastolic blood pressure within 15 min. While systolic blood pressure normalized by 90 min, heart rate and diastolic blood pressure remained significantly elevated.

Similar hemodynamic changes have been reported by others. In a randomized controlled trial involving 34 participants at an average age of 22 years, 1 group noted a significant increase in systolic blood pressure of 5 mm Hg and diastolic blood pressure of 4 mm Hg in those consuming an energy drink (32 oz, 320 mg caffeine) versus placebo [7]. Their peak in systolic blood pressure elevation occurred at 60 min following consumption. Unlike ours, which normalized by 90 min, their systolic blood pressure remained elevated at 6 h post-consumption when compared to placebo. One explanation for this difference could be the dose: their subjects consumed a larger quantity of energy drink compared to our study, 32 versus 24 oz, which results in a larger amount of caffeine being consumed, 320 mg versus 240 mg. A review noted a typical increase in systolic blood pressure, diastolic blood pressure, and heart rate in normal healthy persons 1–2 h following consumption of energy drinks is 6–10 mm Hg, 3–6 mm Hg, and 3–7 bpm, respectively [8].

We did not find any significant changes in electrocardiogram parameters following the consumption of energy drinks. Specifically, no significant effects on the duration of QRS, QT, QTc, and PR intervals were seen; only some nonspecific electrocardiogram changes were noted. However, others have noted changes, especially significant prolongation of the QT interval if caffeine consumption was 320 mg or more in the energy drink group.

In a recent study of 34 subjects (mean age 22 years), who consumed energy drinks containing 320 mg of caffeine, the authors noted a significant increase in the QTc interval of 7 ms when compared to placebo [7]. A review noted that moderate acute energy drink consumption with a caffeine intake of up to 200 mg does not seem to affect the QTc interval, whereas higher amounts may result in significant prolongations of the QTc interval of 3–8 ms than placebo [12]. In our study, there was 240 mg of caffeine in the energy drink consumed. Of note, in patients affected by a preexisting familial long QT syndrome, moderate energy drink consumption with caffeine consumption of only 160 mg is associated with serious QTc prolongation of >50 ms [12].

We noted a significant attenuation (reduction) in endothelial function 90 min following energy drink consumption. Others have reported similar findings.

In one study, subjects consumed either a 250-mL sugar-free energy drink (1 can) containing caffeine (80 mg), taurine (1,000 mg), and glucouronolactone (600 mg), or 250 mL carbonated water (control) [16]. Their findings showed that energy drink consumption acutely increased platelet aggregation and decreased endothelial function in healthy young adults, which was measured 1 h after consumption of the energy drink.

Acute energy drink consumption in young healthy subjects was associated with significant elevations in systolic and diastolic blood pressures, and heart rate 15 min later. Ninety minutes following energy drink consumption, we found a significant attenuation in endothelial function as well as sustained increases in diastolic blood pressure and heart rate. Reported cardiovascular events and complications are often preceded by endothelial dysfunction, as well as elevations in heart rate and blood pressure. Given these findings, it behooves us to further study the safety of energy drinks. In addition, given the effects on endothelial function and hemodynamics, studies on energy drinks of those exercising may help explain some of the adverse effects seen when those consuming them partake in exercise.

This study was performed with the approval of an appropriate Ethics Committee (Institutional Review Board approval, IRB reference number: HSC-MS-13-0043) and with appropriate participants’ informed consent in compliance with the Helsinki Declaration.

There is no apparent conflict of interest for any of the authors. The authors have no financial or other interest in the products or distributor of the products in the manuscript. In addition, the authors have no other kinds of associations, such as consultancies, stock ownership, or other equity interests or patent-licensing arrangements, with any of the products in the manuscript.

No sources of outside support for research were used in the writing and editing of this manuscript.

All authors qualify for authorship as defined by the Uniform Requirements for Manuscripts Submitted to Biomedical Journals and have read and approved the manuscript. Further, all authors were involved in data collection, processing, and manuscript planning and writing.