We should be upfront about a decision we made when starting this publication. We do not print fat reduction percentages, satisfaction rates or outcome figures for non-surgical body treatments. Not even to argue with them. This article explains why, and it should also equip you to handle the figures you meet elsewhere.
Where a percentage actually comes from
Typically from a study of this shape. A group of participants is recruited, usually selected for suitability, which often means being close to a stable weight with a defined pinchable deposit. A site is treated. The thickness of the fat layer at that site is measured before and after, using callipers, ultrasound imaging, or occasionally another method. The change is expressed as a proportion of the starting thickness. The proportions across participants are averaged.
That average, with its confidence interval, its participant number, its follow-up period, its device and its protocol, is a legitimate research output. It tells you something real about what happened in that study.
What appears in marketing is the number, without any of it.
Four things the number is not
It is not a percentage of your body fat. It is a proportion of the thickness of one layer at one site. Body fat is distributed across the whole body and includes visceral fat, which no external device reaches at all. The two quantities are not related in any useful way, and a reader hearing a percentage will almost always map it onto the wrong one.
It is not a prediction for you. An average conceals its range. In a group where some people respond markedly and some barely at all, the average sits between the two and describes nobody. That is not a criticism of averages; it is a warning about using one as a personal forecast.
It is not a measure of appearance. Layer thickness at a measured point is not the same as how an area looks in clothes or in a mirror, and the relationship between the two is neither linear nor consistent between people.
It is not necessarily durable. Follow-up in this literature is often short. A change measured weeks after treatment tells you about the immediate response and much less about where things stand a year later, when weight, ageing and everything else has continued.
“Clinically proven to reduce fat in the treated area by a specific percentage.”
What would have to be true
- That the figure describes what a typical person receiving this treatment can expect, rather than the mean of a wide spread.
- That the measurement used corresponds to something the person will notice.
- That the result was obtained with the same device, settings and protocol being offered to you.
- That the follow-up period was long enough for the figure to describe a lasting change.
What is actually established
- Studies in this category do measure real changes in fat layer thickness at treated sites.
- Individual responses within those studies vary widely, and an average conceals that variation completely.
- A percentage of a thin layer at one site is a small absolute quantity, and it is not a percentage of body fat.
- Follow-up periods are often short relative to the timescale over which a body changes.
The number is usually real and usually measuring something other than what a reader thinks. The move that makes it misleading is not fabrication, it is the transfer of a group average into an individual promise, and the loss of every qualification that made it meaningful.
The transfer problem
There is a second issue that is less discussed and arguably more serious: figures do not transfer between devices.
Body treatment devices are sold under generic category names. Cryolipolysis, radiofrequency and ultrasound are categories, not products. Within each category, devices differ in energy delivery, applicator design, duration, temperature control and protocol. A result obtained with one device does not describe another device in the same category, and evidence generated for one is routinely cited in support of another.
This is worth checking explicitly. Ask whether the study behind the number was conducted with the device in the room. If the answer is that it was conducted on the technology, that is an answer.
Funding, and what it does and does not mean
A large proportion of research in this field is funded or conducted by the parties who make and sell the devices. This is normal. Device research is expensive and there is no obvious alternative funder.
It is also relevant to how you read a result. Industry-funded work is not fraudulent, and across many fields it does tend to produce more favourable results than independent work, for reasons that include which studies get run and which get published as much as anything that happens within a study. The reasonable response is not to dismiss it but to weight it, and to notice when a claim rests on nothing else.
Systematic reviews are the right tool here, because they assess a body of evidence rather than presenting the most favourable individual study. The Cochrane Library is the place to look, and the absence of a review on a question is itself informative.
Questions that dissolve a percentage claim
- A percentage of what? Of the fat in that layer at that site, or of body fat, or of the volume of the area? These are radically different quantities.
- Measured how? Callipers, ultrasound imaging, photographs graded by an assessor, or a tape measure? Each has different reliability.
- Averaged across how many people, and what was the range? An average with a wide range is a different thing from a consistent result.
- Measured when? A few weeks after treatment, or a year later? The two answer different questions.
- With which device and which settings? Results from one device do not transfer to another sold under the same category name.
- Funded by whom? Industry funding is normal in device research and it is still relevant to how you read a result.
You will rarely get answers to all six. Asking two of them is usually enough to establish whether the person quoting the number knows where it came from, which is the actual question.
Satisfaction figures, which are worse
Percentages describing patient satisfaction are common in this market and deserve separate scepticism.
Satisfaction is measured by asking people, usually shortly after they have paid a significant sum for something they chose. That context reliably produces favourable answers, for well documented psychological reasons that have nothing to do with whether the treatment worked. Who was asked, when, by whom, in what setting, and what proportion of those treated responded are all determinative and almost never stated.
A satisfaction figure with no denominator is not a statistic, it is a sentence with a number in it. The appropriate response is to ask what proportion of all patients treated in the period were asked, and how many answered.
What we do instead
We describe evidence in shape: how many studies, what size, how long, how measured, controlled or not, funded by whom, and whether the results point in the same direction. That is less satisfying than a number and it is more useful, because it tells you how much weight the finding can carry.
Where a mechanism is established but the size of the effect is not, we say the effect is not well quantified, rather than supplying a quantity. Where nothing reliable exists, we say the claim is not established. That vocabulary is published in our editorial standards and it is deliberately short.
What to ask when you are quoted a figure
Ask what it is a percentage of. Ask how it was measured. Ask what the range was across the people in the study. Ask whether the study used this device. Ask who funded it. Ask what proportion of their own patients they would expect to be at the lower end.
The point is not to win an argument. It is that a practitioner who can answer these questions is a practitioner who has read the underlying material, and that is worth knowing about the person who is going to treat you.