Tendons are not fat. They are tough, rope-like bands of connective tissue that anchor muscles to bones, and their primary building material is collagen, not adipose (fat) tissue. By dry weight, 60 to 85% of a healthy tendon is collagen, mostly type I collagen arranged in tightly packed, parallel fibers designed to resist pulling forces. A healthy tendon contains essentially no fat cells.
That said, fat can accumulate inside tendons and the muscles attached to them under certain conditions. This is always a sign that something has gone wrong, whether from aging, injury, metabolic disease, or chronic overuse.
What Healthy Tendons Are Made Of
A tendon’s job is to transmit force from a contracting muscle to a bone, so its structure is built for tensile strength. The dominant material is type I collagen, organized into aligned bundles that function like the fibers in a steel cable. Between these collagen bundles sit a small number of spindle-shaped cells called tenocytes, which maintain the tissue. The remaining components include proteoglycans (molecules that help retain water and resist compression), a few blood vessels, and water. Tendons have a notoriously limited blood supply, which is one reason they heal slowly after injury.
There is no meaningful amount of fat in this picture. Unlike muscle, bone marrow, or skin, healthy tendon tissue does not store energy as fat or use fat cells for insulation. Its structure is lean and fibrous by design.
When Fat Shows Up in Tendons
Fat can infiltrate tendon and surrounding muscle tissue through several pathways, none of them normal.
Aging and overuse. Over time, or with repetitive strain, the organized collagen structure of a tendon can break down. This degeneration sometimes involves a mix of changes: the tissue may become disorganized, stiffen, accumulate calcium deposits, or develop pockets of fatty tissue. These changes often happen together and reflect a tendon that is deteriorating rather than healing properly.
Chronic tendon tears. When a tendon tears and remains unrepaired for months or years, the muscle it was attached to begins to waste away. The body replaces lost muscle fibers with fat and scar tissue. This is especially well-documented in rotator cuff injuries. The American Academy of Orthopaedic Surgeons notes that in patients who manage full-thickness rotator cuff tears without surgery, the tear size, muscle shrinkage, and fatty infiltration may all progress over 5 to 10 years. That fatty replacement of muscle directly impacts whether a surgical repair, if eventually attempted, will succeed.
High cholesterol and metabolic disease. Cholesterol deposits called xanthomas can form inside tendons, most commonly in the Achilles tendon, the tendons below the kneecap, and the tendons on the back of the hand. These yellowish lumps are strongly associated with familial hypercholesterolemia, an inherited condition that causes extremely high LDL cholesterol levels. On imaging, xanthomas appear as areas of fat-like signal that spread apart the normal tendon fibers and enlarge the tendon. Finding a tendon xanthoma is sometimes the clue that leads to a diagnosis of the underlying cholesterol disorder.
Obesity and systemic inflammation. Elevated body fat doesn’t just sit passively. It generates low-level chronic inflammation and raises levels of hormones like leptin, both of which can directly damage tendon structure. High cholesterol is linked to tissue damage in tendons through a combination of disrupted fat metabolism and blood vessel dysfunction. In people with diabetes, sugar molecules can cross-link with collagen fibers, altering their mechanical properties and making tendons stiffer and more vulnerable to injury.
How Fatty Changes Are Detected
Fatty infiltration of the muscles and tendons around an injured joint is typically spotted on MRI, where fat produces a distinctive bright signal on certain image sequences. Ultrasound can also pick up these changes: fat-replaced muscle appears brighter (more echogenic) than normal muscle, and the normal striped pattern of muscle fibers gradually blurs and then disappears as fat accumulates. In one study comparing the two methods, ultrasound correctly identified the degree of fatty infiltration in the vast majority of cases when compared to MRI findings.
Radiologists grade fatty infiltration on a scale from none to severe. These grades matter because they help surgeons predict whether repairing a torn tendon is likely to succeed or whether the surrounding muscle has deteriorated too far.
Can Fatty Infiltration Be Reversed?
This is the difficult reality: once significant fatty infiltration has replaced muscle or tendon tissue, it is largely irreversible. Surgical repair of a torn tendon can partially restore the architecture and function of the muscle-tendon unit, but the structural damage from fat and scar tissue tends to persist. In rotator cuff repairs where the surrounding muscle has already undergone severe fatty replacement, retear rates can reach as high as 94%.
Physical therapy improves symptoms and function in patients with rotator cuff tears, and this benefit holds even in the presence of some fatty changes. But therapy does not reverse the infiltration itself. No approved drug currently prevents or reverses fatty infiltration of tendon and muscle tissue in humans, though early-stage animal research is exploring compounds that may slow the process. In one sheep study, a medication reduced fatty infiltration from roughly 53% to 34% over 16 weeks, but this remains experimental.
The practical takeaway: early intervention matters. The longer a tendon tear or chronic tendinopathy goes unaddressed, the more opportunity fat has to replace functional tissue, and the harder it becomes to restore full strength. Keeping metabolic risk factors like cholesterol, blood sugar, and body weight in check also protects tendon health over the long term.