High Protein Low Carb Diet
Dieters following the popular high protein low carb diet are also prone to bad breath. If you're on this type of diet, also called ketogenic diet, an inadequate carbohydrate intake can result in ketosis:

When your body runs low on energy-giving carbohydrates, it first breaks down glucose stored in the muscles and liver in the form of glycogen. But this is only temporary.

After a few hours the body begins to break down its fat stores and the waste products of their metabolism is known as ketones. Bad breath is a side effect of this protein-induced ketosis.

The breath has a distinctive sweet and sickly smell. You can smell this on the breath of anyone who has vigorously worked out, but who has not replenished the body with sufficient carbohydrate.

Also, if your high protein diet is low in fiber and you are suffering from constipation this may be another cause of your bad breath.