How are bad breath and diet linked? To what degree is bad breath caused by diet?

Definite link. Bad breath can have various causes, but diet can definitely influence it. Garlic, onions and coffee are well known to cause bad breath. I'm sure there are many other foods and spices that do as well. Not eating regularly can also influence your breath. When you are low in blood sugar but don't eat, your body produces ketones which are exhaled and have a distinctive odor.
Bad . Bad breath (halitosis) is not necessarily caused by diet. It can be, but mostly is attributed to oral hygiene --how clean is your mouth? Here are some things that can help reduce or eliminate bad breath: brush twice daily at least, though after every meal is important. Brush your tongue too. Use a new toothbrush every 2-3 months, floss between all teeth once daily, get regular dental cleanings and exams. Things that a dentist can detect and treat that can make breath smell bad: cavities (tooth decay/caries), dry mouth, infections in your gums. Things that make your mouth less healthy: smoking/using smokeless tobacco, eating sugar frequently throughout the day, taking medications that cause dry mouth without using water or sugarless candy to replace saliva. Extra things you can do to help have a healthy mouth: drink 8 or more glasses of water per day, spaced regularly from when you get up until 4 hours before bed. Use gum or mouthwash that contains xylitol. This ingredient helps reduce bacteria levels in the mouth.