How much energy is lost converting protein to fat?

Suppose you consume 100 calories excess protein, and the body converts it into fat. How much energy is this process going to take? Obviously you won’t store all 100 cals as some will be used for the conversion.

It varies among different kinds of proteins, but the average figure I’ve heard is that you can metabolize three calories worth of glucose from every gram of protein you ingest. I don’t know how efficient converting glucose to stored fat is, though.

So since there are 4 calories in each gram of protein, I guess this would mean that 25% of the energy is lost in the conversion? wow…

First of all, you can’t just convert “lean” proteins into fatty acids; you actually need fats to modify and reduce some proteins, and carbs to fuel the process. You can convert fats and proteins to carbs via gluconeogenesis, but that is inefficient and produces toxic byproducts. You can also convert excess glucose into certain types of fats, but too much glucose at any given time will overwhelm the body’s blood sugar balance and will eventually cause pancreatic and kidney problems. It’s more than just calorie trade-offs; [post=6610923]here[/post]'s an old post regarding how the body breaks down proteins to create glucose. Then there are essential fatty acids that cannot be synthesized.

So there’s no simple answer to your question, but converting from protein>glucose>fat is highly inefficient; a high protein diet without sufficient fats (or starved of easy fuel carbs) will result in some degree of nutritional starvation, and it’s quite possible to die from having too lean a diet, regardless of how much protein you consume.


what are these toxic byproducts of which you speak?

Just to add to Strangers excellent description, protein is used by the body for energy as a last resort, normally it is used for building muscle etc. When you are starving and the fat is used up, then the protein gets processed next.

“You can convert fats and proteins to carbs via gluconeogenesis”

This is not correct. Humans cannot convert fatty acids to glucose. Specifically we lack the enzyme that converts acetyl CoA into pyruvate.

During lipolysis (in which unbound fatty acids are released from fat cells) ketones are produced; these are processed out by the liver, but when carbohydrate starvation is occuring they’re produced in excess which leads to ketosis. This can reduce pH levels in the body as well as stressing the liver and kidneys.

I general that’s true; however odd-chain fatty acids can yield propionyl CoA which can form oxaloacetate and thus feed into the citric acid cycle, as shown in this diagram.


Stranger, could I ask you to give us the benefit of your expertise in the rabbit starvation thread?
We’ve managed to ascertain for ourselves that eating nothing but lean rabbit (or any other meat) isn’t likely to keep you healthy, or to provide you enough energy to replace what you expend catching the meat, but we’re a bit stuck as to the relative quantity of energy you’d get from digesting it versus that needed to run the digestive tract.
So if we were hypothetically to find ourselves stranded with an inexhaustible supply of ready-to-eat pure protein, we don’t know if we’d be better of eating it or just going hungry, which is keeping me awake at night worrying.

Yeah. But they can only give us very little citric acid cycle intermediate, as compared to the total energy output. But it seems we agree on this subject. It wasn’t that important to your post anyway.