Fast food plays a role in obesity in various different ways. The most important way in which fast food contributes to obesity is by employing ingredients that are unhealthy or that promote weight gain. These are used for a variety or reasons, such as allowing food to be cheaper, to last longer or to be more addictive.
Another problem with fast food is that of portions. Sometimes, fast food comes in portions that are way too large for most people's dietary requirements. This leads people to overeat and consume more calories than necessary.
Finally, a problem with fast food is its availability. For many people, buying fast food is appealing because of its convenience, and the fact that it is easily available everywhere, which means that they tend to eat them more often than they should.
All of these are important reasons why fast food plays a role in obesity.