Albumin
Albumin is the most abundant protein in your blood plasma, made only by your liver. It keeps fluid inside your blood vessels (colloid osmotic pressure) and ferries hormones, fatty acids, calcium, bilirubin, and many drugs. Your serum level reflects your liver's output, your nutrition, inflammation, and any losses through the kidneys or gut. A lower albumin is a strong marker of biological aging. It is consistently linked to muscle loss (sarcopenia), frailty, longer hospital stays, and higher death from all causes. That is why it features in many composite aging indices.
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