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Protein. My favorite topic. You want to see me get my panties all in a bunch. Let’s talk about protein. If you went out right now and asked 10 people why you eat protein and what it does for your body, I bet all but perhaps a couple people will get the question wrong….
Protein is one of the three macronutrients; however, protein is not a sufficient source of energy used by the human body. However, under certain circumstances, dietary protein and/or certain amino acids can have very important roles in muscle metabolism and exercise performance(1). Proteins are similar in molecular structure to fats and carbohydrates, expect for one defining characteristic – proteins contain nitrogen atoms. The word amino literally means “nitrogen containing(1).” Structurally, proteins consist of various lengths and combinations of amino acids that are linked together by peptide bonds.
Proteins have many functional roles in the human body:
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Classification
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Specific Role in Human Body
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Energy
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After a protein is degraded (broken down), some amino acids can be changed structurally to form glucose
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Growth and maintenance
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Proteins are found in numerous body structures, including hair, skin, tendons, muscles, organs, etc.
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Hormones
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Some hormones are classified as proteins, such as insulin, glucagon, prolactin and growth hormones
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Enzymes
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Enzymes are proteins that speed up chemical reactions
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Antibodies
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Antibodies are proteins produced by specific immune cells to help fight infections
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Acid-base balance
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Hemoglobin (a protein) not only carries oxygen, but serves as a blood buffer to help regulate pH
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Fluid balance
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Albumin and globulin (blood proteins) help draw fluid into capillary beds
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Transportation
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Some proteins carry specific substances (i.e. hemoglobin carries oxygen)
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The human body begins to digest protein in the stomach. The enzyme pepsin cleaves the peptide bonds that hold amino acids together creating smaller peptides (short chains of amino acids) and some free amino acids. Once the contents of your stomach reach your intestines, enzymes from the pancreas and intestines will finish cleaving the peptide chains to absorbable amino acids(1). Amino acids are then absorbed by the small intestine into the bloodstream. Studies have suggested that about 95% of ingested animal proteins and about 85% of ingested plant proteins are absorbed by the body from one meal, but no one is really certain for sure(1). Before amino acids can be used for energy by the body, it undergoes a reaction to remove its nitrogen-containing compounds.
There are 20 unique amino acids that make up various proteins. Nine are called essential amino acids, meaning that the human body does not produce these amino acids and we must obtain them through our diets. The remaining 11 are considered nonessential because the human body can synthesize them.
Much of the debate surrounding protein involves how much should you consume and what types. Traditionally, only animal proteins, such as milk, eggs, meat, and fish, have been considered “complete” protein sources (containing all the essential amino acids). Plants are considered “incomplete” because they lack specific essential amino acids. However, soy is considered a “complete” protein(1). Any vegetarian or vegan can obtain an adequate amount of protein (and all the essential amino acids) through their diet by consuming various food choices throughout the day. Interesting enough, greens have the highest percentage of amino acids per ounce of any food, but since they don’t weigh much, they need to be eaten in greater amounts(2).
The amount of daily protein intake is much debated. It really varies depending on your weight and what your daily activities are. Currently, the RDA for protein in healthy adults is 0.8 g/kg body weight per day(3). The International Society of Sport Nutrition suggests the exercising individuals ingest protein ranging fro 1.4 to 2.0 g/kg/day(3). They suggest that endurance athletes consume 1.0 to 1.6 g/kg/day depending on the intensity and duration of the endurance exercise. Recommendations for strength/power athletes typically range from 1.6 to 2.0 g/kg/day(3).
To figure out your protein requirements is quite easy. It’s just a simple math equation. I will use myself for an example. I currently weigh 125 pounds or roughly 57 kg (1 lb = 0.45 kg). I am an endurance athlete with a fairly intense and long training schedule, although it varies day to day. I am going to use 1.3 g/kg/day as my goal protein consumption.
57 kg X 1.3 g/kg = 74 g of protein per day
One relatively new development in sports nutrition is the knowledge that nutrient timing influences the physiological responses to exercise(1). Studies have shown that after exercise a 4:1 or 5:1 carb to protein ratio food or recovery drink is optimal for resynthesis of muscle protein and maintenance of other physiological structures that rely on amino acids, such as the nervous system(4). (This is a topic I plan on discussing in more detail in the future)
I’m not a huge fan of the Paleo Diet, but I did read The Paleo Diet for Athletes. One interesting section I found in the book was about why our ancestors chose to eat 6- to 8-ton elephants when they could have easily eaten prey like rabbits, partridges, and fish. Well, it’s because if you eat just protein and way too much of it, it can kill you. Laboratory studies have found that the maximum amount of protein humans can consume on a daily basis is about 40% of our daily calories(4). Anything above that, you become sick. Our earliest settlers learned that the hard way in what they referred to as “rabbit starvation.” Apparently, after eating enormous quantities of very lean meat, they would become nauseated and irritable, lose weight, develop diarrhea, and eventually die(4). What a way to go, huh? Have you ever wondered why you eat lobster with lots of melted butter? It’s because lobster is extremely lean (84% of its energy is protein) and could easily cause poisoning if that’s all you ate! So break out that tub of butter!
Don’t worry, I will be talking about protein and amino acids in much more detail in the future so stay tuned for some good posts coming up!
References
- Antonio J et al. Essentials of Sports Nutrition and Supplements. Totowa, NJ: Humana Press; 2008.
- Brazier B. Thrive: The Vegan Nutrition Guide to Optimal Performance in Sports and Life. Philadelphia, PA: Da Capo Press; 2007.
- Campbell B et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition. 2007; 4:8.
- Cordain L, Friel J. The Paleo Diet for Athletes. Rodale; 2005.
(Disclaimer: Like always, this is for your information only. If you are concerned about your health and diet please seek out professional help from your medical provider and/or registered dietitian.)
