*4.4. Cell Viability*

The cell viability was evaluated using a MTT assay [57]. The 3T3-L1 preadipocytes were seeded at a density of 1 X 105 cells/mL in the 24 well plate, and incubated for 24 h. Flavonoids such as QU, IQ, and AF were treated in the wells at various concentrations (1, 2.5, 10 μg/mL), respectively, and then incubated at 37 ◦C for 72 h. Cell culture media was removed, and then MTT solution was added in the wells. The formazan crystals were dissolved in DMSO and the absorbance was measured at 540 nm using microplate reader (Thermo Fisher Scientific, Vantaa, Finland).

### *4.5. Oil Red O Staining*

After 8 days of differentiation, differentiated 3T3-L1 cells were washed with phosphate buffered saline (PBS), fixed with 10% formalin for 10 min and washed with PBS and 60% isopropanol. The fixed cells were stained with 0.6% Oil Red O solution for 20 min in the dark, washed 4 times with PBS and 60% isopropanol. The lipid droplets within the differentiated 3T3-L1 cells were visualized and photographed by microscopy. For quantitative analysis under Oil Red O staining was carried out by eluting with 100% isopropanol and measuring absorbance at 500 nm [58].
