When consumed in normal quantities and with a good dental care regime, sugar is not harmful to the body. However, the WHO advises that we should not take in more than ten percent of our daily energy requirement in the form of sugar. For an energy requirement of 2,000 kcal, that corresponds to around 50 grams or 17 sugar cubes per day.
Too much sugar can lead to tooth decay and obesity. The latter is associated with an increased risk of type 2 diabetes, cardiovascular diseases and certain types of cancer such as bowel cancer. Some scientific studies also indicate that very high and heavily fluctuating blood sugar levels in people with an existing insulin resistance (i.e. the cells no longer respond to the normal actions of the hormone insulin) can cause insulin-producing beta cells in the pancreas to die off. Researchers also suspect that a high fructose intake can lead to a fatty liver, which in turn encourages the onset of type 2 diabetes.
Generally you should take care not to consume too much sugar and also to be aware of hidden sugars in food products. A worrying example is soft drinks containing sugar, including juices. These contain around 10 grams of (natural) sugar per 100 ml. A litre thus provides around 400 kcal, which corresponds to the calorific value of a small meal. But because drinks are not filling, it is easy to take in too much energy through sugary drinks, which is then stored in the body as fat. So it’s better to opt for drinks like mineral water or unsweetened tea. Or if you really need a fruity drink, mix your fruit juice with water yourself.
Gisela Olias of the German Institute of Human Nutrition Research (DIfE) in Potsdam-Rehbrücke answered this question.