Higher cardiorespiratory fitness may be linked to greater energy-use capacity in immune cells. Digest
Exercise has long been linked to better immune health, but much less is known about what happens inside individual immune cells as fitness increases. A new study examined how cardiorespiratory fitness is linked to energy use and signaling in cytotoxic T cells, immune cells that help destroy infected or abnormal cells.
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The study included 43 healthy adults aged 20 to 40. Each participant completed a demanding cycling test to estimate their aerobic fitness, meaning how well the body delivers and uses oxygen during hard exercise. Using resting blood samples, researchers analyzed cytotoxic T cells both as a whole and as subgroups representing different stages of their development: cells that had not yet responded to a specific threat, cells that carried a record of past threats, and cells prepared for immediate immune action. For each group, the researchers measured the cells' capacity to use sugar, fats, and amino acids for energy and how many produced a key immune signaling protein after stimulation in the lab. They also measured body fat percentage to account for differences in body composition that could influence how immune cells use energy.
- Higher fitness was linked to a greater capacity of cytotoxic T cells to use a sugar-based energy route. This association remained after body fat percentage was taken into account both when all cytotoxic T cells were analyzed as a single group and in the subgroup of cells prepared for immediate immune action.
- Higher fitness was associated with a greater capacity of cytotoxic T cells to draw energy from a combined fats-and-amino-acids energy route only when body fat percentage was not taken into account.
- Higher body fat percentage was linked to lower capacity of cytotoxic T cells to use fats and amino acids in two cytotoxic T cell subgroups prepared for immediate immune action.
- Higher fitness was associated with a greater share of the whole cytotoxic T cell population producing the key immune signaling protein after stimulation.
- Fitness was not clearly linked to the balance of different cytotoxic T cell subgroups in the blood, the amount of their mitochondria (the main cell structures that produce energy), or a lab marker of mitochondrial function.
Cytotoxic T cells are energy-sensitive cells. When they are mostly waiting for a threat, they can stay in a lower-energy state. When they are called into action, they need to switch quickly to a faster energy program so they can make proteins, send signals, multiply, and kill infected or abnormal cells. One important part of that fast response is the use of sugar. Sugar can be broken down quickly, and it also helps provide raw materials cells need when they are growing or producing immune signals. Higher fitness may matter because it reflects a body with greater capacity to move oxygen and fuel through the bloodstream during physical stress. It is also often associated with better handling of sugar as a fuel. Over time, cytotoxic T cells circulating in that environment may become better prepared to use this fast sugar-based route when they are stimulated. Body composition may shape the same system from another direction. Fat tissue is not just stored energy. It also releases chemical signals that interact with immune activity, inflammation, and oxidative stress.
The main limitation is that this was a small cross-sectional study in healthy young adults using lab-based cell measures, so it cannot show that exercise training improves real-world immune protection. Nevertheless, the findings add to our understanding of how regular exercise supports overall health, including immune function. In Episode #99, I was joined by Dr. Kerry Courneya to explore the remarkable science of exercise oncology, including how exercise mobilizes immune cells to help recognize and attack tumors.