Watermelon contains carbohydrate and of late carbohydrate has become a dirty word. Of course, some carbohydrate-rich foods, such as breads and other products made with white flour, are bad news. But others, including vegetables, whole grains, beans, and fruits, are a very important part of a diabetic diet. It all depends on how a particular carbohydrate is metabolised in the body.
Although all carbohydrates are ultimately broken down into glucose and other simple sugars, the rate at which this happens varies. Some cause a rapid and dramatic rise in blood sugar levels, while others are digested more slowly and their sugars are released into the bloodstream more gradually. Several factors determine where a food falls in this spectrum, including the type of carbohydrate and amount of fibre it contains, how much it has been processed, how long it is cooked, even how acidic it is.
A means of evaluating this and thereby assigning a set of values to foods is called the glycaemic index. The higher the glycaemic index of a particular food, the faster and more dramatic the rise in blood sugar after eating it. However, because it ignores the amount of carbohydrate in an average serving of a food, it needed a little refining. Enter the glycaemic load. The glycaemic load of a food takes into account both the glycaemic index and the number of carbohydrates per serving, giving us a more reasonable indication of a foods impact on blood sugar.
Watermelon, has a high glycaemic index. However, a typical serving, because so much of it is water, contains very little carbohydrate and thus has a low glycaemic load. Three-quarters of a cup of watermelon balls has fewer than nine grams of carbohydrate.
You would have to eat a heck of a lot of watermelon to have the same impact on your blood sugar that a single slice of bread would have. This means that watermelon, carrots, and some other high-glycaemic index foods that diabetics may previously have shied away from are perfectly acceptable.