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CYCLING PERFORMANCE TIPS

  Last updated: 10/21/201

Fructose


Fructose (or fruit sugar) is a monosaccharide (a simple one molecule carbohydrate). Link fructose to a another monosaccharide, glucose, and you now have sucrose (a disaccharide or 2 sugar molecule). Honey, fruits, berries, and melons are high in fructose, usually stored as sucrose.

Another sugar you will often encounter in discussions of energy and sport drinks is maltodextrin. Maltodextrin consists of multiple glucose molecules connected in chains of variable length. It is a complex - versus simple - carbohydrate.

Sucrose is split into glucose and fructose in the small intestine. The glucose is readily absorbed into the blood stream and then transported to cells throughout the body. Over 90% of the fructose molecules are converted to glucose IN the small intestine itself before they too are absorbed. However a small amount of fructose will still be absorbed unchanged.

All the blood draining the small intestine passes through the liver before entering the general circulation. Almost all the fructose that was absorbed unchanged is removed by the liver, while the vast majority of glucose continues onward to be absorbed directly by the peripheral tissues. This difference has a significant impact on energy metabolism in the athlete.

The small intestinal absorption rate of fructose is limited and any excess passes down the small intestine and into the colon where it is metabolized by colonic bacteria (the microbiome) and can lead to gaseousness and bloating. The efficiency of intestinal fructose absorption varies widely between individuals with half the population unable to completely absorb a 25 g fructose load.

It was originally speculated that fructose would be an ideal carbohydrate for an athlete. It is absorbed by a different (from glucose) enzymatic pathway in the small bowel and might be useful as an option to increase the total carbohydrate Calories that could be packed into a sports drink. And as it did not stimulate insulin secretion (it has the lowest Glycemic Index (G.I. = 19) of all the natural sugars, it might be helpful in avoiding the hypoglycemic rebound seen with glucose based drinks.

However we now know that fructose can be toxic to the liver. Its absorbtion stimulates an elevation in the blood level of the protein fibroblast growth factor 21 (FGF21) which might play a significant role in the development of scarring in "fatty liver". In one study, livers of rats on a high fructose diet looked much like the livers of alcoholics.

And the presence of fructose induced fatty liver impairs the normal response to insulin increasing the risk of diabetes (metabolic syndrome).

The use of fructose as an alternative sugar (it is fructose corn syrup that sweetens colas, for example) appears to have definite 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. So any fructose that is destined to support athletic activity would first need to be converted into glucose in the liver. These additional metabolic steps make fructose less efficient than glucose as an energy source for the muscle.

FRUCTOSE IN AEROBIC EVENTS?

Is there a role for fructose in aerobic events?? Does it have any advantages over glucose and maltodextrin alone that would justify its possible risks? What does the science tell us?

Pre Exercise

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.

During Exercise

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.

But an occasional study has suggested that although fructose ALONE has no advantages to glucose ALONE, there maybe a benefit when it is used in combination with glucose in a sports drink. You can read more on those studies in the section on sport drinks.

Post Exercise

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.)

The bottom line

Based on the facts that glucose would be my pick as the preferred carbohydrate energy supplement.

The public health impact of fructose may be huge. Corn syrup (fructose) as a food supplement increased tremendously after WWII. And obesity, fatty liver, and diabetes follwed in lock step. FRom the science presented in prior paragraphs, it's hard to escape the conclusion that fructose is playing a significant, or least major contributing, role.

Thus the only reason I can see to include any fructose in my daily diet is in the form of whole fruit where the fiber appears to dampen the negative fructose effects. Dr. Mirkin makes a good argument as to why that may be the case.

FRUCTOSE IN SPORTS DRINKS - PROBABLY A BAD IDEA

Epidemiologic studies have suggested a direct association with the increasing use of fructose sweeteners beginning after WWII (the 1950s and beyond). And now we have a study demonstrating fairly conclusively that fructose activates a liver enzyme which in turn causes a) resistance in the liver to the effects of insulin and b) increases the production of additional glucose from the liver cells - a positive feedback loop - in essence a prediabetic state.

How does this relate to me as a cyclist?



Q. .....I will be rowing in a sprint race of 500 meters, lasting about 1 to 1.5 minutes, followed by a short rest, perhaps 30 minutes, then another same length race as a final. Since I and my teammates will be anaerobic throughout each race, I wonder if it would it be helpful to drink a bunch of glucose soon before and/or in between the races. Would absorption be fast enough to have any impact on utilization at the cellular level? - DB

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.


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