We certainly believe that God expects His people to keep themselves healthy and fit and to avoid over-eating and other excesses. We do, however, see a fundamental difference between over-eating and using narcotics or other intoxicants or hallucinogens.
Eating food is a normal and healthy aspect of living. To continually over-eat is to go to an extreme in something that is otherwise healthy. Doing drugs is inherently unnatural and unhealthy and distorts a person's ability to thinks straight. God expects us to be alert and in control of our thoughts.
The root cause of both drug use and excessive over-eating is the same. As with all addictive bahaviors, the root cause is the desire to find a pain-relieving relationship. Sadly, such relationships never resolve the core causes of the pain and only perpetuate the need for ever-increasing substance abuse. In other words, a relationship with heroin only provides temporary relief from pain. Once the effects wear off, the need for more of the drug grows stronger and stronger. The same can be true of food once the addictive process gets started.
We are not sure what you mean by the 'average Christian.' All believers are expected to learn self-control, which is described as a fruit of the indwelling Holy Spirit. In the end, all addictions are fundamentally a lack of self-control. To overcome addiction, a person must be honest about his failings and submit himself totally to the Savior. He alone has the power to deliver and to empower self-control.