Every pocket of fat you can pinch is built from fat cells, known medically as adipocytes, and it can change in only two ways. The cells can hold more, or there can be more or fewer of them. Gain weight and the cells you already have can enlarge. Lose some of the cells themselves, though, and adults do not make new ones in significant numbers to replace them. That second dial, the number of cells, is the one fat freezing is designed to turn.
Fat freezing (medically, cryolipolysis) cools a fold of fat to a point that injures many of the fat cells inside it while sparing the tissue around them. “The fat cells die” is only part of what happens, though. Within a treated area, individual cells meet three different fates, and the skin, nerves and vessels nearby have their own experience of the cold. Follow each of them and you can see what the treatment will do for a given pocket, and what it won’t.
In brief:
- Fat cells are more vulnerable to cold than the water-rich tissue around them.
- Many cells in a treated pocket die in an orderly way and are cleared over about three months. Most survive.
- A rare few respond the opposite way, and the area grows.
Meet the adipocyte: two ways a pocket of fat changes
A fat cell’s job is storage, and that makes it lipid-rich in a way that skin, nerve, muscle and blood vessel cells are not. This difference is the whole basis of the treatment.
It also explains why a stubborn pocket can stay put at a healthy weight. When you eat less or exercise more, your body draws on fat stores across the whole body, in a pattern set by genetics, hormones and age rather than by the area you are working on. Training one muscle group does not reduce the fat over it. Fat freezing takes a different route: rather than asking cells everywhere to release a little of what they hold, it aims to reduce the number of cells in one place. That is why it is a contouring treatment rather than a way to lose weight.
Why cold singles out the fat cell
The fats inside an adipocyte crystallise at temperatures well above the freezing point of water. Cool the tissue carefully and the lipid inside fat cells begins to change while the water-based cells around them carry on. The cooling also reduces blood flow through the fold for the length of the cycle, and when circulation returns, the restored blood supply causes a second round of stress that may damage further fat cells.
The idea grew from doctors noticing, over decades, that cold could injure fat in children’s cheeks, before Dieter Manstein and Rox Anderson at Massachusetts General Hospital turned it into a deliberate technique in 2007.
Fate one: the cells that die quietly
Fat cells pushed past the point of recovery do not burst. They undergo apoptosis, a built-in programme a cell runs to dismantle itself in an orderly way. The alternative, necrosis, is a cell rupturing and spilling its contents into the surrounding tissue, which is how some heat-based treatments work. Reviews of treated tissue describe cryolipolysis as the cleaner of the two, producing a contained inflammatory response rather than widespread tissue damage, scarring or ulceration.
That response is the body noticing cells in trouble. Immune cells gather around the dying adipocytes over the following weeks, and macrophages, the large cells whose job is to engulf and digest debris, do most of the clearing. Within about three months they have dealt with most of the damaged cells, and the fat layer is measurably thinner. How that unfolds week by week, and what you will notice when, is covered in our guide to fat freezing results, week by week.
Fate two: the majority that survive
This is the part most marketing skips. The treatment does not empty a pocket of fat cells; it removes a share of them. Across studies pooled in a systematic review of cryolipolysis, the treated fat layer thinned by 14.7 to 28.5% when measured with callipers and 10.3 to 25.5% when measured by ultrasound. Read the other way, the bulk of the fat layer remains, and the cells in it remain with it.
Several things govern how big the share is. The measurement method is one, which is why the ranges differ. The number of treatments is another: in the same review, two treatments of the outer hip area produced an average reduction of 28.5%, against 19.7% for one. Only the fat that is drawn into and cooled by the applicator is affected, so the size and shape of the pocket matter. The device matters too. The best-studied systems have a large published evidence base; many cheaper devices have not been evaluated to the same standard, may not hold the precise temperatures required, and may lack automatic safety cut-offs.
Fate three: the rare cells that multiply instead
Occasionally the treated area does the opposite of what was intended. In paradoxical adipose hyperplasia (PAH), a firm, clearly outlined swelling develops where the applicator sat. Under the microscope, tissue from these areas shows more fat cells, not fewer, along with fibrous scar tissue. In the terms of this article, the cell count has gone up rather than down. Nobody yet fully understands why.
The FDA notes that PAH develops two to five months after treatment, does not resolve on its own and may need surgery. It is not dangerous to your health, but it is permanent without that correction, usually liposuction. How common it is remains disputed. The manufacturer’s original figure was about 1 in 4,000 treatment cycles, while one single-practice report found 0.72%. The most complete estimate so far, a systematic review pooling 28 studies, put it at 0.22% of patients, about 1 in 455, and concluded that it is more common than manufacturer reports suggested. Rare, then, but not so rare that a consultation can leave it out.
Where a dismantled fat cell ends up
The fat inside a cleared cell does not leave through sweat or urine. Macrophages carry it away through the lymphatic system, the network of vessels that drains fluid and waste from your tissues, and it travels to the liver, where it is metabolised through the same pathways that handle fat from food.
A reasonable worry is whether releasing that fat strains the body. It is released slowly, over weeks, and both animal and human studies that looked for an effect have found no clinically significant change in blood fats or liver function.
The neighbours: skin, nerves and blood vessels
The cells around the fat feel the cold too; they just recover. Tissue studies find the outer and deeper layers of skin, the blood vessels and the muscle fibres structurally intact after treatment. That is not the same as unaffected. Redness, swelling and bruising over the treated area are common and usually settle within days to a few weeks.
Nerves are the neighbours people notice most. A numb or tingly patch is one of the most common after-effects, and in the published studies people recovered normal sensation after about three and a half weeks on average, with a nerve biopsy at three months showing no long-term change. Treatment under the chin can briefly affect the nerves that position the lips and tongue, or cause a feeling of fullness in the throat. Rarer problems the FDA lists include temporary nodules, persistent numbness and, very occasionally, freeze burns to the skin, and it has received reports of hernias in treated areas.
Blood vessels are worth a closer look, because they may be part of the PAH story. One mechanistic study suggests the cold may also cause some loss of vessel and supporting cells, which could contribute to the abnormal tissue remodelling seen in PAH. It is a proposal, not a settled explanation.
What the survivors do if your weight changes
The cells that survive are ordinary fat cells, and they respond to weight gain the way all fat cells do: by enlarging. A treated pocket holds fewer cells than it did, but the remaining ones can fill up, so a stable weight is what protects the result. One long-term follow-up found treated areas still comparatively leaner than untreated ones nine years later, despite changes in body weight, though long-term studies are few.
Nor does removing cells from one pocket move the scales much. A meta-analysis of body contouring outcomes found that cryolipolysis reduces local circumference and fat thickness but does not significantly affect overall body weight, with the benefits more pronounced at short-term follow-up.
Who this isn’t for
The biology sets firm limits. An applicator can only act on fat it can draw in, so the deeper fat around your organs, which carries the real metabolic risk, is out of reach. If that is your concern, the right first conversation is with your GP. For most body areas, the US clearance criteria specify a BMI of 30 or under, and people who need to lose a significant amount of weight are better served by weight management first.
It is also the wrong treatment if your main concern is loose skin, which can look worse once the volume beneath it falls. And it should not be done at all in people with cold-related conditions such as cold urticaria, cryoglobulinaemia, cold agglutinin disease or paroxysmal cold haemoglobinuria, or over a hernia. Raynaud’s, pregnancy, breastfeeding and skin conditions in the area all call for caution.
The evidence has gaps of its own. Many studies are small, much of the research has used a single brand of device, follow-up is usually measured in months rather than years, and the PAH figures are still moving.
Where to go next in this series
If you want to know what you will see and when, read your fat freezing results, week by week. If you are working out how a plan fits your body, read how many fat freezing applicators you need, area by area. Before treatment, what happens at a suitability check explains who it suits and who it doesn’t. After it, the two weeks after fat freezing covers what is normal, and paradoxical adipose hyperplasia explained goes deeper into the rare cells that multiply. For the long view, read whether fat freezing results last, and if you are weighing it against an injectable option, fat freezing or fat dissolving injections compares the two.
VIVO Body Studio offers fat freezing. You can read how we approach it on our fat freezing treatment page.
At a glance
What the fat cell story means for you
Pros
- Cells that die are removed rather than shrunk, and adults do not replace lost fat cells in significant numbers, so the change to that pocket is built to last
- The cold singles out fat cells: tissue studies find the skin, blood vessels and muscle in a treated area structurally intact afterwards
- It acts on one defined pocket, which diet and exercise cannot do, because they draw on fat stores across the whole body
Cons
- Most of the fat cells in a treated pocket survive, and studies measure the fat layer thinning by roughly 10 to 28.5%, depending on how it is measured
- The cells left behind can still fill up if you gain weight, and the treatment does not meaningfully lower the number on the scales
- In a small minority of people, cells in the treated area multiply instead, leaving a firm swelling that does not settle on its own and may need surgery
Your questions
Frequently asked questions
If most of the fat cells survive, is a second session worth it?
It can be. A second treatment on the same area can reduce the fat layer further: in one comparison of the outer hip area, two treatments gave an average reduction of 28.5% against 19.7% with one. Because clearance takes around three months, it makes sense to judge the first result before deciding, and re-treatment is usually spaced at least a month apart.
If fat cells are removed from one area, will other areas store more fat instead?
There is no peer-reviewed evidence that fat freezing pushes fat into untreated areas. What can happen is ordinary weight gain, which enlarges fat cells wherever you have them, including the survivors in the treated pocket. The idea of redistribution may come from confusion with a different condition linked to certain HIV medicines, which works in an entirely different way.
My treated area still feels numb weeks later. Is that normal?
Reduced sensation is one of the most common after-effects and can last from days to several weeks. In the studies that measured it, people recovered normal feeling after about three and a half weeks on average, and nerve biopsies showed no lasting change. The FDA lists persistent numbness as a less common effect, so if it continues well beyond that, or you develop new pain, tell the clinic that treated you.
Are some people more likely to get paradoxical adipose hyperplasia?
Published cases point to a few patterns. In one large multicentre study, men made up 55% of cases despite being a minority of patients treated, abdominal treatment stood out as a risk factor, and most cases involved older device models. None of these lets anyone predict it for an individual, which is why it belongs in every consultation.
