Background: Excess white adipose tissue (WAT) in humans is considered as a harmful health index. However, increased brown adipose tissue (BAT) and brown-like adipose tissue activity are associated with increased resting energy expenditure (REE) that may help to control body weight. Exercise may enhance browning formation of WAT and reduce WAT that may lead to health improvements. Aims: a) to examine the effects of physical activity on the link between peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC-1α) and fibronectin type III domaincontaining protein 5 (FNDC5) genes in muscle, circulating Irisin and uncoupling protein one (UCP1) of WAT in humans (study 1); b) to examine the relationship between UCP1 mRNA and protein expression as well as PGC-1α, peroxisome proliferatoractivated receptor alpha (PPARα) and PPARγ genes with physical activity levels in WAT of healthy men (study 2); c) to examine the effects of different types of exercise and de-training on the UCP1 mRNA and protein expression (study 3), and d) on leptin mRNA in WAT of healthy men (study 4). Method: Study 1: A systematic review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta- Analyses. Studies 2-4: The total of 46 healthy men subjected to measurements for physical activity levels, diet, anthropometry, body composition, REE, peak oxygen consumption, 1-repetition maximum and provided subcutaneous fat biopsies to determine mRNA and protein expression of six genes in one cross-sectional study and one randomized controlled trial. Results: Study 1: No link was found between PGC- 1α and FNDC5, circulating Irisin and UCP1 of WAT in response to physical activity. Study 2: The mRNA of, UCP1, PGC-1α, PPARα and PPARγ genes of WAT were not associated with physical activity levels. The UCP1 protein expression however, was negatively associated with physical activity levels. Studies 3-4: Different types of chronic exercise and de-training do not affect UCP1 mRNA and protein expression 3 and leptin mRNA in WAT. However, effect size analyses demonstrated increased UCP1 mRNA and protein expression, PPARγ and leptin in response to chronic exercise. Conclusions: There is no evidence to support the link between PGC-1α and FNDC5 in human muscle or the link between FNDC5 and circulating Irisin and UCP1 in WAT in response to exercise. There are no effects of exercise and de-training on browning formation of WAT and no link between browning formation indices and REE, body weight as well as leptin mRNA in healthy men. Further research is required to elaborate the aforementioned phenomena.