If you are quitting cannabis then you will probably need all the help you can get. While it is not physically addictive it is mentally and habitually, and your body will be used to having it in its system. Thus it is crucial to give yourself a head start with a healthy diet and lifestyle and there are many ways that changing your diet can affect your chances.
First of all you will want to flush all of the toxins out of your system. To do this you should consume a lot of water which will help encourage your body to expel fluid and liquid and which will keep you hydrated. Similarly by eating fibre you can help clear toxins and improve your heart and circulation which may have been damaged by tobacco. Both of these things are also good for your liver which will have been damaged in trying to filter the toxins in your blood. Similarly milk thistle taken as a supplement can do wonders for a wounded liver.
It is also very possible that you will find yourself feeling lethargic and without energy following your marijuana withdrawal. As such it is important to ensure you have lots of high-energy complex carbs in your system as fuel, which will prevent you from crashing and turning to cannabis. One of the best energy foods for this purpose is the humble banana.
Fruit and vegetables in general will be important during this time too in order to keep your immune system high and ensure that you do not leave yourself susceptible to illness during the withdrawal periods.
It is likely during quitting any habit that you find yourself gaining weight as a result. Do not replace marijuana with junk food or you will be swapping one unhealthy behaviour for another.