The plasma bioavailability of nitrate and betanin from Beta vulgaris rubra in humans
The plasma bioavailability of nitrate and betanin from Beta vulgaris rubra in humans
Tom Clifford 0 1
Costas M. Constantinou 0 1
Karen M. Keane 0 1
Daniel J. West 0 1
Glyn Howatson 0 1
Emma J. Stevenson 0 1
0 Water Research Group, School of Environmental Sciences and Development, Northwest University , Potchefstroom , South Africa
1 Department of Sport, Exercise and Rehabilitation, Faculty of Health and Life Sciences, Northumbria University , Newcastle-upon-Tyne NE1 8ST , UK
2 Tom Clifford
Purpose To evaluate the plasma bioavailability of betanin and nitric oxide (NOx) after consuming beetroot juice (BTJ) and whole beetroot (BF). BTJ and BF were also analysed for antioxidant capacity, polyphenol content (TPC) and betalain content. Methods Ten healthy males consumed either 250 ml of BTJ, 300 g of BF or a placebo drink, in a randomised, crossover design. Venous plasma samples were collected pre (baseline), 1, 2, 3, 5 and 8 h post-ingestion. Betanin content in BTJ, BF and plasma was analysed with reverse-phase high-performance liquid chromatography (HPLC) and mass spectrometry detection (LCMS). Antioxidant capacity was estimated using the Trolox equivalent antioxidant capacity (TEAC) and polyphenol content using Folin-Ciocalteu colorimetric methods [gallic acid equivalents (GAE)] and betalain content spectrophotometrically. Results TEAC was 11.4 ± 0.2 mmol/L for BTJ and 3.4 ± 0.4 μmol/g for BF. Both BTJ and BF contained a number of polyphenols (1606.9 ± 151 mg/GAE/L and 1.67 ± 0.1 mg/GAE/g, respectively), betacyanins (68.2 ± 0.4 mg/betanin equivalents/L and 19.6 ± 0.6 mg/ betanin equivalents/100 g, respectively) and betaxanthins 1 3
Beetroot; Betalains; Nitric oxide; Antioxidant; Bioavailability
-
2 Institute of Cellular Medicine, Newcastle University,
Newcastle-upon-Tyne, UK
(41.7 ± 0.7 mg/indicaxanthin equivalents/L and
7.5 ± 0.2 mg/indicaxanthin equivalents/100 g,
respectively). Despite high betanin contents in both BTJ
(~194 mg) and BF (~66 mg), betanin could not be detected
in the plasma at any time point post-ingestion. Plasma NOx
was elevated above baseline for 8 h after consuming BTJ
and 5 h after BF (P < 0.05).
Conclusions These data reveal that BTJ and BF are
rich in phytonutrients and may provide a useful means of
increasing plasma NOx bioavailability. However, betanin,
the major betalain in beetroot, showed poor bioavailability
in plasma.
Introduction
The root vegetable, red beetroot (Beta vulgaris rubra), is a
functional food that has attracted much attention over the
last decade, with particular focus on its potential health
benefits (for review, see [
1, 2
]). The interest in beetroot
has been largely driven by its nitrate content, which is
proposed to be ~1459 mg kg−1 DW [3]. Dietary nitrate may
confer beneficial health effects via its sequential reduction
to nitrite and nitric oxide (NOx), a pleiotropic molecule
that plays a key role in the regulation of vascular
homoeostasis, immune function and metabolism [
4, 5
]. There are
now several reports that acute consumption of beetroot can
stimulate endogenous NOx production and evoke positive
changes in endothelial function and blood pressure [
6–8
].
Consequently, beetroot is currently purported as a health
promoting food that might be useful for reducing the risk
of developing cardiovascular diseases (i.e. hypertension
and stroke) and immune disorders (i.e. inflammatory bowel
disease) [
2, 3, 9
].
Nitrate is not the only constituent of beetroot that may
have beneficial effects for health. As well as being a good
source of polyphenols, beetroot contains a group of
betalamic acid derivatives known as betalains [
10
]. The
betalains derive from the plant order Caryophyllales and are
categorised as either betacyanins, which are responsible
for the red/violet colour of red beetroot or betaxanthins,
which are yellow in colour [
1, 11
]. Betalains are
water-soluble phytochemicals that have been shown to possess
antiinflammatory, antioxidant and chemo-preventive activities
in vitro [
1, 10, 12, 13
]. In recent years, there has been a
particular interest in the biochemical activity of betanin
(betanidin 5-O-b-glucoside; chemical structure depicted by
Fig. 1), which is the most abundant betacyanin in beetroot
(300–600 mg/kg uncooked), and the main constituent of
the food colourant E162 [
10, 14–16
]. As a cationised
compound, betanin is a highly effective scavenger of reactive
oxygen species (ROS) [
14, 17
]. Betanin also possesses
antiinflammatory properties; betanin and its aglycone betanidin
were shown to modulate lipoxygenase and cyclooxygenase
activity in vitro, indicating that betanin might downregulate
pro-inflammatory signalling [
13
]. These findings have led
to interest in the role of beetroot in the protection against
the potentially damaging effects of ROS and aberrant
immune function [
18, 19
].
An increased awareness of beetroot’s potential health
benefits, together with an increased demand for
convenient health foods, has led to the development of a number
of beetroot-based functional food products. Throughout the
world, beetroot can now be purchased as a concentrated
juice drink or as a ready to consume precooked snack,
bypassing the need for prior cooking. Given the wide array
of bioactive compounds present in beetroot, it is possible
that regular consumption of these products could have
favourable effects for health and well-being. Furthermore,
these products might have applications as dietary
supplements in the management of a host of chronic and
degenerative disorders, including hypertension, osteoarthritis and
numerous cancers [
1, 9, 20
]. However, in order to evaluate
the potential usefulness of these products in the promotion
of general health or as therapeutic agents, information on
their in vivo bioavailability and phytochemical content in
humans is required.
Dietary nitrate is believed to be highly bioavailable [
21
],
and increases in the plasma have been observed following
ingestion of beetroot juice [
8, 22
]. Betalains, in contrast,
are thought to have much lower bioavailability [23];
however, no published studies have characterised the
bioavailability of betalains in plasma from consuming commercially
available beetroot products. Collectively, these data would
provide novel and readily accessible information to both
consumers and practitioners (i.e. nutritionists, dieticians,
physiologists and other health professionals) who may be
interested in the potential health benefits of commercially
available functional foods, like beetroot.
Consequently, the main objective of this study was to
determine the acute plasma bioavailability of betanin, the
major betalain in beetroot, and nitrate, a precursor for NOx
activity, in human plasma after consuming both beetroot
juice (BTJ) and beetroot whole food (BF). A secondary aim
was to characterise the antioxidant capacity, polyphenol
and betalain content of BTJ and BF.
Methods
Participants
Ten healthy, non-smoking males (age 23 ± 3 years; height
1.82 ± 0.60 m; mass 78.8 ± 6.7 kg) were recruited to
participate in this study. Participants were excluded from the
study if they had any known food allergies, were taking
medication (including dietary supplements), or were
suffering from or have previous history of renal,
gastrointestinal or cardiovascular complications or any other
contraindication to the study procedures. All participants were
made aware that their participation was voluntary and that
they were free to withdraw at any time. The study
protocol received institutional ethical approval. Each participant
provided written informed consent prior to study entry.
Experimental design
Subjects were required to attend the laboratory on 3
occasions, separated by 7 days. In the 48 h prior to each visit,
subjects followed a low phenolic and betalanic diet. This
included avoiding all vegetables, cured meats, fruits and
their juices, chocolate, wholegrain breads and grains,
caffeinated beverages including all varieties of tea, coffee and
alcohol. To ensure compliance, subjects were given a
written list of foods to avoid and diaries to record their intake
48 h prior to the first trial. They were instructed to
replicate this diet as closely as possible in the 48 h before the
remaining two trials. For each trial, subjects attended the
laboratory between the hours of 07:00–09:00 following a
12-h overnight fast and had a cannula inserted into a vein
at the antecubital fossa. After a baseline blood sample,
subjects were given 1 of 3 treatments; beetroot juice (250 ml),
an isocaloric placebo (250 ml) or cooked beetroot (300 g)
in a randomised, crossover fashion (see Table 1). Further
blood samples were drawn at 1, 2, 3, 5 and 8 h after
ingesting the treatments (see Fig. 2 for schematic) to ascertain the
pharmacokinetics of the compounds of interest. Subjects
were not allowed to consume any food until testing was
complete but were allowed water ad libitum; the amount of
water consumed on the first trial was recorded and
replicated in the subsequent trials.
Blood sampling procedures
At each time point, 10 ml of venous blood was collected
into EDTA treated tubes (Vacutainer, Bendict
Dickinson) and immediately centrifuged at 3000g (4°) for
10 min to separate plasma. Samples were aspirated into
a series of aliquots and stored immediately at −80 °C
until analysis.
Nutritional composition of the 3 treatments is provided
in Table 1. The beetroot juice (Love Beets Super Tasty
Beetroot Juice, Gs Fresh Ltd., Cambridgeshire, UK) and
the whole beetroot food were freshly cooked and
prepacked (Tesco PLC, Hertfordshire, UK). Both the
beetroot juice and beetroot food originated from the same
manufacturer (Gs Fresh Ltd., Cambridgeshire, UK).
The placebo (PLA) beverage contained a commercially
available fruit <1 % squash (Kia Ora, Coca Cola
Enterprises, Uxbridge, UK) with negligible phytochemical
and nitrate content and was fortified with maltodextrin
(Myprotein, Manchester, UK) flavourless protein powder
(Arla Foods, Amba, Denmark) and water, to match the
BTJ for volume and macro-nutrient content. The
quantification of nitrate in the BTJ was performed by Gs Fresh
Ltd; however, data on the nitrate content of BF were not
available.
Antioxidant activity and phenolic content
Extracts of the whole beetroot samples in 80:20
methanol:water, and mixtures of placebo and beetroot juice
in the same solvent were used to determine their total
phenolic content (TPC) and antioxidant activity (TEAC). For
the beetroot food, samples (about 10 g of whole beetroot)
were cryogenically milled in liquid nitrogen using an IKA
A11 S2 analytical mill (IKA Works, Wilmington, USA).
Accurate amounts of frozen powder (500–700 mg) were
extracted five times with 1 mL 80:20 methanol:water,
filtered and combined to a total volume of 5 mL, whilst the
liquid materials (juice and placebo) were diluted (1:10) in
the extraction solvent.
A modified 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay
used for antioxidant activity measurements was adjusted
for use in the present study [
24
]. The DPPH solution was
prepared freshly before the analysis, by dissolving the
DPPH reagent (2.4 mg) in 80 % methanol (100 mL).
Samples were further diluted in deionised water (1:10 or 1:100)
and 10 µL of sample; 40 µL of deionised water and 200 µL
of DPPH solution were added into each well of the
CELLSTAR 96 well plate (Greiner Bio-One, Monroe, USA).
Absorbance readings were taken at 515 nm, at 3-min
intervals over a 30-min period at 37 °C, using a BioTek
Synergy HT Multi-Mode Microplate Reader (BioTek,
Winooski, USA). A calibration curve using Trolox (0–500 µM,
R2 = 0.99) was plotted. Final values are expressed as
means of Trolox equivalents per milligram of sample ± SD
for 6 replicants.
Total phenolic content (TPC) was measured using a
modified Folin–Ciocalteu colourimetric method [
25,
26
]. Samples were diluted in deionised water (1:10 or
1:100) and 50 µL of the diluted extract; 50 µL of Folin–
Ciocalteu reagent diluted in water (1:25) and 100 µL
of 6 % (w/v) sodium carbonate were added into
corresponding sample wells of a 96-well plate (Greiner
BioOne, Monroe, USA). Absorbance readings were taken
at 725 nm, at 5-min intervals, over a 30-min period at
25 °C using a BioTek Synergy HT Multi-Mode
Microplate Reader (BioTek, Winooski, USA). A stock
solution of gallic acid (5.9 mM) was prepared in aqueous
methanol (80 % (v/v), and quantification was performed
on the basis of a standard curve in the range 0–50 mg/
mL (R2 = 0.99). The analysed samples were
measured versus a blank sample. All values are expressed
as means of gallic acid equivalents per gram of
sample ± SD for 6 replicants.
Total betanin and betaxanthin content
The content of betaxanthins and betacyanins in the buffered
extract and 1:10 juice solutions in aqueous McIlvaine buffer
(pH 6.5) was determined at 538 nm and 480 nm with a
UV–Vis spectrometer (Ultrospec 2000 UV/Vis
spectrophotometer, Pharmacia Biotech, Sweden), respectively,
according to the methods of Cai and Corke [
27
] and Mossamer
et al. [
28
]. Total betalains were quantified using the
following equation: BLC [mg/L] = (A × DF × MW × 1000)/
(ε × 1), where A is the absorption value at the absorption
maximum, DF the dilution factor and 1 the path length
(1 cm) of the cuvette. For quantification of betacyanins (Bc)
and betaxanthins (Bx), the molecular weights (MW) and
molar extinction coefficients (ε) of betanin (MW = 550 g/
mol; ε = 60,000 L/mol cm; λ = 538 nm) and indicaxanthin
(MW = 308 g/mol; ε = 48,000 L/mol cm; λ = 480 nm)
were applied, respectively.
Extraction of whole beetroot betalains
A previously described method [
29, 30
] was used for
betalain extraction of whole beetroots. Samples (about 10 g of
whole beetroot) were cryogenically milled in liquid
nitrogen using an IKA A11 S2 analytical mill (IKA Works,
Wilmington, USA). Accurate amounts of frozen powder
(500–600 mg) were transferred in a 15-mL Falcon tube
and homogenised with 1 mL water for 5 min (Whirlimixer,
FisherBrand, Fisher Scientific, UK). The homogenate was
centrifuged for 15 min at 3000 rpm, and the supernatant
was collected. The insoluble part was re-extracted with
1 mL of water for a total of five times. The extracts were
combined, and the water was removed using a rotary
evaporator, coupled to a heated water bath under vacuum
(STUART, Bibby Scientific, Staffordshire, UK) and re-dissolved
in McIlvaine buffer (pH 6.5, 10 mL).
Extraction of beetroot food for betanin determination
Betanin standard was purchased from Adooq Bioscience
(California, USA). Beetroot samples (about 20 g of whole
beetroot) were cryogenically milled in liquid nitrogen
using an IKA A11 S2 analytical mill (IKA Works,
Wilmington, USA). Accurate amounts of frozen powder (200–
300 mg) were transferred in a 15-mL Falcon tube and
homogenised with 1 mL water for 5 min (Whirlimixer,
FisherBrand, Fisher Scientific, UK). The homogenate
was centrifuged for 15 min at 3000 rpm, and the
supernatant was collected. The insoluble part was re-extracted
with 1 mL of water for a total of five times. The extracts
were combined, and the water was removed using a rotary
evaporator, coupled to a heated water bath under vacuum
(STUART, Bibby Scientific, Staffordshire, UK). Finally,
the extracts were re-dissolved in water (10 mL), filtered
and diluted further prior to LC–MS analysis (1:100 in
mobile phase A). All experiments were performed in
sixplicate.
LCMS for betanin determination
Betanin determination of diluted beetroot juice samples
(2.5 mg/mL in 1:1 0.1 % formic acid in water: 2 % HCl
in MeOH) and beetroot food extracts was carried out on
a Dionex UltiMate 3000 RSLC HPLC System (Dionex,
Camberley, UK) equipped with an UltiMate 3000 RS
pump, an UltiMate 3000 RS autosampler and a
QExactive Quadrupole-Orbitrap Mass Spectrometer (Thermo
Fisher Scientific, Waltham, USA). Electrospray ionisation
at negative ion mode was performed with a spray
voltage of 2.00 kV and capillary temperature of 280 °C. The
total ion current (TIC) with a range of 100–1000 m/z and
70,000 resolution was measured. The ion m/z 549 was used
for quantification of betanin. Sample aliquots (3 µL) were
injected on a Phenomenex Luna C18(2) (250 × 2.0 mm,
5um particle size) reverse-phase column thermostatically
regulated at 40 °C. The mobile phase consisted of water
with 1 % acetic acid (solvent A), and acetonitrile with 1 %
acetic acid (solvent B). After a 6-min equilibration with
20 % B, the elution programme was as follows: 0–30 min,
10–100 % B, (0.2 mL/min) followed by a washing stage
(100 % B, 30–36 min, 0.2 mL/min) and re-equilibration
at the initial conditions for 3 min. Betanin with a
retention time of 2.56 min was quantified by external standard
determination.
Extraction of plasma for betanin determination
Several attempts to extract betanin from plasma samples
were performed: (a) 1 mL of plasma was mixed with 4 mL
oxalic Acid (10 nM) and 0.1 mL HCl (12.6 M) in 15-mL
Falcon tubes and centrifuged at 826g for 5 min. The
supernatant was absorbed on to a primed solid-phase
extraction cartridge (Waters Sep-Pak c17 plus short cartridge,
360 mg sorbent per cartridge, 55–105 µm), washed with
methanol +0.2 % trifluoroacetic acid (TFA) followed by
2 × 5 mL of water. The sample was eluted with 3 mL of
MeOH + 0.2 % TFA, dried under N2 at 45 °C. Samples
were then reconstituted in 400 µl of solvent F: 0.1 % formic
acid in water: 2 % HCl in methanol and filtered through
a 0.2-µm polytetrafluoroethylene filter prior to HPLC and
LC–MS analyses; (b) 1 mL of plasma was extracted with
4 mL 1:1 acetonitrile:water for 10 min and centrifuged for
10 min at 3000 rpm. The supernatant was collected,
evaporated to dryness, reconstituted in 1:1 acetonitrile: water and
filtered in autosampler vials. Three types of plasma
samples were analysed using the above two methods as well
as with variations, samples obtained from test subjects after
consumption of beetroot food, beetroot juice and placebo,
at various time points, and analysed using HPLC/UV/Vis/
FLD and LCMS methodologies.
Analysis of plasma NOx
Plasma NOx bioavailability was determined from plasma
nitrate and nitrite concentrations using a standard assay
kit (R&D Systems, Minneapolis, Minnesota). The assay
quantifies plasma NOx by measuring total nitrite after
nitrate has been enzymatically reduced to nitrite via nitrate
reductase.
Data analysis
All data are presented as mean ± standard deviation
(SD). A two-way, repeated-measures analyses of variance
(ANOVA) was used to test for between trial differences in
plasma NOx concentrations; 3 trials (BTJ vs. BF vs. PLA)
by 6 time points (baseline, 1, 2, 3, 5 and 8 h post-ingestion).
In the event of a significant interaction effect (trial × time),
Fisher LSD post hoc analysis was performed to locate
pairwise differences occurred. Statistical significance was set at
P < 0.05 prior to analyses. All analysis was performed with
IBM SPSS statistics 20 for Windows (Surrey, UK).
Results
Inspection of food diaries indicated that participants
complied with the imposed dietary restrictions and that their
intakes did not differ between trials. No adverse events
were reported with any of the supplements.
Betanin content and bioavailability
Betanin was identified in both the BTJ and BF with LCMS
analysis (Fig. 3). Total betanin content for the BTJ and BF
is presented in Table 2. Based on these analyses, each
bottle of BTJ (250 ml) and serving of BF (300 g) contained
~194 and ~66 mg of betanin, respectively. No betanin was
detected in the PLA used in this study. Betanin could also
not be detected in the plasma samples obtained after BTJ,
BF or PLA consumption (data not shown).
Composition of beetroot juice and whole beetroot food
Antioxidant capacity, phenolic content, total betalain and
total betaxanthin content for the BTJ, BF and PLA are
presented in Table 2. According to these data, each serving of
beetroot juice (250 ml) had a TEAC of ~3 mmol/l and
contained ~405 mg GAE equivalents of phenolic compounds,
~17 mg of betacyanins and ~10 mg of betaxanthins. The
analysis of the BF showed that the 300 g serving of BF
fed to participants had a TEAC of ~1.01 mmol/L,
contained ~501 mg of GAE of phenols, ~59 mg of
betacyanins and ~22.5 mg of betaxanthins. The PLA contained a
small number of phenolic compounds (~43 mg); however,
betalains could not be detected and the TEAC was low
(<0.5 mmol/L).
Plasma NOx concentrations
Data are presented in Fig. 4. At baseline (0 h),
concentrations of plasma NOx were similar between trials
(P > 0.05). In the PLA trial, there was no change in plasma
NOx concentrations at any time point (P > 0.05); however,
after ingestion of both BTJ and BF there was an increase in
plasma NOx compared to baseline (time effects; P < 0.001)
and PLA (drink x time interaction; P < 0.001). Plasma
NOx reached peak concentrations 2 h post-ingestion in
both the BTJ and BF groups (P < 0.001; 163.7 ± 46.9 and
189.4 ± 72.8 μmol/L, respectively) and were still elevated
above baseline values at 8 h post with BTJ (P < 0.001) and
5 h post with BF (P = 0.012) (Fig. 4).
Discussion
The present study aimed to determine the plasma
bioavailability of nitrate and betanin after consuming a
commercially available BTJ and BF. Both the BTJ and BF were
rich in betalains, particularly betanin; however, betanin
could not be detected in plasma following consumption.
Conversely, and in line with other research, ingestion of
Fig. 3 a 1 LCMS chromatograms of betanin standard
(RT = 2.57 min). (2) MS output of betanin standard (base peak m/z
548.5–549.5) b (1) betanin in BTJ (RT = 2.55). (2) MS output for
BTJ (base peak m/z 548.5–549.5) c (1) betanin in BF (RT = 2.56) (2)
MS output for BF (base peak m/z 548.5–549.5)
Values are mean ± SD
TEAC BTJ mmol/L, BF μmol/g, TPC BTJ, mg gallic acid equivalent/L, BF mg gallic acid equivalent/g, Betanin BTJ, mg/L, BF mg/kg, Total
betaxanthins BTJ, mg indicaxanthin equivalents/L, BF mg indicaxanthin equivalents/100 g fresh weight, Total betacyanins BTJ, mg betanin
equivalents/L, BF mg betanin equivalents/100 g fresh weight
BTJ and BF evoked elevations in plasma NOx levels
compared to a placebo. Both BTJ and BF were found to
contain significant amounts of polyphenols and antioxidant
compounds. The present findings provide new information
regarding the bioavailability and phytochemical content of
two commonly consumed beetroot products.
Antioxidant capacity of the BTJ, BF and PLA was
measured using the TEAC assay. Although this was not
a primary aim of this study, we felt that this information
would be useful for comparisons with other
antioxidantrich foods. The TEAC assay estimates AC by
comparing the intervention’s scavenging ability to the Trolox
standard [
31
] and is commonly used to provide an index
of a food or beverage antioxidant potential [
31, 32
]. The
analysis revealed the TEAC for BTJ (~11.4 mmol/L) to be
higher than values reported for iced tea, green tea, apple
juice, cranberry juice and orange juice (4–10 mmo/L),
but lower than acai juice, black cherry juice, blueberry
juice and pomegranate juice (12–40 mmol/L) (Seeram
et al. 2008). Based on data from Pellegrini et al. [32], the
BF had a higher TEAC (~3.4 mmol/L) than several whole
vegetables, including tomatoes, radish, potato, onion,
lettuce, leek, green beans, artichokes, avocado, broccoli,
cabbage, carrot and cauliflower. Interestingly, the TEAC of
a number of commonly consumed fruits, such as apples
(~1.45 mmol/L), bananas and pears, were also found to
be lower than the BF [
32
]. However, as seen with the BTJ,
berried fruits such as blackberry, strawberry and raspberry
exhibited a significantly higher AC than BF. Furthermore,
the BF had a lower AC than the reported values for fresh
beetroot extracts (~3.4 vs. ~5.21 mmol/L) but similar to a
cooked beetroot extract (~2.94 mmol/L) [
32
]. This suggests
that some of the active compounds in beetroot are lost or
perhaps degraded during cooking and processing.
Conceptually, thermal treatment, exposure to bacterial agents,
acidification, storage conditions, and modified atmospheric
treatment could all affect phytochemical composition [
33,
34
]. Despite this, the BF and BTJ in particular still possess
reasonably high antioxidant capacities in comparison with
other fruit and vegetables and may therefore be favourable
sources for boosting antioxidant defences and protecting
against conditions associated with oxidative damage.
The antioxidant activity of the BTJ and BF can probably
be ascribed to the high concentration of polyphenols and
betalains they contain (see Table 2) and also to any
synergistic interactions that might occur with these compounds,
as has been suggested previously [
35
]. As our main focus
was on betanin, quantifying individual polyphenols in the
BTJ and BF was beyond the scope of this study. However,
according to data from previous investigations, the main
polyphenols in beetroot are phenolic acids (ferulic acid,
chlorogenic acid, caffeic acid) and flavonoids (epicatechin,
rutin, betagarin) [
29, 35
], many of which possess high
antioxidant potential [
36, 37
]. Furthermore, the polyphenols
in beetroot appear to be well absorbed in humans. Netzel
et al. [
38
] reported that 51 % of the total phenolics (about
338 mg) ingested from a homemade beetroot juice were
detectable in the participants urine, indicating that several
of the polyphenols present in beetroot may be absorbed and
made available in the circulation for physiological effects.
Both the juice and food were rich in betalain compounds
(Table 2). In accordance with studies on fresh beetroot
extracts, the betaxanthin content of BTJ and BF was much
lower than the betacyanin content [
39
]. Betacyanins appear
to be stronger antioxidants than betaxanthins [
10, 39
] and
were likely major contributors to the antioxidant activity
demonstrated by the BTJ and BF. Interestingly, the
betanin content of BTJ was much higher than BF (~194 mg vs.
~66 mg per serving). The reason for this is unclear, but is
probably due to differences in how the products are
processed or possibly the extraction methods used for analysis.
Regardless, the higher TEAC values for BTJ versus BF are
probably due to its comparatively higher betanin content.
The antioxidant potential of betanin is believed to be higher
than other betalains present in beetroot [
10, 14, 17
].
To our knowledge, this is the first study that has
characterised the bioavailability of betanin in human plasma
following beetroot consumption. Despite the relatively high
amount of betanin present in both the BTJ and BF, it could
not be identified in the plasma at any time point after
consumption (1–8 h). Our findings conflict with those of a
previous study, in which betanin was identified in plasma at
relatively high concentrations (~0.2 μmol/l) 2 h after
consuming 500 g of fresh cactus pear fruit containing 16 mg
of betanin [
40
]. However, the discrepant findings between
this study and the present investigation could be related
to differences in the foods analysed (i.e. cactus pear fruit
versus beetroot). This is supported by recent work from
Tesoriare et al. [
41
] who compared the absorption rates of
betanin from cactus pear fruit and red beetroot in a
simulated in vitro model of the intestinal wall. They showed
that epithelial transport was much lower when betanin
was derived from red beetroot, speculating that the rate of
absorption was inhibited by beetroot’s food matrix. This
suggests that the bioavailability of betanin may be lower
after beetroot consumption compared to other sources of
betanin.
Nevertheless, our inability to identify betanin in the
plasma suggests that it may be lost or degraded during
digestive processes. Previous studies investigating the
renal elimination of betalains have indicated that betanin
may instead be absorbed as downstream metabolites [
17
].
Kanner et al. [
17
] found that after consuming a
betaninrich beetroot juice, isobetanin, but not betanin, could be
detected in urine. The authors suggested that betanin
undergoes isomerisation to isobetanin in the intestinal milieu
and may therefore be the major metabolite absorbed after
betanin ingestion. In addition to isomerisation, there are
several other metabolic processes that could degrade
betanin and limit its systemic bioavailability, including
glycosidase enzyme activity from cellulase [
17
] or the presence of
the pancreatic enzyme amylase [
33
]. Such data support the
possibility that betanin is largely metabolised to secondary
compounds prior to entering the circulation, which would
provide a potential explanation as to why we were unable
to detect betanin in the present study. This raises doubts as
to whether the wide array of biological effects displayed by
betanin in vitro can be extrapolated to in vivo conditions.
Instead, the in vivo biological activity displayed by betanin
in some studies [
42, 43
] could be mostly due to the
biological effects of secondary betanin metabolites, although
this remains to be elucidated. At present, data on the
bioavailability of these metabolites or their potential
biological activity are not yet available. Unfortunately, we were
unable to unequivocally identify any metabolites of betanin
in this study due to appropriate standards for HPLC/LCMS
detection not being available. Thus, whether metabolites
of betanin reached the circulation in the present study is
purely speculative until clarified with future research. The
development of new methodologies, analytical techniques
and suitable standards will be required to establish the
presence of these compounds.
Plasma NOx activity was significantly augmented after
consumption of both BTJ and BF compared to the placebo
(Fig. 3). These findings agree with a previous study that
reported a rapid rise in NOx activity 1–3 h after BTJ
ingestion [
8
]. Collectively these data are important, because an
increase in the endogenous NOx pool is associated with
a range of physiological effects that might be beneficial
to health, such as improved endothelial function, reduced
blood pressure, enhanced mitochondrial efficiency and
improved metabolic function [
5
].
We acknowledge that a potential limitation of this study
is the absence of any measures of urinary excretion.
Therefore, we cannot rule out the possibility that betanin would
have been detectable in excreted urine had we collected
samples after beetroot consumption. Because the urinary
elimination of betanin has been described before [
17, 38
],
we chose to focus specifically on plasma bioavailability,
which to the best of our knowledge, has not been
characterised after beetroot consumption. We also acknowledge that
restricting our analysis to 8 h post-consumption is a
possible limitation because betanin might have appeared in the
plasma at later time points. However, we feel that this is
unlikely based on previous work in cactus pear fruit that
showed plasma betanin concentration peaked at 3 h
postconsumption and was undetectable at 8 h post [40]. Finally,
our analysis focused exclusively on betanin and nitrate, and
it was beyond the scope of this study to analyse individual
phenolic and betalainic compounds present in beetroot.
However, we acknowledge that these are not the only
compounds in beetroot that have the potential to exert
beneficial physiological effects. We therefore thought it prudent
to describe the total phenolic, antioxidant capacity and
betalainic content of these foods to demonstrate that there
are in fact other phytonutrients in these foods. However,
further research is required to delineate the individual
bioactive compounds present in these particular foods.
Despite the aforementioned limitations, the data
presented in this study provide new information on the
bioavailability and phytochemical content of commercially
available beetroot juice and whole food and will serve to
stimulate further research into the effects of beetroot, while
also being useful to practitioners interested in the potential
health benefits of these products. Future research on the
bioavailability of betanin metabolites and their potential for
biological activity are required to further exude the
potential usefulness of beetroot foods for health.
Acknowledgments This study was funded as part of a doctoral
degree that receives financial support from Gs Fresh Ltd. The funders
supplied the treatments used in this study but had no role in the
conception of the study, its design, preparation, analysis and writing
of the manuscript. The authors declare no conflict of interest. The
authors wish to thank all the volunteers for their participation.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License
(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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