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Lean woman with prediabetes signs
The Case
The case came up in a New York City practice opened right after a fellowship, where the clinic used an InBody, a scale that estimates body composition through bioimpedance. The woman did not look skinny fat, and her body fat was average. Her InBody showed muscle mass so low that she was close to sarcopenic.
Muscle And Glucose
Muscle is the primary site where the body disposes of glucose, so someone with low muscle mass can show signs of prediabetes even when body fat is not deranged. It is also the site of fatty acid oxidation.
Metabolic Syndrome
The cluster of findings called metabolic syndrome includes high blood pressure, elevated glucose and insulin, lipids out of range, and a larger abdominal circumference. People read it as a body fat problem, yet these are early signs that muscle is unhealthy.
How does a bioimpedance scale estimate muscle?
It sends a weak electrical current through the body. Water-rich lean tissue conducts that current easily, while fat resists it. The device measures the resistance and feeds it into equations calibrated against reference methods such as DEXA. Hydration changes can shift the readings, so the same person can score differently on different days.
Why do some lean people have metabolic problems?
In 1981, Neil Ruderman and colleagues described the metabolically obese, normal-weight person: a normal body mass index alongside insulin resistance and raised triglycerides. Researchers later showed that fat in the liver and around organs tracks with these problems more closely than fat under the skin. Imaging studies led by Jimmy Bell at Imperial College London gave the pattern a nickname, thin outside, fat inside.
Why does muscle shrink with age?
The word sarcopenia was coined by Irwin Rosenberg in 1989, from Greek roots meaning flesh and loss. Muscle mass declines by roughly 3 to 8 percent per decade after age 30, and the pace picks up after 60, according to reviews in the geriatrics literature. Fast-twitch fibers shrink first, and motor neurons that wire muscle fibers die off, leaving some fibers without a nerve supply.
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