There is no scientific evidence that these gels work. From a site reviewing a Hoodia weight loss gel:
There is not much actual evidence that the Hoodia Slender Gel actually works. However, there seems to be quite a bit of sales copy pointing to the fact that this product may be the very best, lowest priced, most successful, and the strongest.
However, not much evidence is offered to this fact. The hugest problem is that there is not yet much evidence that that hoodia Functions trans-dermally (meaning, that it absorbs through the skin).
So save your funds for any Hoodia solution that is established to assist suppress your appetite and shed pounds. Don’t waste it on fad items. It is possible that Slender Gel could be one of those fad items. (Source)
From the Shiseido site, on the "Body Creator Abdomen Toning Gel", if you check the "Technology" tab, you'll find a lot of fluff, and an extremely short list of ingredients:
Contains Caffeine, Houttuynia Cordata Extract, Red Pepper Extract, and Menthol [...] (Source)
- Caffeine, as we all know, is a stimulant. Caffeine is absorbed through the skin. (Source)
- Houttuynia Cordata Extract is an anti-inflammatory. (Source) It is also an anti-oxidant. (Source)
- Red Peppers are also an anti-oxidant, and also have anti-bacterial properties. (Source)
- Menthol has a wide variety of useful properties, most affecting the skin (anti-inflammatory, anesthetic, etc), but also has a few others, including: decongestant, strengthening the heart muscles, reducing muscle and joint inflammation. It has many other uses, but none are associated with weight loss. (Source)
While it is possible that the caffeine in the gel may be able to carry a portion of the houttuynia cordata and red peppers through the skin and into the subcutaneous layer, those elements will effectively be absorbed into the bloodstream and carried and spread throughout the body.
The same is true of liquid nicotine, if you spill it on your leg (in sufficient quantity) it will kill you. Certainly, it absorbs locally, but it doesn't make your leg fall off. Such items as the red pepper and the menthol will be more noticeable at the localized area because of their extreme effects on the nerve endings and their diminished effects as they are spread thinner through the bloodstream.
My advice would be to test this in theory if you really want to see if it works. Simple method: take yesterday's coffee that was left on the burner overnight, mix it into some menthol rub, add some finely ground red peppers (they will extract into the caffeine/menthol suspension), and you'll have to buy some houttuynia cordata extract. Then rub this concoction on your skin. It will sting and tingle a little bit for a few minutes.
The final question is: Will it stimulate caloric burn? In order to achieve weight loss, one must burn more calories than they consume. The body's ability to absorb this mixture will require metabolization within the internal organs of the substance absorbed through the skin. This is also true if the substance were ingested. I would say that given the amount of the mixture one might use, it is unlikely that it would stimulate the metabolism enough to make any detectable change on any sensitivity of equipment used for such testing.
Even from the review you linked in comments on your question:
But then again like many other toning gel, this cannot give you your desired results if you won’t cut your calorie intake.
In summary, you could acquire the same materials that these gels are primarily comprised of for a fraction of the cost of the gels, and the bottom line is that they will have negligible effects on any stimulation of weight or fat loss. There is no scientific data to support any effect of weight loss, fat loss, or metabolic stimulation from the use of these gels. Simply put, no, they have not been shown to be effective.