No WOD yet today, I'm going to the evening class. So instead I'll give you a very nice guest post from Mike. It starts out a little math-y, so if you're not a math person, just skip the blue stuff, and the rest will make sense.
Ok, first I'm going to start with a true statement:
ΔW is weight change
κ is ~0.000285 lbs/Cal (Food Calorie = 1kcal)
Ein(t) is the calories you intake
Eout(t) is the calories you output
To make it simple, lets integrate over one day
No one disputes this, it's the truth, you can't argue with thermodynamics.
The trouble comes because every one thinks that Energy in and Energy out are not correlated, and people say things like: "Eat one less cookie a day (~100 Cal) and you'll lose 1lb/month" or "Stand instead of sit and you'll burn enough calories to lose weight."
Unfortunately our bodies have some really really good feedback mechanisms that make Energy in and Energy out nearly perfectly correlated (correlation time, τ, is about 1 day). So if you skip that 100 Cal cookie, you're going to eat a slightly bigger dinner without thinking about it. Likewise, if you stand instead of sit, you'll eat more too (without realizing it). If you try to force yourself to eat fewer calories and stick to it, then your body will adjust Energy out to maintain homeostasis.
It's nearly impossible to win! Seriously!
Energy in is not only a function of what you eat, but it's a function of what you do.
Likewise, Energy out is not just a function of your activity, but it's a function of what you eat.
It's more like this:
or more correctly, we'd have to integrate over τ too
My favorite quote about this is: "Fat people aren't fat because they eat too much, they eat too much because they're fat."
When you're fat your hormones are all screwed up which makes you hungry and eat more and slows your metabolism down so that you can't burn more. Walking around the office or cutting out a cookie a day isn't going to change a thing. What you need to do is change your diet so that your body gets all the hormones balanced correctly.
As an example, take me (and I have a whole group of people at my gym for more data points), you know I've always worked out really hard all through grad school, but my diet was pure crap, and no matter how hard I worked out I still slowly gained weight that whole time. Even after I was out of the stressful environment of grad school, I still was slowly gaining weight, even with a switch to a higher-intensity workout program. But as soon as I adjusted WHAT I ate (in fact, I eat more now than I ever did before), I dropped nearly 30lbs in a couple of months. I had no change in activity.
Think about it this way: If you run 10km/hr (think Bolder Boulder in 1 hour), that's only about 800 Cal/hr. That's about 1 meals worth if you believe the USDA recommendations. And no one is doing that everyday. Your body is so efficient at turning stored energy into motion that truthfully your activity level barely matters. Your weight is mostly determined by what you eat (macronutrient ratios) and only slightly determined by how active you are. However, how active you are is determined by what you eat. People don't get fat because they sit on the couch, they sit on the couch because they're fat.
I need to go back to grad school so I can actually write a thesis about this stuff, there's so much misunderstanding out there, and I think it's needs a physical chemist's approach anyway. Math doesn't lie.