Does taking antioxidants like vitamin C (Vitamin C) interfere with the adaptive response to exercise?

High-Dose Antioxidant Supplements Like Vitamin C Can Impair Exercise Adaptations

Avoid high-dose vitamin C and other antioxidant supplements (vitamins C and E) during training periods, as they may interfere with beneficial cellular signaling pathways that drive muscle adaptation, mitochondrial biogenesis, and insulin sensitivity improvements from exercise. 1

Key Distinction: Whole Foods vs. High-Dose Supplements

The evidence clearly differentiates between dietary antioxidants from whole foods versus isolated high-dose supplements:

• Fruits and vegetables are beneficial and recommended for athletes, providing polyphenols, vitamins, minerals, and fiber that support health, recovery, and performance 1
• High-dose antioxidant supplementation carries well-established risks of blunting adaptation 1
• Vitamin insufficiencies (e.g., low vitamin C) increase oxidative stress and decrease physical performance, but this does not justify supraphysiological supplementation 1

Mechanism: Blocking Beneficial Signaling

Exercise generates reactive oxygen species (ROS) that serve as essential signaling molecules for training adaptations 1, 2:

• ROS act as signals for mitochondrial biogenesis, antioxidant enzyme upregulation, and insulin sensitivity improvements 2
• High-dose antioxidants can competitively interfere with these redox-sensitive signaling pathways 1
• The adaptive response follows a hormetic pattern—physiological ROS levels are beneficial, while excessive antioxidant supplementation blocks these benefits 2

Evidence on Training Adaptations

Endurance Training

The evidence is mixed but concerning:

• Some studies show antioxidant supplementation (vitamins C and E) can attenuate endurance training-induced improvements in antioxidant capacity, mitochondrial biogenesis, and insulin sensitivity 2
• However, other well-controlled studies found no impairment of adaptations with very high doses (750 mg/kg vitamin C + 150 mg/kg vitamin E), showing normal increases in GLUT4, mitochondrial enzymes, and insulin responsiveness 3
• A 12-week study with combined vitamins C and E showed no differences in VO2max, power output, or muscle enzyme adaptations compared to placebo 4

Resistance Training

The evidence is more consistent for resistance training—antioxidants appear more likely to impair adaptations:

• Supplementation may decrease muscle hypertrophy by blocking cell-signaling pathways 5
• Upregulation of endogenous antioxidant enzymes after resistance training is blocked by exogenous supplementation 5
• Special caution is warranted with resistance training, as muscle remodeling appears more dependent on ROS/RNS signaling 2

Clinical Guideline Recommendations

Strong Recommendations Against Supplementation

The ACC/AHA guidelines for post-MI patients explicitly state: "Antioxidant vitamins such as vitamin E and/or vitamin C supplements should not be prescribed to patients" (Class III recommendation) 1

The UEFA expert statement on elite football nutrition states: "Reducing exercise-induced muscle inflammation and free radical production, particularly with large doses of individual antioxidant vitamins C and E, may interfere with adaptive processes in muscle and is therefore discouraged" 1

Weak Evidence for Specific Conditions

For exercise-induced bronchoconstriction, dietary supplementation with ascorbic acid showed only incomplete inhibition of airway responses, with weak recommendation strength due to small effect sizes and methodological concerns 1

Practical Algorithm

For athletes in training:

1. Prioritize whole food sources of antioxidants through 5+ servings of fruits and vegetables daily 1
2. Avoid high-dose isolated vitamin C and E supplements during training blocks focused on adaptation 1, 2
3. Consider acute supplementation only for immediate pre-competition or recovery scenarios, not chronic use 5
4. Monitor serum carotenoids as a biomarker of fruit/vegetable intake adequacy rather than relying on supplements 1

For individuals with documented deficiencies:

• Correct vitamin insufficiencies through diet or targeted supplementation to restore normal levels 1
• Once replete, discontinue high-dose supplementation 5

Important Caveats

• Baseline antioxidant status matters: Benefits may only occur in individuals with low basal antioxidant levels 5
• Timing considerations: Acute administration immediately before/during exercise may delay fatigue without impairing long-term adaptations, but chronic supplementation is problematic 5
• Dose-response relationship: The negative effects are primarily seen with high-dose supplementation, not physiological amounts from food 1
• Exercise-associated gastrointestinal symptoms: Limited evidence suggests acute ascorbic acid (1000 mg) may modestly reduce endotoxin translocation, but this does not justify routine use 1