The high antioxidant capacity of beets is due not only to the high content of betalains, but also other phenolic compounds—flavonoids and polyphenolic acids. Significant amounts of catechins and polyphenolic acid (ferulic, protocatechuic, vanillic, p-coumaric, p-HBA, syringic, and caffeic) have been detected in beets ^{[29][30][31][32]}[47,48,49,50]. However, the composition of plant extract depends on many factors, including the original plant (variety, maturation state), conditions of its cultivation (climate, harvesting), as well as its storage condition ^[33][51].

Kavalcowa et al. showed how much the total content of polyphenolic compounds depends on the beet variety and the place of its cultivation ^[32][50]. The highest content of total polyphenols was found in the variety of Renova from Sliač, in Slovakia (1280.56 μg·g⁻¹), followed by red beetroot from Zohor. In addition, the variety of Zohor, obtained from Sihelné, had the lowest concentration of polyphenols (820 μg·g⁻¹). In the case of the variety Monorubrawe, the highest value of total polyphenols was determined in samples from the area of Sihelné (1201.6 μg·g⁻¹), followed by red beetroot from Sliač (1023.21 μg·g⁻¹) and Zohor (988.66 μg·g⁻¹). The variety-specific differences between the highest and lowest polyphenol contents were 460.56 μg·g⁻¹ in the Renova variety and 212.94 μg·g⁻¹ in the Monorubrawe variety. The authors concluded that the observed differences may be due to the high humus (4.0%) and potassium (520 μg·g⁻¹) content in the soil. Studies conducted by Kujala et al. clearly showed that the total phenolics content decreases in order: peel (50%), crown (37%), and flesh (13%) ^[34][52]. The comparison of the phenolic composition in betalain extracts from intact B. vulgaris cv. Detroit Dark Red plants and hairy root cultures was described by Georgiev et al. [11]. The authors proved that the concentrations of selected polyphenolic compounds were even more than 100 times higher in extracts from hairy root cultures than in extracts obtained from intact plants. Moreover, rutin was present only in hairy root cultures, while chlorogenic acid was found only in intact plants. While quercetin was not reported in this study, Pratimasari and Puspitasari proved its presence in the purified extract of beetroot leaf ^[35][53]. In the study performed by Płatosz et al., quercetin was detected in fresh red beet, fermented red beet, and commercial fermented beet juice, in concentrations of 0.023 μg·g⁻¹, 0.002 μg·g⁻¹, and 0.009 μg·mL⁻¹, respectively ^[29][47]. The results obtained by Georgiev indicated that the extract of the hairy roots had substantially higher concentrations of the identified phenolic compounds than did the intact plants, as noted above [11]. Moreover, there were differences in the phenolic profiles of the studied extracts. Chlorogenic acid was present only in the extract from intact plants, while rutin was found only in the extract from hairy root cultures. As the authors emphasize, it is a valuable discovery as rutin interacts synergistically with many other biologically active compounds present in the plant material enhancing its antioxidant properties ^[36][54]. The fermentation process caused an increase in the content of phenolic acids and a reduction in the content of their conjugated forms, while the same process caused a decrease in the content of free flavonoids ^[29][47]. In the case of the studies described by Płatosz, significant differences in the concentrations of polyphenolic acids were observed between the extracts from fresh and fermented beetroot ^[29][47]. Such large differences were not observed in the case of flavonoids, their concentrations in most cases were rather similar.

4. Carotenoids

Carotenoids are another group of bioactive compounds present in beetroots; however, they are present rather in small amounts ^[37][65]. The content of these compounds strictly depends on the part of the plant that is being tested; the highest content of carotenoids is found in beetroots peel, followed by pulp, leaves, and stalks [8]. This is because these compounds are accumulated in the green parts of the plants, namely in chloroplasts, as a mixture of α- and β-carotene, β-cryptoxanthin, lutein, zeaxantin, violaxanthin, and neoxanthin ^[38][66]. The main carotenoids in red beets are β-carotene and lutein, which pose strong anticancer properties [16]. Red beets are not a very good source of red carotenoid lycopene in comparison to tomatoes, watermelon, or papaya [8]. The lycopene content was 30.0 ± 0.3 µg per 100 g of beet tubers and was not detected in the leaves. α-Carotene was detected in the leaves and tubers of the beetroot, and its concentration in tubers was almost seven times higher than in leaves. However, the leaves are a good source of β-carotene (11.64 µg per 100 g) and lutein (1.503 µg together with zeaxanthin) ^[39][67]. The presence of these compounds was not confirmed in beet tubers [8]. A higher concentration of carotenoids was found in organic stalks and cooked pulp in comparison to the conventionally grown plants ^[40][68]. This is in line with reports in the literature showing the content of β-carotene is higher in organically cultivated plants in comparison to conventionally grown plants ^[41][69]. However, significant differences were only observed between these two parts. After cooking the beet peel and pulp, a decrease in the carotene content was observed; however, its content was still higher in organic beets after this process ^[40][68].

5. Minerals

Beetroot contains natural minerals that are essential for the proper functioning of the human body. Wruss et al. analyzed the mineral composition of seven different popular beetroot varieties grown in Upper Austria ^[31][49]. Obtained values were in a range similar to those reported in previous studies ^[42][70]. However, as the authors highlighted, data regarding the concentration of trace elements and minerals in beet juices are still limited. It is also crucial to note which part of the plant is tested, as the content of individual elements varies among leaves and tubers [8]. The concentration of copper ions was more than two times higher in leaves, in comparison to tubers (0.191 and 0.075 mg per 100 g of beets, respectively) [8]. A greater difference was observed for iron, for which 2.57 mg was found in leaves and 0.80 mg in tubers (values per 100 g of beets). Also, calcium, sodium, potassium, and magnesium were detected in much higher concentrations in leaves. The concentrations of zinc, phosphorus, and manganese did not significantly differ from each other.

6. Vitamins

Beetroots and beet products are also great sources of vitamin C, vitamin A, and vitamins from the B group. The B-group vitamins are a collection of 8 water-soluble vitamins including thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folate (B9), and cyanocobalamin (B12). According to Odoh and Okoro 100 g of this plant contains: vitamin A (2.6 mcg), K, (3.2 mcg), C (4.36 mg), E (0.18 mg) B3 (0.03 mg), B6 (90 mg), B2 (0.034 mg), and B5 (0.151 mg) ^[39][67]. These values are in agreement with findings published by Neha et al. ^[43][76]. Similar concentrations of vitamins mentioned above were reported by the US Department of Agriculture in the most recent release of the USDA National Nutrient Database for Standard Reference, a major source of food composition data ^[44][77]. Analysis of vitamin C in different parts of the beetroot was conducted by Rosseto et al. ^[40][68]. According to this study, the highest concentration of ascorbic acid was found in the pulp of the beet and the lowest in its leaf. The concentrations of vitamin C, a well-known antioxidant, vary by plant part; in decreasing order of concentration, the parts are: pulp > skin > leaf > stalk for organic beet and pulp > stalk > skin > leaf for conventionally grown beet. This suggests that the method of cultivation influences the concentration of ascorbic acid in a specific part of the plant. Part-specific concentrations were observed in beets from organic and conventional cultivation. However, the highest level of vitamin C was detected in each part of plants fertilized with organic manure ^[40][68].

7. Other Compounds

Nitrate (NO₃⁻) is one of the most important inorganic compounds in red beets, and their ingestion provides a natural means of increasing in vivo nitric oxide (NO) [8]. This can prevent several pathologies such as hypertension and endothelial dysfunction ^[45][46][78,79]. For a long time, nitrates, which can be converted into nitrosamines, were thought to be harmful substances. These, in turn, lead to endocrinological diseases, defects in human fetuses or cancer ^[47][80]. Today it is known that red beetroot nitrates are one of the most important nutrients. In general, nitrates provide ergogenic and cardio-protective properties. Nitrates present in beetroots are converted into nitrite and nitric oxide, which are responsible for lowering blood pressure and vasoprotection ^[48][81]. Jonvik et al. reported that beetroot juice lowers blood pressure to a greater extent than sodium nitrate ^[49][82]. Many studies have focused on the impact of beetroot juice consumption on the body’s efficiency, mainly during intense physical exercise. Volino-Souza et al. performed a randomized, clinical, crossover, double-blind study which has shown that the consumption of 140 mL of juice from red beets improved macrovascular endothelial function ^[50][83]. However, no oxygen saturation parameters in muscle tissue of pregnant women were observed. Other studies showed that one week of supplementation with beetroot juice improved submaximal endurance and reduced blood pressure in older patients with heart failure and preserved ejection fraction ^[51][84]. Subsequent works emphasize the beneficial effect of beetroot supplementation on the body’s efficiency during physical exertion, as well as its impact on the speed of regeneration ^[52][53][85,86]. The content of nitrates in beetroot depends on its variety. Wruss et al. analyzed nitrates content in seven beetroot varieties grown in Upper Austria ^[31][49]. The highest content was detected in the Mona Lisa variety (4626 ± 568 mg·L⁻¹), while the lowest was found in Robuschka (564 ± 129 mg·L⁻¹). The mean value across all analyzed cultivars was 1970 mg·L⁻¹, but the established standard deviation was 1395 mg·L⁻¹, which demonstrates the variability of nitrate content among beet varieties. Gallardo and Coggan studied the nitrate and nitrite content of beet juice products marketed to athletes ^[54][87]. The NO₃⁻ concentration of powders was significantly higher than that of concentrates (174 ± 63 μmol·g⁻¹ and 70 ± 39 μmol·mL⁻¹, respectively). A lower concentration of nitrates was found in mixed drinks (13 ± 5 μmol·mL⁻¹) and bulk juices (18 ± 11 μmol·mL⁻¹). Authors highlighted that regardless of the type of product, there was considerable variability in NO₃⁻ concentration/content between products and, often, between samples of the same product ^[53][86]. The evaluation of nitrate and nitrite contents of beetroot from different regions of Brazil and the USA showed the highest nitrate (31.2  ±  0.010 mmol·L⁻¹) and nitrite (0.45  ±  0.005 mmol·L⁻¹) contents in US beets when compared to beetroots from Brazil ^[55][88]. In Brazil, Rio de Janeiro was the region that showed the highest nitrate content (17.1  ±  0.020 mmol·L⁻¹), while Rio Grande do Norte presented the highest nitrite content (0.13  ±  0.010 mmol·L⁻¹). Red beetroots have more sugar than many other vegetables. According to Wruss et al., its average total content was 77.5 ± 10.2 g·L ⁻¹ (7.8%) ^[31][49]. In the roots, sucrose was the most commonly identified sugar (94.8%), followed by glucose (3.3%) and fructose (1.9%). However, glucose was the major sugar in the shoots ^[56][89]. Red beetroot can be considered as a part of a healthy diet for diabetic patients. It was found that raw, red beetroot consumption by diabetes mellitus type 2 patients for 8 weeks, has beneficial impacts on cognitive function, glucose metabolism, and other metabolic markers ^[57][46]. The list of biologically active compounds in beets and beet products is presented in Table 1. Unfortunately, in contrast to its health benefits, beetroots contain also oxalic acid; high levels of which lead to the production of kidney stones and their related negative health outcomes ^[58][90]. Moreover, oxalate, by chelating metal ions (such as magnesium and calcium), may inhibit their absorption. Thus, the consumption of red beetroots in large amounts may lead to negative health effects.