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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
Food allergies are also known to cause excessive phlegm and certain types of food such as dairy products cause a thickening of mucous. Postnasal drip also commonly causes excess mucus at the back of your nose and throat many times after eating. Excessive, thick phlegm or chest mucus can often be caused by viral or bacterial infections such as influenza, bronchitis and pneumonia, as well as irritants such as those inhaled during smoking.