History continues to repeat itself with Gut/GI distress remaining the #1 reason for DNF (Did not Finish) outcomes in endurance races, with high-carb consumption being the key driver (read about the Carb-Gut-DNF link here).
Manufacturers continue to re-brand and re-hype fructose maltodextrin mixes and gels, while researchers (1) see no evidence of improved oxidation rates, and the same Gut/GI threats continuing.
So, It’s timely that someone also highlight the metabolic downside of Fructose/Maltodextrin for athletes.
We’ve published in other posts the role that exercise and the GLUT4 transporters play in opening muscle cells to glucose in an insulin independent manner. This is a critical metabolic function, which enables the body to simultaneously oxidize fat and carbohydrate in a highly efficient manner, for maximal endurance performance.
After exercising for ~30-60minutes (intensity drives speed of movement) muscle contractions and muscle contraction by-products trigger GLUT4 transporters to move to the muscle cell edge and open the cell to glucose, WITHOUT insulin. This is why exercise is so highly recommended for treating insulin resistance, and Type 2 diabetes. But for the athlete, it’s important to highlight, that this insulin-independent mode of glucose transport and oxidation, also allows for the simultaneous oxidation of fats. Like any physiology of the human body, this simultaneous oxidation of fat and carbohydrate, can be trained for increased efficiency, resulting in a material glycogen sparing effect to the athlete.
Fructose/Maltodextrin effect on GLUT4
Fructose and Maltodextrin are found in higher amounts in the many foods (examples below) and are commonly consumed by the general consumer and endurance athletes in everyday living and during training.
- sports/energy drinks, gels, hydro-gels,
- nutrition bars,
- fruit juices,
- pop/soft drinks,
- fruits like pears, mango, cherries,
- dried/canned fruits (also in muesli),
- sweetened yoghurts,
- sauces (like ketchup, grill marinades etc.),
- salad dressings,
- and, obviously candies (check your granola bars) are full of it, specially high fructose corn syrup blends.
Being a cheap low-cost carbohydrate, and sweeter than table sugar (sucrose), high fructose corn syrup (hfcs) was heavily used by food manufacturers in the 1970-1990 period, and consumption grew by over 1000%. However in more recent decades consumption has began to drop year on year, on account of growing negative public perceptions.
Besides the waves of research (Google it) on fructose and chronic disease states, there’s increasing references directly showing it’s negative effects on the transport and oxidation of carbohydrates and fats during exercise.
In animal studies (2) exercise training was shown to improve GLUT4 content by over 60%. However, researchers found that by adding fructose or maltodextrin to the diet of trained subjects, GLUT4 improvements were significantly suppressed (~20-30%). In further studies (3), again it was shown that while aerobic training would improve exercise capacity, fructose ingestion impaired multiple markers (GLUT4, mitochondrial biogenesis, protein turnover etc.) of the adaptive response of skeletal muscle to exercise. In human studies (4), a higher fructose diet was shown to markedly inhibit lipolysis, fat oxidation and also increase lactate production. It’s also interesting (5) that the cellular transporter of fructose (GLUT5) is actually very low in the newborn, suggesting that in nature, we’re not metabolically setup to be consuming large amounts of refined fructose in foods and drinks. Final comment, is that fructose (unlike Glucose) does not suppress Ghrelin, the hunger hormone, thereby we blunt the negative feedback signal to stop eating/drinking.
Our concern for athletes taking higher amounts of fructose-maltodextrin would be, as you consume them (in diet and in training) you’re suppressing your fat oxidation efficiency, reducing your glucose transporter efficiency, reducing your ability to spare glycogen, and also losing part of your natural hunger suppressant feedback loop. In other words, as you consume Fructose-Maltodextrin, you could be triggering your body to desire more, versus training your body to oxidize its own energy reserves.
Fueling Alternatives: Without the GLUT4 and Gut distress risks?
At SFuels our approach to aerobic training is to use a medium chain triglyceride (MCT C8) based fuel, with low/no carbohydrate (SFuels Train) to train the body in efficient lipolysis and fatty acid oxidation. Then for high intensity interval training (HITT) and racing, SFuels recommends conducting the first 30-60mins without carbs (more on that here), opening up the GLUT4 transporter channels so that you can optimally oxidize fat and carbohydrate (SFuels Race+) simultaneously for the remainder of the session or race.
Besides using MCTs and electrolytes, SFuels Race+ takes a starch (non-gluten, non-soy), and then applies enzymes to branch the starch, creating a molecule with high molecular weight. This dramatically reduces the risks of gastrointestinal distress risks, as SFuels Race+ carbs empty from the stomach more rapidly than Fructose/Glucose.
Supporting research would show this branched-starch format moves rapidly into the blood stream, and possibly offers better time to exhaustion, than simple glucose. In a 2014 (6), double-blind study, researchers found that there was almost no difference in speed to raise blood glucose levels and the Rate of Perceived exertion levels were similar between branched-starch (HBCD) and maltodextrin over a 60min testing period. In 2015 (7), researchers assessed branched starch, glucose and water, against swimming at various sub-maximal Vo2Max intensities. Time to fatigue was dramatically longer in branched-starch fueled swimmers vs. the glucose fueled swimmers. Blood plasma glucose was maintained higher in the branched-starch swimmers also.
Here’s our Training Fueling Guide to learn more about the practical application of optimizing your performance without the risks of Fructose or Maltodextrin.
Clearly Fast. Go Longer. Team SFuels.
- Carbohydrate Hydrogel Products Do Not Improve Performance or Gastrointestinal Distress During Moderate-Intensity Endurance Exercise. Andy J King, Joshua T Rowe, Louise M Burke. Int J Sport Nutr Exerc Metab. 2020 Jul 23
- Suppression of the GLUT4 adaptive response to exercise in fructose-fed rats. Veeraj Goyaram, Tertius A. Kohn, and Edward O. Ojuka. Am J Physiol Endocrinol Metab. 2014 Feb 1
- Fructose ingestion impairs expression of genes involved in skeletal muscle's adaptive response to aerobic exercise. Natalia Gomes Gonçalves, Stephanie Heffer Cavaletti, Carlos Augusto Pasqualucci, Milton Arruda Martins, Chin Jia Lin. Genes Nutrition. 2017 Dec 8
- A high-fructose diet impairs basal and stress-mediated lipid metabolismin healthy male subjects. Andrew Abdel-Sayed, Christophe Binnert, Kim-Anne Leˆ, Murielle Bortolotti, Philippe Schneiterand Luc Tappy. British Journal of Nutrition 2008.
- The role of fructose transporters in diseases linked to excessive fructose intake. Veronique Douard, Ronaldo P Ferraris. J Physiol. 2013 Jan 1
- Effects of ingesting highly branched cyclic dextrin during endurance exercise on rating of perceived exertion and blood components associated with energy metabolism. Takashi Furuyashiki, Hidenori Tanimoto, Yasuhiro Yokoyama, Yasuyuki Kitaura, Takashi Kuriki & Yoshiharu Shimomura. Bioscience, Biotechnology, and Biochemistry, 2014
- Evaluation of Exercise Performance with the Intake of Highly Branched Cyclic Dextrin in Athletes. Takahisa Shiraki, Takashi Kometani, Kayo Yoshitani, Hiroki Takata, and Takeo Nomura. Food Science and Technology Research, 21 (3), 499_502, 2015