Protein in Milk and the Unique Properties of Goat Milk Protein
Protein in milk and milk products is a significant dietary component throughout the world. Protein is one of the six major nutrient groups and is essential to life--it composes the muscle and organ tissues, and is used for processes such as manufacturing hemoglobin and antibodies.
It is so significant, that when dietary intake of protein is insufficient, our bodies will begin to utilize protein from our muscles in order to maintain life function.
At the opposite end of the spectrum, too much protein is also harmful. Since the body can not store extra protein, it is converted by the liver into urea, which is then excreted by the kidneys.
Healthy kidneys can handle the extra workload occasionally, but people with kidney disease are typically advised to lower their protein intake. Long-term, high-protein diets have also been criticized for this potential health concern.
Now that we know the implications, I think it's easy to see that protein isn't just something we should take for granted!
So, let's start learning more about protein (specifically, protein in milk), how our bodies use it, and how goat milk protein fits into the picture.
In this article, we'll cover:
To better understand the implications of nutritional protein, let's look at some of the basics, first. At a structural level, protein is composed of building blocks known as amino acids.
Out of 20 total amino acids used by humans for building protein, our bodies are capable of manufacturing 11 of these, called nonessential amino acids. The essential amino acids comprise the remaining 9, which must be supplied through the diet.
The amino acids are joined together in different combinations to form the various types of protein in our bodies. With the proper supply of essential amino acids, our bodies are able to synthesize the necessary protein for our cells to grow, maintain and function properly.
Since we've already established that essential amino acids must come from dietary sources, let's look at the classifications for these. Dietary protein, is typically categorized as either complete or incomplete.
Complete protein sources, which include animal products such as meat, eggs and dairy products, contain sufficient amounts of the essential amino acids for protein synthesis. Protein in milk, therefore, qualifies as a complete protein source.
Incomplete protein sources, on the other hand, do not, by themselves, supply all of the essential amino acids. These incomplete protein sources are comprised of plant foods.
However, as most vegetarians well know, particular combinations of plant products, such as a legume with a grain product, e.g., beans and rice, will supply a complete set of the essential amino acids.
Note: You can read more than you ever wanted to know about protein and amino acids in nutrition in the World Health Organization Technical Report, Protein and Amino Acid Requirements in Human Nutrition.
Protein and Food Allergies
Although protein is a vital component for life, it unfortunately can also cause problems for some people. Food allergies, ranging from mild to life-threatening, are actually the result of the body's rejection of one or more proteins in a particular food.
Symptoms can include swelling, hives, abdominal pain, nasal congestion, and shortness of breath. The most severe allergic reaction is known as anaphylactic shock, a whole-body response that can result in death.
The allergic symptoms are a direct result of the body's immune system response to the protein. The immune system incorrectly identifies the food protein as being a "foreign invader," and sets in motion processes to destroy it.
Food allergies have also been hypothesized, though not proven, as the cause of other medical conditions, such as arthritis, fatigue, headaches and irritable bowel syndrome.
There is no cure for food allergies. The treatment is to first correctly identify the triggering food, followed by avoidance. Allergy symptoms seem to worsen with each exposure to the allergenic protein, so early detection can potentially lower the risk of a person having an anaphylactic shock episode or other severe allergic reaction.
Protein in milk products is one of the most common allergens, particularly among children. Milk allergy is sometimes confused with lactose intolerance, a much less serious condition in which the body lacks an enzyme needed to properly digest milk proteins. (You can read about the role of goat milk for lactose intolerance at this link.)
Useful information on dealing with food allergies can be found at The Food Allergy & Anaphylaxis Network.
Digestion and Bioavailability of Protein in Milk
Protein digestion is accomplished by enzymes that break down the protein into its constituent parts. The digestive process begins in the stomach and continues through the small intestinal tract. Different forms of protein are digested at different rates, and to varying degrees of completeness.
Protein in milk is nearly completely digested and absorbed before it passes through to the large intestine.
A rating scale known as the Protein Digestibility Corrected Amino Acid Score (PDCAAS) is used by the FDA as a method of evaluating protein quality for humans. The highest possible score of 1.0 implies that the digested protein provides 100% of the essential amino acids for humans.
The protein in milk, with each of its two constituent protein classes, whey and casein, is among the few that are rated at a 1.0. Whey is a liquid by-product of the cheesemaking process, and is used as an additive in quite a few processed foods. Whey has been linked to health benefits, such as the regulation of blood sugar levels for diabetics.
Whey protein, collected from liquid whey, is considered to have the highest bioavailability rating of all protein, and is sold as a nutritional supplement. Whey protein supplements are particularly popular with bodybuilders, who take it before workouts to help build muscle mass.
In liquid milk, casein exists in the form of suspended particles known as casein micelles. The addition of a coagulating substance, such as rennet for cheesemaking (or digestive enzymes in the body), causes the casein to form clots or lumps--the "cheese." Casein accounts for about 80% of the total protein in both cow's milk and goat milk.
In contrast to the rapid protein absorption from whey, the gelatinous lump of casein causes it to pass much more slowly through the digestive tract, providing more of a "time-released" supply of amino acids. The slow entry of the amino acids into the system means that they can not be used as efficiently for building protein as those from whey.
However, bodybuilders typically use casein protein in conjunction with whey protein, because the slow release of amino acids from casein is said to prevent muscle breakdown, while the rapid absorption of the whey compounds is beneficial for building muscle. Used together, then, both classes of protein in milk provide a two-fold benefit.
Goat Milk Protein and Health Implications
Now that we have a basic understanding of protein, along with its role in the body and relationship to food allergies, let's take a more in-depth look at the composition of goat milk protein.
Goat milk provides around 9 grams of protein per cup, compared to about 8 grams of protein per cup for cow's milk. Protein for both are composed of approximately 80% casein and 20% whey.
Of the 18 amino acids listed in the USDA's national nutrient database, goat milk contains higher amounts than cow's milk of 14 of them. Of the remaining four, tryptophan, aspartic acid, glutamic acid and glycine, only tryptophan is an essential amino acid.
View the USDA goat milk table for nutrients.
View the USDA cow's milk table for nutrients.
The higher levels of amino acids in goat milk have been linked to several health benefits. For example, the higher cysteine level has been directly implicated in an increased absorption of copper in rats with malabsorption syndrome.
It's not, however, the amount of protein or amino acids in goat milk that is the focus of most of the health benefits of goat milk protein. Rather, it is the difference in the composition of the proteins from that of cow's milk protein.
Remember, we said that protein is composed of amino acids...Well, many different proteins can be formed by slight variations in the combinations of the amino acids.
Think of a child's construction set, such as Legos. If you choose 20 different pieces from the set, think of the great variety of constructs you could make by varying the combinations of those 20 pieces!
It's the same for proteins. Since goat milk and cow's milk are complete proteins, we know they each supply all the essential amino acids (the Legos pieces) for humans. However, the structures of the proteins (your Legos creations!) that supply the amino acids are different.
They are not all different, however. Goat milk and cow's milk share a great percentage of their protein structures. It is the small percentage of proteins that are different that has the implications for health benefits.
A nutritional study [Morales, et. al] cited the protein in milk from goats as being higher in nutritional value than cow's milk, in part due to the different nature of the proteins, together with the different fat compositions.
This small set of different proteins also has a direct implication on considerations for individuals with cow's milk allergies. Since we know that allergies are direct reactions against proteins in food, we can easily see that goat milk may or may not be helpful in this case.
Because a large number of the proteins are shared between the two types of milk, it does not look very hopeful, at first glance. If a person is allergic to a protein in milk that falls into this group, then goat milk will be an equal offender with cow's milk.
However, it seems that the alpha-s1-casein protein, the major protein in cow's milk, is a primary offending agent for people with adverse reactions to cow's milk.
Goat milk contains much lower levels, and some goats genetically produce milk with no alpha-s1-casein. Studies show that around 40% of people allergic to cow's milk are able to tolerate goat milk.
In summary, protein in milk is a valuable dietary nutrient. Differences in the amounts and structure of goat milk protein, together with digestibility factors, give it an advantage over cow's milk protein.
Haenlein, G.F., "Goat Milk in Human Nutrition," Small Ruminant Research, February 2004,Vol. 51, No. 2, pp. 155-163.
Haenlein, George F., "Why Goat Milk?," http://www.goatworld.com/articles/whygoatmilk.shtml.
Morales, E. Ramos, G. de la Torre Adarve, F.D. Carmona Lopez, F. Gil Extremera, M.R. Sanz Sampelayo and J. Boza, "Nutritional Value of Goat and Cow Milk Protein," Options Mediterraneennes, Series A, No. 67.
Nutritional studies are ongoing, so check back here for any new information related to goat milk protein!
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