Why is inactivity so destructive to health?
To explain why smoking causes harm, it is easy to conjure a picture of tar and other chemicals doing damage to the lungs. It is perhaps less intuitive why the impacts of inactivity are similarly enormous. Koalas, after all, expend little energy, but do not become obese or diabetic.
A 2011 paper by Audrey Bergouignan et al. makes a helpful contribution to understanding the causal steps by which not getting exercise creates so many problems. The authors stitch together evidence to identify feedback loops that were helpful when humans lived the endurance lifestyle of a hunter gatherer, expending more than 2,600 calories per day, but which become harmful when we do not move. The following is a brief summary, drawing on theirs and other work.
1. Muscles save glucose for the brain
The brain’s main food is glucose, the form sugar takes in the blood. Our muscles, however, are happy to eat fat, protein, or glucose, whichever is most convenient. Sensibly, therefore, our muscles burn very little glucose except when they are doing work and really need it.
That system made sense for our ancestors, who had no choice but to run, walk, and tug on roots daily. If the body needed to burn glucose somewhere, there were always muscles being active, ready to burn it. When all muscles are turned off, however, their generous propensity to leave food for the brain causes problems.
2. While inactive muscles resist taking glucose, fat cells are happy to
When people eat large amounts of sugar, glucose levels spike in the blood. The body must reduce the level of glucose quickly, because in excessive quantities, glucose can corrode nerve fibres, including in the brain, which is as destructive as it sounds. The usual method is simple: the pancreas releases insulin, and insulin tells fat and muscle cells to absorb the glucose. Later, when glucose and insulin levels drop, fat cells release it back into the blood, and active muscles happily make use of it.
When absolutely no muscle is active, however, one of the primary places where insulin wants to put all that glucose is unavailable. The pancreas responds by releasing far more insulin to convince recalcitrant muscle cells to accept the glucose anyways.
Fat cells, therefore, get hit with an amplified signal from insulin to absorb far more glucose, and also to release less of it back into the blood. As a result, when glucose levels eventually drop, the remaining insulin discourages the body from using its own fat stores. To get energy, the body perversely needs to seek an outside source, and becomes hungry.
4. Muscle cells become better at burning glucose, but less able to burn fat
After long periods of being asked to burn high quantities of glucose, muscles begin to switch from slow-twitch fibres, which burn more fat, to fast-twitch fibres, which burn more glucose. Under normal circumstances, that switch happens if someone does more sprinting or lifting weights than endurance exercise, but in inactive people who eat large quantities of sugar, it appears to happen for the purpose of burning glucose, without the attendant benefits to sprinting.
As a result of this switch to glucose-burning muscle fibres, the body also becomes less able to use fat in the blood for energy, a central proposal of Bergouignan’s paper. When skeletal muscles do not burn sufficient fat, it starts to collect in places it shouldn’t, such as between muscles, in bone marrow, and on the walls of arteries, where it is associated with many adverse health outcomes, such as heart attack and stroke.
5. The overburdened liver passes glucose straight through to the blood
Usually, the liver plays a central role in using and storing excessive glucose. Once, however, the liver itself becomes full of fat — a condition known as fatty liver syndrome — it becomes less able to manage incoming glucose, and passes more of it directly through to the blood, exacerbating the initial problem.
Over time, fat and muscle cells become less responsive to insulin, so even more of the hormone is necessary to attain the same results. Eventually, the pancreas fails and can no longer produce sufficient insulin, a situation we call diabetes. Being physically active helps reduce the impacts of diabetes, and can actually reverse the condition, by helping muscles return to their original role in helping the body burn glucose.
Physical activity’s keystone role in the body
The way our bodies process food is predicated on the assumption that a certain amount of muscles will be used at any given time. During millions of years of evolution, our bodies could assume such a consistent level of physical activity, and so removing it is like removing a structural pillar from a building.
This explanation does not cover all aspects of why inactivity causes harm. It does not explain why sedentary people experience hardened arteries, a higher risk of cancer, and issues with the immune system. Similar, perverse feedback loops involving other hormones are likely involved, especially since skeletal muscles themselves release hormones.
Identifying such causal mechanisms, and expressing them in intuitive terms to the public, may help build long-term support for increasing physical activity, the way people picturing black tar helps build support for tobacco regulation. Being physically active is not just good for one’s health, but foundational.
- Tristan Cleveland
Bergouignan, A., F. Rudwill, C. Simon, and S. Blanc. 2011. “Physical Inactivity as the Culprit of Metabolic Inflexibility: Evidence from Bed-Rest Studies.” Journal of Applied Physiology 111 (4): 1201–10. doi:10.1152/japplphysiol.00698.2011.
Photo credit: Tom Woodward