Fructose is a naturally occurring sugar that is commonly found in fruits, vegetables and honey. Fructose can be considered a low glycaemic index (GI) carbohydrate with a GI of around 19 and its obvious associations with fruits means this sweet tasting sugar is a great way to add sweetness into a diet without a high calorie content (fruits are for the most part water), not forgetting the fact that fruits are micronutrient dense and typically high in fibre, making them a perfect healthy food for those with a sweet tooth.
Fructose can be classified as a monosaccharide but when it is combined with glucose, another monosaccharide, this creates a disaccharide (unsurprisingly two monosaccharides combined) known as sucrose. The sweet taste of both fructose and sucrose combined with its lower GI than other simple sugars mean that they are often refined and used as sweeteners suitable for diabetics.
Unfortunately, despite these benefits, an understanding of fructose metabolism leads to air caution about including a large amount of fructose or sucrose in the diet, especially from concentrated sources. Fructose and sucrose generate a low GI response because unlike other sugars they need to be metabolised in the liver before entering the blood stream, whereas other carbohydrates when broken down to glucose can enter the blood stream for delivery to tissues directly. This leads to the inhibited insulin release BUT also importantly inhibits another important hormone, leptin. Because insulin and leptin are important in the long-term regulation of energy balance, prolonged consumption of diets high in energy from fructose can lead to either increased caloric intake and/or decreased caloric expenditure potentially contributing to weight gain and obesity and high fructose diets have been associated with the conversion of fructose to fatty acids in the liver which may contribute to liver conditions.
Many processed foods contain sucrose as a sweetener; another common sweetener is high fructose corn syrup (HFCS). HFCS has been linked in some studies with increased risk of obesity when compared to sucrose; however, evidence tends to not be clear cut and other lifestyle factors may be at play including general over-consumption of calories. But for the cautious among you, a diet containing whole food sources of fructose from micronutrient dense sources is recommended and highly unlikely to cause any issues.
In summary, eating a moderate amount of fruit will have multiple health benefits due to the micronutrient profile, low insulin response and due to the relatively low fructose content in the context of the whole food/fruit weight. Although concentrated fructose, sucrose and HCFS have shown some evidence in regard to negatively impacting liver health, this is far from conclusive. Special attention to consumption of these sources should be given to those who are overweight, consuming a large amount of calories and sedentary, with less of a concern for those who maintain healthy body fat levels and exercise regularly.