I started smoking again (I know, I know), and had forgotten some effects. The first one is that it stimulates me to go the bathroom in the morning. Correlation or causation?
The bit about losing weight is anecdotal to me. Models and ballet/modern dancers seem to be pretty often smokers (I worked with a dance company for a number of years). What’s up with that?
While a common belief that nicotine is an appetite suppressant my understand is the topic has not been studied as rigorously as one might believe. It looks like nicotine has an impact on metabolism (link), and there are many studies that vaguely assert it could be an appetite suppressant. It is also common for people who are battling nicotine addiction and have gone cold turkey will compensate by comfort eating, exacerbating the problem.
Knowing the impact of over eating versus other factors, over eating will almost always be a bigger factor in weight gain than anything else, so I think that is probably why people traditionally gain weigh when they stop smoking. The metabolic impact is notable, but probably pales compared to someone eating 3,000 calories a day where before they were eating 2200.
There is a correlation; people who have a high quantity of nicotine in their system can have constipation problems if they quit, though it will go back to normal over time.
Personal anecdote - Smoking tended stimulated a #2 for me back when I was a smoker. Never had constipation problems when I quit, though.
As far as weight loss, nicotine stimulates the pleasure center in the brain. It makes you feel good. Eating sweets stimulate the same pleasure center. So, smoking can effectively eliminate the desire to snack for the enjoyment of snacking. It won’t make you less hungry, it’ll just make you less likely to eat for the fun of eating.
The Master speaks to answer your first question.