**2. Metabolism of tannic acid**

Tannic acid metabolism in male wistar rats was investigated *in vivo*. Tannic acid solutions containing 1 gram of tannic acid in 10 mL of water were administered at a rate of 10 mL per kilogram of body weight. Tannic acid was also given orally at a dosage of 1 g per kilogram, and serum was collected at various time intervals. Similarly, urine and fecal samples were collected at different times.

The most abundant metabolites in blood samples were 4-O-methyl gallic acid (4-OMGA), pyrogallol, and resorcinol. The highest concentration of 4-O-methyl gallic acid (4-OMGA) was found 1.5 hours after injection. Similarly, the highest levels of pyrogallol and resorcinol were found at 4 and 17 hours, respectively.

Urine contains four distinct metabolites: 4-O-methyl gallic acid, gallic acid, resorcinol, and pyrogallol. The metabolite 4-O-methyl pyrogallol was not found in the feces. The presence of gallic acid in the liver, which is eliminated through urine, was confirmed by analysis of metabolites present in serum, urine, and fecal samples (**Figure 1**) [19].

Tannic acid and theaflavin-3-gallate, two natural polyphenols present in black tea, have been shown to inhibit SARS-CoV, with IC50 values of 3 and 7 micromolars, respectively. As a result, investigations on the metabolism of tannic acid and theoflavin-3-gallate are becoming increasingly essential [20].

Recent research suggests that theaflavin metabolites of microbial origin derived from black tea intake are the primary cause of its positive benefits. Microbial

**Figure 1.** *Metabolic pathway of tannic acid in the rat [7].*

#### *Metabolism of Phytochemicals DOI: http://dx.doi.org/10.5772/intechopen.100569*

biotransformation products 3.4-dihydroxybenzoic acid and phloroglucinol, as well as their corresponding sulfate and glucuronide conjugates, have been shown to lower pro-inflammatory chemokine levels, as well as molecules such as TNFalpha, IL-6 in CD4L, oxidized LDL-challenged vascular endothelial cells, and sVCAM-1 [21].

In a reported study, healthy humans were ingested with 1 g of theoflavin extract and their urine and fecal samples were analyzed by high-resolution chromatography and mass spectrometry [22].

Theaflavins from black tea have low bioavailability, according to research. A sufficient amount of theaflavin enters the colon while traveling through the GI tract, where bacteria attack it, resulting in low-molecular-weight metabolites [23].

Additional metabolites for theoflavin have been found as 3-(4′-hydroxyphenyl) propionic acid and gallic acid. According to research, these intestinal metabolites act as agents against the progression of neurodegenerative disorders and have the ability to shield brain cells from oxidative stress. In human colon cancer cells, gallic acid inhibits cell proliferation and causes apoptosis.
