The sugar fructose is found naturally in honey, maple sugar, fruits, berries, some vegetables (e.g., cassava, onions, potatoes, sweet potatoes, sweet corn, etc…) and some grains (e.g., wheat, rice, barley and rye). Table sugar or sucrose is half (50%) fructose and half (50%) glucose and is the most common form of sugar added to foods and non-alcoholic beverages in manufacturing, and at the table, in Australia, and consequently is a major source of fructose in this nation. Pure crystalline fructose is available as a table-top sweetener and is sold in Australian supermarkets under the brand name Fruisana. So called high fructose corn syrups (HFCS) are commonly used as sweetening agents in the United States of America (USA), but are not commonly found in Australian foods or beverages. Despite the name, HFCS are most commonly 55% fructose. Fructose is nearly twice as sweet as sucrose and provides slightly less kilojoules per gram (15.6 kJ/g vs 16.5 kJ/g).
Based on the most recent national dietary intake surveys, 95% of all people living in the USA over 19 years of age consume less than 100 grams (24 teaspoons) a day from all foods and drinks, and 95% of Australian adults (25-64 years of age) consume less than 60 grams (14 teaspoons) of total fructose a day from all sources. The average amount of fructose consumed per person has been increasing in the USA since the 1970’s, primarily through the use of HFCS, and appears to parallel the increase in overweight/obesity in that nation leading some to believe it is one of the major causes. In Australia, however, Australian Bureau of Statistics apparent consumption data indicate sugar consumption peaked just after World War 2 (54 kg per person per year) and reached its lowest point in 1998–1999 (38 kg per person per year). However, rates of overweight/obesity nearly doubled from 37% in the 1980s to 60% in 2000. When considering apparent consumption data it’s important to realise that the average Australian is now significantly older (there are proportionately less children), taller and heavier than they were 30 years ago, so the actual “dose” of sugar per person has likely decreased even further than these crude data suggest.
There are currently no specific recommendations for fructose but there are for sugars in general, with international guidelines recommending we consume less than 25% of energy (kJ) from total sugars. In the last National Nutrition Survey, 95% of Australian adults consumed less than 115 g of total sugars per day, or 19% of total energy, a level well within the international recommendations. The percentage of energy from total sugars remained either the same or decreased from 1983 to 1995, depending on the age group.
A recent review investigated the fate of isotopically labelled fructose as it is metabolised by the human body. It found that nearly half (45%) of all pure fructose consumed is used up within 3-6 hours by the body for energy. If fructose is consumed with glucose (as it typically is in nature), up to 66% of it is used for energy within the same time frame. Roughly a third (29%) to a half (54%) of all fructose consumed is converted to glucose. Less than 1% of fructose appears to be directly converted to blood fat.
Fructose and addiction
It has been suggested that sugars are addictive; however, there are few studies in humans consuming realistic amounts of fructose, sucrose or HFCS in real foods and drinks to support this hypothesis. A recent review of the scientific evidence on sugar addiction concluded that “There is no support from the human literature for the hypothesis that sucrose may be physically addictive or that addiction to sugar plays a role in eating disorders”. Similarly, a recent review of all of the scientific evidence in humans on fructose or HFCS and satiety (feelings of fullness) concluded “On balance, the case for fructose being less satiating than glucose or HFCS being less satiating than sucrose is not compelling.”
Fructose and weight
There have been two reviews of the scientific evidence investigating the effect of fructose on body weight in humans. The first, published in 2008, investigated the effect of consuming between 46 g and 176 g of pure crystalline fructose a day on body weight in 14 trials and determined that no significant effects are seen when adults consume less than 100 g of fructose a day, but that consuming more than 100 g a day of pure fructose may lead to a modest but statistically significant rise in body weight of 0.44 kg a week. The most recent (2012) reviewed 31 trials where participants consumed either pure fructose or non-fructose carbohydrates (e.g., glucose, starch, etc…) in equal amounts and found that fructose had no statistically significant effect on body weight (-0.14 kg) in ideal-weight participants or those with diabetes. However, a small but statistically significant weight loss (-0.55 kg) occurred in the 5 trials on overweight/obese individuals. In 10 high kilojoule trials investigating the effect of a usual diet supplemented with excess energy from high doses of pure added fructose (104-250 g/d or 18% to 97% of energy from pure added fructose) body weight increased modestly overall (0.53 kg). However, the authors concluded that this was likely due to the additional kilojoules, not any unique property of fructose.
Since these reviews were conducted another clinical trial looked at the effect of a low fructose (less than 20 g a day) versus “moderate” (50-70 g a day – high by Australian standards) fructose kilojoule-controlled diet in 107 obese American adults over a 6 week period and found that weight loss was 50% higher in the “moderate” fructose group (4.2 kg) compared to the low fructose group (2.8 kg).
Overall, the evidence suggests that when consumed in normal physiological doses, fructose may in fact help some people to lose weight, consistent with the fact that it has a low GI and slightly less kJs than most other carbohydrates. However, consuming more than 100 g a day of pure fructose on top of a normal diet will lead to modest weight gain, most likely due to the extra energy (1,560+ kJ a day) it provides.
Fructose and blood glucose
There have been two reviews of the scientific evidence investigating the effect of fructose on long-term measures of blood glucose control in humans. The first investigated the effect of consuming between 0 and 90 g of fructose a day on HbA1c (a measure of average blood glucose over 120 days) in 8 trials and found an average 5.7% improvement in HbA1c per 10 g of fructose a day over an average of 10 weeks. The second reviewed 6 trials and found that consuming “catalytic” doses (less than or equal to 36 g a day) of fructose led to significant improvements in HbA1c (-0.4 % points) without adversely affecting fasting insulin, uric acid or triglycerides. Therefore, when consumed in moderate amounts as part of a normal healthy diet, added fructose may help improve blood glucose levels in people with diabetes over the long-term.
Fructose and blood pressure
There has been one review of the evidence from 15 human trials assessing the effect of fructose consumption on blood pressure. It found that swapping equal amounts of fructose for other carbohydrates (e.g., glucose, starch, etc…) significantly decreased diastolic blood pressure (-1.54 mmHg) and mean arterial pressure (-1.16 mmHg) but had no statistically significant effect on systolic blood pressure (-1.10 mmHg). Therefore substitution of fructose for other carbohydrates appears to help decrease blood pressure in humans.
Fructose and triglycerides
There have been two reviews of the scientific evidence investigating the effect of fructose on triglycerides (blood fats) in humans. The first looked at the effect of consuming added fructose on both fasting (60 studies) and post-prandial (25 studies) triglycerides. Consuming 100 g or more of fructose a day significantly increased fasting triglycerides and more than 50 g a day increased post-meal triglycerides. The second reviewed the effect of fructose on fasting triglycerides in 16 trials in people with diabetes. They found that compared to starch, consuming more than 60 g of fructose a day had a triglyceride-raising effect in people with type 2 diabetes. There is therefore good evidence that moderately high doses of added fructose raise triglyceride levels in humans.
Fructose and uric acid
There has been one systematic review of the evidence from 21 controlled feeding trials of ≥ 7 day duration investigating the effect of fructose consumption on uric acid levels in humans. Exchanging fructose for the same amount of other kinds of carbohydrate (glucose, starch, etc…) within regular diets does not affect blood uric acid levels in people with diabetes or those without the condition (average difference = 0.56 mmol/L ). Large amounts of fructose (+35% excess energy (kilojoules)) at extreme doses (213–219 g/d) do significantly increase serum uric acid levels compared to other carbohydrates, however. There is therefore some evidence that very high (unrealistic) levels of pure added fructose raise uric acid levels in humans.
Fructose and fatty liver
A recent study investigated the effect of feeding either a high fructose (25% of kilojoules) or high glucose (25% of kilojoules) diet to 32 healthy but overweight men for 2 weeks. Participants in the high-fructose and high-glucose groups had similar increases in weight (1.0 vs 0.6 kg; P=0.29) and absolute concentration of triglyceride in their livers (1.70% vs 2.05% P =0.73) and blood serum (0.36 vs 0.33 mmol/L; P =0.91), and similar results in biochemical assays of their liver function. The authors concluded that "in the hypercaloric period, both high-fructose and high-glucose diets produced significant increases in these parameters without any significant difference between the 2 groups. This indicates an energy mediated, rather than a specific macronutrient mediated, effect.".
Fructose and physical performance
Fructose is twice as effective at replenishing glycogen stores in our livers compared to equivalent amounts of glucose in cyclists (8.1 g/ hr versus 3.7 g/hr). Therefore, there is some evidence that fructose may provide a performance advantage to athletes during certain forms of exercise.
Added refined sugars like sucrose, fructose and glucose are essentially devoid of nutrients other than kilojoules. It is therefore recommended that we consume no more than 10% of energy from added sugars. For someone consuming 8,700 kJ a day, this is equal to no more than 55 grams, or 13 teaspoons a day. It’s important to note that total not added sugars are listed in a foods nutrition information panel. It is therefore not necessarily appropriate to choose foods with less than 10% (10 g per 100 g) of (total) sugars as this may inadvertently lead to unnecessary exclusion of nutritious foods and drinks like fruits and certain dairy foods from the diet.
The evidence to-date suggests that consuming moderately high (>50 but ≤100 g, or 12 – 24 teaspoons) amounts of pure refined fructose may have an adverse effect on postprandial triglycerides and that consuming large amounts (>100 g, or 24 teaspoons) on top of a regular food and beverage intake will contribute to weight gain and fasting triglyceridaemia. The reality is that few humans eat pure refined fructose, let alone in the amounts required to cause adverse health problems. People should continue to be advised to eat added sugars, including fructose, in moderation.