Cachexia (CC) is the name given to weight-loss (mostly muscle, but sometimes also including fat) associated with cancer and a long list of other severe diseases. It's thought to affect as many as 70 million people worldwide.

It's been found to be linked to abnormal increases in certain cytokines (cell-signalling peptide proteins) circulating in the bloodstream. These specific cytokines trigger protein breakdown of the muscle fibres.

The exact mechanisms by which so many severe diseases cause the over-production of the cytokines - and possible other bio-pathways - is not known.

The mechanisms underlying the development of cachexia are not well understood. However, these mechanisms are gradually being elucidated through recent progress using biochemical and biological analyses. For example, the involvement of proteolysis-inducing factor (PIF), which is released from tumors and reflects abnormalities of the
neuroendocrine system, is gradually being revealed. The activation of inflammatory cytokines resulting from the interactions between the cancer cells and host cells varies."

Anticancer Research, 43, 511-521, 2023

Note: Cachexia is a separate syndrome from Sarcopeniaplugin-autotooltip__plain plugin-autotooltip_bigSarcopenia

"Sarcopenia, the decline in skeletal muscle mass and function without any underlying disease, is a major contributor to physical disability, poor quality of life, and death among the elderly [...] 40%–50% of individuals over 80 years of age suffer from this loss of muscle mass and function [ - which isn't triggered by other underlying diseases. The symptoms appear substantially similar though, and the two syndromes often occur together.