Why do some people easily draw on their fat reserves, during exercise or a fast, while others run out of steam quickly? The answer lies partly in a discreet quality of mitochondria: their ability to switch fuel.
Two fuels, the right switching
Our cells run on two main fuels: sugar and fat. Knowing how to switch from one to the other depending on the moment, after a meal, during exercise, while fasting, is called metabolic flexibility. It is the work of the mitochondria.
A German study published in October 2026 in the Journal of Cachexia, Sarcopenia and Muscle measured it in fifty adults aged 70 on average. The researchers assessed the respiration of mitochondria in simple blood cells, then how the body burned fat and sugar during a treadmill walk. The result: people whose mitochondria breathed best burned more fat during exercise, had a stronger grip and walked faster.
When sugar runs short, ketones take over
The other side of this flexibility appears during fasting. Deprived of sugar, the liver makes ketone bodies, a backup fuel that the brain knows how to use. A laboratory study published in October 2026 in Cellular and Molecular Neurobiology tested the idea on isolated brain tissue: fed only with ketones, with no sugar at all, this tissue kept functional mitochondria and withstood a severe energy stress.
This is work on animal tissue, far from the situation of a person who fasts, and the authors remain cautious. But it illustrates a now well-established principle: the brain is not condemned to run on sugar alone, it has a backup fuel when the mitochondria know how to use it.
Maintaining the switch
These two works converge: good metabolic flexibility goes hand in hand with mitochondria in good shape, and it can be read right down to walking speed and grip strength. It tends to decline with age and a sedentary life.
The known levers are simple, with no miracle promise: move regularly, vary the intensity of efforts, leave the body moments without sugar intake between meals rather than snacking continuously. These are all signals that train the mitochondria to juggle their fuels. In case of illness or treatment, such changes should be discussed with a doctor.
What to remember
- Metabolic flexibility is knowing how to switch from sugar to fat depending on the moment; it depends on the mitochondria.
- In older adults, better mitochondria go hand in hand with more fat burned during exercise, more strength and faster walking.
- When fasting, the liver produces ketones, a backup fuel the brain can use, which remains demonstrated mainly in the laboratory.
- Moving and spacing out sugar intake maintain this flexibility; no result is guaranteed, and any change should be discussed with a doctor.
Sources
- Li D, Herpich C, Norman K et al. Mitochondrial respiration in peripheral blood cells links to metabolic flexibility and physical performance in ageing. Journal of Cachexia, Sarcopenia and Muscle, 2026. https://doi.org/10.1002/jcsm.70361
- Ricks RL, Nevers DS, Parrish RR et al. β-Hydroxybutyrate maintains energetically demanding neural functions during exogenous glucose deprivation. Cellular and Molecular Neurobiology, 2026. https://doi.org/10.1007/s10571-026-01834-3
