THC “staying in one’s system” is a complex issue. Cannabinoids (THC and other molecules) are found both in the body (endocannabinoids) and in the cannabis plant (phytocannabinoids). Endocannabinoids are always in your system, and phytocannabinoids can remain in the blood stream for months! However, their presence does not necessarily mean they are actively bound to their receptors, and it is this binding that creates the medicinal and psychoactive effects of cannabis. To answer your question, yes, the binding of receptors is influenced by diet and inhalation amongst other things- however, weight isn’t really a factor.
The most important thing to understand is that everyone’s endocannabinoid system is unique and always changing. There is no “one size fits all” answer in holistic medicine, and that is particularly true of cannabis. You can’t look at someone and know how sensitive they will be to THC. In fact, on average, women tend to have higher tolerances yet lower body weight.
Absorption of cannabis will determine how long THC will stay “active” in the system:
-When cannabis is inhaled it immediately binds to receptors in the brain and should be active in 0-15 minutes and last about 2 hours. Interestingly, everyone’s inhale is different, so “one puff” can be a drastically different dose between two relatively similar people!
-When cannabis is eaten it passes through the stomach and liver before being distributed to receptors throughout the body. In this case, cannabinoids become active 60-90 minutes after ingestion and stay bound longer, usually 4-6 hours.
-In the middle, sublingual absorption (dissolving in the mouth) with edibles or tinctures will become active in about 30 minutes, and last 2-4 hours.
Diet only really comes into play with the ingestion of edibles. An edible on an empty stomach will absorb and wear off more quickly because of the body’s digestive response. A person’s everyday diet will also effect the efficiency of their metabolism, and therefore their response to ingested cannabinoids. Another factor to consider with edibles is time of day. An edible absorbed in the evening will metabolize more slowly than one eaten during the day.
THC and other cannabinoids can also bind without a “psychoactive effect”. For example, applied topically, cannabis activates receptors in the skin in 0-15 minutes. There is no “high” associated with this binding, but medically it will still be “active”.