There are numerous, inter-connected factors that likely contribute to the loss of muscle mass and function with age, some key ones include:
Reduced physical activity
A major cause of sarcopenia is the simple fact that, as we age, we tend to do less physical activity. Even short periods (2-3 weeks) of reduced daily steps lead to declines in muscle mass in older adults . These periods of reduced activity can occur relatively often (e.g. due to injury/a cold/minor illness, the beast from the East!) and are difficult for older adults to fully recover from .
Compared to younger adults, older adults require more protein in their diets (found in foods like milk, yogurt, fish, eggs, meat, beans, nuts) and not eating enough protein can contribute to muscle loss. Other contributors include not eating enough food (for example due to poor appetite) and vitamin D deficiency .
Imbalance between muscle building and muscle breakdown
Our muscles are constantly undergoing cycles of building and breakdown. When rates of muscle building and muscle breakdown are equal, which is the case in younger adults, muscle mass remains stable. In older adults, however, rates of muscle building are blunted, especially in response to “muscle building triggers” like eating protein-rich foods or performing exercise . This means that the balance between building and breakdown is tipped towards less building and more breakdown.
Changes to nerves and muscles
An important cause of strength loss is age-related changes to nerves and muscles. As we get older we begin to lose nerve cells that are responsible for carrying messages to our muscles to tell them to contract. There is also a decline the number and size of our muscle cells (called muscle fibres), especially the “fast” type of muscle fibres. The “fast” type of muscle fibres generate more force than the “slow” type of muscle fibres .
As we age the levels of a variety hormones change in our bodies. In particular, the decline in the hormone testosterone that occurs with age, especially in males, is thought to contribute to muscle and strength loss . This may explain why men tend to lose muscle faster than women as they age.
Older adults tend to have slightly increased levels of inflammatory chemical messengers in their blood. This state of chronic low-grade inflammation may play a role in age-related muscle loss .
Fat infiltration into muscle
Aging is associated an increase in fat accumulation inside our muscles. Fat infiltration has been linked to reduced muscle mass and quality, as well as reduced physical performance among older adults .
This image shows the inside of a thigh from a young man (left) and older man (right). The dark colour shows muscle and the light colour shows fat. The older man has less muscle in his thigh and more fat, both surrounding the muscle as well as inside the muscle .
Reactive oxygen species (ROS), otherwise known as free radicals, are naturally formed inside our body cells, including our muscle cells. These ROS play an important role in our bodies provided their levels are kept under control. In order to control the ROS levels, our cells have inbuilt anti-oxidant mechanisms that scavenge and quench the ROS. However, in older adults, there is increased production of ROS and the inbuilt anti-oxidant mechanisms are not as effective at dealing with the ROS. This can lead to excessive ROS levels and result in damage inside the muscle cells .