Part 2 of the excellent 3 part series on protein
By Dr. Rob Wagner
By Dr. Rob Wagner
When talking about protein amounts we ingest, they can be viewed in two ways. One is based on a portion of the total amount of calories we take in (Pro=25%, Fat= 30% and Carb = 45%) and the other is based on a grams per bodyweight per day (g/ kg/d) measurement. In this post I will use the latter approach to discuss protein amounts.
Minimum Protein requirements
Protein, it’s not just for Muscles - Part 2
In part one of this post I discussed the digestion and absorption of protein and included some info about the needs of the digestive system and its role in regulating protein absorption. In this post I will give you some insight into the minimal and maximal levels of protein intake and the absorption rates of various proteins.
Staying with the theme of the first post I also would like to address the ways that determine the needs for protein. The key idea is called Nitrogen (N) balance. Nitrogen is the element that sets proteins apart from the other two energy supplying nutrients, Fat and Carbs. All three nutrients are made up of similar molecular skeletons that are formed from carbon, hydrogen and oxygen. Protein also contains N and it is the N that is crucial for our bodies structural components. Nitrogen balance is a matter of keeping the minimal supply of incoming N equal to that of N being excreted or eliminated from the body. Our rate of loss based on the research ranges from .25g/kg/d - .35g/kg/d. The minimal requirements needed to maintain an N balance range much more widely and have been shown to be as low as .39g/kg/d and as high as 1.09g/kg/d with the average being around .65g/kg/d. The difference between loss and protein taken in is a matter of efficiency of protein utilization. Other balance studies have shown the rate to range more specifically between .65 and .83g/kg/d. In viewing the differences between measured loss and intake one can see that protein or N intake easily doubles the loss. The difference is due to the fact that protein or N ingested not only replaces what is lost but is also assimilated into the bodies structural components along with the protein in the form of AA that is in the digestive system or blood stream. Our Recommended Daily Allowance (RDA) in the US falls within the second stated range of minimum requirements and is .8g/kg/d. This is supposed to meet 97.5% of the population’s daily needs to maintain nitrogen balance. Another way to view this is that the RDA meets the needs for structural demands for protein in the body (bone and muscle growth, hormone and enzyme production) and does not consider protein used for energy. Based on this information a healthy American, weighing 80k (176 lbs), would need 64g of protein per day for structural needs or to maintain his N balance.
Max Protein requirements
Knowing the minimal levels of protein intake is helpful in maintaining nitrogen balance or preventing deficiency. RDA’s in general are set for just such a reason. The concern however with most strength athletes and fitness enthusiasts is “what about me don’t I need more protein than a sedentary person?” This usually raises the opposite idea of the minimum levels required and targets the peak level a person could tolerate? Let it be understood that large amounts of Protein could potentially be toxic. From history there is evidence that early explorers who went on meat only diets experienced what is called Rabbit starvation syndrome. If meat was the only food source and individuals were forced onto this diet (due to lack of other food sources) they would suffer from nausea and diarrhea and would die in 2-3 weeks. Keep in mind this is not the same as a person having a high protein intake in a mixed diet. The problem that protein creates is related to an inability of the human liver to up regulate its enzymes to create urea from ammonia (more about that later) a waste product of protein breakdown. Excess protein (not used structurally) in the diet is typically broken down by the liver and used for energy. Certain animals have been shown to be able to regulate this as well as the rate of gastric emptying when large loads of protein are ingested. In humans it is possible that this regulation may take more time to adjust and that going from a low to high intake can challenge the liver in its ability to get rid of ammonia.
Peak levels of protein intake in humans can be determined from knowing a few factors. These are urea synthesis and excretion rates and the minimum Recommended Dietary Allowance (RDA) for protein. The rates of Urea synthesis and excretion include the protein the body uses only for energy, not structural demands. Based on the information from these areas we can assume what would be safe. Let me explain the role of urea. When AA are used for energy they are broken down and parts of the structure are used to form glucose and the others are converted to ammonia. Ammonia is a toxin and the liver converts it to urea. Urea is then sent to the blood stream and is removed from blood by the kidneys and excreted in the urine. The rate of this conversion from Ammonia to urea is limited in the body due to livers inability to up regulate its enzymes. Research does provide the max rate of urea excretion and synthesis and knowing this tells us how much protein you could breakdown and safely use for energy daily. A healthy human can make about 15% more urea based on averages than it can excrete. This means that max excretion rates might be held for extended periods of time to meet the difference. Based on the data about urea formation, a healthy human, weighing 80k, can breakdown 221g to 301g protein per day just for energy usage (2.76 – 3.76/kg/day). If we combine these numbers with the RDA you get 285g/day (3.5g/kg/d) to 365g/day (4.56g/kg/d). The authors of the research did make it clear, and I agree with them as well, that the upper levels should not be used. The research simply shows what is possible. They even stated that the average of 325g/day (4g/kg/d ) and the lower end number of 285g would both be preferable for an individual pursuing a high protein diet. So we have a high and a low end for our 80kg person and it ranges from 64g/day (.8g/kg/d) to 365g/kg/d. We know based off of the research that these numbers can be handled by humans but again the top end recommended by the researchers was between 3.5g/kg/d (1.58g/lb/d) and 4g/kg/d (1.81g/lbs/d).
I would also like to point out that when making conversions from Kg to Lbs. you need to use 2.2046 instead of simply dividing by two. If we look at the last set of numbers, dividing by two gives you 1.75g/lbs/d and 2g/lbs/d respectively. Using our 80k (176lbs) man these values would equal s 308g/d – 352g/d overshooting the recommended levels by 8% and 8.3% respectively. This is why I caution people on the 1g/lbs/d concept, even though these levels are safe based on research, are you really optimizing your dietary needs based on this number? Protein is an expensive fuel source both literally and physiologically. Excess protein in the diet does not stimulate muscle development, performance enhancement, or weight loss. It just simply doesn’t. The excess AA are simply broken down and turned to glucose if needed and anything that is not used gets turned to fat. I will discuss more on the appropriate levels of protein intake for various athletic populations in the final post.
Rates of protein absorption
There is no data on dietary protein absorption rates. In other words there is no data on ingested whole food meals (mixed food) which is the way most people typically eat. However, there is quite a bit of data on selected proteins. They are listed below in Chart 1 which is from research by Billsborough and Mann. The researchers here gathered info on a variety of foods and compared their rates of absorption. The studies looked at the release of either N in the blood or Carbon in exhaled air to determine the rates of absorption. The finding showed quite clearly that free AA and isolates of protein absorb much quicker due to their predigested states (already having free AA and di and tri peptides). Even the Casein isolate, which is typically identified as a slow protein when in its dietary form, absorbed almost as quickly as its AA counterpart. The pork also had a rapid rate of absorption. The researchers in this study used an AA sample that reflected the makeup of pork so it was not actually pork but an AA representation of pork. The problem with this example is that AA will always absorb more quickly than the whole food.
Chart 1 (adapted from Billsborough and Mann)
rate of absorption
> 8 hrs
> 8 hrs
Pea flour (raw purified) globulin
22g gl, 8g al
Soy Protein Isolate
Pork (amino acid equivalent)
Free AA (based on Casein molecule)
After looking at the chart one might think that the myth of 30g is not a myth since the rates of absorption are much slower than 30g/hr. It is also interesting to see that most of the amounts used in the studies are at or below 30g. Actually the chart helps to discredit the myth even further. Again the idea was that you can only ingest 30 gram or less per meal or per hour whatever the case may be. Anything in excess of that will not be used. Based on the chart you can see in 6 hours, 25g cooked egg protein (the amount equal to roughly 4 whole eggs) was 70% absorbed. At the rate listed it takes 8.5 hours for that meal to be absorbed completely. If the person ingested another 4 eggs (~50g total) at the same time it would have simply extended the time to absorb the entire meal. Digestion would have been extended due to the larger volumes and the process would have continued at the same rate of absorption. The same holds true for the faster absorbing proteins like individual AA and Whey Isolate. If you added more volume again the rates would be the same and the time to complete absorption would be extended. While it may be hard to visualize, the digestive processes work in stages, the first food stuff that gets into the system gets treated first and whatever comes behind goes through the same process sequentially. Think of it as an assembly line, without the conveyor belts. The whole system is regulated for efficiency not waste. In the next and final post I will explain what the effects of fast and slow proteins are, the role other nutrients play as well as the research on the needs of athletes and protein’s effects on their performance.