On the marathon course, the elite runners at the front are always the focus. We enjoy watching their running form and analyzing their tactics.

At the same time, the elites' slender physiques are always a topic of discussion. So much so that many people have doubts about the sport:

How do they run so fast while being so thin?

So do you have to be thin to run a marathon?

Does losing weight necessarily make you faster?

First, it’s clear that compared with heavier athletes, being of normal weight or relatively lean is more advantageous for running.

A simple experiment: running normally versus running with a 30-jin (about 15 kg) load on your back. Obviously, you feel much more tired in the latter condition, although the experiment is quite extreme because body fat usually accumulates in specific areas, such as the abdomen. But even this simple test provides a direct comparison of how increased load affects speed.

If you compare the human body to a car — muscles are the engine and food is the fuel — then with the same engine, a heavier car needs more fuel. It's the same for people: greater body weight requires more energy. Using that extra energy lowers performance — because surplus weight only makes you more fatigued.

Moreover, for running this isn’t just a matter of energy expenditure and being more tired. Running relies primarily on lower-body strength, with the hips, thighs, calves and multiple joints all involved. So the heavier the body weight, the greater the load on the lower limbs and joints during movement—especially during the prolonged effort of a 42-kilometer marathon.

So, carrying excess weight not only affects running efficiency, the load on joints—especially the knees, hips and ankles—is greater. Being overweight not only reduces performance but also increases the risk of joint injury, leading to pain, inflammation and even long-term problems.

Put simply, when your weight increases over a period of time, you'll feel noticeably more tired during training, your body won't feel light, and your speed won't improve. As you control and lose weight, your stride will become lighter.

Runners who train consistently over the long term are more sensitive to the effects of weight changes.So, how does weight affect marathon pace? If you want to quantify the impact of weight on performance, there is a rule of thumb expressed as a calculation—the so‑called "1% rule": when body weight increases by 1%, running speed correspondingly decreases by 1%.

Suppose you weigh 70 kg and are currently running at a 5:00 per-kilometer pace. If your weight increases to 70.7 kg (a 1% increase), then by this rule your pace would slow by 3 seconds per kilometer. That number may seem insignificant, but note: a 0.7 kg weight change is simply a normal physiological fluctuation.

For example, if your weight rises from 70 kg to 77 kg (a 10% increase), your pace would slow by a cumulative 30 seconds per kilometer — a fairly significant effect. Over the course of a race you would be about 21 minutes slower.

Another rule of thumb is: for every 1 kg increase in body weight, your time to run 1 mile (about 1.6 km) will increase by roughly 2 seconds. The effect on a single mile may seem small, but over a full marathon (42.295) it's a different story — each additional 1 kg of body weight will add 52 seconds.

It’s important to note, however, that “heavier” doesn’t always mean “slower.” For some runners who naturally have low muscle mass, appropriately increasing muscle weight is vital for fast, powerful running.

More importantlyAn increase in muscle mass can actually improve athletic performance: Stronger muscles let you run faster and more efficiently while reducing injury risk. They better support the joints, coordinate force production, and effectively disperse the load and impact generated during activity.

As recreational runners, if we want to get faster we also need to understand our body type — after all, not everyone is born with the long limbs and low body-fat percentage of elite athletes.

Moreover, if we look at more marathon races, we'll find that this body type isn't the only standard.Having stronger, more powerful muscle reserves, so while we cut weight, weshould place more emphasis on building muscle.

There’s no denying that if you want to perform better in a marathon, sensible weight lossis naturally beneficial both for overall health and for athletic performance. However, to run more safely and to boost performance, alongside appropriate weight loss you need to prioritize building and strengthening muscleto get the most out of your training.

However, losing weight is not easy and it's easy to "go off track." When we overly restrict nutrient intake or create too large a calorie deficit to lose weight, body weight can drop sharply for a time, but what is lost more is muscle rather than fat. At the same time, under extreme weight-loss conditions, the body will compensate by slowing metabolism to conserve energy, so sometimes reducing energy intake can actually lead to storing more fat.

A gradual approach to weight loss is necessary to avoid negative effects on performance, after all we need to focus on improving running efficiency rather than weight loss; weight loss should serve running. Just as we increase training load by 10%-15% per week, weight loss should also be kept to about 0.25 to 0.45 kilograms per week. In addition, choose the timing for weight loss carefully — it's best before formally starting training, during the off-season, or in the base training phase, rather than risking interference with specific training.

When it comes to diet, we need to consider training, nutrition, and body composition — focus on controlling intake rather than dieting. Controlling intake doesn’t mean eating less or simply going hungry; it means reasonably adjusting your daily meals. For example, while ensuring the body gets necessary nutrients — protein, vegetables, carbohydrates, and fats — avoid high-sugar, oily processed foods and cut down on high-calorie snacks.

Dieting alone may produce quick weight loss, but to maintain your target weight long-term you need to pair it with exercise. Running is an excellent starting point for losing weight and is one of the aerobic activities best suited to slimming. When diet and training form a closed loop, the energy you consume is accurately converted into fuel for exercise rather than being stored as excess fat. If we look closely at the training behind top athletes, they never deliberately restrict their diet, because they need sufficient energy to sustain year‑round high‑intensity training.

It's important to stress that our baseline is "health." An ideal body weight is one that allows you to maintain consistent training at your best — without interruption from injury or fatigue — and that provides enough energy for recovery. All of this will enhance your athletic performance.

Text:CC / Editor:CC

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