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Food and weight (4)

Dec 10, 2007

If I eat a 9-oz steak at Outback, shouldn't I be 9 ounces heavier? Maybe I'm missing something here.

Let's say I eat two pounds of food during the course of a weekend (I think this is very possible). Let's assume there's zero outflow (mainly because I don't really want to talk about that, but also because I'd need to weigh it, and that's not cool). Assuming I weight 158 lbs, here's the math:

158 + 2 = 160 lbs

The problem is, that didn't happen. It never happens. I never gain weight based on the mass of food I eat. Though I don't have accurate (or any) measurements for outflow, in my expert opinion, inflow is way more than outflow, and this baffles me.

I'm guessing it has something to do with energy. If E = mc² (does this equation even apply here?), the energy contained in the two pounds of food I ate over the weekend is

0.907 kg * (299,792,458 m/s)² = 8.15x10¹⁶ J

I have a pretty active metabolism, so let's say I burn through 2500 Calories per day. If 1 Calorie (big C) = 1000 calories (small c), and 1 calorie = 4.184 J, I burn through

2 * 2500 Cal * (1000 cal/1 Cal) * (4.184 J/1 cal) = 2.092x10⁷ J

over the course of a weekend. That leaves 8.15x10¹⁶ J unaccounted for (it's actually 8.14999999791x10¹⁶).

My question is: Where did that energy go? Did I just break physics? #food

Comments

#1	Rich	Dec 10, 2007
	"E = mcÂ² (does this equation even apply here?)" No. Your body deals with food using only chemical processes. Chemical processes just move electrons around, which can consume energy or liberate energy depending on what you are trying to do (endothermic vs. exothermic reactions). E=mcÂ² only applies if you start getting into nuclear processes, which I think we all hope your body does not suddenly start doing.

#2	Dave	Dec 10, 2007
	I'm glad you chimed in on this. I was gonna write, "Help, Rich Santillo!" but decided against it. You heard my inaudible call for help anyway. Gotcha. So I'd need to know each of the specific chemical reactions taking place and add all their energies together. That doesn't sound like something I'd be willing to do. Oh well. Thanks for the insight.

#3	Dave Brown	Dec 11, 2007
	I think your mass based analysis is right. The difficult part is keeping up with all the ins and outs. You have to include water in and out. That would include sweat and water vapor in your breath. You should probably also include the weight of the air in and air out. Which is heavier, an inhale or an exhale?

#4	Dave	Dec 12, 2007
	I would say an exhale is heavier because it has water vapor. But I could be wrong. Keeping track of the ins and outs would be the right way to go, but as I said, that's not something I want to get into.