The Unique Adverse Metabolic Effects of Fructose

By James J. Kenney, PhD, FACN

A recent study suggests a lack of satiety and rewarding effects of fructose. Lead authors Dr. Bettina Wölnerhanssen and Dr. Anne Christin Meyer-Gerspach were concerned about ongoing research suggesting differing insulin levels and other metabolic effects associated with the consumption of high-fructose corn syrup (HFCS). There is evidence that, in large amounts and especially when consumed as a beverage, dietary fructose may pose some unique metabolic challenges for the human body. It is suspected that fructose (especially in beverages) may be promoting the development of various metabolic disorders, including obesity, fatty liver disease, elevated TG levels, type 2 DM, and gout (1).

It is well-known that the rise in blood glucose and insulin levels after a meal promotes satiety. Since dietary fructose has much less impact on blood glucose and insulin levels than the same amount of glucose, the results were perfectly predictable. Indeed, by feeding the sugar solutions via a naso-gastric tube directly into the stomach (thereby bypassing the mouth) the researchers took away another source of satiety that occurs when we eat sweet-tasting foods. Fructose is sweeter than glucose and so the satiety signals from the mouth are greater for fructose, but the tube feeding eliminated the taste of the sugar solutions. Of course, the taste sensations generated from eating whole fruit are much greater than those generated by drinking fruit juice, because the former spend far more time in the mouth where the sugar is tasted for a much longer time.

In addition to the prolongation of the sweet taste, there is also evidence that the fiber content and the chewing required by consuming whole fruit versus fruit juice not only results in the sugar impacting the taste buds far longer than occurs with a sugar-sweetened drink, but may also enhance satiety via satiety signals from the stomach. Solid foods sit in the stomach far longer than drinks and when food is in the stomach longer, this suppresses the release gremlin (the hunger hormone) into the blood from cells in the stomach. The soluble fiber (pectins) and the insoluble fibers found in whole fruits are known to not only delay gastric emptying but also result in additional satiety signals being sent to the brain. Dietary fiber, when passing through the intestines, also prolongs satiety after a meal via a variety of mechanisms that are still being worked out. Some of the prolonged satiety effects of whole fruits may include alteration of the gut microbes in ways that not only help reduce fat storage but also may improve health in other ways.

So while Dr. Meyer-Gerspach suggests that her study indicates the use of HFCS may be promoting weight gain obesity because of reduced satiety and results in a dangerous product that promotes obesity in some unique way, this conclusion would be premature. Why? First off, HFCS is not pure fructose but instead is actually a mixture of fructose and glucose. Indeed, it is well known that there is no meaningful difference between the metabolic effects of sucrose (a disaccharide with one molecule of fructose and one molecule of glucose) and HFCS (which is typically about 55% fructose and 45% glucose). Nor is HFCS any different nutritionally than the fruit juice concentrates (FJC) found in so many “health-food” cookies, cakes, breakfast cereals, etc. Why? The commercial FJC are typically made from apple, pear, and grape juices that have had their nutritional minerals and vitamins removed in processing. Like HFCS, they too have a bit higher ratio of fructose to glucose than the 50-50 mix seen in refined sugar or sucrose. But at least those “health foods” are solid and may contain some fiber from whole grains, nuts, or dried fruits so that their satiety per calorie value is likely still much greater than would occur if one were to consume that same sugar (HFCS, FJC, or sucrose) in a beverage. As bad as a soft drinks sweetened with HFCS likely are for those trying to lose excess weight and keep it off, they are not unique in this regard. Most research suggests the impact of HFCS, sucrose, and FJC on one’s weight would likely be very similar. Indeed, the satiety per calorie ratio of HFCS sweetened drinks are likely little different than the same calories from even fresh squeezed fruit juice. Replacing HFCS drinks with fruit juice would simply make one a better nourished overweight person because of their vitamin and mineral content. There is no data showing fruit juice is more satiating per calorie than HFCS sweetened drinks. So the results of this recent study do not demonstrate that HFCS is likely more fattening than sucrose or commercial FJC because the researchers did not use HFCS, but rather compared pure fructose to glucose solutions. Such data tell us nothing about the relative effects of HFCS-sweetened drinks with a similar fructose and glucose content. Recall also that this study fed just fructose or glucose alone via a feeding tube which likely reduced the satiety effects of both sugars, but especially the fructose. Why? The much greater sweetness of fructose compared to glucose may well have seen its satiety per calorie enhanced more had the two sugar solutions had simply been consumed the old-fashioned way -- by drinking them.

Bottom Line: While there is little credible scientific evidence implicating HFCS as being far worse metabolically than refined sugar, honey, commercial FJC, maple syrup, and other concentrated sweeteners, it does appear that all of these sweeteners may have some unique metabolic effects due to their high fructose content. These adverse metabolic effects appear to increase liver fat and uric acid synthesis. When consumed in relatively large amounts, especially in sweetened beverages, it appears that all fructose-rich sweeteners may promote excessive calorie intake, fatty liver, insulin resistance, dyslipidemia, and eventually increase the risk of gout, type 2 DM, and cardiovascular disease.

References:

  1. Bettina Karin Wölnerhanssen, Anne Christin Meyer-Gerspach, André Schmidt, Nina Zimak, Ralph Peterli, Christoph Beglinger, Stefan Borgwardt . Dissociable Behavioral, Physiological and Neural Effects of Acute Glucose and Fructose Ingestion: A Pilot Study. Plos One, published June 24, 2015 -  doi: 10.1371/journal.pone. 0130280.
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