sQuestion: I have done a lot of study in diets and nutrition but to this day I have not been able to get any concrete evidence on what happens with excess protein in the body and I’m hoping you can help.
To make things simple, lets take a theoretical diet consisting of 5000 calories of pure protein for a 60kg, 175cm female.
Many people claim that excess protein will get wasted while others say that all excess calories eventually end up being stored as fat.
I have done my own research on the breakdown of protein into amino acids and I understood it as: some of the amino acids are wasted while others will go through the cycle of conversion and will still be used by the body for energy.
Answer: Ok, first things first. The example given above is absurdly non-physiological. The satiating power of protein would make such a high protein consumption impossible. That is, 5000 calories of pure protein is 1250 grams of pure protein. Can’t be done. Beyond that, while the biochemical pathways for the conversion of protein to fat do exist in humans, the likelihood of it ever happening in any but the most absurdly non-physiological circumstances are effectively nil.
Let me put this in perspective. Despite a lot of claims to the contrary, the actual conversion of carbohydrate to fat in humans under normal dietary conditions is small approaching insignificant (a topic I discussed at least briefly in Nutrient Intake, Nutrient Storage and Nutrient Oxidation).
Make no mistake, the conversion of carbs to fat (a process called de-novo lipogenesis or DNL) can happen but the requirements for it to happen significantly are fairly rare in humans under most conditions (to discuss this in detail would require a full article, interested readers can search Medline for work by Hellerstein or Acheson on the topic).
At least one of those is when daily carbohydrate intake is just massive, fulfilling over 100% of the daily maintenance energy requirements. And only then when muscle glycogen is full. For an average sized male you’re looking at 700-900 grams of carbohydrate daily for multiple days running.
Which means that the odds of protein being converted to fat in any quantitatively meaningful fashion is simply not going to happen. Certain amino acids are processed to a great degree in the liver (as I discuss in The Protein Book) and this can produce glucose, ketones and a few other things. But triglycerides (the storage form of ‘fat’) isn’t one of them.
I imagine that if protein were going to be converted to fat, it would first have to be converted to glucose and only if the amount produced were then in excess of daily maintenance requirements would there be conversion to fat. But as noted above, this simply isn’t going to happen under any even reasonably normal circumstances. No human could eat enough protein on a daily basis for it to occur.
What will happen, as discussed in Nutrient Intake, Nutrient Storage and Nutrient Oxidation. is that amino acid oxidation (burning for energy) will go up somewhat although, as discussed in that article, it’s a slow process and isn’t complete.
So, as noted above, while the pathway exists for protein to be stored as fat, and folks will continue to claim that ‘excess protein just turns to fat’, it’s really just not going to happen under any sort of real-world situation. Certainly we can dream up odd theoretical situations where it might but those won’t apply to 99.9% of real-world situations.