Lactate and Brain Health in Zone 2 Training

A new review proposes that the well-known antidepressant effect of exercise may depend not just on the brain, but on a molecular conversation between muscles and the immune system, initiated by a familiar metabolic signal: lactate.

Beyond BDNF: A New Immunometabolic Hypothesis for Exercise

For decades, the mental health benefits of physical activity have been linked to changes within the brain itself. Scientists have focused on increased brain-derived neurotrophic factor (BDNF), better regulation of stress hormones, and the growth of new neurons. Authors Chuankai Luan, Chuanping Lei, and Min Liu argue that while these mechanisms are important, they may not fully explain depression cases marked by high inflammation, profound fatigue, and loss of motivation.

Their review, published in Frontiers in Psychiatry, shifts the focus from a brain-centered view to a whole-body dialogue. They propose that lactate—the molecule produced in muscles during moderate to intense exercise—is a central messenger in this conversation. The framework suggests that appropriately dosed exercise creates a beneficial “lactate pulse” that communicates directly with the immune system to reduce inflammatory signals associated with certain depression symptoms.

The Lactate-HCAR1 Axis: A Signal, Not Just Waste

Lactate is often mislabeled as a simple metabolic waste product causing muscle burn. In reality, it’s a valuable fuel source and, as this review emphasizes, a potent signaling molecule. The key is its receptor: hydroxycarboxylic acid receptor 1 (HCAR1 or GPR81).

Cells lining blood vessels, certain immune cells, and brain cells like astrocytes and microglia all possess HCAR1 receptors. When lactate from exercise binds to these receptors, it triggers a cascade of effects. In the brain, it may influence how microglia—the brain’s immune cells—behave, potentially calming neuroinflammation. In the bloodstream and bone marrow, the lactate signal might help regulate the production and activity of myeloid cells, which drive systemic inflammation.

This creates a direct biological link between contracting muscles and the immune networks implicated in inflammation-driven depression. It positions exercise as a quantifiable intervention that can modulate the body’s inflammatory tone, a concept explored in related research on lactate’s broader role in exercise and depression.

A Cross-Organ Pathway from Muscle to Mind

The model proposed by Luan and colleagues is notably systemic. It connects several organ systems into a coherent pathway explaining exercise’s antidepressant potential:

1. Skeletal Muscle: Exercise at an appropriate intensity generates and releases lactate into circulation.
2. Bone Marrow & Peripheral Blood: Lactate signals via HCAR1 to help regulate myelopoiesis (white blood cell production) and the function of circulating immune cells, reducing pro-inflammatory activity.
3. Blood-Brain Interface: Lactate crosses or signals at the blood-brain barrier, influencing endothelial cells and perivascular macrophages.
4. Brain Parenchyma: Within the brain, lactate influences microglia and astrocytes, potentially reducing neuroinflammation and supporting metabolic and synaptic functions.

This pathway specifically addresses depressive symptoms like fatigue and cognitive slowing, which are strongly correlated with elevated inflammatory biomarkers. It also complements other whole-body benefits of consistent training, such as the improvements in mitochondrial health from Zone 2 training and the systemic anti-aging effects on the immune system.

Methodology and Current Strength of Evidence

The authors built their hypothesis through a comprehensive review of existing literature. They integrated findings from human exercise physiology, which clearly shows exercise-induced lactate pulses, with preclinical studies that demonstrate lactate’s anti-inflammatory effects via HCAR1 in cell and animal models.

They are careful to note the varying strength of evidence along the proposed pathway. The basic physiology of lactate production and the existence of the HCAR1 receptor are well-established. Some anti-inflammatory effects of lactate signaling have causal support in animal studies. However, the direct connection of this entire axis to reducing depressive symptoms in humans remains inferential and requires targeted validation.

Consequently, the team frames “lactate-informed exercise prescription” as a translational hypothesis. It is a compelling, biologically-plausible model ready for testing, not a ready-made clinical protocol.

Practical Implications and Future Research Directions

If future research confirms this model, it could change how we prescribe exercise for mental health. Instead of relying solely on heart rate zones or perceived exertion, clinicians might one day consider an individual’s lactate kinetics—how their body produces and clears lactate during exercise—to optimize the immunometabolic signal.

This approach could help identify which patients with depression, particularly those with high inflammatory markers, would benefit most from exercise therapy. It also provides a clear biological framework for measuring response, looking at changes in immune cell phenotypes and inflammatory biomarkers alongside symptom scores.

The review calls for specific future studies: measuring lactate dynamics and immune changes in patients with depression during exercise programs, testing the safety and optimal recovery dynamics of lactate-focused protocols, and directly comparing this method to conventional exercise prescriptions. This need for precise measurement aligns with broader trends in fitness science, such as the use of fitness tracker data for heart rate and training analysis.

The ultimate goal is to move exercise from a generic “good for you” recommendation to a targeted, dose-prescribed intervention. By understanding the lactate-HCAR1 axis, we may learn to more effectively use exercise to regulate the immune system and treat the specific inflammatory dimensions of depression.

Source: Luan C, Lei C, Liu M (2026). The lactate-HCAR1 axis as an immunometabolic pathway in exercise psychiatry: a cross-organ hypothesis. Front. Psychiatry. doi: 10.3389/fpsyt.2026.1871773.