Harmful substances from protein burning: what forms in your body (2026)

When proteins are burned, three main substances are released that your body needs to process: ammonia, urea and uric acid. Ammonia is directly toxic and is converted in the liver into urea, which is excreted via the kidneys. At very high protein intakes, ketones can also form. Your body normally handles this well, but excessive intake puts extra strain on liver and kidneys.

What happens when proteins are burned?

Proteins consist of amino acids, the building blocks of muscles, organs and enzymes. When your body needs energy and not enough carbohydrates or fats are available, it breaks down proteins. That process is called catabolism. The amino acid chain is split: the carbon part delivers energy, the nitrogen part has to be removed. That splitting of nitrogen is called deamination, and that is exactly where the waste products are formed that play a role when protein intake is excessive.

Proteins as an energy source

Proteins are primarily intended as building material. Your body only uses them as an energy source when carbohydrates and fats fall short, or when you structurally eat more protein than you need. With strength athletes on a high-protein diet or people eating very few carbohydrates, this happens more often than you might think.

The difference with fat and carbohydrate burning

When carbohydrates and fats are burned, mostly carbon dioxide and water are released — substances your body simply breathes or excretes. With proteins it is different. The nitrogen in amino acids yields by-products that become harmful when they accumulate. That makes protein burning biochemically heavier on your organs.

Which harmful substances are released?

As soon as amino acids are broken down, your body produces a series of by-products. Some are mild at normal intake, others are directly harmful at high concentrations. The three main ones are ammonia, urea and uric acid. Ketones play a side role with extreme diets. source: Voedingscentrum

Ammonia

Ammonia (NH3) is the first waste product released during the deamination of amino acids. At high concentrations it is toxic for brain cells and the nervous system. The liver picks up ammonia almost immediately and converts it into a safer substance. But with liver disease or persistently extreme protein intake, ammonia can still accumulate, with confusion and in serious cases liver failure as a consequence.

Urea

Urea is the packaging the liver makes to transport ammonia safely through the blood. Via the bloodstream, urea travels to the kidneys, which excrete it via urine. A normal urea level lies between 2.5 and 7.5 mmol/L. If you structurally eat a lot of protein, your urea values rise and your kidneys work harder. For people with reduced kidney function this is a serious point of attention.

Uric acid and ketones

Some amino acids are, via a detour, converted into uric acid. Too much uric acid in the blood can form crystals in joints, better known as gout. Ketones are produced when, on an extremely high-protein and low-carbohydrate diet, you burn fats and proteins as primary fuel. With a normal varied diet this hardly plays a role, but on a strict ketogenic diet it becomes a factor to consider.

How does your body process these substances?

Your body has two organs that take charge of protein waste products: the liver and the kidneys. As long as both function well, there is little to worry about.

Liver and the urea cycle

The liver is the central processing factory. Through the urea cycle, a series of five enzymatic steps, the liver converts toxic ammonia into urea. This process costs energy and requires sufficient fluid. A liver disorder, prolonged excessive alcohol consumption or a congenital enzyme defect can disrupt the cycle, allowing ammonia to accumulate in the blood.

Kidneys and excretion

The kidneys filter urea, uric acid and other waste products from the blood and excrete them via urine. The more protein you eat, the harder the kidneys work. With dehydration, urine production drops and waste products circulate longer in the blood. Therefore drink at least 1.5 to 2 litres of water per day, certainly if you are actively exercising.

Eating too much protein: risks according to the Voedingscentrum

For the average healthy adult a high protein intake is not directly dangerous, but structurally eating too much does carry risks.

How much protein do you really need?

For someone weighing 75 kg that comes out at around 60 grams per day. Strength athletes need more, up to 1.6 to 2.0 grams per kilogram, but even then there is an upper limit. Eating more protein than you use produces no extra muscle mass, only extra strain on liver and kidneys.

Signs of too much protein

If you recognise the following complaints, an excessive protein intake could be the cause:

• Persistent fatigue and a heavy feeling after meals
• Dark, strongly smelling urine due to increased urea excretion
• Joint pain or recurring gout attacks due to elevated uric acid

Always discuss long-term complaints with a dietitian or GP.

Athletes and protein: what we see at gym visitors

High-protein diets are wildly popular with strength athletes in the Netherlands. At virtually every Basic-Fit, SportCity or local gym in gyms in Amsterdam, gyms in Rotterdam or gyms in Utrecht, a shake counter with protein shakes is prominently in view. Understandable, but it calls for conscious choices.

Protein shakes in Dutch gyms

Dutch brands such as Body & Fit and XXL Nutrition are popular, both online and at the counter. In our compare gyms overview you can see which shakes offer the best quality for the price. Many athletes overestimate how much extra protein they need on top of a normal diet. Quark, eggs, pulses and lean meat already provide a strong base.

Practical shopping tips

Spread your protein intake across the day, combine animal and plant sources, and drink enough water so your liver and kidneys can dispose of waste products efficiently.