What happens to excess dietary protein in a 70 kilogram adult male consuming over 300 grams of protein per day in an attempt to build muscle?

From the Guidelines

When a 70 kg adult man consumes excessive protein (over 300 grams daily) for muscle building, the excess protein is not stored as muscle tissue and is instead metabolized by the liver, placing additional stress on the kidneys and liver. The body can only utilize a limited amount of protein for muscle synthesis, typically around 1.6-2.2 grams per kilogram of body weight per day (112-154 grams for a 70 kg person) 1. The surplus protein is metabolized by the liver through deamination, where amino acids are broken down, and the nitrogen component is converted to urea and excreted through urine, while the remaining carbon skeletons are either used for energy or converted to fat and stored 1. This process not only places additional stress on the kidneys and liver but may also lead to dehydration, as more water is needed to flush out the nitrogen waste products, and can cause digestive discomfort including bloating, gas, and constipation. Rather than improving muscle growth, protein intake beyond the body's utilization capacity simply represents wasted calories and unnecessary metabolic strain. High animal protein intake, in particular, may negatively influence glomerular hemodynamics, increasing the risk of kidney disease, and counseling patients to moderate animal protein intake may have beneficial effects on GFR decline for some patients 1. Key points to consider include:

• The daily whole-body protein turnover rate is approximately 5·7 g/kg body weight, meaning that about 400 g mixed proteins are turned over every day for a 70 kg adult individual 1
• Excessive protein intake may lead to an increased risk of kidney disease, particularly in individuals with pre-existing kidney issues 1
• A balanced diet with adequate protein intake, rather than excessive protein consumption, is essential for maintaining overall health and muscle mass.

From the Research

Excess Protein Ingestion

• When a 70 kg adult man consumes an excessive amount of protein (over 300 grams of protein per day), the excess protein is not stored as protein in the body, but rather is converted into other compounds 2.
• The amino acids from the excess protein are either used for energy production, converted into glucose or ketone bodies, or excreted in the urine 2.
• The liver plays a key role in processing the excess amino acids, and the kidneys are responsible for excreting the waste products, such as urea, in the urine 3, 4.

Renal Function and Excess Protein

• Chronic high protein intake (>2 g per kg BW per day for adults) may result in digestive, renal, and vascular abnormalities and should be avoided 2.
• High protein intake has been shown to increase kidney mass and function, as well as blood urea nitrogen levels 3.
• The kidneys have a limited capacity to excrete excess nitrogen waste, and high protein intake can put a strain on kidney function 3, 5.

Urea Production and Excretion

• The liver produces urea as a waste product of amino acid metabolism, and the kidneys are responsible for excreting urea in the urine 4.
• The kidneys have specialized transport mechanisms to regulate urea transport and excretion, and high protein intake can affect these mechanisms 4.
• Excess urea production can lead to increased urea levels in the blood, which can be harmful to the body 4, 5.