Can I Get Fat From Protein

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Question

Can I get fat from eating too much protein?

Short Answer

Yes, Calories in Calories Out (CICO). Excess calories will cause weight gain.

Before you go into the long answer below, please make sure you understand how CICO works.

Long Answer

Protein Catabolism

Amino Acid Metabolism

Protein Catabolism is the breakdown of proteins into amino acids. The amino acids that are produced from protein catabolism can be used to form new amino acids, used for energy in the body, or stored as fat.

Let's break this down a little bit more.

Protein Into Fatty Acids

With a caloric surplus, adequate carbohydrate intake, and full glycogen stores, amino acids are going to be moved away from gluconeogenesis & ATP production and instead converted into fatty acids which can then be stored as body fat.[1][2]

Protein breakdown is extremely complex and beyond the scope of this, but a byproduct of protein catabolism is a carbon skeleton which is made up of various amino acids. Different amino acids will have different fates:

  • Alanine, Glycine, Cysteine, Serine, & Threonine will be converted into pyruvate
  • Isoleucine, Leucine, & Tryptophan will be converted into Acetyl-CoA
  • Leucine, Lysine, Phenylalanine, & Tyrosine will be converted into Acetoacetyl-CoA
Protein Catabolism

All of these are or can be converted into a Acetyl-CoA which is the building block of fatty acids.

As you can see, protein does have the ability to be stored as fat. However, this is not likely the main reason that protein will cause fat gain.

Protein into Glucose

Gluconeogenesis is the process in which non-carbohydrates are turned into glucose. Glucogenic amino acids (those that can be converted into glucose via gluconeogenesis)[3] include:

  • Alanine
  • Arginine
  • Asparagine
  • Aspartic
  • Cysteine
  • Glutamic
  • Glutamine
  • Glycine
  • Histidine
  • Methionine
  • Proline
  • Serine
  • Valine
  • Isoleucine
  • Phenylalanine
  • Threoine
  • Tryptophan
  • Tyrosine

Excess protein will be broken down from dietary protein into amino acids into carbon skeletons, into pyruvate, into glucose (via gluconeogenesis), then into fat (via De Novo Lipogenesis (DNL)) - (Note that DNL is very rare in humans and most likely only if you are in a caloric surplus, and have full glycogen stores)[4]. While the pathway does exist for protein to be directly converted into glucose and into fat, this is rare and most likely never going to happen unless under extreme circumstances.

The most common cause for the increase in body fat from excess protein is the increase in amino acid oxidation which caused the body to use fewer carbohydrates and fats for energy and cause excess carbohydrates to be stored & fat to remain stored.

Protein Oxidation

Amino Acids that will convert to Acetyl-CoA

We have seen that while protein catabolism does have a pathway to convert amino acids into fatty acids, it is unlikely. We have also seen that while protein can be converted to glucose and then stored as fat, this is also unlikely.

The most likely reason that someone would gain body fat from excess protein is due to the use of amino acids in the Krebs Cycle after being converted into pyruvate or Acetyl-CoA. This means that the extra protein will be used to generate ATP (energy). This use of excess protein would mean that the use of carbohydrates and fats for energy would not be needed and thus excess carbohydrates would be stored as fat, new dietary fats would be stored, and existing body fat would continue to be stored [5].

References

  1. Gropper, S; Smith, JL; Groff, JL (2012). Advanced Nutrition and Human Metabolism. 6th Edition. Belmont, CA: Wadsworth Cengage Learning. p. 197. ISBN 978-1133104056. 
  2. McGuire, M; Beerman, KA (2012). Nutritional Sciences: From Fundamentals to Food. 3rd Edition. Belmont, CA: Wadsworth Cengage Learning. p. 184. ISBN 978-0840058201. 
  3. Brosnan J (1 June 2003). "Interorgan amino acid transport and its regulation". J Nuitr. 133 (6 Suppl 1): 2068S–2072S. PMID 12771367. 
  4. Bodyrecomposition: How We Get Fat
  5. Bodyrecomposition: Excess Protein and Fat Storage - Q&A