Best I can tell, I lost about 1 pound of body mass during the Tampa Bay Marathon Swim. Actually – probably closer to 1.5 pounds, but we’ll call it a pound. Marathon swimmers often lose substantial weight over the course of a swim, but most of this is water loss that is soon regained. I estimate that I lost a little over a pound of body mass – that is, fat (and possibly some protein cannibalized from my muscles). For convenience, we’ll call it a pound of fat.
Some back-of-the-envelope calculations:
Losing a pound of fat requires an energy deficit of 3,500 calories. I consumed 2,800 calories during the swim. That puts my total energy expended during the 9-hour swim at 6,300 calories — 700 calories per hour.
This tells me a couple of things. First, my energy expenditure is higher than I expected. 700 cal/hr is the typical estimate for “vigorous” swimming – but this was my ultra-marathon pace. Second, I can probably experiment with raising my calorie consumption. I took 311 cal/hr in Tampa, but when I’m burning 700 cal/hr that puts me in deficit within, at most, 5.1 hours (assuming 2,000 calories pre-stored glycogen), and possibly as little as 3.8 hours (assuming 1,500 calories glycogen).…