Incredibly, one in four Americans over age 20 has prediabetes -- and most don't even know it. Being prediabetic means that your blood glucose levels are higher than normal but short of being classified as diabetic levels. Studies show that most people with prediabetes go on to develop type 2 diabetes within 10 years, unless they lose weight and make dietary and exercise changes.

Because prediabetes develops gradually over years, it's often said that there are no obvious symptoms. But it's possible to notice certain warning signs of growing insulin resistance, the inability to process the energy in food properly that's a key aspect of prediabetes, says Beth Reardon, director of nutrition for Duke Integrative Medicine at Duke University.

Paying close attention to such warning signs gives you plenty of time to make changes before the situation progresses to type 2 diabetes, she says.

"These symptoms usually occur in tandem with one another; together they create a bigger picture that says insulin resistance is going on," Reardon says. "Some signs can be measured, some we feel, some we can just see."

If you're experiencing the following signs, you should ask your doctor about an insulin response test to measure your insulin and blood sugar levels. If the tests confirm that your body is starting to have trouble managing its glucose, it may be incentive for you to commit to the diet and exercise changes that can help move you away from the path toward diabetes.

Ready to nap right after a big meal? This is a normal response to an influx of carbs (think of that post-Thanksgiving dinner feeling). But if it happens often, your body may be sending a message that your diet is too diabetes-friendly.

After eating, all carbohydrates -- whether in a doughnut or a carrot -- are broken down into the bloodstream as glucose (blood sugar), the body's main energy source. When the blood containing the glucose hits the pancreas, this organ gets the message to release insulin, a hormone it produces to help the cells throughout the body use glucose. Cells have insulin receptors that allow glucose to enter and either be stored as future energy or used right away.

It's a great system. But a diet that's high in simple carbs like sugar, white flour, and sweet beverages -- especially when consumed in large quantities at one sitting -- overwhelms it. According to Reardon, the cells' insulin receptors eventually stop receiving the insulin, which means they can't take in the glucose. The glucose builds up in the blood while the needy cells don't get any. The pancreas, meanwhile, notes the glucose level is still high in the blood that flows through it, and it pumps out still more insulin in response. Net result: You feel sleepy and may find it hard to think, because your brain and body are depleted until the system rights itself.

"Over time, this cycle can cause someone to become chronically insulin resistant. The body simply can't keep up with the demands that all those simple sugars and fats are placing on it," Reardon says.

What helps: Slow your carb load. Choose more complex carbohydrates, such as whole grains (barley, oats, quinoa, spelt, brown rice), vegetables, and whole fruits (not juices) that the body has to work harder to digest. This means blood sugar stays stable longer. Move around right after eating -- take a 15-minute walk; even washing the dishes helps -- rather than plopping in front of the TV. The activity will help your body begin to process the big glucose intake faster and more efficiently. In fact, a Mayo Clinic study presented at the 2011 American Diabetes Association annual meeting reported blood sugar levels rose only half as much after eating in a group that was moderately active after a meal, compared to a control group that ate, then rested.