Losing muscle as you get older is something that happens to lots of people, but the consequences could be more serious than we realized.
New evidence suggests that pronounced muscle loss in middle-aged and older adults doesn't just increase physical frailty, but may have a significant impact on brain health.
In the International Journal of Food Sciences and Nutrition, scientists found that sarcopenia – an age-related condition characterized by abnormally low muscle mass and strength – was linked to a dramatically higher risk of developing dementia.
Analyzing data from 83 prospective cohort studies following participants with obesity and/or sarcopenia, an international team of researchers found sarcopenia was associated with a 42 percent increased risk of all-cause dementia, which rose to a 48 percent increased risk for middle-aged adults under 65 years of age.
The findings, which also examined the links between dementia and obesity – an association that is generally better understood – show that muscle loss's contribution to brain health may have been overlooked in comparison.
"It's not just about how much you weigh – maintaining muscle strength and function also appears to be important as we age," says co-first author and nutrition researcher Uraiporn Booranasuksakul from Burapha University in Thailand.
"Our findings suggest losing muscle could be an important warning sign for dementia risk."
The association was particularly strong for vascular dementia, with sarcopenia being linked to a 70 percent higher risk of vascular dementia, rising to 74 percent higher risk in people aged 65 and over.
Compared to sarcopenia, obesity's links to dementia risk created a different pattern.
People under 65 with obesity had a 9 percent increased risk of developing all-cause dementia, but those aged 65 or older actually seemed to have a protective effect if they had obesity, being 17 percent less likely to develop dementia than people without obesity.
This phenomenon – called the obesity paradox – doesn't necessarily mean that obesity is actually providing health benefits for older adults that reduce or prevent cognitive decline.
Instead, the paradox might just be a false signal, reflecting body mass index (BMI) measurements in the source studies not adequately distinguishing adiposity from lean mass, or other possible factors.
However, it's also possible that obesity does confer actual neuroprotection, the researchers note.
"A higher BMI may confer a greater metabolic reserve against catabolic processes common in preclinical dementia," the team writes in their paper.
"Adipose tissue also produces estrogen, which may have neuroprotective effects in older adults."
With regards to sarcopenia, given the new study is a meta-analysis of previous observational research, the findings don't demonstrate that muscle loss or sarcopenia actually causes cognitive decline.
As the researchers acknowledge, it's possible that the pathology of neurodegeneration could influence body composition, rather than the other way around.
But there are certainly known biological mechanisms that could explain the associations we're seeing.
"As an endocrine organ, skeletal muscle secretes beneficial myokines such as irisin and brain-derived neurotrophic factor with established neuroprotective, synaptogenic and anti-inflammatory properties," the researchers write.
"Reduced muscle mass and impaired muscle quality may therefore lead to diminished myokine signaling, weakening neuronal resilience and plasticity."
Sarcopenia is also associated with insulin resistance and reduced physical activity, which may impair cerebral blood flow, endothelial function and neurovascular health, which could accelerate damage to the brain's white matter and neurodegeneration overall, the team says.
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While there's still a lot we don't fully understand about how sarcopenia contributes to dementia, it's yet more evidence of how population-level dementia risk is modifiable, and for many people, potentially preventable.
"These findings emphasize the importance of life-course approaches to body composition management for dementia prevention," the researchers write.
"The results support public health strategies focusing on maintaining a healthy body composition and suggest that combined preventative and therapeutic approaches addressing both muscle mass and adiposity may be most effective for reducing dementia risk."
The findings are reported in the International Journal of Food Sciences and Nutrition.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.