Effects of vitamin D supplementation on muscle function and cardiorespiratory fitness in concurrent trained mice
Authors: Huey Kimberly , Conroy Zachary , Temperly MacKenzie 
Published: May 2023

Muscle hypertrophy induced by functional overload (FO) provides an in vivo model to study muscle growth and when combined with running wheel (RW) activity can model concurrent strength/endurance training (CT). Research suggests vitamin D supplementation may stimulate muscle growth and may be associated with increased growth factor levels, such as insulin-like growth factor-1 (IGF-1), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF). However, we previously reported that vitamin D supplementation did not enhance muscle hypertrophy or strength following 14d of FO alone. It remains unclear if vitamin D impacts muscle hypertrophy, contractile function and cardiorespiratory fitness following 14d of CT (FO + unlimited RW access). We hypothesized that vitamin D supplementation would positively impact the ability of skeletal muscle and the cardiorespiratory system to effectively respond to CT as evidenced by improved contractile function, cardiorespiratory fitness, and growth factor levels (IGF-1, FGF, and VEGF) compared to placebo. Adult mice underwent FO of the plantaris and had unlimited RW access (CT) or had sham surgery and remained sedentary (CON) for 14d while receiving daily vitamin D or placebo (n= 5-8/group). Cardiorespiratory adaptations were assessed with maximal treadmill tests before and after CT or CON. In vivo plantaris force and fatigue resistance (% of maximal force after 10 contractions) were measured 14d post-CT or CON. IGF-1, FGF, and VEGF were measured in the plantaris by ELISA. RW activity increased daily over 14d similarly in placebo and vitamin D groups, but values were lower than distances observed in mice performing only RW. Maximal test time was significantly lower post CT in placebo group and remained similar post-CT in vitamin D group. CT increased plantaris mass, independent of treatment (placebo: 23.0±0.86 vs. 13.4±1.3 and vitamin D: 20.2±0.7 vs. 13.2±0.7 mg, for CT and CON, respectively, p<0.05). Maximal force relative to body mass was unchanged with CT or vitamin D. Maximal force normalized to plantaris mass was significantly lower with CT compared to CON independent of treatment. Fatigue resistance was increased with CT, independent of treatment (placebo: 56±7% vs. 40±4% and vitamin D: 61±7% vs. 33±6%, for CT and CON, respectively, p<0.05). IGF-1 levels were increased in the CT plantaris, independent of treatment (placebo: 1,534±80 vs. 99±9 and vitamin D: 1,339±208 vs. 115±10 mg/mg, for CT and CON, respectively, p<0.05). FGF levels were increased in the CT plantaris, independent of treatment (placebo: 462±50 vs. 105±37 and vitamin D: 466±49 vs. 153±82 mg/mg, for CT and CON, respectively, p<0.05). Both IGF-1 and FGF were unchanged in the unloaded tibialis anterior independent of training or treatment. VEGF levels were unchanged with CT and/or vitamin D. Vitamin D treatment in combination with CT did not significantly improve the ability of skeletal muscle to increase ma Research is supported by the NASA Iowa Space Grant under Award No. NNX16AL88H. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.