BEETter AGING: Short-Term Dietary Nitrate Supplementation Enhances Muscle Contractile Properties in Older But Not in Young Adults

Purpose
Dietary nitrate (NO₃^-) supplementation has been shown to improve skeletal muscle contractile function and reduce fatigue, potentially due to alterations in skeletal muscle Ca₂⁺ handling/sensitivity. Since aging muscle can have impaired Ca₂⁺ handling, the aim of the study was to evaluate the effects of dietary NO₃^- supplementation on muscle contractile properties in young and older adults.

Methods
Eleven older (69±4yrs, O) and 11 young (26±2yrs, YG) adults consumed either NO₃^-rich beetroot juice (BR) or placebo (PLA), for 7 days. After supplementations, plantar flexors of dominant leg were evaluated as follow: a) maximal voluntary isometric contraction (MVIC); b) potentiated single twitches (Tw_pot) and double twitches electrical stimulations at the frequency of 100Hz (Db₁₀₀) on the tibial posterior nerve; c) a fatigue isometric (70% of MVIC) test until exhaustion. The force-frequency relationship was assessed with trains of electrical pulses across a wide range of frequencies on the muscle belly of the non-dominant leg.

Results
BR supplementation increased plasma [NO₃^-] and nitrite [NO₂^-] in both O and YG compared to PLA (more than 7-fold; all P≤0.02). No changes were observed in MVC, Tw_pot, and Db₁₀₀ force after BR compared to PLA in both YG and O. Only in O, Db₁₀₀ area under the curve (-7±6 N∙s change from PLA) and half relaxation time (-0.05±0.06s change from PLA) were significantly reduced. and time to exhaustion (+32±43s change from PLA) was significantly longer (all P<0.02) after BR. In O, BR also significantly increased submaximal force produced by trains of electrical pulses (P < 0.001).

Conclusions
NO₃^- supplementation positively affects muscle contractile proprieties, submaximal electrically evoked force production and fatigue resistance in older adults while these positive results were not found in young.