Anything ingested can have an effect on the vocal cords. Even medications such as ibuprofen and aspirin can have a significant effect on the voice and even increase the risk of a hemorrhage on the vocal fold. Some irritants to look out for include antihistamines and diuretics (which are very drying so more water intake is crucial), dairy products (which cause excessive mucous production), drinks with caffeine and alcohol (which dry out the vocal cords), nuts (which can cause anything from excessive mucus production to allergic reactions involving the throat), hormones, allergens and pollutants (such as dust or mold which cause several negative effects on the voice) and fatigue.
Lack of sleep is a huge issue when touring, so singers need to keep their immune systems up. Joss Stone does this by drinking a tea with cayenne pepper, a bit of honey and lemon. She says she eats less before performances to avoid bloating and doesn't drink alcohol while touring.
The best thing a singer can do for his voice is to drink a lot of water. Avoiding foods and drinks that cause excess mucus or acid are also key to smooth singing. A healthy diet is also extremely important to a singer's health and overall energy. The healthier the singer is, the healthier the voice will be and the more energy he will have to sustain notes and maintain the stamina for performances. Sleep is also extremely important to a singer's vocal health and appearance and should not be taken for granted.
Excessive mucus while singing or when you wake up is a sign to pay close attention to your diet. If you are eating or drinking dairy products, it may be in your voice's favor to eliminate them completely. Some people can handle a bit of milk in their tea, but if you notice a difference in your voice, so will everyone else. Also pay attention to hoarseness or a "gritty" sound as this can mean your voice is strained and may need some rest and lots of water. Acid reflux can be a major issue for many singers because proper singing and vocal support involves "supporting the tone," which works against the esophageal sphincter, causing stomach contents to be pushed upwards toward the diaphragm. Many singers will not eat before performing because it interferes with breath support; although many performances are late at night, eating right before sleeping is a bad habit that will cause acid reflux overnight into the next morning. Spicy foods, fried foods, acidic foods (such as citrus fruits or even coffee) and MSG can all cause acid reflux so it is best to avoid them completely. If you suspect acid reflux is the cause of too much mucus or painful irritation while singing, you should see a doctor or gastrointenstinologist as soon as possible; your doctor may prescribe some medication to correct or alleviate the situation.