2 upvotes, 1 direct replies (showing 1)
View submission: Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology
what happens to the rest of the protein we eat?
It goes down the toilet. Ever notice you take a big dump the day after a feast?
Depending on the type of protein, maybe >90% doesn't get used by your body.
One of the most digestable proteins is casein from cow milk, and that is only about 8% usable.
You chew the food, it passes into your stomach where is starts to break apart into smaller peptides - it's still quite big. But that takes time so a large amount of time so your stomach does feel very full.
Then it moves into your small intestive where enzymes chop it up smaller into amino acids. To move into your bloodsteam requires energy from your body called ATP, so it if isn't needed, your body won't do the work and it keeps on passing through you.
Amino acids that do pass in the bloodsteam have to go to the liver to be processed. Your body then puts them into the general pool of amino acids available to build new muscle or storage as fat. However, again, if your body doesn't need more amino acids, it passes it along and you pee it out.
What is the role of protein in providing energy for us?
Main use is growing new or repairing damaged proteins in your body.
A secondary use is making some signalling molecules, hormones and enzymes in your body.
To be used as energy, your body needs to be deficient in glucose or ATP. It takes energy upfront to convert amino acids into a usable form of fuel. The pay-off is not worth it if any other source of energy exists.
In order to use amino acids to make ATP, glucose, or fat, the nitrogen first has to be removed in a process called deamination, which occurs in the liver and kidneys. The nitrogen is initially released as ammonia, and because ammonia is toxic, the liver transforms it into urea. Urea is then transported to the kidneys and excreted in the urine. If you ever have a simple urine test, one marker is always urea. High urea indicates you have health issues.
Comment by aTacoParty at 17/03/2023 at 14:22 UTC
2 upvotes, 0 direct replies
Where are you getting this information on dietary protein absorption? Casein is absorbed at a 75% efficiency (https://academic.oup.com/jn/article/139/9/1707/4670534[1][2]) and from measuring stool nitrogen we can find that most people absorb between 80-90% of dietary protein (https://www.annualreviews.org/doi/pdf/10.1146/annurev.me.41.020190.001025[3]). Eating high amounts of protein may saturate transporters but I haven't seen any sources that claim <10% of dietary protein is absorbed.
1: https://academic.oup.com/jn/article/139/9/1707/4670534
2: https://academic.oup.com/jn/article/139/9/1707/4670534
3: https://www.annualreviews.org/doi/pdf/10.1146/annurev.me.41.020190.001025
As for metabolism, you're absolutely correct that amino acids need to be deaminated before they enter glycolysis (at pyruvate for glucogenic amino acids) or the krebs cycle (at acetyl-coA for ketogenic amino acids with a few others entering at other stages). Generally protein is the last source of energy your body will use because amino acids are needed to keep every cell functioning. But they can come in handy during periods of starvation or when there is an abundance of them.
https://courses.lumenlearning.com/suny-ap2/chapter/protein-metabolism/