Edited by: David Christopher Nieman, Appalachian State University, United States
Reviewed by: Mark Willems, University of Chichester, United Kingdom; Jonathan Peake, Queensland University of Technology, Australia
This article was submitted to Sport and Exercise Nutrition, a section of the journal Frontiers in Nutrition
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Sitting in an upright position for extended periods leads to significant changes in hemodynamics (
Studies demonstrating the benefits of consuming berryfruit on cardiovascular health have ascribed these benefits to their high phytochemical and anthocyanin content (
It is still unclear, however, whether blackcurrants can attenuate the detrimental changes in blood hemodynamics that occur during prolonged sitting. This is because nutritional intervention studies characterizing the effect of consuming berryfruit on resting measures of blood hemodynamics have not elaborated on whether participants remained seated in their resting position for the duration of the trial. This is a critical variable, as minor exercises (
Given the evidence supporting the modulatory effect of blackcurrants on resting hemodynamics, there is interest in their potential as ergogenic aids for exercise performance. By altering arterial vessel tone, blackcurrant supplementation is proposed to enhance exercise performance through improved oxygen delivery, phosphocreatine resynthesis rates and lactate clearance (
While the potential ergogenic effects of blackcurrant extract have been investigated across a range of exercise modalities, it is currently unclear whether supplementation can benefit repeated isometric hand grip exercise, as occurs in a number of sports including rock climbing (
The objectives of this present study were to (1) investigate the hemodynamic effects, including changes in peripheral blood flow, of consuming a single dose of a blackcurrant extract during a prolonged period of sitting, and (2) identify whether a single dose of blackcurrant extract can alter repetitive, isometric handgrip performance. We hypothesized that the intake of blackcurrant extract would improve peripheral blood flow, compared with a placebo, and that this would improve muscular performance during fatiguing exercise.
Ten healthy males (mean ± SD; age = 29.2 ± 6.0 years; mass ± 92.1 ± 19.9 kg; height = 181.6 ± 8.9 cm) volunteered to participate in this study. Participants were non-smokers and were free of cardiovascular disease and conditions that may affect circulation or blood flow. Each participant gave informed written consent and completed health screening prior to being familiarized to the experimental procedures. Ethical approval was obtained from the University's Human Ethics Committee.
For this study, we used a commercial anthocyanin-rich extract made from New Zealand grown blackcurrants, sourced from the New Zealand Blackcurrant Co-operative (Nelson, New Zealand). The total anthocyanin composition of the blackcurrant powder was determined by high performance liquid chromatography (HPLC) using procedures described by Lyall et al. (
The blackcurrant extract was encapsulated in opaque gelatin capsules and were served to trial participants at a dose of 1.87 mg total anthocyanins/kg bodyweight (172.23 ± 37.21 mg total anthocyanins). This dose was based off the findings of Matsumoto et al. (
Participants underwent a familiarization and two subsequent trials, each separated by at least 1 week, in which the hemodynamic response and performance effects of blackcurrant extract were compared to the responses with the placebo at rest and during exercise. Allocation of treatment was double-blinded and randomly assigned so that an equal number of participants were allocated in each treatment intervention in the first trial. Participants were subsequently allocated to the opposite treatment in the second trial. Prior to each trial, participants were instructed to abstain from caffeine, alcohol, exercise and foods containing anthocyanins in the 24 h period; participants were provided with a list of foods to avoid.
At least 1 week after familiarization, having fasted overnight and having consumed a standardized pre-trial meal (8 g protein, 45 g carbohydrates, 11.4 g fat; 1,450 kJ) and bolus of water (600 mL) 120 min prior, participants presented to the laboratory in the morning. To ensure consistency, the time of day was the same between trials for each participant. Participants were seated in an upright position with knee and hip angles at ~90° and both arms supported, at approximately heart height, on a cushioned table. Participants sat at rest for 30 min before a blood sample was collected into an EDTA vacutainer from the antecubital vein of the upper arm and baseline measures of forearm blood flow (FBF), forearm skin temperature, heart rate (HR), systolic (SBP), and diastolic (DBP) blood pressure were made. Participants then consumed their allocated treatment intervention, containing either blackcurrant extract or placebo. Participants then remained seated for 120 min and the same measures were made every 30 min, except blood sampling which occurred again at 120 min only.
Having sat for 120 min, participants completed a bout of intermittent hand-grip exercise to volitional fatigue. The number of repetitions completed was recorded. Forearm blood flow of the exercised arm was subsequently measured immediately after (0), 2, 5, 10, and 15 min post-exercise.
Blood pressure was measured non-invasively via finger photoplethysmography (FinaPres® Medical Systems: Biomedical Instruments, Amsterdam) and periodically checked against a manual sphygmomanometer for accuracy. HR was measured with 3-lead electrocardiogram (ADInstruments Ltd., Australia) and cardiac output
Forearm blood flow was measured using standard procedures for venous occlusion plethysmography (
Forearm arterial blood flow (mL/100 mL/min) was then calculated using the following equation:
200 x increase in forearm circumference (mm/min) / resting forearm circumference (mm)
Total peripheral resistance (TPR) and forearm vascular resistance (FVR) were calculated using the following equations:
To reduce the influence of changes in forearm skin temperature on FBF, attempts were made to maintain forearm skin temperature at 30°C. Consistent forearm skin temperature was maintained using a liquid conditioning garment (CoreTech® tube suit, Delta Temax Inc., Canada) worn on the arm of interest; this garment was fitted and forearm skin temperature adjusted and maintained at the target temperature from 30 min prior to baseline measures until the completion of the trial. Temperature (accurate to 0.1°C) and flow rate (15.7 L/min) of the circulating liquid was controlled and pumped using a water heater/cooler with a built-in pump (Neslab Instruments Inc., USA). Skin temperature was recorded with a skin temperature probe (ADInstruments Ltd.) taped on the forearm, 2 cm below the position of the mercury strain gauge. Ambient temperature and relative humidity were maintained at 21 ± 0.3°C and 55 ± 2%, respectively, for all trials.
Having sat at rest for 150 min, participants completed as many repetitions as possible at 60% of their iMVC force, obtained during familiarization, by squeezing a grip force transducer (MLT004/ST, ADInstruments Ltd., Australia) with their dominant hand. Each repetition was held for 5 s at the target force, with 10 s rest between efforts; this was continued until participants could no longer match the specified force output over 3 consecutive repetitions. Grip force output was collected using a PowerLab data acquisition system and recorded and displayed in LabChart (ADInstruments Ltd.) so that participants had real-time visual feedback of their force output. The number of repetitions completed, excluding the 3 consecutive repetitions below the target force was compared between treatment interventions.
Quantification of endogenous nitrite and nitrate levels in plasma samples, taken prior to and 120 min after treatment ingestion, was performed using a commercially available kit (SKGE001, RnD Systems™), according to the manufacturer's instructions. Endogenous nitrite was quantified by measuring sample and standard absorbance at 540 nm (Fluostar® Omega plate reader, BMG Labtech, Ortenberg, Germany) following the addition of Griess reagents. For nitrate quantification, plasma nitrate was enzymatically converted to nitrite with nitrate reductase followed by quantification of total nitrite as per the manufacturer's instructions. Nitrate concentrations were then calculated by subtracting the endogenous nitrite concentration from total nitrite concentration. Samples were analyzed in duplicate with a coefficient of variation of 5.62%.
Plasma endothelin-1 concentration was measured in samples collected before and 120 min after treatment ingestion using a commercially available endothelin-1 (human) enzyme immunoassay kit (ADI-900-020A, Enzo® Life Sciences). Plasma samples initially were diluted 1:4 in the kit's assay buffer endothelin-1 quantification. Samples were analyzed in duplicate with a coefficient of variation of 11.1%.
Statistical analysis was performed in IBM® SPSS Statistics v21 (NY, USA). Two-way (treatment x time) repeated measures analysis of variance (ANOVA) was carried out to investigate changes in blood measures, hemodynamic variables and handgrip performance. Where significant main effects were identified,
To compare the reliability of baseline measures of FBF, the intraclass correlation coefficient (ICC) and 95% confidence intervals (CI) were calculated with a two-factor mixed-effect model with absolute agreement. Reliability was classified as poor (ICC <0.5), moderate (0.5 to 0.75), good (0.75 to 0.9) and excellent (> 0.9) (
A significant increase in SBP (
Acute effects of consuming either blackcurrant extract (BCE) and placebo powder on resting
Mean arterial pressure (MAP) and TPR also significantly increased over the 120 min period at rest (
Irrespective of the experimental controls put in place prior to each trial, significant differences in FBF between treatments were found at baseline (blackcurrant extract 2.49 ± 1.27 vs placebo 2.84 ± 4.29 mL/100 mL/min;
Acute effects of consuming blackcurrant extract (BCE) and placebo powder on forearm
A treatment x time interaction was found for both raw (
A time effect (
Mean forearm skin temperature was 30.3 ± 1.6°C. No change in forearm skin temperature was observed over the 120 min rest period (
No treatment or time effect (
Plasma
Endothelin-1 concentration in plasma tended to be higher 120 min after intervention intake compared to baseline concentration (
No difference in the number of repetitions completed was evident between trials (blackcurrant = 74 ± 29 repetitions vs placebo = 77 ± 45 repetitions,
Absolute post-exercise FBF results showed a significant treatment (
Percentage change, from baseline, in forearm blood flow (FBF) after a bout of intermittent hand-grip exercise 2-h after ingesting blackcurrant extract (BCE) or placebo powder. Data are mean ± SD. * indicates significant difference from FBF measures immediately post-exercise (0 min) (
The primary aim of this study was to investigate the effects of blackcurrant extract on FBF, and other hemodynamic measures, at rest during a prolonged period of sitting and after exercise. Additionally, intermittent handgrip exercise was performed to determine whether exercise performance increased with blackcurrant extract, compared with a sugar-equivalent placebo control. Our findings show that a single dose of blackcurrant extract maintained both FBF and FVR during prolonged sitting but did not enhance resistance to fatigue from repeated submaximal isometric hand contractions.
Uninterrupted sitting for 120 min also led to significant increases in blood pressure (SBP and DBP), MAP,
Epidemiological and clinical studies demonstrating the benefits of consuming berryfruit on cardiovascular health have ascribed these benefits to their high anthocyanin content (
Although great care was taken to collect data in a highly controlled manner, including standardizing diet, time of day, ambient and skin temperature, arm position and restricting previous day physical activity, a significant difference in FBF was found between treatments at baseline. However, the controls put in place in the present study appear to have been successful as no order effect was found between trial 1 and trial 2. Irrespective of controls put in place, previous studies investigating FBF, using venous occlusion plethysmography, have reported considerable variability between days (
In the present study, 120 min of uninterrupted sitting led to the progressive decline in FBF. Importantly, we show that the decline in FBF following 90 and 120 min of uninterrupted sitting was mitigated by consuming a single dose of blackcurrant extract. The progressive increase in FVR during the 120 min sitting period was similarly moderated by blackcurrant extract so that FVR did not significantly rise above baseline levels. Taken together, these findings support the efficacy of blackcurrant extract for maintaining forearm blood hemodynamics during prolonged sitting. Although anthocyanin bioavailability was not measured in this study, the time points where treatment differences in FBF and FVR were observed corresponded with the expected bioavailability of blackcurrant anthocyanins (
These findings align with similar placebo-controlled research (
Existing studies suggest that the efficacy of flavonoids in modulating FBF during prolonged sitting is independent of secondary hemodynamic parameters. Consuming a single dose of anthocyanin-rich blackcurrant extract and blueberry juice has been shown to have no observable effects on resting blood pressure and HR, despite reported increases in FBF and endothelial function, respectively (
Declining concentrations of nitric oxide, a key vasodilator that regulates cardiovascular function, contributes to venous stasis during prolonged sitting (
Prolonged sitting has been proposed to induce oxidative stress which favors the up-regulation of endothelium-derived endothelin-1 (
The potential for New Zealand blackcurrants to improve isometric muscular performance was demonstrated by Cook et al. (
Hyperemia to exercising muscles rapidly declines after the cessation of exercise. The rate of this decline is dependent upon exercise intensity so that greater elevations in FBF occur following heavier workloads, even after considerable recovery time (
Although great care was taken in the control of this study, there are several limitations, in addition to those previously discussed, that should be considered. Firstly, this study was only done on healthy males with no apparent cardiovascular disease. Ovarian sex hormones produce known changes in vascular tone with estrogen promoting vasodilation via nitric oxide dependent and independent pathways (
Secondly, while the measure of forearm blood flow using venous occlusion plethysmography is considered the “gold standard” for assessing vascular function, particularly in the forearm vascular bed (
In summary, ingestion of a single dose of a New Zealand blackcurrant extract preserved forearm blood flow during a prolonged period of sitting and inactivity. This finding supports the growing evidence that blackcurrant anthocyanins have the ability to alter hemodynamics during rest/inactivity. This may provide health benefits to those unable to move their lower limbs regularly, such as individuals undertaking long-haul air travel, sedentary work, or those with decreased mobility due to illness and disease. However, further research is needed to better understand the implications of altered hemodynamics on lower limb blood flow, as the effects of black currant extract/anthocyanins are likely to be systemic in nature, and not just localized to the upper limbs. Although blood flow was maintained, this did not improve intermittent hand-grip exercise; however, because of experimental limitations, further research is needed to identify whether blackcurrant anthocyanins can improve repetitive muscular work.
The datasets generated for this study are available on request to the corresponding author.
The studies involving human participants were reviewed and approved by Massey University Human Ethics Committee, Southern A. The patients/participants provided their written informed consent to participate in this study.
All authors conceptualized and designed the study. MB, DL, and BP recruited the participants, performed the human trials, carried out the experimental analysis on collected blood samples, the interpretation of results and drafted the manuscript. RH reviewed the manuscript and approved the final version submitted.
DL and RH were employed by company The New Zealand Institute for Plant and Food Research Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors wish to thank the participants who kindly volunteered for this study.