Lipomas form when fat cells grow faster than normal and don’t die off at the usual rate, creating a soft lump under the skin. The exact trigger varies from person to person, but the causes fall into a few well-understood categories: genetic changes inside fat tissue, metabolic conditions that alter how your body handles fat, and sometimes physical injury. Most lipomas are completely benign, and understanding what drives them can help you make sense of why they appeared and whether you should pay attention to new ones.
How Fat Cells Behave Differently in a Lipoma
Normal fat tissue constantly turns over. Old fat cells die, and stem cells in the tissue produce new ones to replace them. In a lipoma, this balance tips in one direction. Research comparing lipoma tissue to normal fat found a large number of small, newly formed fat cells surrounded by actively dividing stem cells. At the same time, the rate of cell death wasn’t any higher than usual. So the lump grows because new fat cells are being produced at an accelerated pace while old ones stick around on their normal schedule.
This is different from what happens in obesity, where existing fat cells swell larger. Lipoma growth involves the creation of extra cells, not just bigger ones. The molecular signals driving that process in lipoma tissue are distinct from those involved in weight gain, which is why lipomas don’t shrink when you lose weight.
Genetic Changes That Drive Lipoma Growth
The strongest known cause of lipomas is a specific genetic rearrangement inside the fat cells themselves. These aren’t mutations you inherit from your parents in most cases. They’re changes that happen spontaneously in a group of cells during your lifetime.
The most common alteration involves a section of chromosome 12, specifically the region 12q13 to 12q15. Rearrangements here affect a gene called HMGA2, which helps regulate cell growth. When parts of this gene get shuffled around or fused with pieces of other chromosomes, HMGA2 loses its normal controls and drives fat stem cells to keep dividing. The most frequent partner in these fusions is a gene on chromosome 3, but researchers have documented fusions involving chromosomes 1, 2, 4, 5, 9, and 13 as well.
These chromosomal swaps appear to be the sole genetic abnormality in many lipomas, meaning a single rearrangement is enough to start one growing. The cells are otherwise genetically normal, which is part of why lipomas almost never become cancerous.
The Link to Metabolic Health
A large multicenter study comparing over 7,800 adults with lipomas to the general population found a striking pattern. People with lipomas had significantly higher rates of every major metabolic condition tested. Among the lipoma group, 83.8% had abnormal cholesterol or triglyceride levels, compared to 23.8% in the general population. Hypertension affected 64.1% of lipoma patients versus 17.9% of the general public. Type 2 diabetes was present in 38.4% of people with lipomas, compared to 9% in the broader population. Even obesity rates were higher: 25.4% versus 17.7%.
The clustering of these conditions was especially pronounced after age 35. By midlife, having three or more metabolic risk factors at once was common in lipoma patients. The researchers concluded that lipomas may actually serve as visible indicators of underlying metabolic dysfunction, essentially a surface-level sign that something deeper is off with how your body processes and stores fat. This doesn’t mean metabolic problems directly cause lipomas, but the two clearly travel together.
Physical Trauma as a Trigger
Some lipomas develop at the site of a previous injury, sometimes years later. One documented case involved a lipoma that appeared 14 years after a blunt force injury to the same area. The proposed explanation centers on what happens to fat tissue during impact. A hard blow can push fat through the connective tissue layers (called fascia) that normally keep it contained. Once fat herniates outside its usual borders, the body’s healing response floods the area with inflammatory signals and growth factors. These chemical signals may then act as a trigger, pushing local fat stem cells into overdrive.
Post-traumatic lipomas are not the most common type, but they’re well-documented enough that clinicians recognize the connection. If you’ve noticed a soft lump forming in a spot where you took a significant hit, the injury may have set the process in motion.
Hereditary Conditions That Cause Multiple Lipomas
Most people who develop a lipoma get one or two in their lifetime, and the cause is a random genetic event in one patch of fat cells. But some people develop many lipomas, and that can point to an inherited syndrome.
Familial multiple lipomatosis runs in families and causes dozens of lipomas, typically on the arms, legs, and trunk. Cowden syndrome, caused by mutations in the PTEN gene, produces lipomas as one of several features alongside skin growths and an increased risk of certain cancers. Lipomas are considered a minor diagnostic criterion for Cowden syndrome. A related condition called Bannayan-Riley-Ruvalcaba syndrome shares the same PTEN gene mutation and also involves lipoma formation.
Madelung’s disease is a distinct condition worth knowing about. It causes large, symmetric fat deposits around the neck, shoulders, and upper arms that look different from typical lipomas because the fat isn’t enclosed in a capsule. It overwhelmingly affects men between 30 and 60 who have a history of heavy alcohol use. The leading theory is that chronic alcohol consumption impairs the body’s ability to break down fat through normal pathways, leading to uncontrolled fat deposition. Madelung’s disease often appears alongside diabetes, hypertension, hypothyroidism, and liver disease. Reducing alcohol intake won’t shrink the existing fat deposits, but it helps manage the metabolic problems that come with the condition.
When a Lump Might Not Be a Lipoma
Lipomas are overwhelmingly benign, but a rare type of fat-based cancer called liposarcoma can look similar at first glance. The differences are fairly reliable once you know what to look for.
Size is the simplest flag. Lipomas are usually well under 5 centimeters. A fatty lump larger than 10 centimeters on or near the surface, or larger than 5 centimeters deep inside the body, raises suspicion for liposarcoma. Growth speed matters too. Lipomas grow slowly over months or years and then often stop. Liposarcomas tend to enlarge more steadily, especially aggressive forms.
On imaging, a lipoma looks like a uniform blob of fat, identical in density to the fat under your skin. A liposarcoma is more likely to show thick internal walls (greater than 2 millimeters), non-fatty nodules, calcifications, or areas of mixed density. Location also factors in: fatty tumors that arise in the abdomen, pelvis, or deep behind the organs carry a higher index of suspicion than a squishy lump on your forearm.
If you have a fatty lump that’s been stable for years and is smaller than a golf ball, it’s almost certainly a lipoma. A lump that’s growing noticeably, feels firm or irregular, or is deep-seated and larger than 5 centimeters warrants imaging to rule out something else.