What can happen if you eat a high simple carb diet and a sedentary lifestyle?

High carb diets can be both good and bad depending on the circumstances. in this case, because you said "Simple" diet (thanks for being specific!), it would result in superior health and a tremendous amount of weight lost. Vitamin A, C, and calcium are all included in this diet, ultimately, boosting metabolism. You first may experience somewhat of a "withdrawal" if you first start, but the headaches and gaseous tendencies you WILL experience should go away within the two week to a month mark. Visible changes won't be notices until around the three month mark. The process would be much more faster if it wasn't a sedentary lifestyle.

Cheers to medical science!