L4 - Proteins and Amino Acids

What essential functions do proteins provide in the body? (7)

• Digestive enzymes

• Antibodies

• Regulation and expression of DNA and RNA

• Provide support to the body

• Muscle contractions and movement

• Hormones

• Move essential molecules around the body

How many kcal and kJ does 1g of protein give?

4 kcal and 17 kJ

Draw the structure of a generic amino acid

H H O

N - C - C

H R O - H

How many amino acids are there in the human body?

20

Describe the 3 classifications of amino acids

• Indispensable (essential) amino acids - cannot be synthesised by the human body, e.g., lysine and valine

• Dispensable (non-essential) amino acids - can be synthesised de novo by the human body, e.g., alanine and glutamic acid

• Conditionally indispensable amino acids - sometimes required, e.g., glycine and glutamine

Describe the events at the mouth, stomach, small intestine, and intestinal lining in the digestion and absorption of proteins, including any enzymes involved

• Mouth - chewing begins digestion and lasts less than a minute

• Stomach - HCL and pepsin break down proteins into polypeptides

• Small intestine - trypsin and chymotrypsin break polypeptides into smaller peptides

• Intestinal lining - brush border enzymes break down polypeptides into amino acids which can enter the bloodstream

Where are undigested proteins moved on to?

The large intestine

What is the reference nutrient intake (RNI) value for protein?

0.75 g/kg of body weight per day

Give 2 examples of populations where protein is needed in higher quantities

• Older adults

• Athletes

Describe the cause, age group, and effects (subcutaneous fat, oedema, liver, and muscle wasting) of kwashiorkor

• Caused by protein deficiency

• Age group is 6 months to 3 years

• Effects - subcutaneous fat is preserved, oedema is present, enlarged fatty liver, muscle wasting is mild or absent

Describe the cause, age group, and effects (subcutaneous fat, oedema, liver, and muscle wasting) of marasmus

• Caused by protein and energy deficiency

• Age group is infants under 12 months

• Effects - subcutaneous fat is not preserved, oedema is absent, no fatty liver, muscle wasting is severe

Describe the potential harms of excessive protein intake (3)

• Inefficient use of protein - excess protein (~ 2 g/kg/day ceiling effect) is stored as fat or excreted

• Metabolic burden - increased calcium excretion leading to a bone health risk. Kidney strain from nitrogen clearance. Liver has an overloaded metabolism

• Increased disease risk - heart disease and cancer, mostly from high animal protein intake (particularly red or processed meat)

What is protein quality?

The ability of a protein to supply essential amino acids in adequate quantities and in a form that can be digested and absorbed

What is protein quality dependent on?

Amino acid composition and digestibility

What are complete proteins and what can they be found in?

• Provide all 9 essential amino acids in sufficient quantities

• Can be found in animal-based foods, plant-based foods (soy and quinoa), or from alternative sources like mycoprotein (fungal derived protein)

What are incomplete proteins and what can they be found in?

• Do not contain adequate quantities of at least one essential amino acid

• Include most plant proteins

What is protein complementation?

Combining incomplete protein sources to provide all 9 essential amino acids

Describe and explain the time frame of protein complementation

• Protein complementation is flexible as complementary proteins don’t need to be consumed within the same meal, but need to be consumed within the same day

• This is because the body stores amino acids, called an amino acid pool, so essential amino acids consumed within a reasonable time frame can still support muscle protein synthesis and other protein-related functions

What are the 2 protein quality scoring systems?

• Protein Digestibility-Corrected Amino Acid Score (PDCAAS)

• Digestible Indispensable Amino Acid Score (DIAAS)

Describe the Protein Digestibility-Corrected Amino Acid Score (PDCAAS)

• Based on faecal digestibility (total GI tract)

• Scores are truncated at 1.0

What are the limitations of the Protein Digestibility-Corrected Amino Acid Score (PDCAAS)? (3)

• Because scores are truncated at 1.0, it cannot reflect proteins with a higher amino acid quality

• Overestimates some proteins due to colonic fermentation (microbes reduce nitrogen in faeces)

• Uses rat-based digestibility values

Describe the Digestible Indispensable Amino Acid Score (DIASS)

• Based on ileal digestibility (small intestine)

• A limiting amino acid (indispensable amino acid present in the lower proportion relative to human requirements) is not a deficiency if it has a DIAAS score of 1.0 or greater

• WHO 2013 recommend that DIASS replaces the PDCAAS

What is a limitation of the Digestible Indispensable Amino Acid Score (DIAAS)?

Typically uses pig models

How does skeletal muscle play a role in metabolic health?

• It accounts for about 50% of total body protein

• It is the largest reservoir for amino acids and is crucial for protein turnover and metabolic health

Describe the 2 key processes of muscle protein turnover

• Muscle protein synthesis (MPS) - building new muscle protein

• Muscle protein breakdown (MPB) - degradation of muscle proteins

What is muscle protein synthesis stimulated by? (2)

• Protein ingestion

• Resistance exercise

What is a potential reason why older adults experience muscle loss?

Older adults have a smaller muscle protein synthesis response to anabolic stimuli (protein ingestion and resistance training)

What is sarcopenia?

A biological ageing process associated with a gradual loss of skeletal muscle mass and function that affects about 45% of adults over 60 years

What are the effects of sarcopenia? (5)

• Reduced mobility

• Loss of independence

• Frailty

• Impaired metabolic adaptation to illness and disease

• Increased mortality

What is anabolic resistance?

A reduced ability of muscle tissue to synthesise protein

What are characteristics of anabolic resistance? (2)

• Blunted muscle protein synthesis (MPS) response to anabolic stimuli

• Increased muscle protein breakdown (MPB)

What can anabolic resistance lead to?

Sarcopenia

How can older adults avoid sarcopenia and muscle mass loss? (2)

• Greater resistance exercise

• Greater protein ingestion

What are the reasons for higher protein needs in older adults and describe them (4)

• Muscle remodelling decline - process of muscle repair and renewing becomes slower with age

• Deceased muscle protein synthesis efficiency

• Anabolic resistance - ageing muscles respond less to dietary protein due to reduced blood flow after meals and reduced amino acid uptake

• Preventing sarcopenia - adequate protein intake helps maintain muscle mass, strength, and mobility, reducing frailty

What is the recommended protein intake for older adults?

At least 1.0 - 1.2 g/kg body weight per day

When can the reference nutrient intake value for older adults increase and what can it increase to?

• Acute or chronic illness, and can be even higher for those with severe illness or injury

• 1.2 - 1.5 g/kg body weight per day

What are the advantages of animal-derived protein sources? (4)

• High quality protein, rich in essential amino acids

• Highly digestible

• Supports muscle maintenance and growth

• Contributed to micronutrient intake

What are the disadvantages of animal-derived protein sources? (3)

• Red and processed meat is associated with colorectal cancer risk, but this could be due to saturated fat or other nutrients, not protein

• Higher greenhouse gas production

• Greater resource use (land, water etc.)

What are characteristics of the Eat Lancet diet? (3)

• Reduced meat consumption

• Restricting added sugar

• Restricting salt

How does the UK average intake compare to the Eat Lancet diet?

The UK consumes over 4 times the quantity of red meat recommended by the Eat Lancet diet

Describe the meat alternative protein options (5)

• Meat and dairy analogues with plant protein - look and taste like meat but made from plants

• Cultured or cultivated meat - growing meat in a lab with animal stem cells

• Fermentation-derived protein - biomass and precision fermentation using microorganisms

• Edible insects - 4 species found in Europe

• Algae - macroalgae (seaweed) and microalgae

What are the advantages of alternative protein sources? (3)

• More sustainable

• Safe

• Affordable

What is mycoprotein?

A whole food produced by the filamentous cultivation of Fusarium venenatum that is a complete protein

How does biomass fermentation work?

It uses the rapid growth of some protein-rich microorganisms to make large quantities of protein

What are the advantages of plant- and fungal-based proteins? (4)

• Lower greenhouse emissions

• Lower type 2 diabetes risk

• Lower cardiovascular disease risk

• Phytochemicals and antioxidants

What solutions are there to overcome the lower digestibility of plant proteins? (6)

• Soaking

• Germination

• Cooking

• Industrial processing

• Use of probiotics and digestive enzymes

• Protein isolate or concentrate

What solutions are there to overcome the lower protein content per serving of plant-proteins? (3)

• Increase the portion size

• Protein isolate or concentrate

• Combine food groups

What solutions are there to overcome the limiting essential amino acids of plant-proteins? (2)

• Fortification of protein supplements with specific amino acids

• Combine food groups