Nutrition of Proteins

Proteins

• Proteins are macronutrients that contain carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) atoms.
• Proteins are essential for energy production, particularly when fat and carbohydrate stores are low.
• Proteins are synthesized in the body based on a genetic code.

Learning Goals

• Proteins are compared and contrasted with other energy-containing macronutrients (carbohydrates and lipids).
• A list of essential amino acids, and how they are quantified is covered (PDCAAS).
• Low-quality protein deficiencies can be overcome through mutual supplementation.
• Roles of body proteins, protein turnover, and nitrogen balance are identified.
• Health Canada's recommendations for total protein intake and challenges for vegetarians are explored.

Comparison to Lipids and Carbohydrates

• Similar to carbohydrates and lipids, proteins contain carbon, hydrogen, and oxygen atoms, but proteins also contain nitrogen.
• Proteins produce energy when there is a shortage of other energy sources like fats and carbohydrates.
• Proteins are synthesized in the body by following the instructions encoded in genes.

Base Unit - Amino Acids

• Proteins are composed of amino acids.
• Amino acids share a common structure, a backbone composed of a central carbon atom bound to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R-group).
• The unique R-group gives each amino acid its distinctive properties.

Amino Acids

• There are 20 different amino acids involved.
• 9 of these are essential amino acids, meaning the body cannot produce them and they must be obtained from the diet.
• The 9 essential amino acids are: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.

Transamination

• Transamination is a process that converts one amino acid into another.
• It transfers an amino group (-NH2) from one amino acid to a keto acid, converting the keto acid into a new amino acid.
• Transamination primarily occurs in the liver, and it plays a crucial role in converting essential amino acids into nonessential amino acids.

Amino Acids (Conditionally Essential)

• Some amino acids are considered conditionally essential. This means they are normally non-essential but become essential during certain circumstances.
• An example is phenylalanine and tyrosine. The body normally converts phenylalanine to tyrosine, but if the conversion process is impaired (as seen in phenylketonuria), phenylalanine becomes essential.

How are Proteins Made?

• Amino acids are linked together through condensation reactions to form peptide bonds.
• A dipeptide is formed when two amino acids are bonded together.
• When multiple amino acids are bonded together, they form a polypeptide, which is a chain of amino acids. Most proteins are 30-300 amino acids in length.
• The sequence of amino acids determines the protein's unique shape and function.

Protein Synthesis

• The sequence of amino acids in a protein is determined by the genetic code in DNA.
• This process begins with transcription, where DNA is copied into mRNA.
• The mRNA then travels to ribosomes, where translation takes place.
• During translation, the ribosomes use the mRNA code to assemble amino acids into a polypeptide chain.

Protein Synthesis Limiting Factors

• The synthesis of proteins may be limited by the availability of specific essential amino acids present in lower concentration in dietary protein than needed
• Inadequate energy intake can also limit protein synthesis, as amino acids might be used for alternative metabolic processes, such as gluconeogenesis.

Protein Organization

• The unique sequence of amino acids in a protein determines its three-dimensional structure.
• The structure then determines its function.
• Proteins vary in complexity ranging from around 10,000-50,000 different kinds.

Protein Denaturation

• Proteins can be denatured by various agents such as heat, acid, changes in pH, and changes in salinity.
• Denaturation results in a loss of protein function and the proteins' three-dimensional structure is disrupted.

Protein Digestion

• Protein digestion begins in the mouth with mechanical breakdown and mixing with saliva.
• In the stomach, hydrochloric acid denatures proteins and activates pepsin, an enzyme that breaks down proteins into smaller polypeptides.
• In the small intestine, tripeptidases and dipeptidases further break down these smaller polypeptide chains into amino acids.

Protein Absorption

• Intestinal cells absorb amino acids and some di- and tripeptides from the small intestine's lining.
• Amino acids are transported directly to the liver through the portal vein to be utilized for multiple processes.
• Excess amino acids are rapidly metabolized, and nitrogen is excreted if exceeding body need.

Protein in Foods

• Complete proteins contain all essential amino acids and are typically animal-derived.
• Incomplete proteins are lacking one or more essential amino acids and are mostly plant-derived.
• Protein quality is determined by the amount and digestibility of essential amino acids.
• PDCAAS, the Protein Digestibility-Corrected Amino Acid Score, is a method to quantify protein quality.

Mutual Supplementation

• Mutual supplementation involves combining incomplete protein sources, providing needed essential amino acids missing from each other.
• For example, legumes provide abundant isoleucine and lysine but are deficient in methionine and tryptophan, while grains offer the opposite profile.

Roles of Body Proteins

• Proteins have many roles in the body, including forming structures (tendons and ligaments), facilitating chemical reactions (enzymes), transporting molecules, regulating fluid balance, and maintaining acid-base balance.
• They also create blood clots (fibrin), form scars (collagen) and participate in vision (opsin).
• Antibodies help protect the body and hormones regulate bodily processes.

Protein Turnover and Nitrogen Balance

• Protein turnover is the continuous process of protein synthesis and breakdown within the body's cells.
• Amino acids released during protein breakdown can be recycled, or used as energy.
• Nitrogen balance is the difference between nitrogen intake (from proteins) and nitrogen excretion (primarily in urine).
• Positive nitrogen balance indicates growth or tissue repair, whereas negative nitrogen balance can occur in situations of starvation, injury, or illness.

How Much Protein Should We Eat?

• The Acceptable Macronutrient Distribution Range (AMDR) for protein intake in adults is usually 10% to 35% of daily caloric needs.
• Individual needs vary based on factors such as age, activity level, and health status. Based on body weight, estimated requirement rates for proteins are higher for athletes, and children and adolescents. Lactating and pregnant women require even more than the normal daily requirements.
• There are differences in the exact amounts required based on specific factors.

Calculation of Energy Content

• The method for converting grams of nutrients to kilocalories is provided and the calculation of percentages is shown for an example.

Dietary Protein Risks

• Too little dietary protein results in protein-energy malnutrition (PEM), often affecting children most severely.
• Signs of PEM can include stunted growth and weakened immunity.
• Too much dietary protein may contribute to risks such as high cholesterol, heart disease, and possible bone loss, particularly if excessive saturated fat from animal products is consumed. This risk is especially higher for those with a family history of kidney disease.

Vegetarianism

• Vegetarianism involves restricting the diet to exclude meat or other animal products.
• Several types of vegetarianism exist, each with different restrictions. Vegetarians may avoid red meat, poultry, seafood, eggs, and dairy.
• Vegetarians may need to carefully plan protein intake and supplement diets to ensure they obtain nutritional requirements as complete proteins are hard to obtain through plant-based proteins alone. Protein sources such as beans, tofu, and nuts should be prioritized to maintain optimal health.
• Health advantages of vegetarianism include generally reduced fat and calorie intake, and therefore a reduced risk of obesity, heart disease, and certain cancers.