This article is from Racing Weight, Matt Fitzgerald’s proven weight-management program designed specifically for endurance athletes.
It’s the most obvious reason. We’ve all felt the heaviness that comes with fatigue while running or riding uphill. Swimmers that have to move larger limbs get tired more quickly, too.
Competition for Oxygen
One of the most crucial underpinnings of endurance performance is the ability to deliver oxygen to the working muscles at a high rate. As body-fat levels go down, aerobic capacity goes up, because muscle has less competition from fat tissue for oxygen and fuel.
The primary function of body fat, of course, is insulation. An athlete’s ability to dissipate heat is an important performance factor in all forms of long-distance racing. While this ability is partly a function of the ratio of body surface area to body volume, this ratio is smaller in bigger athletes. So excess body fat impedes heat dissipation. It’s easier to stay cool on a long ride on a hot summer day if you’re very lean.
Fat-Burning vs. Carb-Burning
Sure, heavier athletes have more mass to move, but body fat isn’t just dead weight. Fat is a metabolically active organ that affects exercise metabolism in important ways that are not yet fully understood. One thing we do know is that athletes with larger amounts of body fat burn less fat and more carbohydrate at lower exercise intensities. Since the body burns carbs during racing but can only store small amounts of carbs, less lean athletes will burn through their valuable carb stores before leaner athletes.
Lighter cyclists can accelerate more efficiently. Good criterium riders are often smaller riders who can match surges in the race more quickly than others.
Swimmers with heavier limbs have to use more energy to move their arms and legs.
But small athletes are often at an advantage on flat courses. Why? Inertia and hydrodynamics.
In swimming, the fastest athletes tend to be tall and rangy instead of broad. Their narrower bodies present less frontal area in the water, making them more hydrodynamic.
But Get This…
Excess fat hurts performance, but excess muscle is in fact even more detrimental because it is far more dense, which is why we’re as unlikely to see a muscle-bound Tour de France winner as an obese one.
But what constitutes excess muscle is very different from what constitutes excess body fat since muscle is the engine of movement whereas body fat makes no contribution to endurance performance beyond providing energy for low-intensity exercise. Even the skinniest runner carries enough body fat to fuel 24 hours of continuous exercise.
Too Much Fat Is Bad for Athletes in Many Ways
Not only are top-level athletes quite lean, but also body composition is an excellent predictor of performance at all levels of endurance sports.