Here’s something most fitness articles never mention, and it’s part of what separates how I think about body composition from the standard advice. There’s a category of fat that doesn’t sit in your belly and doesn’t sit under your skin. It sits inside your muscles. Two forms of it.
Intermuscular adipose tissue, or IMAT, accumulates between muscle fibers and between muscle groups. On an MRI scan of an older sedentary person’s thigh, you can literally see fatty streaks running through what should be solid muscle — the marbled-steak appearance. The muscle is structurally infiltrated with fat, which reduces the muscles capacity, strength and endurance.
Intramyocellular lipid, or IMCL, sits as tiny fat droplets inside the muscle cells themselves. Athletes have some of this and use it as fuel during exercise. But when someone doesn’t train aerobically and develops insulin resistance, these droplets accumulate beyond what’s useful and contribute to metabolic dysfunction at the cellular level.
Both reduce muscle quality — the ability of muscle to produce force and respond to insulin — independent of how much muscle you have. The InBody tells you quantity. It doesn’t tell you quality.
Here’s the part that surprised researchers when they first looked carefully: an endurance-trained athlete can have MORE fat inside their muscle cells than a sedentary diabetic, and still be far more insulin-sensitive. The scientific literature calls this “the athlete’s paradox.”
The simplest way to put it: the fat inside an athlete’s muscle is fuel ready to burn. The fat inside a sedentary person’s muscle is sludge that’s slowing the engine. Same molecule. Different metabolic context.
Whether intramuscular fat helps you or harms you depends almost entirely on whether the muscle around it is being used.
The practical implication: you don’t fight intramuscular fat by dieting. You fight it by training the muscle that contains it. Resistance training is the single most powerful tool for reducing IMAT. Aerobic exercise is the most powerful tool for keeping IMCL in healthy ranges. Neither one alone is enough.
Next week: A deeper dive into which of the four fitness groups you belong to:
- Group 1 – Thin, Cardio Fit, and Undermuscled
- Group 2 – Strong and Undertrained Aerobically
- Group 3 – Doing Little of either Cardio or Strength
- Group 4 – Doing both well
With some thoughts to your health and fitness priority regardless of with group you identify with the most.
