I wondered about this myself. I've done a VERY LITTLE research on the matter via internet postings. Here are a couple of sources...information that I found.
University of California - Berkeley's Campus
Molecular Neurobiology Manuscripts
In humans, withdrawal symptoms are noted as being: irritability, restlessness, difficulty concentrating, aggressiveness, depression, increased hunger, and cravings. Biological symptoms include: changes in EEG, drop in heart rate and blood pressure, increased circulation, drop in urinary adrenaline, noradrenaline, and cortisol, increased weight, and decreased performance on mental and psychomotor tests6. Most of these symptoms disappear within a few weeks, with the exception of hunger and body weight. Although there is yet not quantifiable data to support this, these symptoms are believed to last a great deal longer (depending on duration of tobacco use), even permanently.
Medical News Today
One of the common fears associated with giving up smoking is that it will lead to uncontrolled weight gain. While it is true that some people can gain 9 kilos during the first year of abstinence, other more encouraging studies have shown that initial gains tend to peak after six months and that many people return to their normal body weight within twelve months of quitting the habit.
Some of the tendency to develop stores of body fat is blamed on the slowing of the metabolism when nicotine is withdrawn from the diet because, like caffeine, nicotine is capable of slightly increasing the metabolic rate. Smoking and nicotine may also have other physiological effects that affect behavioural and sensory mechanisms that in turn affect food intake and trigger off an increased desire to eat, when withdrawn.
The mechanisms have not yet been identified but research does indicate that 70 per cent of the weight gains measured can be accounted for by increased calorie intakes. In one particular study, women were shown to have increased their food and drink consumption by an average of 227 calories per day.
