Something like this:

http://www.webmd.com/food-recipes/news/20130710/could-artifi...

"After a while, Swithers said, it's like the mouth keeps crying wolf, and the brain and gut stop listening. As a result, when real sugar and real calories come along, the body doesn't respond to them as strongly as it normally might. Calories don't end up making you feel as full as they should. They aren't as rewarding. So you don't get the signals that might stop you from eating when you should."

I think the parent is referring to this: http://www.bbc.com/news/health-36772303

> Artificial sweeteners can boost appetite by activating hunger pathways in the body, scientists have found in animals.

> In fruit flies and mice, the mismatch between sweet taste and fewer calories sends the body into "feed me" mode, the journal Cell Metabolism reports. But UK nutritionists say this does not mean the same is true in humans.

With all the artificial flavoring, there can end up being very little nutrients in the food. So if you're always having sweet but very low nutritional/caloric foods, there's a chance your body will subconsciously learn this.