Long COVID, Mitochondrial Dysfunction & Exercise Intolerance: What New Research Reveals

For many individuals living with Long COVID, the most frustrating symptom isn't always fatigue itself—it's the inability to tolerate physical activity.

Walking up stairs, completing a gym session, or even performing everyday tasks can trigger disproportionate exhaustion, dizziness, muscle fatigue, breathlessness, and prolonged recovery times. Yet standard cardiac and pulmonary testing often returns normal results.

A growing body of research suggests the answer may lie deeper at the cellular level.

A New Perspective on Long COVID

Researchers from National Jewish Health investigated whether patients experiencing Post-Acute Sequelae of SARS-CoV-2 infection (PASC), commonly known as Long COVID, demonstrated abnormalities in mitochondrial function during exercise.

The findings were striking.

Despite largely normal resting cardiac and pulmonary function, participants displayed:

• Significantly reduced fatty acid oxidation during exercise
• Excessive lactate accumulation at relatively low exercise intensities
• Evidence of altered metabolic function within skeletal muscle
• Signs consistent with impaired mitochondrial efficiency

The researchers concluded that mitochondrial dysfunction may be an important contributor to exercise intolerance in Long COVID.

What Is Fatty Acid Oxidation?

Under normal conditions, the body generates energy from a combination of carbohydrates and fats.

During lower-intensity activity, healthy mitochondria efficiently utilise fatty acids as a primary fuel source. As exercise intensity increases, the body gradually shifts toward greater carbohydrate utilisation.

This process is known as metabolic flexibility.

In the Long COVID cohort, researchers observed significantly impaired fat oxidation compared with healthy individuals. This suggests that patients may be unable to efficiently access stored fat for energy production during physical activity.

Why Does Lactate Matter?

Lactate is a normal by-product of energy production.

In healthy individuals, mitochondria continuously recycle and utilise lactate as fuel. However, when mitochondrial function becomes impaired, lactate accumulates prematurely.

Researchers observed elevated blood lactate levels in Long COVID patients at exercise intensities where such elevations would not normally occur.

This suggests that energy production may be shifting toward less efficient pathways much earlier than expected.

What Does This Mean For Long COVID?

The findings support a growing hypothesis that many Long COVID symptoms may not be solely related to the heart or lungs.

Instead, abnormalities in:

• Mitochondrial function
• Cellular energy production
• Fat oxidation
• Lactate clearance
• Metabolic flexibility

may contribute significantly to ongoing fatigue and exercise intolerance.

Importantly, these abnormalities were observed even in individuals who had experienced relatively mild acute COVID-19 infections.

The Mitochondrial Connection

Mitochondria are often referred to as the powerhouses of the cell.

They are responsible for converting nutrients into usable cellular energy through oxidative phosphorylation. When mitochondrial function becomes compromised, the consequences can affect multiple body systems simultaneously.

Researchers continue to investigate how viral infections may alter:

• Mitochondrial biogenesis
• Fatty acid metabolism
• Redox balance
• Cellular signalling pathways
• Exercise performance and recovery

Areas of Emerging Research Interest

Growing scientific interest surrounds compounds and interventions being investigated for their potential role in supporting mitochondrial health and cellular energy metabolism.

Current areas of investigation include:

• Mitochondrial signalling pathways
• Oxidative stress regulation
• Cellular repair mechanisms
• Fat oxidation and metabolic flexibility
• Exercise recovery and adaptation

As our understanding of Long COVID evolves, mitochondrial dysfunction remains one of the most compelling areas of ongoing research.

Final Thoughts

Many Long COVID patients describe feeling as though their bodies have "lost the ability to produce energy normally."

This study provides important evidence that those experiences may have a measurable biological basis.

By demonstrating reduced fat oxidation and altered lactate metabolism during exercise, researchers have identified a potential mechanism that could contribute to fatigue, post-exertional symptoms and reduced exercise tolerance in Long COVID.

Further research is needed, but these findings represent another important step toward understanding the complex biology underlying Post-Acute COVID Syndrome.

Reference

de Boer E, Petrache I, Goldstein NM, et al. Decreased Fatty Acid Oxidation and Altered Lactate Production during Exercise in Patients with Post-acute COVID-19 Syndrome. Respiratory Care Medicine. 2021.