Old 03-24-05, 02:09 PM
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JamesV
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Originally Posted by Paul W.
If you put any high glycemic food/sugar into your blood while exercising, i.e. Gatorade, Bananas, Clif Bars, etc., shouldn't you have a corresponding low blood sugar episode shortly thereafter? Correct me if I'm wrong, but doesn't a sudden spike in blood sugar also cause a spike in insulin, AKA hypoglycemia? Wouldn't this cause a "bonking" effect?

It would seem to me that high sugar foods would be counter-productive to endurance activities like biking where you spend a long time in the saddle. Am I missing something?
These two studies confirm that the glycemic index is not just for diabetics only, it's important for athletes as well:

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Carbohydrate feeding before exercise: effect of glycemic index.

Thomas DE, Brotherhood JR, Brand JC.
Department of Biochemistry, University of Sydney.

Low glycemic index (GI) foods may confer an advantage when eaten before prolonged strenuous exercise by providing a slow-release source of glucose to the blood without an accompanying insulin surge. To test this hypothesis, eight trained cyclists pedalled to exhaustion one hour after ingestion of equal carbohydrate portions of four test meals: lentils, a low GI food (LGI); potato, a high GI food (HGI), and glucose and water. Plasma glucose and insulin levels were lower after LGI than after HGI from 30 to 60 min after ingestion (p less than 0.05). Plasma free fatty acid (FFA) levels were highest after water (p less than 0.05) followed by LGI and then glucose and HGI. From 45 to 60 min after ingestion, plasma lactate was higher in the HGI trial than in the LGI trial (p less than 0.05) and remained higher throughout the period of exercise. The rank order from lowest to highest for total carbohydrate oxidation during exercise was water, lentils, glucose and potato. Endurance time was 20 min longer after LGI than after HGI (p less than 0.05). These findings suggest that a low GI pre-game meal may prolong endurance during strenuous exercise by inducing less post-prandial hyperglycemia and hyperinsulinemia, lower levels of plasma lactate before and during exercise, and by maintaining plasma glucose and FFA at higher levels during critical periods of exercise.

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Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings

L. M. Burke, G. R. Collier and M. Hargreaves
Department of Sports Medicine, Australian Institute of Sport, Australian Capital Territory.

The effect of the glycemic index (GI) of postexercise carbohydrate intake on muscle glycogen storage was investigated. Five well-trained cyclists undertook an exercise trial to deplete muscle glycogen (2 h at 75% of maximal O2 uptake followed by four 30-s sprints) on two occasions, 1 wk apart. For 24 h after each trial, subjects rested and consumed a diet composed exclusively of high-carbohydrate foods, with one trial providing foods with a high GI (HI GI) and the other providing foods with a low GI (LO GI). Total carbohydrate intake over the 24 h was 10 g/kg of body mass, evenly distributed between meals eaten 0, 4, 8, and 21 h postexercise. Blood samples were drawn before exercise, immediately after exercise, immediately before each meal, and 30, 60, and 90 min post-prandially. Muscle biopsies were taken from the vastus lateralis immediately after exercise and after 24 h. When the effects of the immediate postexercise meal were excluded, the totals of the incremental glucose and insulin areas after each meal were greater (P < or = 0.05) for the HI GI meals than for the LO GI meals. The increase in muscle glycogen content after 24 h of recovery was greater (P = 0.02) with the HI GI diet (106 +/- 11.7 mmol/kg wet wt) than with the LO GI diet (71.5 +/- 6.5 mmol/kg). The results suggest that the most rapid increase in muscle glycogen content during the first 24 h of recovery is achieved by consuming foods with a high GI.

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Bottom line: eat low-GI prior to a ride, eat or drink high-GI right after a ride.
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