Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
9 Jul

Dear Mark: How Much Protein Can You Absorb and Use from One Meal?

steakEarly last week, I received an email from a reader. The subject was protein absorption, and it referred to something I’d written several weeks past in this post on Leptin Reset among other things. I had suggested that your body can only deal with about thirty grams of protein in one sitting. I immediately realized that this statement wasn’t nuanced enough and might give the wrong impression, so I explained what I meant in a bit more detail in the comment board. After receiving the following email (thanks for keeping me honest, Brandon!) I figured I would revisit this topic and further articulate what we know about the human body and dietary protein utilization.

Let’s go.

Hi Mark,

I had a question about protein intake. In an older post, you mentioned something about anything beyond 30 grams of protein in a single meal likely being turned into glucose or fat, but I’ve heard different things from others. So, how much protein can you really use in one meal?

Brandon

First off, “meal” is the key word, one that I omitted in my original statement. Eating 50 grams of straight protein – and protein alone, whether it’s whey protein or 99% fat-free lean sirloin – is probably going to have a different effect than eating a mixed meal which contains 50-some odd grams of protein in addition to fat and digestible, fermentable, and insoluble carbohydrates. In other words, protein is rarely eaten alone. It’s eaten atop salads, alongside veggies, with starches, and it often comes imbued with animal fat (as meat). It’s eaten as a meal.

Digestion takes a long time, and it’s not a segmented procession of different meals through the gastrointestinal tract. Food isn’t separated into “meals” in your stomach. It’s just all food, all mashed together. If you still have breakfast in your stomach when your lunch enters the picture, lunch and breakfast will meet and mingle. Stomach acid breaks the food down into a big semifluid mass of partially digested food components, water, digestive enzymes, and hydrochloric acid – all referred to as chyme.  When the chyme is “ready,” it’s pushed through the duodenum (where nutrient extraction begins) and into the intestines.

Once the protein part of the chyme has been broken down by enzymes into amino acids, amino acid absorption by the intestines can begin. Amino acid transporters  grab amino acids and transport them through the cells lining the intestinal wall and from there into the bloodstream to be delivered to other parts of the body. But because the pool of amino acid transporters is limited, amino acids are typically absorbed by the small intestine at a rate of 5-10 grams per hour. This would suggest, at first glance, that the original “30 grams of protein per sitting” figure is close to correct. If you can only absorb, say, 7 grams of protein per hour, and the bolus of digested protein and other foods takes 4 to 5 hours to wind its way through the intestine, you’re gonna absorb maybe 28-35 grams. Right?

Not exactly. It turns out that our digestive process is fairly fine-tuned and regulated by the composition of whatever’s being digested. The presence of protein in chyme actually causes the secretion of CCK, a hormone that slows down the intestinal contractions that move food along, thus giving the protein more time to be transported by amino acid transporters. So, while it’s technically true that we aren’t absorbing 30 grams (let alone more) of protein all at once, it will eventually be absorbed.

Clearly, a fair amount is directed to normal tissue growth and repair. If it’s much more than that, a few possibilities enter the picture. Some will be directed towards a short term storage option referred to as the “labile protein reserve.” In this case, skeletal muscle tissue will take up some of the excess amino acids and simply store then for possible near future use (like within a day or so or for emergencies). Beyond that, your body will want to oxidize excess amino acids directly for fuel or convert them to glucose in the liver via gluconeogenesis. It will convert the nitrogen from the amino acids into ammonia and you will excrete that in your urine. If levels get too high in the bloodstream, your pH will shift (more acidic) and calcium will be called upon to balance pH out again. As we have seen with some people going full Primal, larger intakes of protein can offset lowered carb intake by generating significant rises in glucose through gluconeogenesis. Indeed, there are a lot of options the body has when you eat a high protein meal.

I still wouldn’t “stuff” myself on protein. If you have to force feed that lean chicken breast, perhaps your body’s trying to tell you something. Upon digestion, excess carbs and fat can be limitlessly and easily absorbed and assimilated (as body fat or glycogen) or burned off, but excess protein still requires extra work. Protein digestion produces toxic metabolites that we can usually get rid of, even on higher protein intakes, but there’s clearly a limit. Our bodies finding protein to be extremely filling? Our intestines actually slowing down the digestive process when protein is present? This is just how our bodies absorb protein at their own pace. They take their sweet time, but they get it all.

Okay, so we do absorb most of the protein we eat, whether it’s a 30-gram whey shake that’s absorbed in a couple hours or a large porterhouse whose protein is absorbed in ten hours. We’re not pooping amino acids. But are we using all that protein? Is it doing uniquely proteiny stuff, like building muscle? Or will everything above 30 grams get converted into glucose?

That depends. Many factors affect how your body utilizes a given amount of protein:

Body Size

If you’re a bigger person (longer limbs, more potential spots for amino acids to be utilized), you can handle more absolute protein, and that protein will be more likely to go toward muscle protein synthesis.

If you’re smaller, the reverse is true. You simply don’t need as much absolute protein for structural demands, and you’ll have a lower threshold before protein becomes an energy substrate. If you eat the same steak as the bigger guy (all else being equal), you won’t “use” as many of the amino acids as he will.

Activity Level

Are you sedentary? You require less protein. Your muscles aren’t getting the signal to build and grow stronger and adapt to an imposed demand. The RDA of 0.36g/lb bodyweight should suffice.

Are you moderately active, jogging here and there, maybe riding the bike to work once or twice a week, and doing some basic lifting in the gym? You can use a bit more, as physical activity increases protein demand.

Are you actively trying to pack on muscle mass, and working out accordingly? You can use a lot more protein, up to and perhaps even over 1g/lb bodyweight.

Age

Muscle maintenance in the elderly seems to require larger relative amounts of dietary protein. In one study, the RDA of 0.36g/lb bodyweight wasn’t enough to prevent some muscle catabolism.

Stress Level

Chronic stress, as indicated by chronically elevated cortisol levels, reduces muscle protein synthesis (protein doing proteiny stuff) and increases gluconeogenesis (conversion of protein into glucose).

It’s also worth noting that amino acids do other stuff, too, beyond building muscle and providing energy substrates when in excess. It’s not as if amino acids that don’t go toward rippling pecs are immediately converted into glucose. No, they’re also:

  • Taken up by the intestines, both as an energy source for the small intestine and to form a reserve pool of amino acids that the body can draw upon during fasting or starvation (when there’s little to no dietary amino acids input) instead of breaking down skeletal muscle to turn into energy, at least in rats.
  • Precursors for various neurotransmitters (like serotonin and dopamine).

So, to answer the question – yes, we’re probably absorbing all the protein we eat, but, depending on what we’re doing with our time, how old we are, how large or small we are, how active we are, how much we’re lifting, and how much stress we’re under, we’re all using the protein we absorb in different ways and proportions. As is often the case, the answer brings up even more questions. But that’s okay, because that’s just something else to explore.

Until next time, Grok on!

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. LOL digestion requires takes a long time?
    Yeah if you eat not natural food for humans like meat, fish, eggs, etc..
    Try eating the only natural food for us (fruits and some greens) and you’ll digest in minutes!

    Attilio wrote on July 21st, 2012
  2. I resently started working out. I don’t know how much protein I should have. I work out either jogging, a body pump class, weight lifting and zumba classes. I do have eggs in the morning (6:00am) with tomatoes with my smoothie and Whey protein powder after my morning work out. Sometimes I switch this routine to the afternoon. About 10:30 I do eat fruit. In the afternoon, about 1pm I eat piece of meat with veg. and about 5 I have another smoothie with protein powder.
    Do you think this is enough to build muscles. I am 69 very very healthy and I am not a your normal 69. In March 2013, I will start training for my first 5k.

    A'lice Portas wrote on September 18th, 2012
  3. so on my tub of maximuscle it says 41g of protein, im now reading your body can only absorb 20g in one hit? so surely false advertising??

    venetian plaster wrote on September 25th, 2012
  4. You said “If you can handle exercising without having to carb-load, you’re probably fat-adapted. If you can work out effectively in a fasted state, you’re definitely fat-adapted” Ok, no problem exercising in fasted state or skipping meals. Yet, I don’t seem to burn fat. No weight loss (been paleo for 7 months). Also I note that even when trying to get into ketosis, that I can barely make the Keto strips change color at all. Am I just amazingly efficient at deriving energy from fat that I burn so little? Or does my body steal protein for gluconeogenesis even though I don’t eat large amounts? Or is the problem high cortisol levels (I do have major trouble getting to sleep)and what the heck do I do about any of this?

    nic wrote on September 27th, 2012
    • Check your thyroid. Maybe it is too low and that’s why you can’t lose. If it’s not that, definately your cortisol. Both cause sleep issues.

      RM wrote on September 27th, 2012
  5. All of the complex metabolic pathways f the body are so interesting, and many we still don’t know exactly how they work. This is why it’s great to eat a diet rich in veggies- we don’t need to know how the many different phytochemicals work in order to gain the benefit from them.

    Something that was blatantly absent from this article was any mention of nitrogen balance. It is a test used in labs to determine how much protein is absorbed. The body ‘deanimates’ proteins by ripping of the nitrogen, sending it out via urea. So there is no ‘pooping out’ of protein but if you’ve ever noticed how smelly your urine is the morning after a heavy protein meal, it’s pretty clear the body does not use all of the protein we throw at it. And gluconeogenesis is considered a minor pathway, but in any case metabolic pathways are still quite mysterious, so I think we can say there is ‘conjecture’ at best.

    Mandy wrote on October 13th, 2012
  6. Hi Mark, have you read any of Donald Layman’s research? Jimmy Moore had an interview with him, URL below:

    http://livinlavidalowcarb.com/blog/atlcx-episode-24-all-things-protein-protein-101-dr-donald-layman/15359

    His research seems to disagree with your explanation.

    Gerogette wrote on October 23rd, 2012
    • That’s because this article, while well written, does not accurately describe what happens in the body.

      To be fair, no short article ever could. It would require at least 15-20 pages to give an overview and explain the nuances as well as how to apply them, and I give that number because I have typed that out before and I wasn’t completely satisfied with it.

      That’s without references or special concerns.

      Joshua Naterman wrote on December 21st, 2012
  7. dear mark
    ok i am a 43 year old 288 pounds.fixing to have the gastric sleeve.i know doctors know a lot,but i would still like to research the protein i intake.after my sleeve i am wondering what my protein intake will be.doctor says 60 to 90 grams a day.i cant exercise right now as its hard to walk and bend,for now.after i lose 40 pounds then i will be able to walk more.i will only be able to eat about 4 ounces of food at a time.starting with liquids,then puree,then soft foods,then its a try and see.but our requirements are sip water all day try to get in 80oz. of that 64oz at least.then 60 to 90 grams of protein,plus our vitamins.i feel it is to much protein for a person that just sits around all day.what do you think please.thx

    angela wrote on April 20th, 2013
  8. Erm, but protein digestion only takes place in the small intestine with a transit time of 1-2 hours. Will the small intestine really hold on to the porter house steak you mentioned for 10 hours???

    Dan wrote on May 3rd, 2013
  9. I would like to know after a 40 or so gr. meal of protein how long does aminos stay in the blood. Thanks Mario

    mario wrote on February 12th, 2014
  10. End result might be another issue

    100 gms of steak to get 30 gm protein, fibre, and waste products) will take 4-5 hrs to digest (60cal) and leave you only 60 cal for exercise.

    30 gms protein powder will give you 120 calories.

    This is only a rough guide and Mark may like to follow up with more accurate numbers. Thanks

    Peter

    Peter Smernos wrote on March 16th, 2014

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