Body weight as a performance factor: Less is not always more
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Whether an endurance athlete can deliver good performance also depends on body build and weight. But when does a lower body weight actually lead to better performance? And where are the limits?
Anyone who has ever done a mountain hike with a heavy backpack knows that every extra kilo quickly becomes a noticeable burden. This exact physical effect also applies in endurance sports. The less weight the body has to move, the more efficiently it can work. Especially when working against gravity, such as running uphill or during the final phase of a marathon.
Less mass, more efficiency?
The principle is simple: a lighter body must lift, slow down, and accelerate less weight with each step. Especially in endurance sports like marathon running, cycling, or mountain running, a low body weight is often an advantage. Among women in long-distance events, the best are usually lightweight under 50 kilograms, and top male athletes are also light, hardly weighing more than 60 kilograms—while maintaining high relative muscle strength.
Body weight also plays a role in amateur sports: a rule of thumb says that losing one kilogram can save up to two minutes on a marathon distance. However, it is important to note: in endurance sports, it’s not weight alone that matters, but body composition. That is the ratio of muscle mass to fat mass.
Reduce body fat but not at the expense of muscle
Targeted reduction of excess fat mass can improve endurance performance. But as soon as valuable muscle mass is lost during preparation due to too large a calorie deficit or a one-sided diet, this effect can quickly reverse, leading to a significant and often long-term drop in performance. Muscle mass plays a central role in strength, running economy, and injury prevention.
Athletes benefit most when they maintain a balanced ratio between low body fat and sufficient functional muscle mass. This requires time, targeted training management, and a sports-appropriate diet that meets energy needs. Definitely not any radical diet.
REDs: When less becomes too much
In extreme cases, a too large calorie deficit or an excessive pursuit of the so-called “ideal weight” can lead to a condition known in sports as REDs (Relative Energy Deficiency in Sport). In this state, the body receives insufficient energy over a prolonged period, potentially causing serious health and performance consequences.
Typical symptoms of REDs:
- Menstrual disturbances (up to amenorrhea in women)
- Reduced bone density (osteopenia, osteoporosis)
- Chronic fatigue, susceptibility to infections
- Performance declines, injury susceptibility
- Psychological problems, irritability
REDs affects all genders and occurs particularly often in endurance or weight-class sports. Particularly insidious: performance may initially improve due to the lower weight until the body falls into an energy deficiency. This often triggers a vicious cycle of energy shortage, mental pressure, and frustration, which is very difficult for affected individuals to escape.
BMI is only a rough guideline
The Body Mass Index (BMI) is often used to classify body weight relative to height. However, BMI is only of limited relevance for athletic performance because it does not differentiate between muscle and fat mass. Endurance athletes with a BMI of 19 can be very capable if they have a high proportion of muscle and low body fat. Sprinters often have a BMI of 22 or higher due to their more pronounced musculature.
Balance is key
In practice, long-term successful endurance athletes often settle at a BMI of about 19–20, with individually varying distributions of fat and muscle mass. Much more important than an ideal “target BMI” is the balance between energy intake, training load, and recovery.
Those who are already very light and well-trained risk more by further weight loss than they gain. Performance does not increase indefinitely with every lost kilo. Eventually, there is a risk of strength loss, increased susceptibility to infections, or even burnout. Young athletes who are still growing or undergoing hormonal changes should be especially vigilant.
Conclusion: Slim is good, strong and healthy is better
A slim, light body can enhance endurance performance if it is strong, well-nourished, and balanced. The best time is not achieved by the lightest athlete but by the one who moves with energy, efficiency, and stability.
Therefore, body weight should never be viewed in isolation. What matters is the interplay of many factors: oxygen uptake, muscle mass, mental strength, coordination, nutrition, and recovery. And not least: a good dose of body awareness and self-care.
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