Two very interesting pieces of research related to junk food:

First, a study out of Australia found that eating junk food (high fat, sugary foods) actually rewires the brains of rats. After two weeks of eating “cafeteria food” the reward circuitry in the rats’ brains was altered.  The rats sought out the junk food, cared less and less about eating a balanced diet and their sense of satiety was impaired. This behavior in the rats persisted for  after the junk food was withdrawn.  Thus, eating high fat, sugary food is a vicious cycle – the more you eat it, the more you crave it and the less you want to eat other foods. Link to study: http://journal.frontiersin.org/article/10.3389/fpsyg.2014.00852/full

Second, another study found that low-fiber diets kill off large amounts of healthy gut bacteria in mice. Study of gut bacteria, otherwise known as the microbiome, is still in its infancy, but there is evidence that our gut bacteria holds the key to many aspects of health and illness. The researchers reiterated the importance of humans consuming a diverse diet containing ample fiber for the health of our microbiomes.  More info here: https://www.theatlantic.com/science/archive/2016/01/fiber-gut-bacteria-microbiome/423903/