Pounds of flesh
In kindly sponsoring my effort in the Noosa Triathlon, where I’m supporting HeartKids (click on the button at the right to help) long-time commenter Jack Strocchi made a demand for a “pound of flesh” in return. Sad to say, I’m going to shortchange him. Based on past performance I expect to burn about 2500 calories (or about 10 Megajoules, just to make life hard for some of the computationally-challenged media figures we’ve been poking fun at lately). That corresponds to about 10 ounces (300g) of fat, most of which will be replaced in advance with a big pasta meal the night before the race. Of course, if I allow fluid loss, and weigh in just after the race, it will be more like 2kg.
One of the side benefits of taking up exercise is that I can now do all sorts of conversions of this kind. For example, a glass of red wine is about 150 calories (600 kJ), and running uses about 75cals/km so I have to run 2k to burn it, which seems like a fair deal. By contrast, despite their healthy image, a typical muffin is about 450cal/6km, definitely not worth it to me.
Perhaps I’m a bit too obsessed with numbers. But on matters of this kind, I’m with Lord Kelvin who observed
I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
Update A fun theoretical observation that just came to me, and which I don’t remember seeing anywhere else. It’s obvious and well known that, the heavier you are, the more energy you need just to move yourself about. In fact, a 50 per cent increase in body weight implies a 25 per cent increase in the energy intake needed to sustain a given level of activity (try this calculator). What this means is that there is a linearly increasing relationship between body weight and the energy intake consistent with maintaining that weight. Turning that around, any given energy intake is consistent with a unique stable weight, for given activity level. So, whatever your starting point, if you eat the amount consistent with your target weight, and change nothing else, you will end up there, sooner or later.
fn1. A "standard drink" is more like 100, but that's a small glass. If you are keeping count for driving purposes, two drinks of any kind usually amount to three standard drinks.
fn2. Surprisingly, so does walking. Energy consumption is determined mainly by distance travelled and body mass – the speed at which you go affects the rate of energy use, but not (much) the total over a given distance.