Sarcopenia is not caused by one single factor. It is the result of several changes that tend to overlap as you age, and it is often the combination of these that accelerates muscle loss rather than any one issue on its own.

Hormonal changes
Declines in testosterone, oestrogen, growth hormone, and IGF-1 reduce the body’s ability to maintain and rebuild muscle. These hormones play a key role in muscle protein synthesis, recovery, and adaptation to training.

For women, menopause adds another layer, with declining oestrogen affecting muscle mass, strength, and tendon health. This can also increase injury risk and slow recovery between sessions.

These hormonal shifts also affect connective tissue. Tendons become less elastic and slower to adapt, which can increase injury risk and reduce how efficiently you transfer force when you run. This is another reason why maintaining strength becomes important, not just for muscle, but for overall movement quality.

The practical implication is that your body becomes less responsive to the same training stimulus. Sessions that once built strength may now only maintain it, or in some cases, not even do that. Maintaining muscle requires more intentional loading, not less, along with better recovery and nutrition to support it.

Insulin resistance
As you age, your body becomes less efficient at moving glucose and amino acids into muscle. With less muscle mass, there is less space to store glucose, so more remains in circulation. Over time, this creates a cycle of increased insulin production and reduced sensitivity.

For runners, this affects more than just long-term health. It shows up in how well you recover, how effectively you replenish glycogen, and how easily your body can use the nutrients you consume. You may find that the same fuelling strategy no longer supports your training in the same way, and recovery between sessions becomes less consistent.

Low protein intake
Older muscle requires more protein to stimulate growth and repair due to reduced anabolic sensitivity. This means that even if your intake has not changed, it may no longer be enough.

If protein intake is low or inconsistent, muscle protein synthesis is not fully stimulated, and over time this leads to a gradual loss of muscle. This is especially relevant for runners, who often prioritise carbohydrates for performance but may overlook total protein intake across the day.

Spreading protein intake evenly across meals becomes more important, rather than relying on a single larger serving.

Lack of strength training
Running alone does not provide enough stimulus to maintain muscle mass, particularly as you get older. While it supports endurance and cardiovascular fitness, it does not create the level of mechanical load needed to preserve strength.

Without resistance training, the body has little reason to maintain muscle tissue, especially in an environment where hormonal support is already declining. Over time, this leads to reduced strength, poorer running economy, and a higher risk of injury.

Including regular strength work gives your body a reason to hold on to muscle and helps maintain the force production needed for efficient running.

Energy deficits
Consistently under-fuelling makes it harder to maintain muscle, especially when combined with regular training. If your body does not have enough energy available, it will prioritise essential functions over muscle maintenance.

For runners, this often happens unintentionally. High training loads combined with inadequate intake create a long-term energy deficit, which increases muscle breakdown and limits recovery.

What may have worked earlier in life, such as running on lower calories, becomes less effective and more damaging over time.

Visceral fat
Even if you are running regularly, there is a tendency to gain more visceral fat with age, particularly if strength training is limited.

This type of fat is not just stored energy, it is metabolically active. It releases inflammatory markers that interfere with muscle maintenance and insulin sensitivity, making it harder for your body to use nutrients effectively.

Over time, this creates another negative loop. Increased visceral fat worsens insulin resistance, which then impacts muscle repair and growth, accelerating muscle loss.

From a running perspective, this is why staying active is not always enough on its own. Without strength training and proper nutrition, body composition can shift in ways that affect both health and performance.

Reduced responsiveness to training
Your body becomes less sensitive to both exercise and nutrition, meaning you need a more deliberate approach to get the same effect. This is often referred to as anabolic resistance.

This does not mean progress is not possible, but it does mean that consistency, intensity, and nutrition all need to be more aligned. Small gaps in training or fuelling have a bigger impact than they used to.