Swimmers tend to be fatter than other athletes, but most swimmers and their coaches believe that slimming down can lead to swifter swim performances. In fact, they seem to think that swimmers should be just like top-level runners and cyclists, with equally low levels of body fat. The knock on fat is that it is simply dead weight which must be lugged along as an athlete moves - weight which cannot provide even a smidgen of propulsive force.
However, propelling one's body through water is quite different than moving through air, and it's possible that pudginess might be an advantage in the water. For one thing, fat has a lower density than water (muscle and bone have higher densities). Thus, while muscle and bone make you sink like a rock, fat can make your body more like an unsinkable buoy. As a result, corpulent swimmers can use their muscular power to drive their bodies forward and don't have to waste energy providing vertical lift in the water column. Also, as fat converts an athlete's body into a sort of fishing bobber, it also reduces the 'drag' (friction) with which water slows down swimming speed. To put it simply,
with more fat there's less of the body in the water, less drag, and therefore (one might hope) higher performance. Perhaps swimmers should eat like sumo wrestlers!
Supporting the view that fat is fine for swimmers, studies have shown that triathletes who wear wet suits have better flotation and improved performance times during the swim portions of triathlons, compared to triathletes who compete in the raw. One interpretation is that a wet suit may act like a layer of subcutaneous fat, retarding sinking and diminishing drag.
To see if feasting and fattening is really a good idea for swimmers, scientists at the University of Miami artificially increased body fat levels by 2 per cent or more in a group of 10 male and female swimmers who had been swimming competitively for at least three years. 'Fatness was enhanced by fitting latex pads under a spandex triathlon suit in the swimmers' adominal, hip, thigh, chest, back, and buttock areas. Microscopic balloons were added to the latex so that the pads had the same density as actual body fat. Male swimmers attached a total of 3.3 pounds of artificial fat, while females donned an extra four pounds. Each athlete swam a 50-yard freestyle race as fast as possible, with and without the pads.
While the latex pads did improve flotation, they also slowed the swimmers down considerably. The athletes could rip through their 50-yard sprints in about 26.6 seconds without the added 'fat' but required around 27.4 seconds with the additional fat on board. Thus, each additional pound of fat slowed 50-yard swim times by approximately .2 seconds.
Why did added 'fat' slow performance, even though it improved buoyancy? While supplemental fat can reduce friction drag, it can actually expand something called 'form drag,' which is determined by the dimensions of a swimmer's body. Specifically, as a swimmer fattens up in the abdomen, thigh, and buttock areas, swirling eddy currents form around these protruding areas and can slow swimming velocity appreciably.
A second kind of drag - 'frontal surface resistance' - can also make it harder for corpulent swimmers to get through water. Frontal surface resistance is a function of how much body you actually have. If you have a big body, you have more frontal resistance, because there's more body to push against the water ('more front for the water to confront'). Instead of slipping through the water like a torpedo, you constantly bump into it. As water crashes against your large surface, it slows you down. Thus, even though fat helps by getting you out of the water, the part of a fatty body which remains in the water impedes progress. Perhaps the ideal free-style swimmer would have a fat back and sides but a slim anterior, while a backstroke specialist would have a paunchy tummy and flat buttocks.
The lack of benefit from fat doesn't mean that wearing a wet suit is bad during competitive swimming, however. In fact, studies have shown that slipping into a wet suit during the swim portions of triathlons can improve performance by about 7 per cent, primarily because a wet suit can reduce drag on a swimmer's body when swimming velocity is between 1.1 and 1.5 meters per second. Neoprene, the material used in wet suits, is less dense than fat, improves flotation more than fat, and significantly reduces the frontal area that a swimmer presents to the water. In addition, since a wet suit covers the trunk and legs but not the head, it shifts a swimmer's centre of buoyancy toward the feet and puts a swimmer in a more horizontal position, which further reduces the frontal surface area which makes direct contact with the water. With wet suits, swimmers simply become more streamlined.
Also, the Miami study doesn't necessarily mean that fat is always bad for swimmers. The swimmers in the Miami research were not bone thin; males possessed 11-per cent body fat while females checked in at 21 percent. It's possible that thinner swimmers might have actually benefited from increased body fat. The idea would be that an ultra-slim swimmer might slip too far down in the water column, hiking frontal drag to performance-hampering levels. Also, given the prevalence of eating disorders in athletes, it's unwise to put pressure on swimmers to lose large chunks of weight. The bottom line is that we don't yet know the true impact of body fat on swimming performance - or which level of fat produces the fastest times.
(The Effect of Varying Body Composition on Swimming Performance,' Journal of Strength and Conditioning Research, vol. 8(3), pp. 149-154, 1994)