Bioactivities of Red Pitaya Fruits: Comparison
Please note this is a comparison between Version 1 by Yanyi Huang and Version 3 by Catherine Yang.

Pitahaya, or the pitaya fruit, is a well-known member of the Cactaceae family and is widely cultivated in tropical and subtropical areas. Pitaya fruit is classified based on the colour of pulp and peel, namely white-pulp with pink peel pitaya (Hylocereus undatus), red-pulp with pink peel pitaya (Hylocereus polyrhizus) and white-pulp with yellow skin (Hylocereus megulanthus).

  • Hylocereus
  • Glycaemic Control
  • Inflammation

1. Pitaya Species and Maturation

Pitaya fruit (Hylocereus) or dragon fruit is indigenous in Mexico and Central America [1][12]. Its attractive colour appearance, low-calorie values and sweet flavour have been very attractive to the consumer. Hylocereus comprises 14 species, and they have been mainly classified by their skin and flesh colour. Hylocereus undatus (white flesh and pink skin), Hylocereus polyrhizus (red flesh and red skin, shown in Figure 1), and Hylocereus megulanthus (white flesh and yellow skin) have been the three most commonly consumed and studied pitaya species in recent years [2][13]. Pitaya fruit is highly perishable throughout postharvest storage due to the high moisture content present in the pulp. A study indicated that melatonin treatment effectively delayed fruit deterioration by reducing respiration intensity and oxygen radical production [3][14]. However, little information is known regarding the physicochemical changes that occur during fruit growth and maturation
Figure 1. Red pitaya fruit.

2. Antioxidant Activity

2.1. Glycaemic Control

Blood glucose levels reflect the amount of glucose released into the blood stream following food consumption, which is an important parameter to measure, since high blood glucose levels can cause diabetes, high blood pressure and other chronic diseases [4][68]. Red pitaya is a potential medicinal plant for attenuation of blood glucose due to the regeneration of pancreatic β-cells that synthesise and secrete insulin and amylin (two hormones that regulate blood glucose levels) [5][4]. There is growing evidence to suggest that beta-cyanins derived from red pitaya have a significant effect on attenuating blood glucose and preventing fatty liver, as well as cardiovascular disease.
A study investigated the effect of red pitaya on lipid metabolism and glycaemia, in which mice were treated with different diets, namely standard diet, hyper-cholesteraemic diet, lipid-lowering diet and red pitaya pulp diet (100, 200 and 400 mg/kg) [6][47]. Results showed that the total cholesterol and blood glucose level in red pitaya treated animals significantly decreased compared to their counterparts due to the presence of betalains, oligosaccharides, quercetin, and other bioactive compounds. However, the mechanism contributing to hypo-cholesterolaemia and blood glucose modulation was not discussed in detail.
Song, Chu, Yan, Yang, Han and Zheng [7][41] isolated beta-cyanins from red pitaya, which were orally administrated to high-fat diet mice (200 mg/kg/day) for 14 weeks. The betacyanin incorporating diet significantly reduced mice body weight gain, although no difference was observed in the daily calorie intake between the betacyanin treated and high-fat treated diet mice. Lower blood glucose and insulin levels were detected in the pitaya betacyanin group, in addition to the modulation of gut microbiota in mice. They showed a decrease in the content of Firmicutes and Bacteroidetes and an increase in the proportion of Akkermansia, which safeguards the healthy state of the human gastrointestinal tract by regulating host energy balance. It was indicated that understanding how betacyanin from red pitaya fruit targets the gut microbiota is an area of research that needs expanding to understand the mechanism involved in these health benefits. This was supported by the observation that pitaya betacyanin lowered serum triglyceride, total cholesterol and LDL cholesterol but increased HDL cholesterol levels couple, with an overall improvement of the lipid profile. This study is in line with the findings by Lugo-Radillo, et al. [8][69], arguing that betanidin reduces the glycaemic response in mice compared to the mice fed with an atherogenic diet, accounting for 50.94%, and limits the expression of quinone reductases and activity of DNA methyltransferase. It appears that betacyanin present in red pitaya fruit is an important component for the regulation of blood glucose levels, enhancement of lipid profile, and prevention of lipid accumulation. The antidiabetic effect of beta-cyanins derived from red pitaya is strongly correlated with betacyanin-starch and betacyanin-amylase interactions, which require further investigation.

2.2. Anti-Inflammatory Activity

Inflammation is a defensive response of the immune system to harmful bacteria and viruses in order to combat infections and repair tissues and wounds. It is the main cause to different human diseases, including cancer, type II diabetes, and cardiovascular diseases [9][70]. Scientific evidence has reported that polyphenols and flavonoids derived from fruits and vegetables exhibit strong anti-inflammatory activity and decrease the formation of reactive oxygen species (ROS) [10][11][71,72].
A study demonstrated that betalains from red pitaya fruit peel showed up to 92.06% of irritation inhibition on the sodium dodecyl sulphate-induced vascular irritation of duck embryo chorioallantoic membrane due to the strong scavenging ability of free radicals [12][73]. In another study, Saenjum, Pattananandecha and Nakagawa [13][25] determined the inhibitory effects on both reactive oxygen and nitrogen species to analyse the anti-inflammatory activities of red pitaya. The cell-based study reported that the fresh red pulp extract (containing anthocyanin and catechin) had the highest inhibition of both reactive oxygen and nitrogen species, and higher numbers of free hydroxyl groups around the pyrone ring of anthocyanin positively affected the antioxidant capacity. The concentration of 25–100 mg/L showed active inhibitory effect on both reactive species. It was also reported that the cyanidin 3-glucoside suppressed the generation of tumour necrosis factor and modulated gene expression. Montiel-Sánchez, et al. [14][74] observed a similar result and stated that the extracts from red pitaya pulp showed higher in vitro anti-inflammatory activity, representing 82.81%, and betalains were the main effective phytochemicals.

2.3. Other Bioactivities

The polyphenol extracts of red pitaya with high anti-microbial activity were reported by some researchers. Zambrano, et al. [15][75] studied the anti-microbial activity of pitaya extracts (phenolic compounds) against foodborne pathogens and spoilage bacteria. They found that the growth of Bacillus subtilis, Staphylococcus aureus and Salmonella enterica were significantly inhibited compared with Listeria monocytogenes and Pseudomonas strains. Except for phenolics, beta-cyanins were reported to control the reproduction of the S. aureus, Enterococcus spp. and Bacillus spp. with an antimicrobial activity (MIC) range from 12.5–25 mg/mL [16][76].
Anti-mutagenicity of betacyanin extracts was studied by Thaiudom, Oonsivilai and Thaiwong [17][26] and Salmonella typhimurium TA98 was used as the test strain. The results revealed that the colony amounts of S. typhimurium TA98 decreased with the addition of betacyanin extracts and inhibited the mutation. IC50 of the antimutagenicity activity was observed at 0.522 mg GAE/mL of the total phenolic content. However, this study did not explain the inhibitory mechanism and did not deal with safe use in human diet.
Collectively, many researchers have confirmed the bioactivities, including anti-hyper-glycaemia, anti-inflammatory, anti-microbial and anti-mutagenicity, of the polyphenol extracts from red pitaya, but the mechanism has yet to be elucidated.