Most of the people I talk to seem to think that they’d be healthier if they ate less carbohydrate. Most of them seem convinced that a high-carbohydrate diet makes people fat. They know that if you eat starch, it gets broken down into sugar. They know that when sugar flows into your bloodstream, your pancreas is supposed to release insulin to enable the sugar to enter your cells, where it can be burned for energy. That part's true. However, they think that if you eat a lot of sugar or starch, you’ll somehow wear out your body’s ability to make or respond to insulin and thus you’ll end up diabetic. They couldn’t be more wrong. In reality, a high-carb, low-fat diet cures the most common type of diabetes.
If eating a starchy, low-fat diet made people fat and caused diabetes, then we’d see lots of fat, diabetic people in populations that eat a starchy, low-fat diet. We don’t. Instead, we see that the people of China and Japan, whose diet is based heavily on rice and vegetables, tend to be slim and remarkably free of diabetes and heart disease. We see the same thing in other populations that base their diets on other starchy staples. For example, the indigenous people of Peru eat a diet based heavily on potatoes. The Tarahumara of Mexico eat mainly corn and beans. The people in the New Guinea Highlands eat practically nothing but sweet potatoes. The story is the same wherever we look. In reality, the populations that eat low-fat, starchy, high-fiber diets are thin and healthy. The people who eat lots of fatty animal-based foods are the ones at risk for obesity, diabetes, and heart disease.
If eating a lot of sugar caused diabetes, then the people who eat the most sugar would be more likely than the average person to develop diabetes. On the contrary, a study of nearly 40,000 women age 45 and older in the United States found that the women who were eating the most sugar were no more likely to get diabetes than the ones who were eating the least sugar . The women who were most likely to get diabetes were the ones who were eating the most meat! 
1. Janket SJ, Manson JE, Sesso H, Buring JE, Liu S. A prospective study of sugar intake and risk of type 2 diabetes in women. Diabetes Care 2003;26:1008-1015. http://care.diabetesjournals.org/content/26/4/1008.long
2. Song Y, Manson JE, Buring JE, Liu S. A prospective study of red meat consumption and type 2 diabetes in middle-aged and elderly women: the women's health study. Diabetes Care 2004;27:2108-2115. http://care.diabetesjournals.org/content/27/9/2108.long