Many people will experience some fat gain initially after quitting smoking. The nicotine contained within the cigarettes causes your liver to release glycogen, raising your blood sugar levels and suppresses the appetite. Your heart rate is artificially elevated and your metabolism is increased. As soon as you stop smoking, your bodies metabolic rate will lower. As a result, if you eat exactly the same as what you were eating previously you will quite possibly notice some fat gain - as your body is now storing more energy in the form of body fat. Apart from this, after quitting, food taste is more prominent and also many smokers will consume extra to compensate for the lack of cigarettes.
That said, your 12 week challenge is much more than a weight loss challenge. It's about recreating your lifestyle, becoming healthy and feeling great. Quitting smoking should be at the forefront of your priorities.
There was a study published in the American Journal of Physiology Endocrinol metab. 2003. whereby it was determined that smoking could inhibit carbohydrate uptake into muscles - resulting in significantly lower glycogen levels. Low glycogen levels will negatively impact your performance in the gym as your muscles will lack the energy to perform the work required.
Here's a plan of attack. Number 1 - Quit smoking. Now, devise a sound diet and exercise plan for your challenge. Monitor your results, and if you see significant fat gain, adjust your diet and exercise plan accordingly. Exercise will help assist and cravings you get and help relieve stress. Ensure you receive adequate rest - 7-8 hours of sleep every night. Also if you have cravings, a great tool to use is sugarless gum.
Think about the big picture - where will you be at the end of the 12 weeks? No doubt you want to be fit, healthy and happy - without the burden of smoking.