How fat hidden inside our muscles is ‘critical’ to health outcomes

This can be the result of hormonal changes, poor diet, weight loss without maintaining muscle mass or a lack of exercise. Being injured can also affect IMAT.

In the case of an injury, the authors suggest that when the muscle tissue degenerates, it causes dysfunction in a type of cell known as fibro-adipogenic progenitors or FAPs. Instead of supporting muscle repair, FAPs morph into IMAT within the damaged muscles.

“It’s a distinct pathological process, highlighting that muscle health can deteriorate due to factors intrinsic to the muscle itself, independent of the body’s overall fat distribution,” Contreras explains.

And while we start to lose muscle very gradually from our 30s onwards, this process may be accelerated if we spend our days seated (the average worker spends about 10 hours a day sitting, which is about four hours more than experts recommend for good health) and do not meet the minimum exercise guidelines (more than 30 per cent of adults).

“We used to think that muscle loss was [an issue] if you were 60 or older,” says Contreras. “But now we know that everyone could be experiencing this without noticing.”

A hidden problem leading to ‘ghost diseases’

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The invisible changes happening inside our bodies and the lack of immediate impact on our lives have a name, Contreras explains: ghost diseases.

“It’s not like having a heart attack where you wake up that morning, you were feeling awesome, and then you have a heart attack.” Rather, according to Contreras, ghost diseases progress slowly and silently in the background of our lives.

“They don’t really damage you immediately, but they will deteriorate your general body health, and they will, therefore, play a major role once you age,” he says. “And they come in different flavours: hypertension, obesity, cardiovascular disease, metabolic disorder type 2 diabetes.”

The role of IMAT in these ghost diseases is “particularly important” and “poorly recognised”, agrees Associate Professor David Scott of Deakin University’s Institute for Physical Activity and Nutrition.

“We tend to measure only the decline in muscle quantity and not the decline in muscle ‘quality’,” adds Scott, who was not involved with the new paper. “Some research suggests that the amount of IMAT in muscle is a stronger indicator of poor physical function than the amount of muscle itself.”

How to offset IMAT

Given its importance, understanding how to keep IMAT at levels that support our health is key to preventing and treating a range of health conditions. It underscores the role of physical activity, nutrition, and injury prevention and management, says Contreras.

By increasing our lean muscle mass through exercise that includes resistance training and breaking up sitting all day (take the stairs, walk at lunch, move more in any way you can), we can offset muscle loss and IMAT accumulation. Doing this also minimises the likelihood of various conditions, including obesity.

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Our lean muscle, which accounts for 30 to 40 per cent of our body weight if we are healthy, helps us maintain healthy blood sugar and burns more energy than fat.

“It’s so important to recognise the value of skeletal muscles in general, not just for moving, but also for metabolic maintenance,” Contreras says.

Along with regular physical activity, specific interventions to target IMAT include physical therapy to strengthen the specific muscles when injured, learning proper techniques to avoid injuries, and adequate protein intake. These interventions could possibly include nutritional supplements that support muscle repair and reduce inflammation, though there is a lack of good evidence for what works.

As for the theory around how FAPs contribute to excess IMAT, Scott says it opens up questions for future research studies to explore the development of drug treatments to reduce IMAT.

“These would be likely to extend years of healthy life and independence.”

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