Fat metabolism during exercise

Fat metabolism changes noticeably during prolonged physical exertion — the body gradually switches from burning carbohydrates to burning fatty acids. The longer and more moderate the workout, the bigger the role of fat. This is not a myth; it’s physiology.

How does fat metabolism work during physical work?

During short, intense effort, the body prioritizes glucose — more precisely pyruvic acid, which is formed when sugars are broken down. In prolonged work, however, free fatty acids come into play. They are the other key substrate that gets oxidized.

The simple truth: the more fatty acids there are in the blood, the more your muscles use. Fatty acid uptake and oxidation depend directly on their concentration in the blood.

Where do these fatty acids come from? The body’s stored fat — adipose tissue — is broken down and releases fatty acids into the blood. This process is called lipolysis, and it doesn’t happen by chance.

The hormones that trigger lipolysis

During prolonged training, several hormones kick in that together ensure efficient fat mobilization:

• Adrenaline — released from the adrenal glands under training load; it activates the breakdown of adipose tissue.
• Noradrenaline — a hormone released from the endings of sympathetic nerves; it acts similarly to adrenaline and strengthens lipolysis.
• Cortisol — a hormone of the adrenal cortex whose level rises during prolonged training load. It has lipolytic activity.
• Growth hormone — a hormone produced by the pituitary gland that also stimulates the breakdown of adipose tissue, especially after intense training.

These hormones don’t work in isolation — they form a coordinated system that responds to the duration and intensity of the training load.

Fat metabolism and the drop in insulin during prolonged work

Here we have one of the body’s most elegant regulatory mechanisms. During prolonged physical work — lasting several hours — the concentration of insulin in the blood falls. This sets off a series of changes.

First, the conditions for mobilizing stored fat become favorable precisely when blood insulin drops. Second, at the same time, glucose transport into muscle cells becomes harder — cell membranes become less permeable to glucose.

So what is actually happening? The drop in insulin is a kind of signal to the body: switch from carbohydrates to using fats as the substrate for oxidation. It is a regulatory measure that optimizes energy use under conditions of prolonged training load.

Calculations show that during prolonged work, fat accounts for 60–90% of total energy expenditure. That is a remarkable figure. For much of the time you spend on moderate, prolonged exercise — jogging, cycling, hiking — your body is burning mainly fat.

At the same time, glucose uptake by the muscles decreases. Glucose is directed preferentially to nerve cells — the ability of their cell membranes to let glucose through does not depend on insulin. The brain and central nervous system have to keep working regardless of what is happening at the muscle level.

Fat metabolism in practice — what every trainee should know

A few concrete conclusions follow from these physiological mechanisms:

• Duration matters. The share of fatty acids as an energy source grows as training goes on. In the first minutes glycolysis dominates; in prolonged work fat takes over.
• Intensity affects the balance. In very high-intensity training there isn’t enough oxygen to fully oxidize fatty acids — the body uses mainly glucose. At a moderate pace, fat can play a bigger role.
• Meal timing affects insulin. A large carbohydrate-rich meal right before training raises insulin, which slows the mobilization of stored fat. That is why many trainees prefer long morning cardio before eating.
• Hormones activate on their own. The body’s hormonal system starts fat breakdown automatically during training — adrenaline, cortisol and growth hormone work without any extra help. What matters is sufficient duration and a moderate pace.

FAQ: fat metabolism and training

Why is prolonged training better for burning fat?

During prolonged work, the level of insulin in the blood falls, which creates favorable conditions for mobilizing stored fat. Calculations show that during prolonged work, fat accounts for 60–90% of total energy expenditure.

Which hormones control the breakdown of adipose tissue?

Mainly adrenaline and noradrenaline, which is released from the endings of sympathetic nerves. Cortisol, a hormone of the adrenal cortex, and growth hormone, produced by the pituitary gland, also have lipolytic activity.

Why does the body conserve glucose during prolonged training?

Glucose is the primary fuel for the brain and nerves — their cell membranes let glucose through independently of insulin. During prolonged training load, the body reserves glucose for the central nervous system and switches the muscles to fatty acids.

Supplements for recovery: https://turg.fitness.ee/et

Author: Ero Viik, based on various sources

Source: WHO – healthy eating.

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