Obsessive thoughts about food can occur in people of all body sizes. Often it is a component of disordered eating. Obsessive and ruminative thoughts about food usually cause the person significant distress. They want the thoughts to just go away so they can continue on living their life. They want to know how to stop thinking about food all the time.
People who fear gaining weight may spend their day deciding what they will eat next, thinking about where they will get it from, and calculating exactly what is "safe" to eat. They might also spend excessive time reviewing their past meals and evaluating if their food was in line with what they are "allowed to" eat.
People with binge tendencies often have similar thoughts. They might plan where they can get the food they are craving, figure out how to purchase it without others noticing, and mentally arrange where they will eat it privately. Then they might spend hour deliberating over their past eating occasions, looping through thoughts of guilt over their "bad" eating.
These obsessive and ruminative thoughts can make a person miserable! People don't want their lives limited by their mental gymnastics about food. They want to have brain space to think about other things.
There is this wonderful idea in the field of psychology about living in line with your values. That means doing behaviors that you want to be doing, even if they are not comfortable. For people with disordered eating who find that their food thoughts push them away from doing what they value, this can mean doing those things anyway, no matter what the brain decides to hang onto at that time.
As people, we don't really have much control of our thoughts. There is no guaranteed way of how to stop thinking about food all the time. We can made decisions about our actions and behaviors. We can have a thought to recalculate all the carbohydrates we ate today, and then we can get up and go hang out with friends despite those thoughts creeping in.
The goal is not to push away thoughts. Rather to focus your attention on what is important to you. It can be pretty painful sometimes. For people with obsessive tendencies, there is a safety in ruminating in the hopes of a satisfactory mental conclusion. It feels unsafe to not rethink.
In truth, I don't have the magical way to stop obsessing and ruminating about food and thinking about food all the time. But I want to remind you that you can direct your attention to what you really want to be doing--what is really in line with your values. You can go through the behaviors of how you want to live, and leave your mind to do it's thing.