Health Nutrition

The fine distinction: Body fat gain vs. Weight gain

Gaining weight does not necessarily mean that you have put on body fat. Indeed, the numbers on the scale don’t always tell the whole story. In this article, we want to show why, for example, right after starting to run, you might gain weight, and what other reasons there are for weight gain.

Types of weight gain

1. Increase in body fat

The increase in body fat typically occurs due to a caloric surplus, meaning when you consume more calories than you burn. To gain approximately 1 kilogram of fat, you need to consume about 7,000 kcal more than your body burns. The reduction of 1 kilogram of body fat requires a similar caloric reduction. This can vary depending on your daily caloric deficit (when we consume fewer calories than we burn). For example, a daily deficit of 500 kcal would theoretically result in losing 1kg of fat in about two weeks.

2. Gain in muscle mass

Gaining muscle mass requires both training and adequate intake of proteins and calories. Your body needs to be in a slight caloric surplus to effectively build muscles. The increase in muscle mass is a slower process compared to fat gain. For beginners, a gain of 1 kilogram of muscle mass can occur over several weeks or months, depending on training intensity, diet, training experience, and genetics. Loss of muscle mass can occur quite rapidly due to inactivity or inadequate nutrition, especially if you’re not training regularly.

3. Water retention

Water retention (accumulation of excess water in your body’s tissues) can quickly lead to weight gain, especially due to dietary factors such as high salt intake, carbohydrate intake, and hormonal changes. A weight gain of 1 kilogram caused by water can occur in a short period, often even overnight. Reducing water retention can be accelerated by reducing sodium intake, increasing water intake, and regular exercise, often within a few days.

4. Glycogen storage

Carbohydrates are stored in your body as glycogen, with each gram of glycogen binding approximately 3-4 grams of water. An increase in carbohydrate intake can thus quickly lead to weight gain. This type of weight gain is mostly temporary and decreases with the use of glycogen stores through physical activity or reduced carbohydrate intake.

How training changes your body: muscle building and calorie consumption

As mentioned above, training can build up muscle mass and thus result in temporary weight gain. On the other hand, regular running training also increases calorie consumption, which, with a balanced diet, can create a calorie deficit and thus lead to a reduction in body fat. In addition, running improves the efficiency of the cardiovascular system, which paradoxically increases the basal metabolic rate – the amount of energy the body uses at rest.

  1. Strengthening of cardiac tissue: A stronger heart pumps more blood with each contraction, improving blood circulation efficiency.
  2. Increase in mitochondrial density: More mitochondria in your cells increase energy production.
  3. Improvement in capillarization: A greater number of capillaries improve muscle oxygen and nutrient supply and optimize metabolic waste removal.
  4. Muscle building during running: Running strengthens especially the muscles in your legs, buttocks, and lower back. This muscle strengthening is a natural adaptation to the effort. Muscles, which are metabolically more active than fat tissue, also burn more calories at rest, which can lead to weight gain on the scale even as body fat volume decreases. This promotes a tighter and more defined body.
  • Increased muscular activity: More muscle mass means higher energy expenditure, as muscles also require energy at rest.
  • Increased recovery processes: The state of increased post-workout repair (EPOC) boosts oxygen and energy requirements.
  • Increased heart and lung work: A stronger heart and more efficient lungs also require more energy at rest.


It is important to understand the various causes of weight gain and not to hastily assume an increase in body fat. For example, when starting to run, the development of new muscles can even result in short-term weight gain, or weight can be lost during a longer training break when muscle mass disappears. But even if sport and exercise can lead to weight gain in the short term, these effects are generally positive and a sign of healthy physical adaptations.

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