Animal Nutrition: Methods, Enzymes, Glucose Transport, and Energy Calculations

Calculations based off feeding experiments

Direct measurement of intake and fecal/urinary losses

Gold standard, but labor-intensive and expensive.

What are the 5 methods to calculate ME of the diet?

Calculations based off of feeding experiments

Calculations using FACTORS

Atwater

Modified Atwater

NRC

Calculations Using FACTORS

Uses average digestibility coefficients applied to nutrients

Quicker/more practical but assumes GE of nutrients and nutrient digestibility

Atwater method

Uses human-based fixed factors (4 kcal/g protein, 4 kcal/g CHO, 9 kcal/g fat)

Assumes digestibility is high.

Modified Atwater method

Adjusted for dogs and cats (3.5 kcal/g protein, 3.5 kcal/g CHO, 8.5 kcal/g fat)

Accounts for lower digestibility of pet diets.

NRC equation method

Uses species-specific equations and corrections

More accurate (because accounts for fiber) but more complex.

Difference between ME of the diet and MER of the animal

ME of the diet = energy available to the animal from food after accounting for losses (feces, urine, gas)

MER of an animal = actual daily energy requirement the animal needs to maintain its body functions and lifestyle (maintenance, activity, growth, reproduction)

Importance of calculating ME

Ensures proper feeding amounts, prevents under- or over-feeding, avoids obesity or malnutrition, and supports disease management.

Cachexia in dogs with cancer

Altered carbohydrate metabolism (↑ glycolysis, ↓ glucose tolerance)

Tumor cells compete for glucose (Warburg effect)

Increased lactate production and Cori cycle inefficiency

Net energy deficit even if caloric intake is sufficient.

Describe the difference of glucose uptake in most tissues vs enterocytes

Most tissues = facilitated glucose transporters

Enterocytes = Glu/Na+ are coupled

Irreversible enzymes of glycolysis

Rxn 1: Hexokinase/Glucokinase (Glucose → Glucose-6-phosphate)

Rxn 3: Phosphofructokinase-1 (PFK-1) (Fructose-6-phosphate → Fructose-1,6-bisphosphate)

Rxn 10: Pyruvate kinase (Phosphoenolpyruvate → Pyruvate).

Importance of glycolysis enzymes

They are irreversible steps that regulate glycolysis and require bypass in gluconeogenesis.

Difference between hexokinase and glucokinase

Hexokinase: Found in most tissues, high affinity (low Km), inhibited by G6P

Glucokinase: Found in liver and pancreas, low affinity (high Km), not inhibited by G6P, allows regulation of blood glucose.

GLUT transporters

Facilitative glucose transporters in most tissues

GLUT1: Ubiquitous, basal glucose uptake

GLUT2: Liver, pancreatic β-cells, kidney, intestine (bidirectional transport; glucose sensing)

GLUT3: Neurons (high affinity, ensures brain uptake)

GLUT4: Skeletal muscle, adipose (insulin-responsive)

GLUT5: Intestine (fructose transport).

SGLT1

Sodium-glucose cotransporter in enterocytes (small intestine)

Uses Na+ gradient to actively transport glucose against concentration gradient.

Pentose phosphate pathway purpose

To generate NADPH (for biosynthesis and antioxidants) and ribose-5-phosphate (for nucleotide synthesis).

Tissues most active in PPP

Liver, adipose tissue, and mammary gland (high biosynthetic activity).

PPP activity in muscle tissue

Muscle prioritizes ATP production over NADPH/nucleotide synthesis.

Energy needs variation (MER)

Energy needs vary with life stage and activity; the factor adjusts for conditions like growth, reproduction, weight gain, or weight loss.

Weight loss initiation factor

Multiplying RER by a factor of 1.0 initiates weight loss because it only covers the animal's resting metabolic needs with no allowance for activity, growth, or other processes.

Weight gain promotion factor

Multiplying RER by a factor of 1.8 promotes weight gain because it accounts for higher energy needs (growth, reproduction, activity).

Physiological state energy demands

Each physiological state has different energy demands: Weight loss → provide just enough for survival; Maintenance → balance energy intake and expenditure; Growth/lactation → supply extra energy for tissue deposition or milk production.

Role of pyruvate dehydrogenase complex

Links glycolysis, gluconeogenesis, and fatty acid oxidation to the TCA cycle by converting pyruvate to acetyl-CoA.

End products of pyruvate dehydrogenase complex

NADH

Acetyl-CoA

Regulation of pyruvate dehydrogenase complex

Active when dephosphorylated; inhibited by high acetyl-CoA and NADH; activated by CoA and NAD+.

Role of glycogen phosphorylase

First enzyme in glycogenolysis; cleaves glycogen into glucose-1-phosphate.

Activation of glycogen phosphorylase

Activated by glucagon and epinephrine (in the fasted state).

Glycogen phosphorylase function in liver vs. muscle

Liver: maintains blood glucose; releases glucose to circulation

Muscle: provides glucose only for its own energy needs (not exported)

Glycolysis

Breakdown of glucose to pyruvate with small energy yield (ATP, NADH).

Gluconeogenesis

Synthesis of glucose from non-carbohydrate precursors (pyruvate, lactate, amino acids, glycerol).

Glycogenesis

Storage process converting glucose to glycogen.

Glycogenolysis

Breakdown of glycogen into glucose.

TCA cycle

Oxidation of acetyl-CoA to generate ATP, NADH, and FADH₂.

Pentose phosphate pathway

Pathway that produces NADPH and ribose-5-phosphate for biosynthesis and antioxidant defense.

Body condition scores assessment

Visual evaluation plus palpation of fat cover at key anatomical sites.

Application of body condition scoring

Applied throughout management for monitoring nutrition, reproduction, and health.

Difference between BCS and MCS

BCS = fat coverage assessment

MCS (muscle condition score) = muscle mass evaluation (important in animals with normal BCS but muscle wasting).

Importance of body weight estimations

Important for proper dosing of medications, monitoring growth, and assessing nutritional needs when a scale isn't available.

Overlap of management and environmental factors

Timing, frequency, delivery method, and competition at feeding can overlap as both management and environmental factors