When you have a cold or flu, the doctor (or Dr. Mom) may tell you to drink lots of fluids. But did they ever explain why you need to guzzle all that water and juice? The extra fluid helps prevent dehydration, which can make you feel even worse, and make it harder to get well. If you have a runny nose, a feverish sweat, or vomiting and diarrhea, it may seem obvious why you'd need to drink more to replace those lost fluids. But these aren't the only reasons the body needs extra fluid when you're sick.
If you have a fever, that increase in body temperature increases your metabolism--in other words, all the biochemical reactions that keep you alive speed up. A fever of 101 degrees Fahrenheit increases your body's metabolic rate by about 10%. What difference does that make? Two key ingredients of metabolic reactions are water and oxygen. Increased metabolism uses more water and also makes you breathe faster, to supply the extra oxygen. And guess what? As you breathe faster, you breathe out more moisture, along with more carbon dioxide.
So, even if you don't have symptoms that cause obvious water loss--like a runny nose, sweating, vomiting or diarrhea--a cold or flu can dehydrate you in hidden ways. Just a slight rise in body temperature requires more water for metabolic reactions and breathing. On top of all that, when you're sick you might not feel like eating or drinking as much as usual. That's why Mom keeps reminding you to drink your fluids!