There’s an old saying about cold-water marathon swimming:
Either be fat, or be fast.
Is it oversimplified? Probably. Crass? Definitely. But there’s a kernel of truth worth examining. Thin swimmers have made it across the English Channel, but they’re usually fast. Slow swimmers have made it across the Channel, but they’re usually… carrying a healthy layer of bioprene.
The common factor: Core temperature must be preserved. Either generate heat, or retain it. Fast swimmers are good at generating heat. Fat swimmers are good at retaining it.
In the English Channel (from what I gather), it’s considered prudent for non-overweight swimmers to put on some weight, even if they’re “fast.” A Channel attempt is expensive and, unless your name is Petar Stoychev, just getting across is the main priority. Bioprene increases the probability of success.
But at what cost? How much does the extra weight slow you down? Swimming is a gravity-less activity, so obviously it matters less than in running or uphill cycling. Further, the flotational benefits of fat may improve your body position in the water.
In running, the rule of thumb is 2 seconds (faster) per mile per pound (lost). Is there a similar rule of thumb for swimming?
Out of curiosity, I asked my coach to estimate the benefit of losing 10 pounds of body fat on threshold pace per 100m (assuming stable fitness & muscle mass). He said 2-3 seconds per 100m. Some quick conversions: 32-48 seconds per mile, 10-16 minutes per 20-mile channel swim. Or, for an apples-to-apples comparison with running: 3.2-4.8 seconds per mile, per pound.
And actually… that accords fairly well with my own experience. I do a lot of threshold (a.k.a. CSS) training - so I’m intimately familiar with my basic pace per 100m. Also, my weight has fluctuated a bit in the past couple years - giving me some data to draw on.
Can we do better than a rule of thumb? Scientists being scientists, it turns out someone has actually studied this question. In a paper published in the Journal of Strength and Conditioning Research, Ilka Lowensteyn and two colleagues artificially varied the body fat of competitive swimmers by fitting them with weighted latex pads under a spandex triathlon suit. The swimmers were timed at 50-yard sprints at various weights.
Lowensteyn et al. estimated the swimmers were slowed by 0.2 seconds per 50 yards, per pound. That’s 4 seconds per 100, per 10 pounds - not far off Coach AB’s estimate. And it makes sense there would be a larger effect in a sprint (compared to threshold pace), because in water, drag increases exponentially with speed.
Bottom line: Let’s say you gain 20 pounds for your English Channel attempt. You might be looking at about an extra half-hour in the water. Given the thermal-protective benefits of those 20 pounds, though, it seems like a small price to pay.