Fructose and metabolic health

Carbohydrates | Sugars | 02 December 2014

Fructose has undergone a lot of scrutiny recently regarding its impact on metabolic indicators of health. We looked at how fructose is metabolised and the current evidence of how this affects health.

What is fructose and where does it come from?

Fructose is the main naturally found sugar in honey and fruits (e.g. dates, raisins, figs, apples, and fruit juices) and in small amounts in some vegetables (e.g. carrots). Fructose is also bound to glucose in table sugar (sucrose) which is half (50%) fructose and half (50%) glucose. Table sugar is used at home, ‘at the table’ and for cooking and baking, and is used as a sweetening agent in the manufacturing of foods and non-alcoholic beverages. Another source of fructose is Glucose-Fructose syrups, which are made from maize and wheat and used as sweeteners in a variety of foods such as jams, preserves and confectionary. The fructose content can range from 5% to 50%. If the fructose makes up more than 50% of the syrup, the name on the ingredient listing should read “Fructose-Glucose Syrup”. Fructose provides the same caloric energy per gram as any other sugar or digestible carbohydrate, i.e. 4 kcal/g.

Fructose metabolism

Ingested fructose is metabolised in the liver to produce mainly glucose (~50%), and minor amounts of glycogen (>17%), lactate (~25%) and the small remainder to fatty acids; the latter via a process called de novo lipogenesis.1 Glucose travels through the bloodstream to all the tissues, and cells transform glucose into energy. Lactate and fatty acids are also energy sources.

In comparison to glucose, fructose provides a lower glycaemic response, as it has a very low Glycaemic Index (GI). Therefore, the consumption of foods in which fructose, replaces glucose, sucrose, or starches, leads to a lower blood glucose rise compared to foods containing only glucose and sucrose. A reduced glycaemic response may be beneficial to people with impaired glucose tolerance (high glucose levels).2 Blood glucose fluctuations are also influenced by the chemical and physical nature of foods/drinks consumed, and by individual factors.

Some studies show, however, that high intakes of fructose may lead to metabolic disturbances. Many of these studies are done in animals, or are short-term overfeeding trials in humans, with levels of fructose much higher than normally consumed (for example 100-150 g pure fructose/day). This approach called hyper-dosing provides energy above normal needs. For example, a recent study found that 7-day overfeeding with high levels of either fructose, glucose, or saturated fat, all increased fat in the liver to the same extent, and that both fructose and glucose overfeeding decreased liver insulin sensitivity (making the liver less sensitive to insulin).3 Many short-term overfeeding trials have also shown that fructose can raise triglycerides (fats in the blood), within the normal range in healthy people.4,5 When fructose replaces other carbohydrates (containing similar levels of energy), it does not appear to cause more weight gain than the other carbohydrates, adversely affect blood pressure, or raise blood triglycerides.6-8 These effects, may therefore not be unique to fructose and may in fact be due to excess energy intake.9 Increased dietary intake from any energy source above energy needs will eventually lead to weight gain, unless balanced by increased physical activity. Obesity, particularly excess abdominal fat, is clearly associated with metabolic disease. Interestingly, the effect of very high fructose intake on blood fat levels was not seen in a study in healthy young men that cycled two times 30-minutes a day, which highlights the benefits of exercise.10

Studies on fructose typically use pure fructose, rather than in combination with glucose as would be ingested within a food or beverage. Further research is clearly needed to determine the consequence of high dietary fructose intake in humans, in the long-term, and the differences between individuals and between different population groups, i.e. overweight/obese.4

Fructose and athletes

Sport drinks are designed to support athletic performance by replacing fluids, salt and carbohydrates lost during physical activity of high intensity or long duration. High concentrations of fructose are slowly absorbed, resulting in reduced plasma volume, and higher occurrence of gastrointestinal distress. However, fructose ingested in small amounts in combination with sucrose and/or glucose does not delay fluid absorption.11 Solutions that contain both glucose and fructose appear to improve sodium and fluid absorption better than those containing either glucose or fructose alone.11,12 The combination of glucose and fructose also increases fructose uptake by the body.12 It has been shown that when athletes consume fructose and glucose in combination, energy is released at relatively high rates and thus beneficial effects on exercise performance and reduced fatigue were observed.13 Athletes often have a higher than normal fructose intake but tend to have less metabolic and cardiovascular disease than sedentary individuals.4

Meatbolic health advice

There is currently little evidence that fructose itself causes metabolic diseases when consumed in amounts consistent with current average dietary habits in Europe.4 There is need for a better understanding of the factors, such as genetics, which may regulate the effect of high fructose intake on health. To protect metabolic health, avoiding excessive energy intake, engaging in regular physical activity, maintaining a healthy body weight and eating a healthy, varied diet is still the best advice. Also do not hesitate to ask your doctor or healthcare professional for precise advice that fits your personal health condition.

References

  1. Tappy L & Le KA (2010). Metabolic effects of fructose and the worldwide increase in obesity. Physiol Rev 90:23-46.
  2. European Food Safety Authority (EFSA) (2011). Scientific opinion on the substantiation of health claims related to fructose and reduction of post-prandial glycaemic responses. EFSA Journal 9(6):2223.
  3. Lecoultre V, Egli L, Carrel G, et al. (2013). Effects of fructose and glucose overfeeding on hepatic insulin sensitivity and intrahepatic lipids in healthy humans. Obesity 21(4):782-785.
  4. Tappy L (2012). Q&A ‘Toxic’ effects of sugar: should we be afraid of fructose. BMC Biology. 10:4.
  5. Silbernagel G, Machann J, Unmuth S, et al. (2011). Effects of 4-week very-high-fructose/glucose diets on insulin sensitivity, visceral fat and intrahepatic lipids: an exploratory trial. British Journal of Nutrition 106(1):79-86.
  6. Sievenpiper JL, de Souza RJ, Mirrahimi A, et al. (2012). Effect of fructose on body weight in controlled feeding trials: a systematic review and meta-analysis. Annals of Internal Medicine 156(4):291-304.
  7. Ha V, Sievenpiper JL, de Souza RJ, et al. (2012). Effect of fructose on blood pressure: a systematic review and meta-analysis of controlled feeding trials. Hypertension 59:787-795.
  8. Wang DD, Sievenpiper JL, de Souza RJ et al. (2014). Effect of fructose on postprandial triglycerides: A systematic review and meta-analysis of controlled feeding trials. Atherosclerosis 232(1):125-133.
  9. Johnston RD, Stephenson MC, Crossland H, et al. (2013). No difference between high-fructose and high-glucose diets on liver triacylglycerol. Gastroenterology 145(5):1016-1025.e2. Doi: 10.1053/j.gastro.2013.07.012.
  10. Egli L, lecoultre V, Thevtaz F, et al. (2013). Exercise prevents fructose-induced hypertriglyceridemia in healthy young subjects. Diabetes 62(7):2259-2265.
  11. Currell K & Jeukendrup AE (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine & Science in Sports & Exercise 40(2):275-281.
  12. Johnson RJ & Murray R (2010). Fructose, exercise and health. Current Sports Medicine Reports 9(4):253-258.
  13. Jeukendrup AE (2013). Multiple transportable carbohydrates and their benefits. Sports Science Exchange 26(108):1-5.