FGF21 plays an important role in carbohydrate and lipid metabolism, and energy balance [1)]. However, in humans under physiological conditions, its functions are not clear. Our group identified that FGF21 levels are associated with the amount of physical activity recorded using a validated questionnaire 
. Here, we investigate if a period of supervised exercise altered serum FGF21 levels in healthy adults. Results showed that serum FGF21 levels significant increased after two weeks of daily physical activity, but no change was observed after a single bout of exercise. The increment in FGF21 showed a positive correlation with the changes in serum FFA levels. Interestingly, METs, RHR, MHR and delta epinephrine (markers of adrenergic activation) correlated positively with the increase in FGF21.
Exercise increases lipolysis through the agonist effect of epinephrine on beta 3 adrenergic receptors in adipose tissue 
. The amount of exercise considered in our protocol induced a significant adrenergic response measured clinically (with heart rate and blood pressure) and biochemically (with epinephrine). The adrenergic response is greater in women who are not physically fit. These characteristics allow us to evaluate with certainty the acute and medium-term effects of exercise on FGF21 serum levels.
Baseline fasting glycemia, delta FFAs, epinephrine, heart rate and METs were associated with an exercise –induced increment in FGF21 levels. The FFA response may be the main signal for increasing FGF21 concentrations. The FGF21 gene has a PPAR alpha response element that is activated by the complex FFA/PPAR alpha/RXR 
. The central role of PPAR alpha activation is demonstrated by the almost complete elimination of the effects of fasting on FGF21 expression in the PPAR alpha knockout mouse 
, and the increment of hormone in response to an IV infusion of fatty acids 
. However, in order to become a biologically relevant signal for increasing the expression of the FGF21 gene, supraphysiological concentrations of FFAs are needed 
. This phenomenon may explain the increased FGF21 concentrations reported in prolonged fasting 
, lactation 
, and growth hormone therapy 
, conditions associated with remarkably high FFA plasma levels. The same observation may explain the weak correlation observed between FGF21 and baseline serum FFAs (r
0.03) reported in both our study and by other groups 
. Thus, it is likely that chronic exposition to increased FFA concentrations may have a stronger association with FGF21 serum levels than fasting FFA. This proposal may explain the strong association reported between serum FGF21 levels and liver fat content 
. On the other hand, another determinant of the change in FGF21 levels in our study was baseline fasting glycemia. This association is explained by the presence of a carbohydrate response element binding protein (ChREBP) response element in the FGF21 gene 
. Previous reports have shown that fasting glycemia is associated with FGF21 serum levels 
. Our report provides confirmatory evidence and shows that the prominent role of glycemia continues under conditions of stimulated lipolysis. The other predictors of the change in FGF21 levels (i.e. epinephrine, heart rate and METs) are markers of the exercise induced adrenergic response and the subsequent increment of FFAs. The variables discussed above determined more than 50% of the increment in FGF21 after two weeks of physical activity. Thus, a large percentage of the variability in the FGF21 response to exercise can be explained by the variables evaluated here.
Exercise improves glucose disposal by increasing insulin action and by activating the AMPK pathways, causing GLUT4 translocation to the muscle cell surface and glucose uptake 
. The FGF21 response to exercise could be involved in the beneficial effects of increased physical activity in lipid and carbohydrate utilization, in healthy and obese individuals. The increment in serum FGF21 may aim to decrease the level of FFAs through the inhibition of lipolysis 
. In addition, both chronic exercise and FGF21 induce expression of the peroxisome proliferator-activated receptor c coactivator protein-1a (PGC1alpha), a transcriptional coactivator protein that interacts with several different DNA-binding proteins resulting in increased gluconeogenesis, fatty acid oxidation, and ketogenesis 
. These actions lower serum FFAs concentrations and may protect against the ectopic deposit of lipids in the liver and the muscle. Lipotoxicity plays a major role in the pathogenesis of the obesity-related metabolic complications. This interpretation is in agreement with the increased FGF21 levels reported in patients with the metabolic syndrome, obesity, and impaired glucose tolerance 
. Probably, the increment of FGF21 is a mechanism to counteract the chronic exposition of increased FFAs plasma levels and lipotoxicity. Exercise may reinforce this compensatory pathway. However, the FGF21 increment only happens after a chronic stimulus capable of inducing adrenergic activation. A single bout of intense exercise did not alter FGF21 serum levels. A potential explanation for this finding is the increased insulin response observed after a single session of exercise. Insulin inhibits lipolysis and blocks the main mechanism by which exercise stimulates FGF21 secretion (i.e. increased FFAs plasma levels). Thus, increased FGF21serum levels may be an additional mechanism by which exercise improves carbohydrate and lipid metabolism in the medium term.
Some limitations of our study should be recognized. We cannot fully control other potential sources of physical activity. We fully relied on the adherence of the participants to our recommendations. However, the second evaluation with the questionnaire suggests that participants considerably increased the kcal/day consumed because of the study. In addition, the results may not be applicable to men. Finally, we did not register the caloric consumption during the study. However, no major changes in BMI and body composition were observed.
In summary, serum FGF21 increased significantly after two weeks of daily physical activity. This increment is related to clinical and biochemical parameters of adrenergic response and serum levels of FFAs.