CYCLING PERFORMANCE TIPS
Although they are both monosaccharides of approximately equal Caloric content, fructose and glucose are metabolized differently. All blood leaving the small intestine passes through the liver before entering the general circulation. 90% of the fructose absorbed is then removed from the blood draining the small intestine on the first pass of this blood through the liver, while the vast majority of glucose is absorbed directly by the peripheral tissues with only a small amount is taken up by the liver. This difference has a significant impact on energy metabolism in the athlete.
Oral fructose absorption is limited in the small intestine. Any additional fructose then passes down the small intestine and into the colon where it is metabolized by colonic bacteria and may lead to gaseousness (and possibly bloating symptoms). The efficiency of intestinal fructose absorption varies widely across individuals and approximately half the population cannot completely absorb a 25 g fructose load.
Whether used as a sweetener in a drink, or in food, fructose does not stimulate insulin secretion, and as a result has the lowest Glycemic Index (G.I. = 19) of all the natural sugars. As far as we know, the only hormonal response to fructose absorbed is an elevation in the blood level of the protein fibroblast growth factor 21 (FGF21) which may play a significant role in obesity and liver cell fat accumulation- popularly called "fatty liver". In one study, livers of rats on a high fructose diet looked much like the livers of alcoholics suggesting that it can overwhelm the normal metabolic machinery of this organ. It has been suggested that it is the lack of control (by insulin) of the movement of fructose into the liver cells that is a key factor in the development of the medical condition called Metabolic Syndrome. Thus the use of fructose as an alternative sugar (it is fructose corn syrup that sweetens colas, for example) appears to have potential health risks.
While all human cells can metabolize glucose, only a few tissues (e.g., sperm cells and some intestinal cells) can use fructose directly, thus any fructose that is destined to support athletic activity will first need to be converted into glucose in the liver. And these additional metabolic steps make fructose less efficient than glucose as an energy source for the muscle.
Is there a role for fructose in aerobic events?? Does it have any advantages over glucose and maltodextrin alone? we know After conversion to glucose in the liver, we know it can lessen fatigue on a long ride, at an aerobic pace. And it has been suggested that it might be even more useful for that purpose when combined with glucose in sport drinks.
What does the science tell us?
Burelle Y et al (Int J Sport Nutr 1997 Jun;7(2):117-27) looked at the metabolism of glucose versus fructose as a preexercise meal eaten between 180 and 90 min before exercise in 6 subjects. They found that glucose provided more available energy than fructose, and concluded that for a pre-exercise meal, glucose should be favored over fructose.
Although it does not appear to have any advantage as a preexercise carbohydrate, what about the role of fructose as a glycogen sparing drink during exercise? Massicotte D et al (J Appl Physiol 1989 Jan;66(1):179-83) compared the oxidation of 13C-labeled glucose, fructose, and a glucose polymer (1.33 gm/kg)ingested during cycle exercise (120 min, 50% max O2 uptake) in six healthy male subjects. Oxidation of the ingested glucose and glucose polymer (72% and 65 %, respectively, of the 100 gm ingested) was similar and both were metabolized at SIGNIFICANTLY GREATER rate than the exogenous fructose (54%). And, as expected, all 3 lessened glycogen utilization (internal stores were protected) compared with the water control. Thus it appeared that oral fructose supplements alone had no advantage (and perhaps even a disadvantage) as a carbohydrate energy source and glycogen sparing drink when ingested during exercise.
This was confirmed by Gautier JF et al. (J Appl Physiol 1993 May;74(5):2146-54). They measured the metabolic fate of fructose in a carbohydrate drink and also concluded that when ingested repeatedly during moderate intensity prolonged exercise, fructose was metabolically less available than glucose. Glucose is again the favored monosacharide.
Although fructose ALONE has no advantages to glucose ALONE, there have been a few studies that suggested a benefit when used in combination with glucose in a sports drink. Read more about this in the section on sport drinks.
Does fructose offer advantages as a post ride replenishment for your muscle stores? The answer again is no. To quote the authors "Glucose and glucose/fructose (2:1 ratio) solutions, ingested at a rate of 90 g/hour are equally effective at restoring muscle glycogen in exercised muscles during the recovery from exhaustive exercise." (Chocolate milk remains my personal favorite based on cost, taste, and efficacy.)
But I will offer one caveat. Although fructose appears to be harmful when used in large daily doses as a dietary supplement (you can find fructose as an additive in many unexpected places - ketchup for example) there appears to be at least some protective benefit from regular aerobic exercise. This study suggested exercise protective benefits that ".....minimize these fructose-induced metabolic disturbances. ....and decreases susceptibility to cardiovascular risk factors associated with elevated fructose consumption."
Thus the only reason I can see to include any fructose in my daily diet (whole fruit excluded - the fiber appears to dampen the negative fructose effects) is if it happened to be used in a product (a sports bar for example) that I loved and there were no equally tasty alternatives.
FRUCTOSE IN SPORTS DRINKS - PROBABLY A BAD IDEA
In prior posts I've written on my interest in metabolic syndrome - the clinical triad of a) glucose intolerance, b) fat accumulation in the liver, and c) high blood pressure. And with the number of questions I have received over the years about treatment of this clinical condition, I had pulled together my ideas on a diet that made sense to me to include in a treatment program, and if that was the case, it also qualified as a healthy diet approach for all of us. Here's the link.
Why are pre diabetes (and diabetes) so much more common now than 50 years ago? Epidemiologic studies have suggested an association with the increasing use of fructose sweeteners after WWII (the 1950s and beyond). Now we have a study suggesting that fructose specifically activates a liver enzyme that then causes a) resistance in the liver to the effects of insulin as well as b) increases the production of additional glucose from the liver cells - a positive feedback loop. And in essence a prediabetic state.
How does this relate to me as a cyclist?
A.This is a multipart question, so I'll address each part separately.
For aerobic exercise (< 100% VO2max), it appears that fructose in combination with glucose provides an advantage in maximizing metabolism of orally ingested carbohydrate and preserving your internal glycogen stores. This may have minimal impact on a sprint event, but if you were going to be doing multiple events during the day, or in the case of cycling, riding 50 to 100 miles, carbohydrate supplements would delay the time to "bonking" if you were not paying specific attention to replace the Calories you were expending in the event or on the ride.
The data support a 50/50 glucose/fructose mixture as potentially being the most effective at supporting sprint performance. Did this combination improve the anaerobic power production? To quote the authors "although the effect of fructose quantity on overall sprint power was unclear, the metabolic responses were associated with lower perceptions of muscle tiredness and physical exertion, and attenuated fatigue (power slope)......fatigue and the perception of exercise stress and nausea are reduced with moderate-high fructose doses."
My recommendation? There is no down side, so adding fructose to your supplement program is worth a try.
For sprint performance, oral carbohydrate availability is probably not going to be the issue, rather it will be the fact that anaerobic pathways are being utilized and lactic acid build up will need to be addressed. Thus it is not about glucose versus fructose, but other options such as creatine and perhaps Na bicarb and its effect to buffer the lactic acid build up.