"Animals that we eat are raised for food in the most economical way possible, and the serious food producers do it in the most humane way possible. I think anyone who is a carnivore needs to understand that meat does not originally come in these neat little packages." - Julia Child
Proteins are simply incredible. They are the chicken or steak that sits on your plate to be consumed as a source of energy. They are the muscles that move your body through the world. They transport and deliver oxygen throughout our bodies. They are even the precursors to our neurotransmitters that enable us to interact with the world or get lost in our own thoughts. They are the driving force of our metabolism, some acting as gateways absorb nutrients into the cells for transfer to the bloodstream, while others catalyze reactions sometimes irreversibly and other times they help reactions go in either direction. Through this influence on biochemical reactions, proteins influence our development. In fact, proteins are what make us all unique individuals, because the information stored in our DNA is essentially a blueprint for our cells to assemble a plethora of proteins. In this section, I want to discuss what proteins are, how our body processes them, and some great sources of protein.
What Are Proteins?
Proteins are long strands of amino acids. Ok, great, but what are amino acids? They are molecules that contain both an amino group and a carboxylic group. As seen below, these two groups do not vary between amino acids. Every amino acid has a central carbon atom. Since carbon likes to have four bonds, the amino and carboxylic acid groups take up two of these. The remaining two bonds are to a hydrogen atom and then a group called the side chain. This side chain group is what makes different amino acids unique from one another. In total there are 20 amino acids. While they are all unique, they fall into different categories based on their side chain. Some love water, while some aren’t. Some act like acids, while others act like bases. And some of them have charges on them, which makes one side more positively charged and the other side more negatively charged. These characteristics affect the way that these amino acids interact with their environment, which will come into play later.
So, as I mentioned, proteins are long strands of amino acids. These amino acids are all connected by something called a peptide bond. These are formed by a chemical reaction between the amino group of one amino acid with the carboxylic group of another amino acid to form the peptide bond. When proteins are made, these amino acids are added on, one-by-one. But how do our cells know what the order of the amino acids should be? The answer is DNA.
The Central Dogma: DNA to Protein
Ancestral DNA is all the rage these days. DNA makes us unique, but it also allows us to track lineage, as genes are passed down from parents to children, generation to generation. But how do these genes play a role in shaping us into the individuals that we are? Proteins. Think really hard back to your days in high school biology. DNA is found in the cell’s nucleus. When we talk about “gene expression” we are referring to the process of taking the gene, a stretch of DNA that codes for a particular protein. The process of DNA’s code being shaped into proteins is the central dogma of biology. This occurs through two processes called transcription and translation. Transcription is when DNA is converted into something called messenger RNA (mRNA), which is like a copy of DNA that is small and able to leave the nucleus. Outside the nucleus and in the cell’s fluid (cytosol), translation takes place. During translation the mRNA is read by special molecules that work their way down the mRNA, turning the code that was copied from the DNA into a string of amino acids. The order of these acids is dictated by the DNA code. As this sequence grows longer, a protein is constructed, until it reaches a special signal that the protein is finished.
Protein Structure
The structure and shape of proteins often dictate how they function. If you are really interested in how DNA’s code and the order of the amino acid chain can lead to such a variety of proteins, below I’ve provided a brief explanation of the four levels that dictate a proteins shape.
The primary structure is simply the order of the amino acids along the chain. As mentioned above, this is dictated by the DNA code of the gene. Check out the other levels of structure to see how the amino acids within the strand and their order ultimately determine the shape of the final product.
Based on the order of the amino acids, the primary structure, some very basic shapes can form along the chain. These are called alpha helices and beta-pleated sheets and they are created due to the formation of bonds between hydrogen bonds on the amino acids interacting with each other.
Tertiary structure is where it really starts to get interesting. As I mentioned above, the amino acids all have different characteristics, whether they love or hate water, are positively or negatively charged, acidic or basic. These characteristics influence the ultimate shape that the protein takes, which is the tertiary structure. Because our cells are basically tiny sacs of water, those amino acids that don’t gel well with water hide inside the protein and those that love water tend to gather on the outside. Those that have charges or are acidic or basic can play an important role in the proteins function or even cause it to change shape when the environment of the cell changes.
So, if tertiary structure is the ultimate shape of the protein, what could be left? Well, sometimes there is another layer. This occurs when multiple protein subunits are created individually and must come together to form a fully functional protein. One of the best examples of this is hemoglobin, which is what carries oxygen in our red blood cells and gives it that beautiful red. Four separate protein subunits called heme come together with iron to produce hemoglobin. This is an example of quaternary structure.
Digestion of Proteins
Unfortunately our body is not built to simply absorb proteins whole. Like the other macronutrients, proteins must be digested. The process of digestion for proteins involves breaking those peptide bonds that were formed to hold the amino acids together. Protein digestion is a process that has three major phases.
The gastric phase starts once that chewed up protein reaches your stomach. Proteins stimulate the release of a hormone called gastrin. Gastrin acts to cause the stomach to produce more stomach acid, hydrochloric acid. This causes the stomach to become more acidic. This increase in acidity causes the proteins to denature. Denaturation is when proteins unfold and lose their shape, leaving the strand of amino acids. Gastrin also causes the release of pepsinogen, which is a protease that has yet to be activated. The increase in acid within the stomach leads to the activation of pepsinogen to pepsin, an active protease. Proteases such as pepsin cleave or cut those peptide bonds discussed above that hold amino acids together. Therefore, pepsin begins the process of breaking up long chains of amino acids into small peptide fragments.
As the contents of the stomach move into the first portion of the small intestine called the duodenum, the stomach acid triggers the release of another hormone called secretin. Secretin causes your pancreas to release bicarbonate into your small intestine. Bicarbonate is the base that is found in baking soda and this helps to neutralize the strong stomach acid. This change in pH also triggers the activation of another protease trypsin from its inactivated form trypsinogen. Trypsin is really good at chewing up those peptide bonds. So, trypsin goes to work breaking up all of those fragments into single amino acids.
Now that the proteins have been broken down into individual amino acids, they move through the small intestine. This is where these amino acids will be absorbed by the cells lining the intestines. Once absorbed they are transported into the bloodstream and distributed throughout the body.
Metabolism of Proteins
Once absorbed, amino acids can be used for a host of things. As you might imagine, they can be recombined by the body to make various kinds of proteins. But they can also be broken down further. The carbon portion of the amino acid can be converted into pyruvate and acetyl CoA, which were described in Macronutrients & Metabolism. When this happens the nitrogen-containing amino group is converted into something called urea, which is then excreted in the urine.
The reverse can also happen with some amino acids, meaning our body can actually synthesize them. So you might ask yourself, if our body can make amino acids, why do we need to consume protein? One of the main reasons is that our body is only able to produce 11 amino acids, which we call the nonessential amino acids. Six of these are considered to be conditional, which means that our ability to synthesize them is limited during certain states, such as during an illness. You may have heard people reference essential amino acids. There are 9 essential amino acids. What this means is that because our body is not able to synthesize them, they are a required part of our diet. They are an essential part of our diet, because without them we would not be able to make many proteins, which as you can tell by the many roles that proteins fill in our bodies this could be very problematic.
Essential Amino Acids
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Nonessential Amino Acids
Alanine
Aspartic acid
Asparagine
Glutamic acid
Serine
Conditional
Arginine
Cysteine
Glutamine
Glycine
Proline
Tyrosine
How Much Protein?
So how much protein should we be eating? The answer is that it varies based on your activity, age, and sometimes gender. Athletes or those who are very physically active also have increased requirements for protein. Children also require more protein compared to their body size, because they are growing. Generally speaking, the average adult requires 0.8 grams per kilogram per day (g/kg/day). Athletes, by comparison, require 1.2-1.7 g/kg/day. So, the average 70kg person would require 56 grams/day. These requirements equate to about 15% of your daily caloric intake. Here is how protein requirements generally change as we age.
| Age | Recommended Daily Serving |
|---|---|
|
1-3 years |
13 grams/day |
|
4-8 years |
19 grams/day |
|
9-13 years |
34 grams/day |
|
14-18 years |
Women: 46 grams/day | Men: 52 grams/day |
|
Over 19 years |
Women: 46 grams/day | Men: 56 grams/day |
Americans typically overconsume protein. To compare with the recommended values above, on average American children consume 56 grams per day, adults consume 91 grams per day, and the elderly eat 66 grams per day. It’s easy to see how this can happen when you start to look at the number of grams of protein in various foods. If someone has an egg (12 grams) for breakfast, a 4-oz piece of chicken (33 grams) at lunch, and a 4-oz serving of quinoa (5 grams) with dinner, you can see that in one day it is easy to achieve 50 grams of protein.
Sources of Protein
Animal Products
So, where should you get your protein and essential amino acids? Animal products are the most well-known source. This includes poultry, beef, pork, and seafood. However, this also includes animal products, such as milk, yogurt, cheese, and eggs. Animal sources will provide you with all nine of the essential amino acids and thus are considered complete proteins.
Generally with any meat, you are looking for a leaner cut. Ideally this would be low in saturated fat, about 1-3 grams per serving. Ground meats will usually be displayed as something like 90/10, which means that this would be 90% lean meat and 10% fat. You should aim for ground meats that are at least 90/10, but something more like 93/7 (93% lean meat, 7% fat) would be a better choice. Avoid or try to limit meats that are heavily processed, such as hot dogs and bologna.
Fish and other seafood are a big part of the Mediterranean diet, especially fattier fish that are high in omega-3 fatty acids.
Serving size: Males – at least 1 ounce, females – at least 0.75 ounces; at least 2 servings per week
Fish
Examples: Fish can be thought of in one of two categories: fatty or lean. Those that are lean tend to be white, whereas those that are fattier and full of the healthy omega-3 fatty acids tend to be deep red to pink.
- Fatty fish (deep red to pink) – salmon, tuna, mackerel, blue fish, herring
- Lean fish (white) – cod, flounder, red snapper, drum, halibut
Buying: For starters, wild-caught fish tend to have more nutrients compared to fish that has been farmed. This is primarily due to the feed that farmed fish are fed. People tend to shy away from buying fish, because they don’t know how to tell if it is fresh or not. When buying fish rely on three key senses: smell, sight, and touch.
- Smell – Fresh fish will smell like the ocean. If it smells like fish, then it should be avoided.
- Sight – Whole fish should have clear not cloudy eyes. Fillets and steaks should be a bright color free of spots that are brown or gray.
- Touch – Fresh fish will be firm to touch. Whereas those that are past their prime will tend to be slimy or mushy.
- Internal temperature should reach 145°F for thicker cuts.
- Thin fillets should flake when they are ready.
- Avoid overcooking fish, as this will result in a rubbery and dry texture.
Other Seafood
Examples: Lobster, crab, shrimp, crawfish, clams, and mussels
Buying: The same rules above for fish also apply to other forms of seafood.
- Young, smaller lobsters will yield a fresher and sweeter flavor.
- Shrimp should have a hard shell.
- Clams and mussels should also be alive and closed when purchased.
- Crustaceans (lobster, crab, shrimp, crawfish) will be done when their shell is bright red or pink. When done the meat will be firm and have just turned from translucent to white.
- Lobster antennae should come off easily when they are finished.
- Crabs will float in boiling water when they are done.
- Small shrimp will cook very fast, so add these to the heat last to avoid overcooking them.
- Clams and mussels will be done cooking when their shell opens.
Serving size: Males – less than 3.9 ounces per day, females – less than 3.25 ounces per day; about the size of your palm or a deck of cards
Examples: Skinless chicken breast or skinless thighs trimmed of fat, skinless turkey breast, lean ground chicken or turkey
Buying: Look for lean cuts of chicken or other poultry. Ground chicken or turkey should be 90/10 or less.
Cooking: Trim meat of skin and fat before cooking to decrease the amount of saturated fat. The healthiest techniques for cooking chicken and poultry are sauteing, baking, and grilling. As always avoid frying or cooking with the skin on.
- While internal temperature checked in the thickest portion should reach 165°F, remove meat from the heat when a temperature of 160°F is reached. Cooking will continue off the heat in a process called carry-over cooking.
- The juices should be clear, not bloody.
- To evenly cook chicken breast, cover in plastic wrap and pound to an even thickness with the flat side of a meat mallet. You can also butterfly breasts by cutting them in half almost all the way through so that the breast opens like a book, or cut it all the way through to make two thinner cutlets.
Serving size: Males – less than 3.9 ounces per day, females – less than 3.25 ounces per day; about the size of your palm or a deck of cards
Examples: Tenderloin, rib chop, loin chop, loin, Canadian bacon
Buying: Look for lean cuts of pork like those listed above. If buying ground pork, pay close attention to the percent of fat to lean meat, as they are typically high in fat. As with other ground meats, ideally you want no more than 10% fat (90/10).
Cooking: Trim meat of skin and fat before cooking to decrease the amount of saturated fat. The healthiest techniques for cooking pork are sauteing, baking, and grilling. Avoid frying.
- Pork is tricky, because you should not eat pork raw, but when overcooked it is incredibly dry.
- Internal temperature checked in the thickest portion should reach 145°F. Remove meat from the heat when a temperature of 140°F is reached for chops and 135°F for larger cuts of pork. The temperature will increase with carry-over cooking.
- Allow meat to rest for 5 minutes after cooking. This not only allows for carry-over cooking, but the juices will tend to settle back into the meat, leaving a juicier end product. Cutting too soon will let the juices escape and result in a dry piece of pork.
Serving size: Males – less than 3.9 ounces per day, females – less than 3.25 ounces per day; about the size of your palm or a deck of cards
Examples: Tenderloin, top round, top sirloin, chuck, ground beef 93/7 (no less than 90/10)
Buying: Look for lean cuts of beef like those listed above. As with other ground meats, you want no more than 10% fat (90/10). A 93/7 ground beef would be ideal.
Cooking: Trim meat fat before cooking to decrease the amount of saturated fat. The healthiest techniques for cooking beef are sauteing, baking, and grilling. Avoid frying. Beef is about 60-75% water, which is the less you cook it the juicier your end result will and be overcooking it leads to a dry result. When cooking beef we usually describe the level of cooking on a scale of rare to well-done, which can be achieved by targeting different internal temperatures and result in varied appearance, texture, and water content.
- Rare – 125°F | bright purple/red | warm, tender, juicy
- Medium-rare – 130-135°F | bright red | warm, tender, very juicy
- Medium – 135-140°F | rich pink | slightly juicy
- Medium-well – 140-150°F | tan, slight pink | firm, fibrous, some juice
- Well-done – 155+°F | tan-brown | no pink, chewy, little juice
Serving size: One egg
Buying: To get the freshest eggs, be sure to look at the expiration date on the eggs and buy the eggs that have the expiration date furthest in the future. If you are making hard-boiled eggs, older eggs will peel easier, as more air will have leaked into the egg through pores in the shell.
Cooking: There are several techniques for cooking eggs. Some of the healthier ways are those that avoid an excessive amount of fat and oils, so baking, poaching, and hard-boiling. If scrambling eggs or pan-frying, try to use a non-stick skillet or a well-seasoned cast-iron skillet to limit the amount of butter or fat necessary to prevent them from sticking to the pan.
Serving size: 1 cup milk or yogurt, 1.5 ounces cheese
Buying: As with eggs, pay close attention to the expiration date. The freshest dairy products will have the latest expiration date.
Plant Products
Plant products are also a great source of protein. These include whole grains, nuts, seeds, and legumes. In case you’re wondering, because I know I was, a legume is any seed that grows in a pod. The vast majority of plant sources are classified as incomplete proteins. Plant sources are great, because not only can they provide you with protein, but they are also low in saturated fat and high in fiber.
Grains
Whole wheat
Brown rice
Corn/cornmeal
Bulgur
Oats
Buckwheat
Spelt
Barley
Quinoa
Amaranth
Legumes
Beans
Peas
Peanuts
Lentils
Tofu
Okra
Nuts
Walnuts
Almonds
Cashews
Pecans
Hazelnuts
Chestnuts
Pistachios
Seeds
Sesame
Poppy
Flax
Sunflower
Safflower
Pumpkin
Hemp
As a general rule, a single plant sources won’t be able to provide you with all nine essential amino acids. The two exceptions to this rule are quinoa and amaranth, whole grains that are actually complete proteins, thus providing you with all essential amino acids. The key to getting all of your essential amino acids from plant sources is to combine at least two different sources, which should then provide all nine. This can be as simple as adding lentils to whole wheat pasta or adding dressing a salad with chickpeas (legume) and sunflower seeds (nut). Previously it was recommended that these combinations be consumed in the same meal, but this is no longer the case and consuming a combination from these categories throughout the day will provide you with all of your essential amino acids. Check out how some other simple dishes can easily be made from combining protein from two plant sources.
Red Beans & Rice
+
Brown Rice
(Whole Grain)
Hummus
(Legume)
+
Tahini
(Seeds)
Peanut Butter Sandwich
(Legume)
+
Whole Wheat Bread
(Whole Grain)
Black Bean Taco
(Legume)
+
Corn Tortilla
(Whole Grain)