The sensation of cool and hot after ingesting certain foods or substances really isn’t an illusion. However, the cause is more biological than it is chemical. This is the reason some people feel varying degrees of coolness or heat.
Mint has long been used as a refrigerant. That is, it physically produces cooling. One of the active ingredients that produces this reaction is menthol. When a chemical such as this is consumed or even rubbed on, it tends to constrict the blood vessels and slow blood circulation. If blood flow is restricted, the result of not having the additional heat of the blood is to feel coolness.
Foods that are considered ‘hot’ can have the opposite effect, however the heat can also be brought on by a slightly different body reaction. An example is a substance called capsaicin, which is what gives peppers their ‘bite’. Capsaicin dilates the blood vessels, which would be the opposite of what menthol does, however there is also some indication that it also changes the body’s temperature regulation. (http://www.botanical.com/products/learn/c/cayenne-chili-p.html)
It goes a step beyond this, however. It also simulates damage to the body that would normally result in a reactive response of elevated body temperature. No actual body damage usually exists, but the body reacts to the substance in a way to protect itself from the perceived damage, giving the feeling of heat. This property also causes the body to release endorphins, which are natural pain killers in the body.
Since capsaicin also acts by absorption, in some cases it is used in rubs, such as for relief of arthritic pain.
Herbalists have used both substances for a very long time for treatment of specific pain. Other hot foods, though, cause a different reactive response due to the action of specific chemicals they contain. Examples include garlic and onion, which some people feel are hot. In this case, however, the result of the heat is more likely to be a consequence of expanding blood flow and a greater amount of heat because of it.
The chemicals contained in menthol are the cause of the reactive response that cause it to feel cool, while the substances in spicy food are responsible for the feeling of heat. In the latter case, it is more difficult to explain, because spicy isn’t always the same thing as ‘hot’. Almost certainly, chemicals are the root cause of any that are perceived as hot, but the chemicals involved can be quite varied. It would be unfair to state that it is due to a specific chemical, as in the case of menthol.
Sources:
Botanical.com
http://www.3dchem.com/molecules.asp?ID=105
