Rounding errors in Running Foods

(When I wrote this I was unaware of certain things. A better analysis is given in a subsequent blog post: Not all Calories are created equal.)

Rusty gave me a enormous bag of cliff blocks the other day, so I’ve been eating those on long trail runs rather than GU gels as I usually do. I think it takes me longer to get nauseous when eating these blocks than when eating gels. I also seem to notice that I am hungrier when eating the blocks than when eating gels. This might be part of not being nauseous (if I’m nauseous then I don’t notice hunger, and if I’m not then I do; or maybe I don’t feel nauseous so I run harder and get hungrier?). Or maybe the blocks and the gels have a different number of Calories?

So I came home and checked. The blocks are said to have 100 Calories/serving of 3 blocks, and the GU gels claim 100 Calories/serving of one packet. That was what I thought. So I should be consuming at the same rate…

But then I looked a little closer. A “Mountain Berry Cliff Block” is said to have: No Fat, No Protein and 24g Carbs. A “Salted Watermelon GU gel” is said to have: No Fat, No Protein and 22g Carbs. Now Carbohydrates burn at at rate of about 3.8 Calories/gram.
3.81*24=91.5 Calories/3block serving
3.81*22=83.8 Calories/gel serving
So you’d think I’d get more hungry from eating GU than blocks? But I don’t…

Ah! Not all GU flavors are the same. “Chocolate Outrage GU” has 15Cal from Fat, No Protein and 21g Carb=80Cal. So that one has 95 Calories/serving.

Flavor Cal Fat Cal Carbs Tot Cal Claimed
Salted Watermelon GU 0 84 84 100
Chocolate Outrage GU 15 80 95 100
Salted Caramel GU 0 84 84 100
Strawberry Banana GU 0 95 95 100
Peanut Butter GU 15 76 91 100
Mountain Berry Cliff 0 92 92 100
Chocolate Cherry Cliff 0 88 88 100
Citrus Cliff 0 92 92 100
Margarita Cliff 0 92 92 100
Mountain Huckle Hammer Gel 0 84 84 90
Orange Hammer Gel 0 80 80 90
Raspberry Gel 0 80 80 90
Apple-Cinanamon Hammer Gel 0 88 88 90

Those were all the flavors I had on hand to test. And, of course, I have no idea how accurate the carbohydrate/fat readings are. Could there be something else in these concoctions which would increase the Calorie content? (Alcohol is the only other thing I know of that can be metabolized but there are probably others, and I guess it doesn’t need to be metabolizeable to count toward the Calorie content).

Still it does seem to me that there is likely to be significant error in the Calorie labeling of these products.

Different carbohydrates have slightly different heats of combustion in a bomb calorimeter.

Glucose 3.81 Cal/gram
Fructose 3.79 Cal/gram
Sucrose 3.94 Cal/gram

However maltodextrin (the primary component of GU, and a component in cliff blocks) is a variable length polysaccharide, so heat of combustion is going to be variable too. At any rate I can’t find any data for it. I used 3.81 as a factor in my calculations above (but even had I used 3.94 none of the totals would reach 100 Calories).

Now within the body the energy currency is glucose so all carbohydrates are converted to glucose before being used. Within the cell the energy currency is ATP and one mole of glucose is converted into theoretically 38 (but in the real world about 30~32) ATP molecules. In the theoretical case only about 50% of the glucose energy is captured in ATP (in the real world it’s even less).

To use a molecule of fructose it must first be converted to glucose which takes up 2ATP molecules, so fructose is less efficient.

To use a molecule of sucrose it must first be broken into a glucose and a fructose (the energy released in this break down is too small to make an ATP molecule — I think). Then the two simple sugars would be oxidized as above. So the efficiency here is again slightly less than 50%.


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