Basic mental arithmetic for activity and weight

19.05, Tuesday 4 Oct 2022

Lockdown plus having a kid really did a number on my personal fitness.

Having never really thought much about my weight, suddenly in my mid 40s I have found myself having to acquire new mental models so I can manage it.

What happened? Routines changed I guess. For years I have been in a happy orbit of food, time in the day, activity, and weight, and it all just sort of kept itself steady.

Then I was training a bunch before the first lockdown, running probably 30+ miles/week, in happy equilibrium. Then lockdown meant the training stopped… completely… but the eating not so much…

And with kid timetable stuff (which was new), and not leaving the house because of remote working stuff, and snacking more in the evenings because I’m at home stuff – I guess I got knocked sufficiently far across phase space that I left the attractor basin and didn’t automatically fall back into the “healthy weight” orbit.

Long story short I put on 15–20 lbs, and I’m still 15 lbs over. Whoops.


People older than me in the UK think of personal weight in stones and pounds (there are 14 lb in a stone). People younger use kilograms. I’m transitional, first gen metrification. So I learnt both at school and can feel the heft of both in my hands, but my intuition is stones. Here’s the cheat: 1 kg ~= 2 lb.

So I now use some ballpark figures when I’m correlating weight and activity. Here they are.

Roughly speaking, I burn

  • 100 calories per hour being awake (actually 75/hr being alive, but I find the maths easier this way)
  • 100 calories per mile run or walked
  • 100 calories per 15 minutes hard exercise (like HIIT)

In a day, I have a calorie surplus or deficit. That converts directly to fat, which is either added or used up. The equation is this:

  • 1 lb fat = 3,500 calories

(Yes I know it’s more complicated than this given muscle and metabolism etc, but it gets me close enough.)

Let’s say I’m 15 lb over my target weight. That means I need a cumulative 52,500 calorie deficit to shed the fat. Or 287 calories/day over 6 months.

Or, to put it in more useful terms: a deficit 2,000 calories/week, which is 20 miles/week running assuming fixed energy input. Ok, I can train towards that!

EXCEPT: the final factor is that I also have upward velocity to shed.

For example if I’m putting on a pound every couple of weeks then that means I’m eating at an average 250 calorie daily surplus. I don’t want to cancel that upward velocity with training (that feels like fighting against myself) but I can watch what I eat instead.

Calorie counting every meal doesn’t appeal, but I’m fine with trimming and paying attention to the second time derivative of my weight. So that’s what I do: adjust my snacking and my portion sizes until my weight is stable.

What’s neat about this approach is I’m not really having to pay close attention to what I eat, or sum daily intake, or go on a “diet” or anything like that. I’m just tweaking habit parameters based on current dynamics.

My overall goal, I suppose, is to build an intuition of the local shape of the manifold in order to move from one trajectory to a nearby but different one, ideally a homeostatic orbit. I don’t want to have to think too hard about keeping healthy.

I imagine for other people this is one of (a) unnecessary; (b) obvious; (c) not nuanced enough. For me this basic arithmetic hits the sweet spot.

It helps that I love running! Though finding time is hard. And it’s disheartening to have to really push through only 3 miles when I didn’t even blink at 13 not too long ago.

And of course there’s all the other related considerations about nutrition, like cholesterol and heart health, and glycemic index and afternoon slumps, and calorie absorption rate and glycogen on long runs, etc, but this isn’t about that.

If you enjoyed this post, please consider sharing it by email or on social media. Here’s the link. Thanks, —Matt.