Fat vs. Fast

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 Coach AB 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.

6 thoughts on “Fat vs. Fast”

  1. Interesting post, Evan, and a topic I’m really interested in from a sociology perspective. But I think that are a number of unsupported assumptions at work here. Firstly, while I realise that you’re simply proposing a broad ‘rule of thumb’ and it’s not really fair to hold you to it, the straight multiplying up of presumed speed gains fails to take into account the massive physiologically different processes and systems involved in 100m vs 20 miles. I’m not convinced about this as a meaningful projection.

    Secondly, there is an assumption that it is the fat itself that makes the difference. But for example, if in the process of losing weight, a swimmer trains more intensively or extensively, then this may also improve swimming performance; if they follow an ultra-low calorie diet to get to that lighter weight, they will probably be too exhausted to even get changed. Similarly, if a swimmer gains weight and slows down at the same time, it could just as easily be due to other factors such as a change in diet quality, or other temporary lifestyle changes, responses to stress etc. Weight gain could be a symptom, not a cause. So how particular weights / levels of fatness or leanness are arrived at are potentially as important (or more so) than weight per se. In a similar vein, while a swimmer who purposefully gains weight for a Channel swim may attribute slowing sprint times to added fat, might it not also be that their training focus has changed, impacting upon sprinting power. (These are all speculations – I’m just trying to challenge the idea that fat is necessarily the explanation).

    Thirdly, and with the huge caveat that I’ve only been able to access the abstract of the paper you cited, the study seems to me to be massively flawed in that (1) it assumes that weight gain heads directly for particular body parts rather than being dispersed throughout the body; and (2) that it fails to recognise the likely impact of a sudden change in body composition / shape, which will impact upon stroke dynamics etc. Someone who has been at that size / shape consistently will experience that very differently to those who experience it momentarily. It raises interesting questions, then, about the relative impacts of newly acquired body fat versus body fat that was there all along. It would be impossible to research, but the more telling study of Channel swimmers might be to compare changes in marathon swimming performance between those who purposefully change their weight during training, and those who don’t.

    I don’t have any of the answers to any of this, but just wanted to throw in a few alternative ideas and ways of thinking about fat. Thanks for raising such an interesting topic.

    1. Fascinating comment as usual, Karen – thanks. I definitely made some assumptions and simplifications, and I appreciate how you parsed them. Addressing each of your points:

      1. I agree it would be silly to extrapolate Lowensteyn et al.’s finding of “0.2 seconds per 50 yards, per pound” to a channel swim. However, I found it interesting that it corresponded reasonably well with an experienced swim coach’s off-the-cuff estimate of the effect of losing/gaining weight on threshold pace. (“Threshold pace” meaning: the fastest pace I can maintain for a long time – somewhat faster than channel pace, but not far off.) I also found it interesting that it corresponded to my own experience. In the past couple years, my weight has fluctuated as much as 15 pounds, though my training load has remained similar. 2-3 seconds/100m – or 30-45 seconds per mile (per 10 lbs) – seems pretty spot-on to me.

      2. I appreciate that there are other factors to consider when people lose or gain weight, but… the idea that – all else being equal – extra weight will slow you down just seems like basic physics, right? The question is how much. I thought my estimate of 20 pounds adding half an hour to a channel swim was a remarkably small effect, actually. Presumably 20 extra pounds would add much more time to an equivalent ultra-marathon run (e.g., 80 miles).

      3. I agree completely.

  2. Hi Evan
    I agree, of course, with the basic physics….but I wonder about the effect of bouyancy (especially later on in a very long swim) too, and also, especially on bigger bodies, the impact depending on fat distribution etc. I take your point about your focus on threshold pace though, which I treated as 100m sprinting in my comment – I think I was just projecting because I know how fast you are compared to me! Does the calculation hold for you in OW?

    But ultimately, I think that the broader point for me is that while the calculation may hold “all things being equal”, it is so rarely the case that all else is equal – but then, I’m a sociologist, so I would say that! Best, K

    1. I never time myself in OW training – I’ve found it an exercise in futility. Any differences in swim speed are overwhelmed by differences in conditions & navigation. I also wonder how the buoyancy effect of body fat would differ between saltwater & freshwater…

  3. Hey Evan
    Do you by any chance have the pdf of the Lowenstyn article we were discussing in this post? I can’t access it through my library, but would love to read it in full. If you have it, can you e-mail?

    Hope all’s well.

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