## 2 Bioactive Compounds Commonly Found in Green-Colored Fruits and Vegetables
### General Characteristics, Structure, and Food Sources
In addition, CQA ameliorates obesity and obesity-related metabolic disorders through the PPAR?2 and NF-kB signaling pathways in rat adipose tissue [171]. Both CQA and CGA have antioxidant properties in humans and animals [172; 173; 174], which contribute to excellent functions in treating cardiovascular and metabolic diseases. In human bioavailability, one-third of the CGA from foods is absorbed in the small intestine, and a part enters into the blood circulation, while most reaches the colon [175]. The pathways of CGA are as follows: in humans, a part of the 5-CQA (approximately 33%) from food is absorbed intact in the upper GIT without hydrolysis and enters into circulation; a small amount of the 5-CQA (approximately 7%) is absorbed throughout the small intestine involving hydrolysis into CA and QA; colonic microbiota-mediated 5-CQA metabolism with the absorption of metabolites occurs in the colon; after the intact 5-CQA and its metabolites enter the bloodstream, they are absorbed and/or metabolized in the liver (reviewed in ref. [172]). One should point out that the small molecular metabolites produced by gut microbes degrading CGA and its derivatives also play a vital role in regulating physiological and pathological status of host. **FA** plays an antioxidant role by inhibiting ROS production and aldose reductase activity and by activating the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway. The antioxidant property of FA can be further enhanced by the nuclear shift of heme oxygenase-1 (HO-1) via activation of the Nrf/HO-1 pathway. In addition, FA can inhibit the extracellular signal-regulated kinase (ERK)1/2 signal, regulate the expression of NO/endothelin-1 (ET-1) and vascular endothelial factors, protect endothelial cells, and enhance angiogenesis to maintain the normal function of the vascular endothelium [176]. Moreover, FA can significantly eliminate the negative effects of antibiotics, including upregulation of a variety of antioxidant genes, increase the total antioxidant levels of mice compared with the antibiotic group, reduce the lipid oxidation levels in vivo, regulate NO synthesis through NOS, and stimulate intestinal contraction and peristalsis to promote digestion. Moreover, FA can regulate the intestinal metabolism, intestinal microecological balance, and inflammatory response of piglets by increasing the abundance ratio of Proteobacteria and Firmicutes, significantly increasing the abundance of _Clostridium_ and decreasing the abundance of _Diplococcus_ and _Enterobacter_[162]. Various investigations have shown that _p_-CA and its conjugates exhibit various bioactivities, including antioxidant, anti-inflammatory, cardioprotective, and anti-diabetic activities. In animal models, _p_-CA decreased basal oxidative stress levels more effectively than vitamin E, as assessed by DNA damage in rat colonic mucosa [177]. _p_-CA showedanti-inflammatory effects in adjuvant-induced arthritic rats, reducing the levels of TNF-\(\upalpha\) and macrophage phagocytic index, while increasing serum immunoglobulin levels [155]. 4.2 Hydroxybenzoic Acids: Gallic Acid, Protocatechuic Acid, Syringic Acid and Vanillic Acid General Characteristics, Structure, and Food Sources  
**Gallic acid (GA)**, 5\(A\),3-trihydroxybenzoic acid, is one of the most important and abundant phenolic acids. Gallic acid and its derivatives are present in fruits and vegetables such as grapes, mango, avocado, and green tea [178]. It is an organic acid containing one benzene ring structure with three adjacent -OH groups at the positions 3,4,5 and one -COOH group at position 1. Its molecular formula is C\({}_{7}\)H\({}_{6}\)O\({}_{5}\), and its molecular weight is 170.12 g/mol [179]. It is soluble in water, alcohol, ether, and glycerol. In nature, it exists predominantly as hydroxybenzoic acids and in different forms of ester and catechin derivatives [180].