Zeashan and colleagues documented the antioxidant activity of amaranth extract (obtained from the whole plant). In the study conducted by Zeashan et al., (2009), this extract showed significant antioxidant activity in the DPPH assay. In the next study by Lucero-Lopez et al., antioxidant properties of Amaranthus hypochondriacus seed extract were also examined. The study was conducted on the liver of rats sub-chronically exposed to ethanol. The results obtained in the experiments confirm the beneficial effect of the tested extract, which as a rich source of polyphenols, had a protective effect on the livers of rats ^[13][23]. Sarker and Oba’s work characterized the phytochemical composition of Amaranthus gangeticus L. species. They particularly focused on the identification of phenolic compounds responsible for the antioxidant activity of these plants. Twenty-five different phenolic compounds were identified in the plant. Antioxidant components of A. gangeticus genotypes exhibited good radical scavenging activities ^[14][24]. In another study, the same researcheuthors presented chemical compounds found in amaranth A. tricolor (betaxanthins, betalains) that exhibit antioxidant activity ^[15][25]. In the study by Al-Mamun et al., the antioxidant activity of the methanol extract derived from the seed and stem of A. hybridus and A. lividus was tested. The DPPH radical scavenging assay showed that both extracts examined possessed significant dose-dependent antioxidant potential, exhibiting IC₅₀ values of 28 ± 1.5 and 93 ± 3.23 μg/mL, respectively ^[16][26]. In a subsequent scientific paper, two polysaccharides from A. hybridus named AHP-H-1 and AHP-H-2 were characterized and examined as potential antioxidant factors. The results obtained in the study confirmed that the two polysaccharides purified from A. hybridus have strong antioxidant activity (DPPH radical scavenging activity and superoxide anion free radical scavenging activity) ^[17][27]. Kumari and colleagues confirmed the antioxidant properties of another amaranth species, A. viridis. Aqueous, chloroform, methanol, and hexane extracts were examined in several in vitro model systems. A. viridis exhibited dose-dependent effective antioxidant properties. Major components responsible for his antioxidant activity are gulonic and chlorogenic acids and also kaempferol ^[18][28]. In another paper describing the antioxidant activity of amaranth, the phenolic composition of the aerial part of Amaranthus caudatus was tested using ABTS+, DPPH, and O₂ scavenging activity, ferric-reducing antioxidant power (FRAP), and Fe₂+ chelating ability methods. Different levels of antioxidant activity were observed depending on the stage of plant development and the content of biologically active substances (mainly a wide range of phenolic composition) responsible for generating such activity [3]. Studies focusing on the antioxidant capacity of amaranth over the period 2015–2020 were collected and summarized in Park et al.’s work. In Herethis review, current knowledge on the antioxidant properties of different amaranth species was systematized and consolidated. These properties resulted not only from the presence of phenolic compounds but were also derived from hydrolysates and active peptides with superior antioxidant activity ^[4][8].

3.3. Anticancer Potential

Water extracts of two amaranth species (A. lividus and A. hybridus) were examined as anticancer factors. Female Swiss albino mice divided into a few groups were injected with EAC cells and received 25, 50, or 100 μg/mL/day/mouse of test extracts after 24 h of EAC cells injection. The measurement of cancer cells growth inhibition was conducted. Administration of A. hybridus and A. lividus extracts led to 45 and 43% growth inhibition of EAC cells ^[16][26]. The seed extract of A. hybridus possessed higher growth inhibitory activity than the stem extract of A. lividus and exhibited 14, 26, and 45% growth inhibition at 25, 50, and 100 μg/mL, respectively. In animals treated with amaranth extracts, morphological changes suggestive of apoptosis were also observed in EAC cells. Amaranth preparations can be considered as a potential target for cancer cure studies ^[16][26].

3.4. Antihyperglicemic and Hypolipidemic Activity

There are scientific papers in the databases on the sugar-lowering and cholesterol-lowering effects of amaranth-containing products. Methanolic extract of Amarantus viridis leaves (at the dose of 200 mg/kg and 400 mg/kg per day, 21 days) reduced blood sugar levels in streptozotocin-induced diabetic rats. The administration of the extract also reduced serum cholesterol and triglyceride levels ^[19][29]. Girija et al. investigated the anti-diabetic and anti-cholesterolemic activity of the methanol extract of leaves (200 and 400 mg/kg, for 21 days) from three species of amaranth: A. caudatus, A. spinosus, and A. viridis ^[20][30]. Experiments were conducted in streptozotocin-induced diabetic rats. Methanol extracts of all three species of amaranth showed significant glucose and cholesterol-lowering activity at a dose of 400 mg/kg ^[20][30]. Similar issues are presented in another paper published in 2011. Antihyperglycemic and hypolipidemic activity of the methanolic extract of leaves of Amaranthus viridis was investigated. Normal and streptozotocin-induced diabetic rats were fed with 200 mg/kg and 400 mg/kg of extract per os for 21 days. HThere authors of this study proved that the tested extract showed antiglycemic activity and improved the lipid profile in rats ^[19][29]. Studies on the activity of selected proteins from amaranth (Amaranthus cruentus) suggest hypocholesterolemic activity of this plant. Manolio Soares and colleagues showed that proteins from the plant affect the action of a key enzyme in cholesterol biosynthesis, 3-hydroxy-3-methyl-glutaryl-CoA reductase ^[21][31]. The hypolipemic effect of amaranth oil is associated with its significant squalene content. The mechanism of activity of squalene relies on the inhibition of HMG-CoA activity—a liver enzyme responsible for cholesterogenesis. Such activity has been demonstrated in both rat and clinical studies ^[9][7]. In another paper, the effects of consumption of the Amaranthus mangostanus on lipid metabolism and gut microbiota in high-fat diet-fed mice were examined. Amaranth powder supplementation significantly reduced the levels of triglycerides, total cholesterol, and phospholipids in the liver of rats and also downregulated the expression of a few lipogenesis-related genes ^[22][32]. Recent research findings suggest that the aqueous extract obtained from steamed red amaranth leaves might be used as a potent nutritional supplement to prevent diabetic retinopathy. Anti-glycative and anti-oxidative action of that extract against a high glucose-induced injury was examined in a human lens epithelial cell line HLE-B3 ^[23][33].

3.5. Neuroprotective and Antidepressant Action

An attempt was made to determine the neuroprotective effect of A. lividus L. and A. tricolor L. extracts against AGEs-induced cytotoxicity and oxidative stress. Advanced glycation end-products (AGEs) caused oxidative stress and cytotoxicity in neuronal cells. It was found that examined extracts protect human neuroblastoma SH-SY5Y cells against AGEs-induced cytotoxicity ^[24][34]. The researcheuthors suggest that amaranth may be useful for treating chronic inflammation associated with neurodegenerative disorders ^[24][34]. In another paper by the same researcheuthors, the neuroprotective effect of amaranth was again described. The methanol extracts of A. lividus and A. tricolor leaves were found to decrease cell toxicity and intracellular ROS production in human neuronal immortalized SH-SY5Y cells. Examined extracts decreased oxidative stress by suppressing gene expression of HMOX-1, RAGE, and RelA. Because of such activity and the high content of antioxidant substances, amaranth extracts may be a potential neuroprotective factor ^[25][35]. The methanol extract of Amaranthus spinosus (100 and 200 mg/kg, orally) was investigated for antidepressant activity. In the study, forced swimming test (FST) and tail suspension test (TST) models were used in experimental rats. The results of the tests prove the antidepressive potential of the methanol extract of this plant. The researcheuthors indicate that the mechanism of this activity has not yet been understood and its explanation requires further in-depth studies ^[26][36].

3.6. Anti-Inflammatory Activity

A. lividus and A. tricolor extracts possess anti-inflammatory activity and can reduce pro-inflammatory cytokine gene expression. An increased amount of proinflammatory cytokines, such as IL-1, IL-6, and TNF was observed ^[24][34]. In 2021, information on bioactive peptides with anti-inflammatory activity from germinated amaranth released by in vitro gastrointestinal digestion was described in the scientific literature for the first time ^[27][37].

3.7. Antimicrobial and Antiviral Effect

A new antimicrobial peptide with strong activity against E. coli was found in the medicinal plant Amaranthus tricolor. This peptide was selected after analysis of the protein fraction from A. tricolor and characterized as being highly antimicrobial ^[28][38]. The antimicrobial activity of ethanolic and aqueous extracts of Amaranthus caudatus was also examined in a study by Jimoh and colleagues ^[29][39]. Streptococcus pyogenes, Staphylococcus aureus, Bacillus subtilis, Streptococcus pneumoniae, Escherichia coli, and Pseudomonas aeruginosa were tested in this study. The used strains of fungi were: Candida albicans, Penicillium chrysogenum, Candida glabrata, and Penicillium aurantiogriseum. The ethanolic extract of amaranth showed stronger antimicrobial activity than the aqueous extract. The extracts also showed antifungal activity with an MIC in the range of 0.675 to 10 mg/mL ^[29][39]. A new application of amaranth seed oil (apolar fraction from Amaranthus cruentus L. seeds extract) as an agent against Candida albicans was examined by De Vita and colleagues. Amaranth oil in combination with an antifungal drug named terbinafine possesses synergic fungistatic and fungicidal activity and can be a potentially important ingredient of antifungal formulations ^[30][40]. In the next study, stem and seed methanol extracts of A. lividus and A. hybridus were examined as antimicrobial factors. In vitro susceptibility of five pathogenic bacteria (E. coli, P. aeruginosa, B. subtilis, S. typhi, S. aureus) was confirmed in the disk diffusion assay ^[16][26]. There have also been recent reports of the antiviral activity of amaranth. Chang and colleagues investigated the antiviral properties of betacyanin fractions from leaves of red spinach, Amaranthus dubius ^[31][41]. Betacyanin fractions from A. dubius inhibited DENV-2 in vitro. Betacyanin fractions exhibited antiviral activity against DENV-2 after virus adsorption to the host cells in a dose-dependent manner. For betacyanin fractions from red spinach, the IC50 value was 14.62 μg mL⁻¹, with an SI of 28.51. The researcheuthors point out that the mechanism of infectivity inhibition by the betacyanins must be confirmed by rigorous scientific studies ^[31][41]. In other experimental work, the antimicrobial activity of A. tricolor crude extract against S. aureus was assessed by disk diffusion, minimum inhibitory concentration (MIC) determinations, and growth curves. The researcheuthors of the experiment proved that the extract has antibacterial activity and the mechanism of this activity was connected with cell membrane depolarization, reduction in intracellular pH, decrease in bacterial protein content, DNA cleavage, and leakage of cytoplasm. The plant extract has the potential to be a good food preservative that improves meat quality ^[32][42]. The major biological effects of amaranth are summarized in Table 1.

Table 1.

Key biological effects of amaranth.

Activity	Active Agent	References
Astringent	Saponins, protoalkaloids and betacyans	[5][8]	[14,20]
For skin problems	Naphthalene, squalene, sulfonates of	Amaranthus	spp.	[6]	[19]
Hypolipidemic, antihyperglycemic	Methanol extract of	A. viridis	leaves Methanol extract of	Amaranthus	spp. Proteins from	A. cruentus	squalene Leaves aqueous extracts	[19] [20] [21] [9] [23]	[29] [30] [31] [7] [33]
Action against microorganisms	A. tricolor	isolated peptide Ethanolic, aqueous extract of	A. caudatus	Seed oil from	A. cruentus	Methanol extract of	A. lividus	and	A. hybridus	Betacyanins isolated from	A. dubius		A. tricolor	crude extract	[28] [29] [30] [16] [31] [32]	[38] [39] [40] [26] [41] [42]
Neuroprotective or antidepressant	A. lividus, A. tricolor	extracts	A. lividus	and	A. tricolor	leaves methanol extract	A. spinosus	methanol extract	[24] [25] [26]	[34] [35] [36]
Anti-inflammatory	A. lividus	and	A. tricolor	extracts Bioactive peptides	[24] [27]	[34] [37]
Antioxidant	Whole plant extract Seed extract Phenolic compounds Betaxanthins, betalains Seed or stem methanol extract of	A. hybridus	AHP-H-1, AHP-H-2 polysaccharides from	A. hybridus	Different extracts of	A. viridis	Phenolic compounds	A. caudatus	[10] [13] [14] [15] [16] [17] [18] [3]	[21] [23] [24] [25] [26] [27] [28] [3]
Anticancer	Water extract of	A. lividus	Water extract of	A. hybridus	[16]	[26]