beetroot

The well-documented health benefits of a diet high in fruit and vegetables has led to a growing interest
in so-called “functional foods” and their application in health and disease. In recent years, the root
vegetable Beta vulgaris rubra, otherwise known as red beetroot (herein referred to as beetroot) has
attracted much attention as a health promoting functional food. While scientific interest in beetroot has
only gained momentum in the past few decades, reports of its use as a natural medicine date back to Roman
times [1]. Today, beetroot is grown in many countries worldwide, is regularly consumed as part of the
normal diet, and commonly used in manufacturing as a food colouring agent known as E162 [2,3].
The recent interest in beetroot has been primarily driven by the discovery that sources of dietary nitrate
may have important implications for managing cardiovascular health [4]. However, beetroot is rich in
several other bioactive compounds that may provide health benefits, particularly for disorders characterised
by chronic inflammation. Consequently, the potential role for beetroot as an adjunct treatment in several
clinical conditions will be presented; Specifically, the aims of this review are twofold: (1) to highlight
evidence from recent studies showing the physiological and biological actions of beetroot; and (2) to
evaluate its use as a nutritional intervention in health and disease, with a special emphasis on experimental
studies relating to oxidative stress, inflammation, endothelial function and cognition.
Recent studies have provided compelling evidence that beetroot ingestion offers beneficial physiological
effects that may translate to improved clinical outcomes for several pathologies, such as; hypertension,
atherosclerosis, type 2 diabetes and dementia [1,5–8]. Hypertension in particular has been the target of
many therapeutic interventions and there are numerous studies that show beetroot, delivered acutely as
a juice supplement [9–11] or in bread [12,13] significantly reduce systolic and diastolic blood pressure.
Further discussion of beetroot’s anti-hypertensive potential is summarised in several reviews: [14–16].
Beetroot’s effect on the vasculature is largely attributed to its high inorganic nitrate content (250 mg∙kg−1
of fresh weight; [17]). Nitrate itself is not considered to mediate any specific physiological function; rather,
nitrates beneficial effects are attributed to its in vivo reduction to nitric oxide (NO), a multifarious messenger
molecule with important vascular and metabolic functions [14,18]. The generation of NO via nitrate involves
a series of sequential steps that have been well described in the literature [4,19]. Briefly, ingested nitrate
is first absorbed through the upper part of the small intestine into the systemic circulation [4,15]. It is
then estimated that 25% of the circulating nitrate enters the entero-salivary cycle where bacterial species
located at the posterior aspect of the tongue bioactivate or reduce salivary nitrate to nitrite [16,19]. Because
salivary bacteria facilitate the reduction reaction that converts nitrate to nitrite, spitting out saliva or
taking oral anti-bacterial treatments, like dental mouthwash for example, has been shown to diminish
nitrate-nitrite conversion [10,18]. Under normal circumstances, however, salivary nitrite is re-absorbed
into the circulation via the stomach where it is metabolised to NO and other nitrogen oxides by a variety
of reductase enzymes [4,10,13].
However, as previously mentioned, nitrate is not the only constituent of beetroot proposed to have
beneficial effects in health and disease. Beetroot is a rich source of phytochemical compounds (Figure 1),
that includes ascorbic acid, carotenoids, phenolic acids and flavonoids [2,20,21]. Beetroot is also one of
the few vegetables that contain a group of highly bioactive pigments known as betalains [22,23]. Members
of the betalain family are categorised as either betacyanin pigments that are red-violet in colour or
betaxanthin pigments that are yellow-orange in colour [1]. A number of investigations have reported

betalains to have high antioxidant and anti-inflammatory capabilities in vitro and a variety of in vivo
animal models [3,23–26]. This has sparked interest in a possible role for beetroot in clinical pathologies
characterised by oxidative stress and chronic inflammation such as liver disease [1,23], arthritis [27] and
even cancer [28–31].

For a food component to be considered beneficial for health it must be bioavailable in vivo, that is,
following ingestion, the active compounds are absorbed through the gastro-intestinal tract and made available
in the circulation, in sufficient quantities, to be utilized by cells [21,32]. However, in order to reach the
systemic circulation and exert any salubrious functions, a food component must maintain its molecular
structure through several phases of digestion that each present a significant metabolic challenge for the
molecule and affect its eventual rate and extent of absorption [33,34]. It is therefore critically important
that any alleged health benefit of a food source be firstly verified with well-designed bioavailability
studies that characterise the extent of its in vivo absorption [34]. In this respect, the bioavailability of both
inorganic nitrate and the betalains, the major bioactive components of beetroot, have been considered in
the literature. The high bioavailability of inorganic dietary nitrate is well established and there are reports
of close to 100% absorption following digestion [35]. The extent to which betalains are absorbed is,
however, less clear.
Two studies have directly investigated betalain bioavailability by measuring their appearance in human
urine after ingesting a single bolus of beetroot juice [36,37]. Kanner et al. [37] identified 0.5%–0.9% of
the ingested betacyanins (betanin and isobetanin) in volunteer’s urine in the 12 h after consuming 300 mL
of beetroot juice. This indicates that although in small amounts, betacyanins can be successfully absorbed
in humans. They also showed that the peak urinary elimination rate of betacyanains (indicative of absorption),
occurred 2–4 h after ingestion; however, there was a high level of inter-individual variability within this
time period. Frank et al. [36] reported similar findings while investigating betacyanin bioavailability.
After providing six healthy participants with 500 mL of beetroot juice, they identified betacyanins in urine.

at concentrations equivalent to ~0.3% of the ingested dose over a 24 h period. These studies might be
interpreted to suggest only small level of bioavailability; however, it is important to realise that betacyanins
are unlikely to be exclusively eliminated via the renal pathway [36] Indeed, the use of urinary excretion
as a sole indicator of bioavailability has received criticism because it does not account for the biliary and
circulatory clearance of compounds, thus underestimating true bioavailability [33]. In addition, the extent
to which betalains are metabolised and structurally transformed to secondary metabolites is yet to be
characterized, but should be taken into consideration when examining their bioavailability [36].
Given these limitations, Tesoriere et al. [38] employed a different approach to investigate the
bioavailability of betalains. Tesoriere and colleagues developed a simulated in vitro model of the human
intestinal epithelium using Caco-2 cell monolayers to mimic a functional barrier. This model allowed them
to examine whether betalains can be absorbed through a functioning intestinal barrier and hence give an
indication of their bioavailability. They demonstrated that two betalains; betanin and to a greater extent
indicaxanthin were well absorbed through the simulated model of the intestinal lining (Caco-2 cell monolayer)
and mostly in their unmetabolised form via paracellular transport. The latter finding is particularly
important, because it reveals that betalains can be absorbed into the systemic circulation in their unchanged
form, allowing them to retain their molecular structure and high biological activity [34]. There was some
evidence that betanin may be absorbed through transcellular transport as well. Nevertheless, it is important
to note that results from in vitro experiments, even when designed to mimic the biological milieu of the
human GI tract, do not necessarily translate in vivo, given that several other factors (i.e., first pass metabolism,
interactions with gut microflora and protease enzyme degradation) have a significant influence on the
concentration of the nutrient that eventually reaches the circulation [33,34].
In addition to the betalain family, other aforementioned plant derived antioxidants have been identified
in beetroot, including epicatechin, rutin, and caffeic acid [2], which to varying degrees appear to be well
absorbed and bioavailable in humans [39]. Although, the bioavailability of these compounds and other
phenolics from beetroot have not been individually determined, there are data describing the bioavailability
of the total phenolic compounds present in beetroot. Netzel et al. [40] measured the urinary excretion of
total phenolic substances following a single 500 mL bolus of beetroot juice. They identified ~685 mg of
phenolic compounds in participant’s urine ≤24 h following beetroot juice ingestion; 97% more than the
~347 mg identified after consuming water (i.e., basal concentrations). While the relative bioavailability
from the individual compounds could not be determined, these findings clearly show that beetroots phenolic
constituents are extremely well absorbed and likely increase beetroot’s in vivo antioxidant power.
Taken together, the results of the aforementioned studies provide a good base of evidence that beetroot
is a bioavailable source of bioactive compounds in humans. With that said, further work is still required
to firstly; elucidate the bioavailability of beetroot’s individual bioactive components and secondly; to
establish the extent that plasma, biliary and other metabolic pathways contribute to the excretion of these
components. Together, these data would give a better understanding of beetroots phytochemical
bioavailability and thus elucidate the potential as a health-promoting intervention for humans.