# A Physics-Based Diet Plan

The holiday season is upon us, and for many, this translates into a marked uptick in the consumption of tasty food treats. I’m no different, and can really pack it in on such occasions. For instance, the day after Thanksgiving this year, I stepped on the scale to find myself about 5 pounds (~2 kg) above normal weight. I kicked in my diet plan, and by Monday morning (3 days later) I was back to normal. Resume course. I use a simple formula, backed by physics, that works every single time. The topic is Do-the-Math-relevant for two reasons: it applies quantitative physics to everyday life, and it touches on attitudes relevant to energy/resource conservation.

# Burning Desire for Efficiency

Ever wonder how efficient it is to heat water? Of course you have! Ever measured it? Whoa, mister, now you’ve gone too far!

I recently devised a laser-phototransistor gauge to monitor my natural gas meter dial—like ya do. As a side benefit, I acquired good data on how much energy goes into various domestic uses of natural gas. Using this, I was able to figure out how much energy it takes to heat water on the stove, cook something in the oven, or heat water for a shower. Together with the knowledge of the heat capacity of water, I can compute heating efficiency from my measurements. What could be more fun? I’ll share the results here, some of which surprised me.

# Flex-Fuel Humans

If you’re one of those humans who actually eats food, like I am, then a non-negligible part of your energy allocation goes into food production. As an approximate rule-of-thumb, each kilocalorie ingested by Americans consumes 10 kilocalories of fossil fuel energy to plant, fertilize, harvest, transport, and prepare. The energy investment can easily exceed a person’s household energy usage—as is the case for me. But much like household energy, we control what we stick in our mouths, and can make energy-conscious choices that result in substantial reductions of energy consumption. I now call myself a flexitarian, a term acknowledging that my body is a flex-fuel vehicle, but also that I need not be rigid about my food choices in order to still make a substantial impact on the energy front.

An earlier post on how many miles per gallon a human gets while walking or biking touched on the fact that fossil fuels undergird our food supply. As a result, walking to the grocery store effectively uses as much fossil fuel as would a typical sedan. The lesson is not to walk less, but to change that 10:1 ratio for the better by eating more smartly. Once upon a time, we put less than one kilocalorie of energy into food production per kilocalorie obtained (or else we and our draft animals would have starved to death). So the 10:1 ratio is not at all inescapable, and depends strongly on the foods we choose to eat.

# MPG of a Human

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On Do the Math, three previous posts have focused on transportation efficiency of gasoline cars, electric cars, and on the practicalities of solar-powered cars. What about personal-powered transport—namely, walking and biking? After stuffing myself over Thanksgiving, I am curious to know how potent human fuel can be. How many miles per gallon do we get as our own engines of transportation?

Okay, the “miles” part is straightforward. And we can handle the “per.” But what’s up with the gallon? A gallon of what? Here we have all kinds of options, as humans are flex-fuel machines. But food energy is not much different from fossil fuel energy in terms of energy density.