Author(s): TARRY, E.K.1,2, MIKKELSEN, S.S.1, D’AMBROSIO, M.3, TRANBERG, C.1, FALS, E.B.1, LUNDBY, S.M.1, DELA, F.1, SHAW, C.S.2, LARSEN, S.1,4, HELGE, J.W.1, Institution: UNIVERSITY OF COPENHAGEN, Country: DENMARK, Abstract-ID: 1411

INTRODUCTION:
Prolonged fasting is an underutilised method to assess the mechanistic limitations of peak fat oxidation (PFO) during exercise, given it induces considerable energy deficit, reduced glucose and glycogen availability and thus subsequent adaptations facilitating fat handling. This study examined the fasting-induced changes in substrate availability and resultant alterations in fat oxidation. It was hypothesised that PFO rates would be substantially increased after a prolonged 3.5 day fast in healthy young and older individuals.
METHODS:
22 healthy, moderately trained men were recruited into two groups: young (n = 12) (26 ± 4 yrs, BMI 24.1 ± 1.9, VO2peak 53.6 ± 4.8 ml/kg/min) and older (n = 10) (63 ± 2 yrs, BMI 24.2 ± 1.2, VO2peak 42.9 ± 6.5 ml/kg/min) (mean ± SD)) and underwent 3.5 days of water-only fasting. Fasting blood samples and an incremental exercise test to determine PFO and maximal oxygen uptake were conducted before and at the conclusion of the fasting period. Two-way, repeated measures-ANOVA were used to assess differences between groups over time.
RESULTS:
Whole-body PFO increased from (0.45 ± 0.09 to 0.87 ± 0.14 g/min) and (0.41 ± 0.12 to 0.73 ± 0.15 g/min) in the young and older groups respectively, with PFO rates significantly higher after prolonged fasting in the young compared to older subjects (post-hoc: p < 0.05). No significant time by age interaction was observed when PFO was expressed relative to fat free mass. Rates of weight loss were similar in the young (84 ± 9 to 81 ± 9 kg) and older groups (76 ± 6 to 73 ± 6 kg). VO2peak values were significantly reduced after fasting, regardless of age (main effect; time: p <0.001). As expected, older subjects had significantly lower VO2peak (main effect; age: p <0.001). Similarly, RER values captured at maximal exercise capacity were significantly reduced following fasting, regardless of age (main effect; time: p <0.001), and older subjects had significantly lower RER values regardless of time (main effect; age: p <0.01).
CONCLUSION:
Despite observed age attenuation in PFO and diminished anaerobic metabolism at maximal exercise output, prolonged fasting persists as a potent enhancer of peak fat oxidation. Further analyses of sampled blood and muscle are targeted towards elucidating the mechanisms underlying the fasting-induced adaptations associated with improved fat utilisation.