Fasting
If your diet regime involves fasting or food abstinence for several hours, you'll be prone to bad breath. Here's why:

Chewing food stimulates our body to produce saliva. Saliva not only helps digest food but also acts as a natural mouthwash, washing away the plaque bacteria that produce foul smelling volatile sulphur compounds, and also dissolving those compounds themselves.

When we diet, we produce far less saliva leading to a dry mouth, which encourages odor causing bacteria to flourish. This is why halitosis is more common in people who miss meals or are dieting.