Taking fructose along with glucose during exercise improves lactate and glucose utilization by muscles, researchers from Switzerland report in the September 8th online issue of American Journal of Clinical Nutrition.
In this study, researchers showed that fructose ingested during exercise is mostly converted into lactate, which is then released into the systemic circulation and oxidized in extrasplanchnic tissues, presumably muscle. Dr. Luc Tappy from the University of Lausanne told Reuters Health by email that this is consistent with an improved substrate delivery to muscle when fructose is co-ingested with glucose during exercise.
Dr. Tappy and colleagues assessed the metabolic effects of ingesting equimolar amounts of glucose or glucose+fructose mixtures in seven well-trained male endurance cyclists during exercise.
After a preliminary session during which maximal oxygen consumption (VO2max) and maximal aerobic power (MAP) output were assessed, the athletes underwent three 120-minute submaximal exercise trials, one with glucose and two with the glucose+fructose mixture.
Plasma lactate concentration decreased slightly and remained stable during exercise with glucose but increased significantly with glucose+fructose, whereas plasma glucose and insulin concentrations didn’t differ during exercise between the two solutions.
With glucose ingestion, measures of lactate oxidation during exercise showed that liver gluconeogenesis from lactate was negligible. Lactate kinetics and plasma lactate concentration were higher with glucose+fructose ingestion, but liver gluconeogenesis from lactate remained negligible.
In contrast, gluconeogenesis from fructose and fructose conversion into plasma lactate increased significantly with glucose+fructose ingestion.
Respiratory exchange ratio was also significantly higher during exercise with glucose+fructose than with glucose.
Dr. Tappy said more research is needed to determine who would benefit from glucose+fructose co-ingestion and to assess performance. But for now, he says it should be used in “athletes performing high-intensity endurance exercise, such as cycling or long distance running.
He added that these results “may also be of interest in the nutritional support of critically ill patients, since lactate may be an efficient substrate for wound and inflammatory tissues,” since fructose doesn’t increase glycemia.
When asked whether these findings have any bearing on the high fructose corn syrup (HFCS) debate, Dr. Tappy said, “HFCS is enriched with fructose when compared to traditional, high glucose corn syrup. However, the fructose content of HFCS (42-55%) is quite comparable to the fructose content of sucrose, or table sugar (50%). Total fructose intake is therefore very similar when ingesting food sweetened with sucrose or HFCS.