Running: Understanding the 'Skinny' Feeling After a Workout

Why do I feel skinny after running?

The sensation of feeling "skinny" or lighter after running is a common, transient physiological response primarily due to acute fluid shifts, muscle glycogen depletion, temporary changes in blood flow, and neurochemical effects, rather than immediate, significant changes in body composition.

Acute Fluid Loss and Dehydration

One of the most immediate and significant contributors to feeling lighter after a run is the acute loss of body fluids through sweating. During intense physical activity like running, your body produces sweat to regulate its core temperature. Depending on the intensity, duration, and environmental conditions, you can lose substantial amounts of water and electrolytes.

• Sweat Production: A typical runner can lose 0.5 to 2.0 liters of sweat per hour. This immediate reduction in total body water volume directly translates to a temporary decrease in body weight and a feeling of being "lighter" or "drier."
• Temporary Effect: This is not true fat loss but rather a transient state of dehydration. Rehydrating after your run will restore your fluid balance and your body weight.

Muscle Glycogen Depletion

Your muscles store carbohydrates in the form of glycogen, which is the primary fuel source for moderate to high-intensity exercise like running. Each gram of glycogen stored in your muscles and liver is bound to approximately 3-4 grams of water.

• Glycogen as Fuel: As you run, your body taps into these glycogen stores to power your muscles.
• Water Release: When glycogen is metabolized, the water bound to it is released and can be excreted or redistributed.
• "Flat" Muscle Appearance: Depleting these glycogen stores can lead to a temporary reduction in muscle volume, contributing to a "flatter" or less engorged feeling in your muscles, which might be perceived as feeling "skinny." This is distinct from the muscle "pump" experienced immediately during resistance training, which is due to increased blood flow and fluid accumulation within the muscle.

Blood Flow Redistribution and Visceral Effects

During running, your body prioritizes blood flow to working muscles and away from less immediately critical functions, such as digestion.

• Splanchnic Vasoconstriction: Blood vessels supplying your digestive organs (splanchnic circulation) constrict, shunting blood to your active skeletal muscles. This temporary reduction in blood volume in the abdominal region can alleviate any pre-existing bloating or distension, contributing to a feeling of a "flatter" stomach.
• Reduced Gut Content: If you ran on an empty or partially empty stomach, the movement and the temporary redirection of blood flow can also contribute to a feeling of emptiness or lightness in your abdominal area.

Neurochemical and Hormonal Responses

Exercise induces a cascade of neurochemical and hormonal changes that can influence your perception of your body.

• Endorphins and Mood: The release of endorphins and other neurotransmitters post-exercise can elevate mood and create a sense of well-being, potentially influencing a more positive self-perception and body image.
• Hormonal Shifts: Hormones like adrenaline and noradrenaline are elevated during exercise, which can transiently affect metabolism and fluid balance, further contributing to the post-exercise sensation.

Perception vs. Actual Body Composition Change

It's crucial to distinguish between the immediate, temporary sensation of feeling "skinny" and actual, lasting changes in body composition.

• Acute vs. Chronic: The feeling described is an acute physiological response that resolves as you rehydrate and refuel. It does not represent immediate fat loss or significant muscle mass reduction.
• Long-Term Adaptation: While a single run won't make you "skinny," consistent, well-structured running combined with a balanced diet can indeed lead to long-term improvements in body composition, including reduced body fat and increased lean muscle mass. This is a gradual process of adaptation, not an instantaneous outcome.

In summary, the "skinny" feeling after a run is a natural, temporary consequence of your body's impressive ability to adapt and perform under stress. It's a sign of a successful workout that has challenged your physiological systems, leading to acute fluid and fuel shifts.