CONSIDERED by many to be the worst of the worst, fructose is the fall guy of sugar.
Anti-sugar campaigners focus on fructose because it is metabolised differently to sucrose (table sugar) and glucose; fructose is broken down by the liver and, the theory goes, overloads it leading to a plethora of health conditions including diabetes, cancer, obesity and non-alcoholic fatty liver disease.
But new research out of the University of Canberra suggests it is precisely the fact that fructose is processed by the liver that makes it a healthier option.
"Conflicting evidence exists on the effects of fructose consumption in people with type 1 and type 2 diabetes mellitus," said the authors of the study published in The American Journal of Clinical Nutrition.
Dr Kerry Mills, the lead author, and her team examined the existing research to see if they could distinguish between the effects of fructose, glucose and sucrose.
Many studies were of rats, whose metabolism is very different to humans. "The researchers often extrapolate the results erroneously from animals to humans," she said.
Additionally, many studies on fructose were "poorly designed", Mills said, and didn't control for differences in the type of sugar. Instead, many gave one group fructose and the other no sugar.
"That would mean that the second group were consuming more calories and more carbohydrate than the first group," she explained. "The researchers often attribute things like weight gain or raised triglycerides to the fructose, but it could have been the extra calories or the extra carbohydrate."
For their study, Dr Mills and her team analysed 58 studies that compared different types of sugar.
They found that, contrary to the findings of some other studies, there is "strong evidence" that using fructose instead of glucose or sucrose, lowers blood sugar and insulin levels. This was particularly the case for those with prediabetes and type 1 and type 2 diabetes.
They reasoned that it takes time for the liver to process fructose so it slows the release of sugar into the bloodstream resulting in less of a spike.
Concerns that this overloads the liver are unfounded, Mills said.
"No, this is the liver's bread and butter," Mills said. "The body sees the fructose as a source of energy, and will use it to top up blood glucose levels, and will store the rest of the energy for later use. This is why eating too much of any carbohydrate will increase your body fat – not just fructose."
Mills reiterated that too much of any type of sugar is not good, and certainly, most of us eat too much.
"Fructose is by no means a health food in itself – like all carbohydrates it contains a lot of energy, and eating it in the absence of other nutrients like vitamins, fibre, minerals, protein etcetera is a waste of calories," she said.
But, fructose shouldn't be sugar's fall guy.
"We would like people to consider the facts and consult an expert when changing their diet," Dr Miller said.
"It would seem that where sugar consumption is unavoidable, it may be healthier for people, especially those with diabetes, to use fructose instead of sucrose or glucose. People should follow the nutrition guidelines, that emphasise higher intakes of vegetables and fruits, whole grains and legumes, and much lower intakes of saturated fats and sugars."
Fructose: The facts
Dr Mills believes their research provides answers to these questions:
1. Does fructose lower postprandial (the time immediately after eating) blood glucose and insulin compared with sucrose or glucose? Yes.
2. Do people with excess body weight, pre-diabetes or diabetes benefit more than people with normal blood glucose? Yes.
3. Does fructose raise postprandial triglycerides more than the same amount of sucrose or glucose? No.
4. Does long-term substitution of fructose for glucose or sucrose lower fasting blood glucose? It does, but not by much.
5. Does long-term substitution of fructose for glucose or sucrose raise triglycerides more than glucose or sucrose? No.
6. Does long-term substitution of fructose for glucose or sucrose raise "bad" cholesterol or lower "good" cholesterol? No.
7. Does long-term substitution of fructose for glucose or sucrose increase or decrease body weight? It lowers body weight slightly.
"So overall, for the outcomes we measured, it is clear that fructose is not responsible for any negative effects, and indeed has a number of positive effects (such as reduced blood glucose and insulin 'spikes' after eating)," Dr Mills said.
"Although as a scientist we never 'prove' anything, I do think our study design (systematic review and meta-analysis) is extremely robust – indeed this is the gold standard for drawing conclusions about the effects of interventions. We have combined the results of all the individual studies that measured these outcomes, thus providing a much more reliable result than any individual study."
Glucose is the body's main source of energy. It is found in potatoes, pasta, whole grain bread, legumes, milk and a variety of vegetables.
Fructose is "fruit sugar", which occurs naturally in fruit (apples, dates, pears, blueberries, lychees and grapes are all high in fructose), honey, maple syrup, some vegetables and soft drinks.
Sucrose is otherwise known as "table sugar" and is equal parts glucose plus fructose. It is a common form of sugar added to many processed foods including cakes, biscuits, lollies, ice cream, chutneys, sauces, drinks and sweetened yoghurts. It also occurs at naturally high levels in some fruits, including mandarins, mango, pineapple, apricots and papaya.
