From a nutritional point of view, we do not actually need pure sugar at all. The liver is quite capable of producing the 250-300 grams of glucose that a healthy adult normally needs per day. Small amounts of glucose along with a meal do not cause an increased rise in blood glucose according to several studies in which a small amount of starch has been exchanged for glucose at a meal. This means that you can add 5 grams (1/5 ounce) of sugar to a meal without risk, for example in the form of ketchup.
Where sugar is an integral part of the meal, it should be balanced by a comparable reduction in carbohydrate, or an appropriate increase in insulin. Dietary sugar does not increase blood glucose more than an equivalent amount of starch. This is great news for people with diabetes as it makes following the food plans much easier. However, sugar eaten between meals affects the blood glucose level much more. Your blood glucose level will rise just as quickly if you eat sweets or white bread (without butter or something on it) in between meals.
The important factor is whether the snack contains fibre or fat (like chocolate-covered biscuits), which delay stomach emptying. Far too many people in the UK, Australia, US, Canada and other countries are now becoming overweight, often seriously so. Therefore, you should be very careful to avoid high fat snacks if you have a weight problem.
The recommendation to decrease the sugar content in food is based on more general factors:
Sugar gives "empty calories", i.e. sugar gives only energy and contains no other nutrients. This energy will cause you to gain weight, while reducing your appetite for more healthy foods.
Sugar is bad for your teeth.
In an American study where children took insulin twice daily, there was no difference in their blood glucose levels when they had a diet with 2% of the carbohydrates as pure glucose (in fruit and bread) compared to 10% (in fruit and bread, cereal and toast with jam for breakfast, chocolate chip cookies with lunch, chocolate for an afternoon snack and chilled milk with dinner). This may be surprising, but can be explained by the fact that all the meals contained both fat and protein. The total carbohydrate content was the same for both types of meal.
It used to be common practice to decrease the carbohydrate content in a diabetes meal plan at all costs. The problem with this approach is that the fat content usually increases instead, and this results in the diet becoming inferior to the diet of many children without diabetes. It is much more important to eat regularly and to adjust the insulin dose according to appetite and the content of carbohydrates in the meal.