Ask the Tri Scientist: I’m already skinny. Will losing more weight make me faster?

Racing Weight runner's body composition body fat

Paul A. asks, “If I am 5′ 10″, 148 lbs, and about 10% body fat, will losing more weight help me get faster or am I already at the limit?”

The Scientist responds:

Let’s tackle this in two parts: first by talking about how generic weight influences your speed, then how weight from body fat affects you.

Weight’s Effect on Speed

Weight of any kind will hamper your performance in each of the three disciplines of a triathlon. In swimming, it will obviously cause you to sink further into the water, which is bad for equally obvious reasons. While running, it requires you to produce greater force to get your body to achieve flight and also increases the force of impact when you land. From a performance perspective, this is bad for the joints coming down and bad for the muscles on both takeoff and landing. On the bike, we are all aware of how weight pulls us backwards on a steep hill, but it also contributes to increased rolling resistance on flat surfaces as well.

Losing weight, be it from your body, your bike or your gear, yields a performance benefit. The problem is that small losses only yield small benefits. For most of us, the place where we can shed the most weight is from our own bodies through training and diet.

FASTER science of triathlon weight loss
Think about it: small weight losses = small performance gains.

Losing body fat has a dual speed benefit. Not only does it lighten the load you have to carry, it contributes to an increase in your physiological ability to maintain an aerobic effort. Your VO2 is a measure of your body’s aerobic efficiency.  As such, it is measured as the amount of oxygen you use per kilogram of body weight per minute. Holding your oxygen uptake the same, your efficiency increases any time you decrease your weight. That’s not factoring any actual increase in oxygen consumption you’ll develop as an adaptation to training.

So it seems like lighter is always better, but it’s not quite so simple. How much weight can athletes lose before it becomes worse for performance?

Racing Weight runner's body composition body fat
Used with permission |

It’s not just that too much weight loss can be bad for you, but also that losing weight the wrong way can be detrimental to your performance. When you look at the bodies of elite endurance athletes, keep in mind that they achieve their remarkable physical appearance through a regimen of long, intense training. Competing is their job, and many of them spend as much time on their bike or in the pool as the rest of us do at the office. Their fat loss occurs due to extraordinary physical activity. They train so much that their metabolism works at a higher rate than ours throughout the day. To put this in perspective, in 2008 a journalist attempted to eat like Michael Phelps for a day.

He didn’t make it through lunch.

The reason is that he wasn’t burning nearly as many calories as Phelps. The point here is that people draw dangerous conclusions about athletic performance when they use an athlete’s appearance as their only benchmark. If you think of the human body as an engine, pro athletes are F1 race cars. They are slender, go really fast, and burn through fuel at a tremendous rate. The rest of us are like a Corvette or Mustang. Yes, we’re faster than a lot of the other cars on the road, but we still have more “extra” weight and we go a little slower.

But we do ourselves a disservice if we try to substitute starvation diet methods for physical activity in an effort to achieve that pro athlete look. You’re essentially trying to run the car on an empty tank, and that can be more detrimental to your training and racing than any extra body fat. Athletes who go below a certain threshold body fat experience an interruption in their menstrual cycle, an inability to process fat-soluble vitamins (A, D, E & K), and in extreme cases hindered brain function.

In sum, even professional athletes need some body fat to maintain healthy function of their bodies, as do we all. But beyond that, most of us should not attempt to get that low-fat, high-speed body of the pros unless we can achieve it through the same methods.

Whether it’s a car or a human body, a racing machine has to be built and fueled correctly. Train smart and healthy, and consult your coach, dietician or other medical professional before trying to cut substantial amounts of weight.

If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make simple changes that add up to free speed and faster racing.