Dietary fiber-rich by-products can bind to bile acids, fat and glucose, thereby reducing the rate of absorption of the fat and glucose, consequently leading to hypoglycemic and hypolipidemic effects. Guava leaf extracts are well studied for the in vivo hypoglycemic and anti-hyperglycemic activity [91]. Guava fruits also have similar hypoglycemic and hypolipidemic effects, as shown in Table 3. This provides the basis in using guava or guava by-products as a natural therapy to cope with increasing cases of metabolic syndroms, such as obesity, dyslipidemia and hyperglycemia. Obesity directly links with a chronic energy imbalance of individuals caused by combined factors, including genetics, hormone levels, behavioural patterns and their environmental determinants [92]. Too much energy consumption results in copious levels of triglycerides in the adipose tissue. The accumulated triglycerides lead to increased circulatory free fatty acids and further accumulation in other tissues, like liver, as a lipid burden. Abundant lipid droplets in blood circulation also causes the formation of mature adipocytes from preadipocytes, resulting in increased fat cell size (hypertrophy) and number (hyperplasia), thus leading to obesity. High concentration of fatty acids in adipose tissue leads to secretion of adipokines from adipocytes and results in lowered vascularization with development of hypoxia (low amounts of oxygen) and inflammation. Adipokines consist of adiponectin, leptin, γ-peroxisome proliferator-activated receptor (PPAR), resistin and other proinflammatory cytokines [93]. Resistin induces insulin resistance which links obesity to diabetes. Type 2 diabetes mellitus refers to an abnormally high
blood glucose level as a result of inadequate insulin, resistance to insulin, overproduction of glucose by the liver, defects in pancreatic β-cell function and decreased β-cell mass [94].
Specific chemical constituents have been isolated from plants and the bioactivities are identified. We summarize the evidence of hypoglycemic and hypolipidemic effects based on the main class or subclass of some chemical constituents (Table 4) that are discovered in guava fruits [95,96]. It is noteworthy to research into the bioactives from guava by-products and unravel the mechanisms that leads to hypoglycemic and hypolipidemic effects. World Health Organization estimated a total of 171 million (2.8%) people with diabetes mellitus from the global population, and this figure has been projected to increase to 439 million by 2030 [97]. Guava by-products have high potential to act as natural remedies for these syndroms to reduce the medical burden in the long run.
P
ROCESSINGE
FFECTS ONP
HYTOCHEMICALS ANDD
IETARYF
IBER OFP
LANTF
OODPlant food undergoes processing, such as peeling and desizing, to be marketed as ready-to-eat food or chilled fruit juice. For extending the shelf life further, more severe unit operations are imposed to produce dehydrated fruit, canned fruit in syrup, sterilized fruit juice, concentrate and jam. Unit operations with elevated temperatures, such as pasteurization, blanching, evaporation, drying, roasting, extrusion and microwave heating, are commonly used for inactivating microorganisms and enzymes in processed food.
However, thermal processing also reduces sensory and nutritional compounds of plant-based foods. The effect of common processing steps on fruit has to be well understood, so the by-products’ targeted use as ingredients or feed supplements can be optimized and meet the specific requirements for applications in various sector.
Table 3. Some tested bioactivities of guava and chemical constituents
Extract from Parts of guava
Bioactivity Chemical constituents Reference
Type Mechanism/System
Fruits Anticancer Inducing apoptosis/Acute promyelocytic leukaemia cells. Possibly due to flavonoids such as guaijavarin and quercetin. [75]
Therapy for cardiovascular and gastrointestinal disorders
Spasmolytic activity, against high K+ and phenylephrine pre-contractions/Isolated rabbit aorta preparations
The essential oil consists of butanoic acid methyl ester, 3-methyl glutaric anhydride, butanol, 3-hexenal, cinnamyl alcohol, 1-hexanol and hexane as the major components.
[76]
Cardiovascular benefits
Inhibitory effects/ex vivo collagen-induced platelet aggregation in healthy human subjects
Possibly due to flavonoidcompounds exist in guava fruit including myricetin, apigenin, leucocyanidin, quercetin,
Inhibition effects/5-lipoxygenase (essential oil) β-Caryophyllene, limonene, β-caryophyllene oxide, α-selinene and β-selinene
[78]
Anti-ulcer Inhibitory effects/Urease Quercetin and two quercetinglycosides, avicularin and uaijaverin [79]
Hypoglycemic Restored the loss of body weight caused by Streptozotocin (STZ) and reduced blood glucose levels in a dose-dependent manner/Rats
Possibly due to flavonoids, guaijaverin, quercetin, and other chemicals. Other phenolic compounds, such asmyricetin, ellagic acid, gallic acid, apigenin, and rutin
[80]
Hypoglycemic and hypolipidemic effects
A significant reduction inglycemia/Wister rats fed with guava pulp juice for 40 days.
Possibly due to high concentration of carotenoids, lycopene, vitamin C and polyphenols in guava pulp.
[81]
Peel Anticancer Inducing CD11c expression/acute promyelocytic leukaemia cells Possibly flavonoids such as guaijavarin and quercetin [75]
Anti-hyperglycemic The maximum fall of 20.8 and 17.5% in fasting blood glucose and post prandial glucose levels/STZ-induced severely diabetic rats
Possibly due to several flavonoids, terpenoidsand their glycosides
[82]
Hypolipidemic and hepatoprotective effects
A significant decrease intriglyceride (TG), total cholesterol (TC), high density lipoprotein (HDL), very low densitylipoprotein (VLDL) and low density lipoprotein (LDL)/STZ-induced severely diabetic rats, 21 days treatment
Possibly due to the high amount of polyphenols and lucocynadines
[83]
Seed Antimicrobial Inhibitory effects/The growth of Klebsiella sp. and Proteus sp. Peptide Pg-AMP1 [84]
Cytotoxic A high inhibition activity/Ehrlich ascites carcinoma (220 to 240%) in female Swiss albino mice
Phenylethanoid glycoside:
A significant reduction in the levels of triacylglycerides and total cholesterol, and augmented levels of HDL/ Wister rats fed with rat food containing guava seed for 40 days.
Possibly due to significant levels of β-carotene and totalphenolic compounds
[81]
Table 4. Active chemical constituents and tested hypoglycemic and hypolipidemic effects
Active compound Bioactivity Reference
Hypoglycemic Hypolipidemic
-Reducing digestionof sugars in the small intestine
-Inhibits HMG CoA reductase therefore inhibits cholesterol synthesis
-Inhibits the absorption of cholesterol -Promotes liver cell LDL-R expression -Promotes the reverse transport of cholesterol -Regulates the metabolism of triglyceride
-Stimulating the uptake of glucose without functional insulin receptors
-Inhibits theaggregation of islet amyloid polypeptide (IAPP) (responsible in the death of pancreatic β-islet cells in type II diabetes) -Improves insulin resistance
-Induced antihyperglycemic and renal protective activities in STZ-cadmium-induced
diabeticnephrotoxic rats
-Decreasing the intracellular accumulation of triglycerides in 3T3-L1 adipocytesin high-fat diet-fed rats.
-Increasing plasmaadiponectin in animal model of type 2 diabetes
-Lowering the plasma triglyceride level -Decreasing plasma total cholesterol
-Increasing HDL-cholesterol in animal model of type 2 diabetes
-Decreases serum glucose, glycosylated protein, and serum urea nitrogen
-Decreases of urinary protein and renal-AGE (advanced glycation end- products) in STZ-induced and in db/db type 2 diabetic mice.
-Lowering levels of triglycerides, total cholesterol, and non esterified fatty acid (NEFA)
-Repressing intestinal lipid absorption, chylomicron secretion by the intestine and VLDL secretion by the liver
[103-105]
Sesquiterpe-noids
Abscisic acid (ABA), turpinionoside A -ABA decreases fasting luminal glucose concentrations in mice
-ABA stimulates the secretion of insulin in pancreatic β-cells
-ABA reduces obesity-related inflammation, and mRNA expression of PPARg and its responsive genes in white adipose tissue (WAT) of db/db mice fed high-fat diets -ABA intake attenuates adipocyte hypertrophy, TNF- expression, and macrophage infiltration in adipose tissue
[106]
Triterpenes Pinfaensin,pedunculoside, guavenoic acid,
madecassic acid, asiatic acid
Farnesol terpenoid is classified as the precursor of sesquiterpenoids and triterpenes. This ligand has great potential to activate PPARγ, which attenuates obesity and type-2 diabetes.
[106]
Pectic polysaccharides [16] Pectin intake decreases blood glucose and triglyceride levels in diabetic rats. Its anti-inflammatory properties are probably involved in its antidiabetic action.
[107-108]
Phytochemicals and dietary fiber are the main useful components in guava fruits. Guava fruits are often processed into puree or dehydrated fruit [109]. In addition, guava powder was developed recently to further extend shelf-life [110, 111]. Unit operations involved for puree production include cutting, crushing, sieving, decanting, pasteurization and freezing; whereas cutting, seed separation, osmosis dehydration and drying are involved in dehydrated fruit production. Most phytochemicals contribute to significant antioxidative activity. Food looses nutrients, including phytochemicals, during food processing at increased temperatures [112]. Some phytochemicals in fruits and vegetables are water soluble. Unit operation involving washing after cutting probably causes the leaching of these compounds from the plant material. For example, minimally processed, cut-washed carrots and parsnips showed less retention of polyacetylenes [113]. Pokorný and Schmidt [114] emphasized that very different effects of antioxidant changes may occur during food processing and storage. In fact, not all the phytochemicals and antioxidant compounds are heat labile. Phytochemicals in Brassica, in some cases, were retained during processing and even transformed to novel compounds with biological activity [115]. Processed sweet corn has increased antioxidant activity despite the loss of vitamin C during processing [116]. Thermal processing elevated total antioxidant activity and bioaccessible lycopene content in tomatoes, although a loss of vitamin C was observed [117]. It is interesting to note that dietary fiber is also affected by processing, but mostly positive effects, such as enhanced solubility and functionality [118]. There are some exceptional cases like moisture sterilization on pectin, a soluble dietary fiber, that led to the production of cytotoxic residues [119].
There are a few reports on how processing affects the composition and functional properties of guava. Red guava, (Psidium cattleyanum Sabine) which is a rich source of carotenoids and anthocyanins, showed a decrease in pigment level after undergoing hot air drying. However, the carotenoid concentrations as well as the DPPH scavenging activity were retained by freeze-drying [120]. In contrast, there was a significant loss in total phenolic, esters and aldehydes as well as antioxidant capacity after both oven and freeze-drying to produce guava powder [121]. Terpenes and insoluble flavonoids were predominant in guava powders, which may contribute to the antioxidant capacity of guava powders as high as other tropical fruit powders.
In fact, Alves and Perrone [122] have reported that guava flour as a source of phenolic compound has been successfully incorporated into white bread, which led to an increment of phenolic compounds up to 2.4-fold. The raise in
antioxidant capacity of bread melanoidins was due to the enrichment with guava flour and was partially attributed to the bound phenolics.
The effects of preservation techniques on dietary fiber in guava fruits have not been studied, but we predict the functionality of fibers would not be as susceptible as phytochemicals upon processing. Since guava by-products are rich in total phenolics, especially flavonoids, we propose the pomace, peel and seed to be frozen or freeze-dried for extending the shelf-life while keeping most of the constituents and properties.