What’s the perfect body composition for triathletes?

Can fat make you faster? What does science say about the optimum body composition for swimmers, and what does that mean for triathletes? Andrew Hamilton assesses the implications

Female triathletes may benefit from a touch more body fat than men

For running and cycling, the optimum body composition is relatively straightforward. Since the propulsive force required to overcome gravity and generate movement comes from muscular contraction, it follows that a high power-to-weight ratio (plenty of lean muscle tissue and a minimum of body fat) is beneficial. But in the water things aren’t so simple because, unlike most other body tissues, body fat is less dense than water.


The upshot of this is that the more body fat a swimmer carries, the more buoyant they’ll be in the water. Higher buoyancy means that less of the body will be under the waterline, which in turn means less drag to overcome during movement through the water. But it’s not quite as simple as that because piling on pounds of fat can result in slower movement through the water due to ‘form drag’.

Excess fat on the human body tends to be carried unevenly, largely in the abdomen, thigh and buttock areas. Movement through the water produces swirling eddy currents around these protruding areas and can slow swimming velocity. This is what’s known as 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 presents an additional frontal area that increases form drag.


Increasing body fat in swimmers improves buoyancy (aiding performance) but also increases form drag (a hindrance). To see which effect dominates as body fat levels rise, scientists artificially increased body
fat levels by 2% or more in a group of swimmers who’d been swimming competitively for at least three years. This was achieved by fitting latex pads beneath a spandex tri-suit in the same areas where swimmers might be expected to gain body fat.

While the ‘artificial fat’ improved buoyancy, it also increased the 50-yard 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 extra fat increasing buoyancy.

Does this mean that lower body fat is always beneficial for triathletes? Not necessarily. Triathletes typically have very low body fat in the lower body, which can make it harder to keep the legs horizontal in order to maintain a streamlined position. A little extra fat (and extra buoyancy) could help aid a streamlined position, resulting in a net drop in overall drag.

Gender may also be important in this respect. Female fat tends to be disproportionately distributed in the lower half of the body, giving a bit more lift to the legs that, in turn, reduces form drag. By contrast, males tend to put on excess body fat in the abdominal region; an expanding waistline shifts their buoyancy forward, which tends to make the legs sink, increasing form drag.

How important is body fat percentage for athletes?


What constitutes the optimum body composition for triathletes is tricky to answer because it depends on so many other factors such as body distribution, body shape and the nature of the swimming event. Some researchers claim that optimum body fat levels range from 10% to 20% for male swimmers and from 15% to 25% for women, while others have suggested that body composition may not be particularly relevant in sprint performance and that – in
men especially – muscular power is what really counts.


Excess body fat hinders cycling and running performance and increases form drag in the water. This may be less important for female triathletes or for very short swimming events.

Triathletes with lean and muscular legs should pay particular attention to optimising form in the water.


Don’t get too hung-up on optimum body composition. Monitor it using callipers or a body fat monitor; your optimum composition will be one that accompanies your best times.