Do You Lose Fat When Doing Long-Distance Swimming?

During prolonged periods of physical exertion, fat is the main source of energy

It is well known that fat is used as the main source of energy during endurance exercise. A substantial loss of subcutaneous fat is observed after cycling and running efforts that went over several hours.

In a scientific study set out to determine how much fat and muscle mass a swimmer would loose during a long distance swim (in German only. Knechtle et al.: Einfluss von Langstreckenschwimmen auf die Körperzusammensetzung - das Marathonschwimmen 2006 von Rapperswil nach Zürich. Praxis 2007; 96:585-589). The 42 years-old swimmer swam the 26 km race in 8 hrs and 52 mins in the Lake of Zurich (22-23 °C).

The swimmer drank 1.25 litres during the race and ate 2393 kcal. He used 7037 kcal, resulting in a deficit of 4644 kcal. Theoretically, the swimmer should have lost about 1 kilo fat or 2 to 4 kilos muscle mass.

His body weight remained unchanged (74.9 kilos before and 75 kilos after the race), but he lost720 grams fat and 380 grams muscle mass, resulting in a loss of about 1 kilo of body mass.

It has been reported that the weight can be unchanged or even be increased after long endurance events. This is explained by fluid replenishment and, in swimming, water swallowing during swims [How much water do we swallow when we swim?]. Another explanation is water retention in body tissue (formation of oedema).

The loss of body mass can be explained by high demand in energy that was not covered by food. The swimmer used 781 kcal per hour, which is in accordance with the literature. He took up 265 kcal per hour.

Can you lose weight by swimming?

• Does Swimming Make You Fat?
• Swimming in Cold Water Makes You Hungry – Does It Help You Lose Weight?.