Proteins and Amino Acids

What Are Proteins and Why Are They Important?

Proteins are the predominant structural and functional materials in every cell of the body. They are composed of carbon, hydrogen, oxygen, and uniquely, nitrogen. Each protein is made from chains of amino acids, which are the building blocks of proteins.

• Amino acids have an acid group (COOH), an amine group (NH2), and a unique side chain.

• There are 20 unique amino acids that combine in various sequences to form proteins.

• Peptide bonds connect amino acids, forming dipeptides, tripeptides, and polypeptides.

Essential, Nonessential, and Conditional Amino Acids

Amino acids are classified based on whether the body can synthesize them:

• Essential amino acids (9): Cannot be made by the body; must be obtained from the diet.

• Nonessential amino acids (11): Can be synthesized in the body from other amino acids or by adding nitrogen to carbon-containing structures.

• Conditionally essential amino acids: Become essential under certain conditions, such as illness or in premature infants.

Denaturation of Proteins

Denaturation is the process by which a protein's shape is altered, affecting its structure and function. This can occur due to heat, acid, or other factors.

• Cooking meat or eggs denatures proteins, changing their texture.

• Stomach acid denatures dietary proteins to aid digestion.

Protein Digestion and Absorption

Dietary proteins are digested and absorbed in the stomach and small intestine:

• Stomach acids denature proteins and activate pepsin, which breaks proteins into shorter polypeptides.

• In the small intestine, polypeptides are broken down into tripeptides, dipeptides, and amino acids.

• Amino acids enter the blood and travel to the liver.

Protein Turnover and Amino Acid Pools

The body continuously breaks down and synthesizes proteins from amino acids. Amino acids come from the diet, breakdown of body proteins, and are stored in amino acid pools in blood and cells.

• Protein turnover: The ongoing process of protein breakdown and synthesis.

• Amino acids can be used to make body proteins, nonprotein substances (e.g., hormones, melanin), or, after removal of the amine group, can be burned for energy, stored as fat, or converted to glucose.

DNA and Protein Synthesis

DNA in the cell nucleus contains instructions for protein synthesis. Genes are DNA segments that code for specific proteins. Messenger RNA (mRNA) and transfer RNA (tRNA) carry out these instructions.

• Abnormalities in protein synthesis can lead to medical conditions, such as sickle-cell anemia.

Functions of Proteins in the Body

Proteins serve many vital functions:

• Structural and mechanical support: Collagen and connective tissue provide strength and flexibility.

• Enzymes and hormones: Most enzymes and many hormones are proteins.

• Fluid balance: Proteins help maintain fluid distribution in the body.

• Acid-base balance: Proteins act as buffers to maintain pH.

• Transport: Proteins transport oxygen, waste, lipids, vitamins, sodium, and potassium.

• Immune function: Antibodies are specialized proteins that defend against pathogens.

• Energy: Proteins provide 4 calories per gram.

• Satiety: Proteins improve appetite control.

Determining Daily Protein Needs

Healthy adults should be in nitrogen balance, where nitrogen intake equals nitrogen excretion. Positive nitrogen balance occurs during growth, pregnancy, or recovery; negative balance occurs during starvation, injury, or illness.

• Recommended Dietary Allowance (RDA): 0.8 grams of protein per kilogram of body weight per day for adults.

• Protein should comprise 10–35% of total daily calories.

• To calculate daily protein needs:

Protein Quality and Food Sources

Protein quality is determined by digestibility and amino acid profile:

• Complete proteins: Contain all essential amino acids (e.g., animal proteins, soy, quinoa).

• Incomplete proteins: Lacking one or more essential amino acids (e.g., most plant foods).

• Plant proteins can be "upgraded" to complete by combining complementary sources.

• Protein Digestibility Corrected Amino Acid Score (PDCAAS) measures protein quality.

Meeting Daily Protein Needs

Protein is abundant in meat, fish, poultry, dried beans, nut butters, nuts, and soy. Protein supplements are generally unnecessary with a balanced diet.

Health Consequences of Protein Intake

Both excessive and insufficient protein intake can have health consequences:

• Too much protein: May increase risk of heart disease, kidney stones, calcium loss, and displace other nutrient-rich foods.

• Too little protein: May reduce lean body mass, increase frailty, impair healing, and decrease immune function.

• Protein-energy malnutrition (PEM): Includes kwashiorkor (protein deficiency), marasmus (calorie deficiency), and marasmic kwashiorkor (both).

Vegetarian Diets: Benefits and Risks

Vegetarians can meet protein needs by consuming a variety of plant foods and protein-rich alternatives such as soy, legumes, nuts, eggs, and dairy (for lacto-ovo vegetarians).

• Benefits: Reduced risk of heart disease, hypertension, stroke, type 2 diabetes, cancer, and obesity; diets are rich in fiber and low in saturated fat and cholesterol.

• Risks: Potential deficiencies in protein, iron, zinc, calcium, vitamin D, vitamin B12, vitamin A, and omega-3 fatty acids.

Nutrients That Could Be Missing in Vegetarian Diets

Vegetarians need to plan their diets to avoid deficiencies. Key nutrients and their sources include:

• Protein: Soybeans, quinoa, tofu, legumes, nuts, dairy, eggs

• Iron: Iron-fortified cereals, legumes, soybeans, dried fruits, vitamin C-rich foods

• Zinc: Soy products, legumes, nuts, fortified cereals, dairy

• Calcium: Dairy, fortified soy milk, tofu, green vegetables

• Vitamin D: Fortified milk, soy products, eggs, supplements

• Vitamin B12: Dairy, eggs, fortified cereals, soy milk, supplements

• Vitamin A: Beta-carotene-rich foods (apricots, mangoes, pumpkin, kale, spinach)

• Omega-3 fatty acids: Walnuts, flaxseed, soybean oil, canola oil, fish (for pescetarians)

Nutrition in the Real World: The Joy of Soy

Soy is a high-quality protein source, low in saturated fat, and contains isoflavones (phytoestrogens) that may lower cholesterol and reduce risk of heart disease and certain cancers.

• Soy foods include tofu, soy flour, edamame, soy milk, tempeh, soy meat analogs, miso, and textured soy protein.

High-Protein, Low-Carbohydrate Diets: Two Points of View

There are differing opinions on high-protein, low-carbohydrate diets:

• Yes: Lean meats and vegetables, avoids refined grains, helps with weight loss, may promote satiety.

• No: Restrictive, limits carbohydrates and fiber, weight loss not long-lasting, may decrease kidney function.

Additional info: These notes expand on brief lecture points with academic context, definitions, and examples to provide a comprehensive mini-study guide for nutrition students.