NTDT310 Midterm: Exam 3
11 Jan 2017
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Chapter 6: Proteins
• Physiological Functions of Protein
o Structure of body tissues
▪ BCAA constitute a significant amount of muscle tissues and is
metabolized in the muscles (versus in the liver with other
amino acids)
o Formation of enzymes – metabolic regulation
▪ Enzyme and hormone formation
▪ Maintenance of H2O and acid-base balance/buffers
▪ Carriers for nutrients
▪ Regulates blood clotting
▪ Prevention of infection/antibodies
o May provide energy
▪ Energy use takes precedence over tissue building
• Amino Acids
o Dispensable/nonessential amino acids: may be formed in the body
▪ 6 become conditionally indispensible, meaning they must be
obtained from the diet when endogenous synthesis from an
essential amino acid cannot meet demands
• Catabolic state: deficiency of another amino acid or
stress from illness, injury, or prolonged endurance
exercise
o Indispensible/essential amino acids: must be obtained from food in
the diet
▪ Histidine
▪ Phenylalanine
▪ Isoleucine (BCAA)
▪ Leucine (BCAA)
▪ Lysine
▪ Methionine
▪ Threonine (BCAA)
▪ Tryptophan
▪ Valine (BCAA)
• Animal protein is a complete protein (contains all essential amino acids)
whereas plant protein is incomplete
o The quality of a protein source is in it’s ability to provide nitrogen and
amino acid requirements for growth, maintenance, and repair of
tissues
• AMDR: 10-35%
• RDA: 0.8 g/kg (.36 g/lb) for ages 19+
o No upper limit for specific amino acids but take caution in using any
single amino acid in amounts greater than normally found in food
▪ Free amino acids are manufactured as drugs or dietary
supplements, but excessive amounts may cause health
problems like interfering with absorption of other amino acids
and cause gastrointestinal distress
• Athletes should obtain about 15% or more of daily energy intake from
proteins
o Some athletes may need more than others:
▪ Weight-control sports
▪ Young athletes to support growth and development
▪ Endurance athletes (overtraining/fatigue associated with
decreased plasma amino acid levels)
▪ Female endurance athletes with low energy intake
(amenorrhea)
o Sedentary individuals: 0.8 g/kg
o Strength-trained, maintenance: 1.2-1.4 g/kg
o Strength-trained, gain muscle mass: 1.6-1.7 g/kg
o Endurance-trained: 1.2-1.4 g/kg
▪ Need to recognize carbs are the main fuel, and more dietary
protein is recommended to restore protein used for energy,
promote synthesis of oxidative enzymes and mitochondria, and
help prevent sports anemia
o Intermittent, high-intensity training: 1.4-1.7 g/kg
o Weight-restricted: 1.4-1.8 g/kg
• Sports Anemia: the body uses dietary protein for other purposes at expense
of hemoglobin formation leading to anemia
• Vegetarians should consume 10% more protein than recommended amount
because it takes into account the lower digestibility of plant proteins
• Should consume 70% of protein from plant foods and 30% from animal
(society consumes it the other way around)
• Protein Digestion
o Begins when protein comes in contact with gastric juice in the
stomach, because the HCl denatures the protein
o HCl activates pepsin-enzyme that breaks down the polypeptides into
smaller units
o Digestive enzymes are activated as the broken down polypeptides
move from the stomach to the small intestine
• Protein Absorption
o Takes place in jejunum and ileum
o Protein from food provides 2/3 of amino acids absorbed from the
small intestine
o 1/3 of the amino acids is endogenous
o Once absorbed, the body does not distinguish between amino acids
obtained exogenously/endogenously
o Absorption rates vary according to the protein source – only a certain
amount of amino acids can enter the blood stream (rate limited) (20-
35 g is maximum effective dose at one time to repair, maintain, and
grow)
• Protein transportation
o Liver monitors the supply of amino acids and dictates which amino
acids will be transported to which tissues, except for BCAAs which
circulate immediately in the plasma and are taken up by the muscle
• Liver is the major site of amino acid metabolism
• Deamination
o Fate of nitrogen
• Alpha-ketoacid fate: oxidized for release of energy
• Formation of Carbs and Fats from Excess Proteins
o Glucogenic amino acids
▪ Form pyruvate
▪ Gluconeogenesis
▪ Only leucine and lysine are not glucogenic
▪ Alanine via the glucose-alanine cycle
o Ketogenic amino acids
▪ Form acetyl CoA
▪ Energy production via Krebs cycle
▪ Converted to fat
• Transamination: transfer of NH2 group to another carbon skeleton → new
amino acid is created
o Liver making dispensable amino acids from indispensible ones
• Excess Protein
o Cannot be stored as amino acids – converted to carbs and fats
o There is no storage form for protein
• Anabolism: metabolic processes involving synthesis of simple molecules into
complex molecules
o Skeletal muscle proteins are synthesized using amino acids from the
amino acid pool
o “Anabolic state” occurs when the synthesis of proteins is greater than
the breakdown of proteins
o Amino acids from the amino acid pool are incorporated into synthesis
of proteins
o Major live function: pro anabolism
o Liver makes plasma proteins (albumin)
o Albumin is a protein that helps transport nutrients in the blood to
tissues
• Catabolism: metabolic processes involving the breakdown of complex
molecules into simpler molecules
o Ex: amino acids providing energy
• Excess protein
o Excess NH3 cannot be eliminated in urine and is sweat out – sweat
with NH3 odor may indicate protein intake that is too high
o The body tries to prevent toxic levels by having a rate that food leaves
the stomach – slows it down
