Another sugar you will often encounter in discussions ofenergy and sport drinks is maltodextrin. Maltodextrin consists of multipleglucose 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 readilyabsorbed 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 thegeneral circulation. Almost all the fructose that was absorbed unchanged isremoved by the liver, while the vast majority of glucose continues onwardto be absorbed directly by the peripheraltissues. This difference has asignificant impact on energy metabolism in the athlete.

The small intestinal absorption rate of fructose is limited and anyexcess passes down the small intestine and into the colon where it ismetabolized by colonic bacteria (the microbiome) and can lead to gaseousnessand bloating. The efficiency ofintestinal 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 foran athlete. It is absorbed by a different (from glucose) enzymatic pathway in the small boweland might be useful as an option to increase the total carbohydrate Caloriesthat 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 naturalsugars, 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 absorbtionstimulates an elevation in the blood level ofthe protein fibroblast growth factor 21 (FGF21) which might play a significant rolein the development of scarring in "fatty liver". In one study, livers of rats on a highfructose diet looked much like the livers of alcoholics.

And the presence of fructose induced fatty liver impairs the normal responseto 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 havedefinite 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 fructosethat is destined to support athletic activity would first need to be convertedinto glucose in the liver. These additional metabolic steps make fructoseless efficient than glucose as an energy source for the muscle.

FRUCTOSE IN AEROBIC EVENTS?

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 3lessened 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 isagain the favored monosacharide.

But an occasional study has suggested that although fructose ALONE hasno advantages to glucose ALONE, there maybe a benefit when it is usedin combination with glucose in a sports drink. You can readmore 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

• glucose is delivered directly to the muscle while fructose needs additionalprocessing into glucose (intuitively less efficient),
• the data shows glucose to be more effective than fructose pre exercise,while exercising, and post exercise,
• and there is strong suggestive evidence that fructose can be harmful tothe liver,

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

Thus the only reason I can see to include any fructosein my daily diet is in the form of whole fruit where the fiber appears todampen the negativefructose effects. Dr. Mirkin makes agoodargument as to why that may be the case.

FRUCTOSE IN SPORTS DRINKS - PROBABLY A BAD IDEA

How does this relate to me as a cyclist?

• First, most free carbohydrates (that is simplesugars that are the logical choices for a quick energy boost) in sports drinks arefructose rather than glucose or sucrose (table sugar). If it is fructose thatactivates a specific enzyme (whose effect is harmful to me) and this increasedenzyme activity might last long after the ride is completed (remember that bloodsugar is lowered by the actively exercising muscle so a high blood sugar whileriding is not as undesirable as at rest where insulin is required to take care ofthis extra blood sugar) I am going to avoid it whenever possible. Except for cost,there is no benefit of fructose over glucose or sucrose (table sugar) as anenergy source for the active muscle. So why take any risk that provides no benefit.Time to read those content labels.
• Second, to be on the safe side, (we do not know forsure that table sugar is safer in the long term) I try to avoid as much free sugaras possible (except while actively exercising). That includes all foods with a highglycemic index. I've tried to eat more starches (complex carbohydrates), trying toeat them mid day when I am more active when less insulin is needed to move theblood sugar into active muscle cells.
• Finally, does that mean all sweets have to go? No, while exercising eatall the sweets you need to offset the Calories being expended (within reason) butthen after your ride go for complex carbohydrates or sugar that is complexed withthe fiber in whole fruits (juices are basically a free fructose sugar drink).The complexed fructose in fruits is absorbed more slowly and appears to be lessharmful than the surge of fructose absorbed after a sugary drink.

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 supplementprogram 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.