Fructose

Sucrose (table sugar) is a fructose molecule linked to the monosaccharide glucose. Sucrose is split into individual glucose and fructose molecules in the small intestine. The glucose molecules are absorbed directly into the blood stream and transported to cells throughout the body but fructose is first converted into glucose in the small intestine lining cells before being absorbed.

Another sugar often encountered in discussions of energy and sport drinks is maltodextrin, a complex carbohydrate formed when multiple glucose molecules are linked in chains of variable length.

In small amounts, over 90% of the fructose molecules in our diet are absorbed. But there is a limit for maximal absorption. This upper limit varies from individual to individual, and over half the population are unable to completely absorb the 25 grams of fructose in a single test meal or drink. Any excess fructose passes on into the colon where it is metabolized by colonic bacteria (the microbiome) resulting in occasional gaseousness and bloating.

The blood leaving the small intestine drains into the liver, where liver cells remove any remaining fructose and convert it to fat and glucose before it enters the general circulation and on to peripheral tissues.

Fructose can be toxic to the liver. Large amounts increases the amount of stored liver fat which in turn impairs the normal response to insulin, increasing the risk of diabetes and in turn a condition labeled "metabolic syndrome". Fructose also elevates the blood level of a protein which might play a role in the scarring of livers already injured by too much stored fat. In one study, livers of rats fed a high fructose diet looked much like the livers of alcoholics. In this detailed study of the liver effects of fructose, the authors proposed an upper daily limit of fructose of 50 grams.

The possible role of fructose in the obesity epidemic has been controversial since at least 2013 with the publication of this paper highlighting its effect on mitochondrial energy production, an effect much like that of alcohol. This effect on mitochondrial ATP production (the body's energy molecule) could explain the increase in the number of cases of both metabolic syndrome (fatty liver) and obesity that have coincided with the entry of High Fructose Corn Syrup (HFCS) into the food supply in the 1960's. The calories diverted from energy production appear to be shunted preferentially into fat. So not only is more fat formed from these fructose calories, the production of fewer ATP molecules per gram of fructose short circuits a natural energy balance regulatory feedback loop. Over time we eat just a few more calories than we need and weight gradually goes up.

This study demonstrated fairly conclusively that fructose activates a liver enzyme which causes a) resistance in the liver to the effects of insulin and b) increases the production and release of additional glucose from the liver cells - a positive feedback loop which is in essence the prediabetic state, a key component of metabolic syndrome.

Where do we get the bulk of our daily fructose? The major offender is sweetened beverages (colas and fruit drinks) but HFCS is an additive in many processed foods. We also get some fructose from table sugar (1 sucrose molecule = 1 glucose plus 1 fructose molecule). Add the extra fructose from excess simple carbohydrates in our daily diet to those from a sweetened drink or two and you are quickly approaching the 50 gm/day threshold for harm.

Fructose as an alternative sugar appears to have definite health risks and it may be time to take the approach used for trans-fats in 2018 and eliminate HCFS as an acceptable food additive. When I make a choice for our next sport drink, I'm going to spend a little more time on the label to find one that is sweetened with sucrose (the disaccharide in cane sugar and sugar beets) rather than fructose alone. This will limit how quickly I approach my daily threshold of 50 grams. And I plan to avoid all colas and beverages sweetened with the pure fructose in HCFS.

FRUCTOSE IN AEROBIC EVENTS?

Pre Exercise

Burelle Y et al evaluated 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

What about the role of fructose as a glycogen sparing drink during exercise? Massicotte D et al compared the oxidation of equal amounts of a 13C-labeled glucose, fructose, and a glucose polymer taken while cycling (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 fructose (54%). And, as expected, all 3 lessened the need to tap glycogen stores compared to the water control. Thus it appeared that oral fructose supplements alone had no advantage as a carbohydrate energy source and glycogen sparing drink during exercise.

This was confirmed by Gautier JF et al. who 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 was 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 muscle glycogen stores? The answer is again 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 as a post ride recovery drink based on cost and taste.)

The bottom line

• glucose is delivered directly to the muscle while fructose needs additional processing into glucose (intuitively less efficient),
• the published 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 to the liver

The potential negative public health impact of fructose is huge. The use of corn syrup (fructose) as an inexpensive carbohydrate supplement increased tremendously after WWII. And obesity, fatty liver, and diabetes follwed in lock step. From the science presented above, it's hard to escape the conclusion that fructose is playing a significant role.

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

How does this affect me, personally, as a cyclist?

• First, simple sugars that are the logical choices for a quick energy boost in a sports drinks. With the potential health risks of fructose with little hard evidence for a performance benefit, I am going to avoid fructose whenever possible. Except for cost, there is no benefit over glucose or sucrose (table sugar) as an energy source for the active muscle. It's time to read those content labels on the sports supplements and energy drinks.
• There are definite long term risks of a diet that contains too many carbohydrate Calories. And with its impact on glucose/insulin metabolism, I try to avoid as much free sugar (or foods with a high glycemic index) as possible except while actively exercising. Instead my diet has more starches (complex carbohydrates) which I try to eat mid day when I am more active when less insulin is needed to move the blood sugar into active muscle cells.
• Have I eliminated all sweets? No, I take them with me on a ride to satisfy the Calories being expended on the bike. After a ride I switch to complex carbohydrates or the fructose that is complexed with fiber in whole fruits (juices are basically a no-no, a free fructose sugar drink) which are absorbed more slowly and appear to be less harmful 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 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.