This is true if you are rehearsing thoughts of self-loathing, negative body image, and obsessing about food.

It is also true if you are rehearsing thoughts of confidence, body appreciation, and mindfulness.

Your brain solidifies neural pathways that are most frequently used, which results in the generation of automatic thoughts.  So if you are constantly scrutinizing your body and the food you eat, this is what your brain will automatically focus on.

The good news is that because of what's been coined as "neuroplasticity" you can change your thought patterns.  A good starting point is mindfulness – the non-judgmental awareness of what you are thinking and feeling in the moment.  It is possible to notice that you are having the thought that you are fat without accepting it as truth.  Just because we think something doesn't mean it's true.  Try observing these kinds of thoughts without latching on to them.  Note that they are there and let them pass on by, like clouds in the sky.

The practice of mindfulness takes just that – practice.  Our "monkey minds" will divert attention to other things, so once you notice this has happened gently redirect your thoughts back to the present. 

An eating disorder is like a "monkey mind" that constantly draws your attention to food and weight/shape.  Spending your days rehearsing this thought pattern is like a dog chasing its tail.  It doesn't get you very far.  

Instead, rehearse mindfulness and self-awareness of the present moment.  Train your brain to think differently about food and your body by practicing different thoughts.  For example, instead of, "I shouldn't eat this cookie because it will make me fat," tell yourself, "It's ok to eat a cookie and enjoy it.  My body can handle it."   Over time your brain will start to naturally generate these thoughts – if you keep practicing them and allow yourself to trust your body enough to believe it. 

No doubt this is a difficult task.  Consider the extensive pruning that has been done on your brain to this point when it comes to thoughts regarding food and body image.  We are constantly bombarded by messages about good/bad foods, weight loss, fat phobia, and so much more.  To counteract a lifetime of these messages will take some work.  But it's worth it to have freedom from obsessing about food and weight.