The fat controller: should swimmers fight fat for fitness?

What does the research say about optimum body composition for swimmers? Andrew Hamilton looks at the evidence

In most sports, achieving optimum body composition is relatively simple and governed by two simple rules. Firstly, lower levels of body fat equate to less ‘dead weight’ and improved performance; secondly, providing the dietary fundamentals are correct, training for that sport will help to bring about optimum body composition. For swimmers however, things aren’t quite so straightforward.

On land, superfluous body fat acts as a ‘dead weight’ that blunts acceleration and makes work against gravity (ie any sport than involves running or moving around on foot) more energy demanding for the muscles having to do the work. Since all the propulsive force required to overcome gravity and inertia comes from muscular contraction, having a high power-to-weight ratio (ie plenty of lean muscle tissue and a minimum of body fat) makes moving around on land much easier!

Not so simple

In the water however, things aren’t so simple and that’s because unlike most other body tissues, body fat is less dense than water. In simple terms, a given volume of fatty tissue weighs less than the same volume of lean muscle tissue, because fat is inherently less dense than water, and lean muscle tissue contains much more water that fatty tissue.

Applying Archimedes’ principle of displacement, a body of lower density than water immersed in water is buoyant (ie will rise to the surface), whereas bodies of higher densities than water will sink. It follows therefore that the more body fat a swimmer carries, the more buoyant he or she will be in the water. Given that swimmers need to stay horizontally aligned on the water’s surface for maximum speed through the water, and that they expend energy in doing so, it seems intuitive that unlike land-based sports, higher levels of body fat could be advantageous.

There’s also another reason why higher buoyancy has been regarded as a plus for swimmers; any body moving through water creates ‘drag’, which acts to slow that body down. For any given body weight, the higher the body fat %, the more buoyant the swimmer will be. This in turn means that less of the body will be under the waterline, which will in turn mean less drag to overcome – ie more of the propulsive force can be turned into forward motion.

Hydrodynamics and form drag

Like many things in life however, it’s not that simple. While extra buoyancy in the water confers real advantages, piling on pounds of fat on the human body can result in slower movement through the water, and that’s down to something called ‘form drag’. As anyone who’s ever watched a wildlife program on seals will testify, the transformation of the seal from a fat, lumbering and awkward looking creature on land to the epitome of beauty and grace in the water is amazing to witness. But while seals carry typically around 30-50% body fat(1), it’s distributed evenly around the body and in a manner that doesn’t impede its extremely efficient hydrodynamics.

However, when humans carry extra body fat, it’s carried unevenly. As a swimmer fattens up in the abdomen, thigh, and buttock areas, movement through the water produces swirling eddy currents around these protruding areas and can slow swimming velocity appreciably – this is form drag, which works against the buoyancy gains.

In the natural world, fast swimming fish and mammals such sharks and dolphins all have exceptionally low form drag. A marlin can weigh nearly a tonne yet clock up underwater speeds approaching 50mph! This is largely due to its exceptionally low form drag that results from its almost ‘Concorde’ shaped frontal profile, combined with powerful musculature. It also explains why boats have slim smooth hulls with tapered ends rather than hulls that are brick-shaped or with protrusions sticking out!

Effects of form drag

Form drag is also increased by poor alignment when moving through the water. For example if your legs begin to drop or your head rises in relation to your trunk, this present an additional frontal area, which increases form drag (see figure 1). There’s also ‘skin drag’, which is a frictional drag caused by turbulence as water moves over the skin’s surface. However, for the purposes of this discussion, we’ll restrict ourselves to considering only those aspects of swimming performance directly related to body composition.

Figure 1: Form drag and alignment in the water

Increasing body fat in swimmers increases buoyancy (aiding performance) but also increases form drag and buoyancy (a hindrance). The obvious question therefore is which exerts a more powerful effect as body fat levels rise? To answer this, 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(2).

This was achieved by fitting latex pads under a spandex triathlon suit in the same areas where swimmers might be expected to gain body fat – ie the abdominal, hip, thigh, chest, back, and buttock areas. Microscopic balloons were also 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. Before and after the ‘artificial fat gain’, each athlete swam 50yd freestyle at maximum effort in a counterbalanced design, swimming twice under each condition (to ensure any differences were due solely to ‘fat’ changes and not fatigue).

While the artificial fat improved buoyancy, it also slowed the swimmers down considerably, increasing the 50yd time by around 0.8secs or about 0.2secs per additional pound of fat added. In other words, the detrimental effects of increased form drag greatly outweighed any benefits of increased buoyancy.

Does this mean that lower body fat is always beneficial to a swimmer? Not necessarily. As the authors of the above study cautioned, their subjects were chosen to have 25% or less body fat for females and 15% or less for males. However, these are not particularly lean values for athletes; it could be that their buoyancy may have been adequate already with little to gain by adding more. However, the same might not be true for very lean swimmers, where very low body fat in the lower body can make it harder for swimmers to keep their legs horizontal in order to maintain a streamlined position. Where this is the case, extra buoyancy may help aid a streamlined position, resulting in a net drop in overall drag.

Gender may also be important in this respect. All other things being equal, women tend carry more body fat than men, and thus will tend to be more buoyant in the water. Moreover, female fat tends to be disproportionately distributed in the lower half of the body, giving a bit more lift to the legs, which in turn reduces form drag.

But while a very lean male swimmer may gain a net advantage by increasing his body fat somewhat, increasing buoyancy, this certainly doesn’t provide a licence to pack on the pounds. That’s because males tend to put on excess body fat in the abdominal region; an expanding waistline shifts their buoyancy forward, which in turn tends to make the legs sink, increasing form drag. This is easily demonstrated by the fact that most men can swim faster with a float between their legs than without it. However, most women experience little or no improvement when swimming using a leg flotation device!

Optimum body composition

All of this begs the question of what is the optimum body composition for efficient swimming? That’s a tricky one to answer, as it can depend on so many other factors such as body distribution, body shape and the nature of the swimming event.

Emeritus Professor David Costill, a highly respected exercise physiologist and masters swimmer, has suggested that in masters swimming at least, optimum body fat levels range from 10% to 20% for men and from 15% to 25% for women(3). However, US researchers at the highly regarded Councilman Research Lab at Indiana University claimed that body composition may not be particularly relevant in sprint performance and that (in men especially), muscular power is what really counts(4).

By contrast, a more recent study on male water polo swimmers suggests just the opposite(5). In the study, Greek researchers took anthropological and physiological measurements from nineteen professional water polo players. These included body composition (using a highly accurate X-ray based technique known as DXA), lactate threshold, the energy cost of swimming, peak oxygen consumption, anaerobic capacity and shoulder strength.

The researchers first set out to determine the polo players’ average lactate threshold and found that it occurred at a swimming velocity of around 1.33m per second and at a heart rate of 154bpm. They then measured the average energy cost was for swimming at this ‘lactate threshold velocity’ and discovered that it was in the region of just over 1kJ per meter. When they then looked at how this was affected by body composition, they discovered that the higher body mass indexes (BMI – weight in kilos divided by height in meters squared) of the players were correlated with higher energy costs – ie the higher the players’ BMIs, the more inefficient they were at moving through the water at lactate threshold pace.

However, a word of caution; higher BMI and body fat % are not the same thing. Although high BMIs are often associated with high levels of body fat, this is not always the case – for example in athletes who are very lean but have large bone structure and who carry large amounts of lean muscle. That said, lower BMIs tend to point toward slimmer, leaner physiques and in this study at least, more efficient movement through the water at higher velocities.

Body composition pressures

Competitive swimmers are often young and therefore impressionable. This makes them vulnerable to pressures to conform to the ‘ideal’ notion of body composition. These pressures can come not only from their coaches who may have pre-determined (and often unscientific) ideas about what weight/body composition their swimmers should be, but also from the fact that competing in swimsuits in the public arena can add further pressure.

Studies in the 90s reported that swimmers often feel pressure to drop weight(6) and that many coaches of female Olympic swimmers encouraged their swimmers to lose body fat in order to cut times(7). More generally many swimming coaches routinely advocate around 15% body fat as an upper-end cut-off for elite female swimmers.

However, when double Olympic gold medallist Tiffany Cohen won her 400m and 800m golds at the 1984 Los Angeles Olympics, her body fat was reported at 22%. This is not to say of course that 22% is the ideal level of body fat for females swimmers. It merely illustrates that there are no hard and fast body composition rules about what constitutes the optimum body fat % for a particular swimmer because every individual possesses a unique blend of physiological and anthropological characteristics.

In a study on 62 female swimmers from 7 US college swim teams, researchers set out to assess the pressures to conform to weight ‘norms’ experienced by the swimmers(8). Over half (51.6%) of swimmers agreed with the statement, “There are weight pressures in swimming.” The most commonly cited pressures were as follows:

  • *Wearing a revealing swimsuit – 45.2%;
  • *A perception that lower weight helps swim performance (42%);
  • *Teammates noticing my weight (16.1%);
  • *The crowd scrutinising my body (12.9%);
  • *The belief that the lightest swimmers have a performance advantage (9.7%).

The focus on achieving a headline body composition figure rather than achieving improved swimming performance is not only unhealthy and unproductive, it can signal the start of more serious problems of self-perception, and may result in eating disorders.

Interestingly, although competitive suits are typically one-piece styles, many participants reported that they wore swimsuits two or more sizes smaller than their typical size and some even wore youth sizes in order to prevent drag. This is consistent with the belief that decreased weight and body fat are associated with increased performance. Unfortunately young swimmers then preach the same beliefs when they become coaches themselves. Coaches therefore should approach this issue with caution and recommendations are given in panel 1 below.

Panel 1: Recommended coaching strategies to minimise ‘weight pressures in swimmers (Reel and Gill, 2001)(8)

  • Eliminate weight requirements and weight-related goal setting.
  • Avoid group weigh-ins.
  • Allow team members to choose team suit whenever possible.
  • Educate swimmers about muscle weighing more than fat.
  • Encourage swimmers to meet caloric intake needs.
  • Discourage team members from making weight-related comments to other swimmers.
  • Evaluate your beliefs about weight-performance relationship.
  • Monitor swimmers’ eating behaviour/body concerns and look for “at-risk” swimmers.
  • Listen to swimmers’ concerns about weight and body.
  • Encourage “at-risk” swimmers to keep a food log to ensure adequate caloric intake.
  • Be prepared to refer an athlete as needed.
  • Watch comments that suggest swimmers should drop weight to cut times.


Unlike most other weight-bearing sports, there’s no simple answer as to what constitutes an ‘optimum body composition’ for swimmers. Both excessively high and low levels of body fat appear to be detrimental to performance, yet striving for the ‘perfect’ level may not only fail to produce performance gains, but can also lead to unhealthy body image problems and even eating disorders.

Aiming for a ‘perfect’ body composition measurement is probably unproductive; a far better solution is to monitor body composition data (eg using skinfold measurements) in a training diary alongside your performance times. There’s a high likelihood that your optimum body composition will be the composition that accompanies your best performance times!


  1. Marine Mammal Science 10 (3), 332–341, 1994
  2. Journal of Strength and Conditioning Research, vol. 8(3), pp. 149-154, 1994
  3., 1994
  4. Highlights from the USA Swimming Conference on Sports Medicine and Sports Science in September 2003
  5. J European Appl Physiol, vol 95, (1), September 2005
  6. Journal of Health Education, 24(6), 360-368 1994
  7. Physician and Sportsmedicine, 18(4), 116-122 1990
  8. Sport Journal, vol 4(2), Spring 2001

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