Digestion begins the minute you take a bite of food and ends a day or two later with a trip to the bathroom. The whole process is handled by the same part of your nervous system that controls other body functions that happen automatically, like your heartbeat and breathing.
But over time, high blood sugar can damage the tiny blood vessels and nerves in your body, including your digestive system. A speed-up or slow-down of the process in your intestines could result in diarrhea or constipation. Diabetes medications, certain foods, and related illnesses can cause diarrhea, too.
When diabetes damages the nerves going to your stomach and intestines, they may not be able to move food through normally. This causes constipation, but you can also get alternating bouts of constipation and diarrhea, especially at night.
Misfiring nerves may not contract the muscles that mix and move the stuff in your intestines, so everything slows down. Your colon absorbs more moisture from the waste, which makes your poop harder -- and harder to pass. Constipation that lasts a long time can cause other health problems, such as fecal impaction, a hard lump of poop that blocks your rectum so nothing can get out.
Nerve damage in your large intestine may let fluids move through too fast, or cause problems with absorbing and releasing fluid. If that's the case, your poop could be more watery, and you'll need to go more often and urgently.
Metformin is in medicines many people take for type 2 diabetes. It helps lower your blood glucose and makes your body more sensitive to insulin, but it can also cause nausea and diarrhea when you first start taking it or raise the dose. Those side effects usually go away in a few weeks.
Diarrhea is a possible side effect of other diabetes drugs, too, including:
Eating a lot of sugar-free sweeteners -- like maltitol, mannitol, sorbitol, and xylitol -- can cause diarrhea. They're from a family of compounds called sugar alcohols. Because your body doesn't break them down and absorb them completely, they pull extra water into your intestines.
If you have type 1 diabetes, you are at higher risk for having celiac disease. People with this disorder can't eat gluten (a protein found in grains like wheat, rye, and barley) because it damages the small intestine.
What You Can Do
Talk to your doctor about unpleasant changes in your digestion. They can help you figure out what's going on and how to treat it.
You may be able to manage diarrhea and constipation by having smaller meals more often, eating fiber-rich foods, or taking medicine. Keeping your blood sugar at your target level may help lessen symptoms and stop nerve damage from getting worse.