While all cells can metabolize glucose, only a few tissues (e.g., sperm cells and some intestinal cells) can use fructose directly. Fructose is almost entirely metabolized into glucose in the 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. (more on fructose)

What is it's role in providing carbohydrate fuel for the athlete?

Preexercise

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 PREEXERCISE meal, glucose should be favored over fructose.

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, internal carbohydrate utilization was significantly lower (internal stores were protected) with glucose, glucose polymer, and fructose than with the water control.

Thus it appeared that oral fructose supplements alone had no advantage (and perhaps even a disadvantage) to glucose 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.

Although fructose ALONE has no advantages to glucose ALONE, there was a single study that suggested it was of benefit when used IN COMBINATION with fructose in a sports drink. Adopo E et al (Appl Physiol 1994 Mar;76(3):1014-9) studied the effects of a combination 50 gram fructose/50 gram glucose drink compared to a pure 100 gram glucose supplement. The cumulative amount of ingested carbohydrate metabolized in the combined carbohydrate drink was 21% greater than that observed when 100g of pure glucose alone was used. They speculated that this might be related to differing routes for absorption and metabolism of exogenous glucose and fructose, resulting in less competition for oxidation when a mixture of these two sugars is ingested than when an isocaloric amount of glucose alone was ingested. They concluded that from a practical point of view this data provided experimental support for using mixtures of carbohydrates in the energy supplements for endurance athletes. This was confirmed in a more recent study, again at moderate exercise intensity. In the right combination, the study demonstrated a maximal rate of metabolism of 1.3 grams/minute of external carbohydrate provided as a glucose/fructose mixture. It appears that this limit, about 70 grams of carbohydrate per hour equal to 280 Calories per hour, is the upper limit of what can be achieved with oral carbohydrate drinks.

Any advantages (or disadvantages) for a gel form of fructose versus a liquid? None. Just as with other carbohydrates, how it is packaged is for your convenience of use during the vent/ride, not for superior performance with a liquid versus a gel.

Post Exercise

Does fructose offer any advantages as a post ride replenishment for your muscle stores? The answer 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."

My interpretation? Chocolate milk remains my favorite based on cost, taste, and efficacy.

What is the Role of Oral Fructose in Aerobic Events?

Although fructose does not have a clear role to play in anaerobic events (it has been shown to be associated with less fatigue and nausea), it is does have a role to play in nutritional supplementation of the competitive cyclist - especially for that long ride, at an aerobic pace, with the risk of muscle carbohydrate depletion limiting a sprint at the end of the event. For the average cyclist, it remains to some degree the fad of the day, with your decision based on the taste of the product being proffered and the cost to you versus more traditional carbohydrate snacks.

But one word of caution - and this applies to all carbohydrates. An unbalanced diet, too high in any type of carbohydrate (including fructose), puts one at risk of developing insulin resistance and metabolic syndrome. So keep your carbohydrate replacement Calories equal to the Caloric needs of your ride - and for the remainder of your daily Caloric needs, focus on a balanced diet.

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.