Small Animal Nutrition: Micronutrients

Micronutrient Fundamentals and Classification

• Definition & Core Distinctions:

  • Vitamins: Organic compounds that function primarily as essential cofactors, coenzymes, or regulators in metabolic and biochemical reactions.

  • Minerals: Inorganic chemical elements required for structural integrity, neural transmission, fluid and electrolyte balance, and enzymatic activities.

• Major Categories of Micronutrients:

  • Fat-Soluble Vitamins: Include Vitamins A, D, E, and K. They are absorbed in association with dietary fats and stored in lipid-rich body tissues, primarily the liver and adipose tissue.

  • Water-Soluble Vitamins: Include the B-complex vitamins and Vitamin C. They are readily excreted in the urine (with exceptions for Vitamin B12 and Folate) and function extensively as coenzymes in metabolic pathways.

  • Major Minerals: Inorganic elements required in larger dietary quantities to maintain structural, fluid, and electrochemical homeostasis (Sodium, Potassium, Calcium, Phosphorus, Magnesium, Chloride, Sulfur).

  • Trace Minerals: Inorganic elements required in minute quantities that serve as vital catalysts, structural components of metalloenzymes, or hormonal precursors (Iron, Zinc, Selenium, Iodine, Manganese, Chromium, Cobalt, Copper).

Fat-Soluble Vitamins

• Vitamin A:

  • Dietary Sources: Animal fat, fish oil, milk, and plant matter containing yellow or orange pigments.

  • Molecular Forms: Retinol (pre-formed Vitamin A, derived from animal tissues) and $\beta\text{-carotene}$ (Pro-Vitamin A, derived from plant pigments).

  • Physiological Functions: Essential for visual phototransduction, epithelial tissue growth and differentiation, reproductive function, and immune competence.

  • Deficiency Manifestations: Cats possess a strictly limited enzymatic ability to convert $\beta\text{-carotene}$ into active retinol, making them obligate consumers of pre-formed Vitamin A. Inadequate intake yields visual impairment and epithelial lesions.

  • Toxicity (Hypervitaminosis A): Most frequently encountered in cats consuming high-volume liver diets or fish oils. Leads to severe skeletal pathology due to overstimulation of osteoclast activity and induced Vitamin D resistance.

• Vitamin D:

  • Synthesis & Sources: Endogenous synthesis via the cutaneous steroid precursor 7-dehydrocholesterol is extremely limited in dogs and cats. Main dietary sources include animal fat, fish oil, and milk.

  • Species Requirements:

  • Dog Requirement: $15.2\,\text{IU per kg BW}^{0.75}$

  • Cat Requirement: $6.25\,\text{IU per kg BW}^{0.67}$

  • Dietary Forms: Ergocalciferol (Vitamin D2, plant-derived) and Cholecalciferol (Vitamin D3, animal-derived).

  • Metabolic Activation Pathway:     $\text{Cholecalciferol} \rightarrow (\text{Liver, 25-hydroxylase}) \rightarrow \text{Calcidiol} \rightarrow (\text{Kidney, 1}\alpha\text{-hydroxylase}) \rightarrow \text{Calcitriol (Active Form)}$

  • Physiological Functions: Primary regulator of systemic calcium and phosphorus homeostasis, modulation of inflammatory responses, and immune function regulation.

  • Deficiency Manifestations: Results in Rickets in growing puppies and kittens, and Osteomalacia in adult animals due to failure of bone matrix mineralization.

  • Toxicity: Causes severe hypercalcemia and hyperphosphatemia, precipitating widespread soft-tissue mineralization and secondary acute or chronic renal failure.

• Vitamin E ($\alpha\text{-tocopherol}$):

  • Dietary Sources: Animal fat, fish oil, nuts, leafy green vegetables, seeds, and vegetable oils.

  • Species Requirements:

  • Dog Requirement: $1\,\text{IU per kg BW}^{0.75}$

  • Cat Requirement: $0.95\,\text{IU per kg BW}^{0.67}$

  • Physiological Functions: Major lipid-soluble antioxidant protecting cell membranes from lipid peroxidation, supporter of immune integrity, regulation of hematopoiesis, vasodilation, and maintainer of ocular and cutaneous health.

  • Deficiency Manifestations:

  • Cats: Consuming diets high in unsaturated fish oils or fatty fish without supplemental Vitamin E develop Pansteatitis (Yellow Fat Disease), characterized by adipose tissue inflammation and necrosis.

  • Dogs: Deficiencies manifest as erythrocyte fragility leading to hemolytic anemia.

  • Toxicity: Exceptionally rare, but high doses can interfere with Vitamin K-dependent coagulation pathways.

• Vitamin K:

  • Dietary Sources & Forms:

  • Phylloquinone (Vitamin K1): Plant sources, especially leafy greens.

  • Menaquinone (Vitamin K2): Synthesized by intestinal microbiota and present in animal tissues.

  • Menadione (Vitamin K3): Synthetic form used in feed preparations.

  • Physiological Functions: Essential cofactor for the post-translational $\gamma\text{-glutamyl carboxylation}$ of blood coagulation factors II, VII, IX, and X, as well as anticoagulant proteins C and S.

  • Deficiency Manifestations: Clinical coagulopathies and uncontrollable hemorrhage. Commonly secondary to anticoagulant rodenticide ingestion, which inhibits Vitamin K epoxide reductase, blocking necessary recycling of Vitamin K.

  • Toxicity: Extremely rare under normal nutritional conditions.

Water-Soluble Vitamins

• General Characteristics: Readily excreted via the kidneys in urine (excluding Folate and Vitamin B12, which undergo hepatic and enterohepatic retention), and function predominantly as essential coenzymes in intermediary metabolism.

• Vitamin B1 (Thiamine):

  • Dietary Sources: Meat, organ meats, fish meal, eggs, brewer's yeast, whole grains, seeds, and commercial feed powders.

  • Species Requirements:

  • Dog Requirement: $0.06\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $0.11\,\text{mg per kg BW}^{0.67}$

  • Active Form & Functions: Thiamine Pyrophosphate (TPP). Serves as a cofactor for the oxidative decarboxylation of pyruvate, $\alpha\text{-ketoglutarate}$, and branched-chain amino acids, as well as the transketolase enzyme in the hexose monophosphate (HMP) shunt.

  • Deficiency Manifestations: Seen most severely in cats due to higher metabolic demand and susceptibility to heat-labile degradation or thiaminases in raw fish diets. Results in acute neurological dysfunctions, including ventroflexion of the neck, ataxia, seizures, and central nervous system lesions.

  • Toxicity: Extremely rare due to rapid renal excretion.

• Vitamin B2 (Riboflavin):

  • Dietary Sources: Leafy greens, milk, liver, eggs, whole grains, and feed-grade powders.

  • Species Requirements:

  • Dog Requirement: $0.17\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $0.08\,\text{mg per kg BW}^{0.67}$

  • Active Coenzyme Forms: Flavin Adenine Dinucleotide (FAD) and Flavin Mononucleotide (FMN).

  • Physiological Functions: FAD-mediated redox reactions (e.g., conversion of Succinate to Fumarate in the TCA cycle), activation of Vitamin B6, conversion of Tryptophan to Vitamin B3 (Niacin), and acting as a cofactor for Methylenetetrahydrofolate Reductase (MTHFR).

  • Deficiency & Toxicity: Clinical deficiency is uncommon; toxicity is exceptionally rare.

• Vitamin B3 (Niacin):

  • Dietary Sources: Brewer's yeast, fish, organ meats, bran, alfalfa, and feed-grade powders.

  • Species Requirements:

  • Dog Requirement: $0.45\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $0.79\,\text{mg per kg BW}^{0.67}$

  • Active Coenzyme Form: Nicotinamide Adenine Dinucleotide (NAD) / Nicotinamide Adenine Dinucleotide Phosphate (NADP).

  • Physiological Functions: Essential coenzyme in cellular cellular respiration, electron transport, and redox reactions.

  • Deficiency Manifestations: Cats have a strictly limited capacity to synthesize Niacin from the amino acid Tryptophan due to high activity of picolinic carboxylase. Deficiency in cats leads to poor growth, severe ulcerative stomatitis, dermatitis, and gastrointestinal disturbances.

  • Toxicity: Rare.

• Vitamin B5 (Pantothenic Acid):

  • Dietary Sources: Brewer's yeast, distiller's grains, fish, meat, seeds, and legumes.

  • Species Requirements:

  • Dog Requirement: $0.39\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $0.14\,\text{mg per kg BW}^{0.67}$

  • Active Form & Functions: Essential structural constituent of Coenzyme A (CoA). Required for the citric acid cycle (Acetyl-CoA, Succinyl-CoA), fatty acid $\beta\text{-oxidation}$, and the biosynthesis of steroid hormones, cholesterol, and acetylcholine.

  • Deficiency & Toxicity: Deficiency is clinically uncommon; toxicity is rare.

• Vitamin B6 (Pyridoxine):

  • Dietary Sources: Liver, fish, poultry, eggs, whole grains, and feed-grade powders.

  • Species Requirements:

  • Dog Requirement: $0.04\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $0.06\,\text{mg per kg BW}^{0.67}$

  • Active Form: Pyridoxal 5'-phosphate (PLP).

  • Physiological Functions: Transamination and decarboxylation of amino acids, conversion of Tryptophan to Niacin, neurotransmitter synthesis (GABA, serotonin, dopamine), glycogen phosphorylase activity, heme synthesis, immune function, and sphingolipid metabolism.

  • Deficiency Manifestations: Uncommon, but specific deficiency impairs heme biosynthesis, yielding Sideroblastic Anemia.

  • Toxicity: Rare.

• Vitamin B7 (Biotin):

  • Dietary Sources: Brewer's yeast, fish, organ meats, bran, alfalfa, and feed powders.

  • Species Requirements:

  • Dog Requirement: Not established

  • Cat Requirement: $1.5\,\mu\text{g per kg BW}^{0.67}$

  • Physiological Functions: Functions as an essential coenzyme for five mammalian carboxylase enzymes (e.g., pyruvate carboxylase, acetyl-CoA carboxylase).

  • Deficiency Manifestations: Uncommon under typical dietary regimes. Ingestion of raw egg whites containing high concentrations of Avidin (a biotin-binding glycoprotein) prevents biotin absorption, inducing severe dermatitis and alopecia.

  • Toxicity: Rare.

• Vitamin B9 (Folate):

  • Dietary Sources: Fish, meat, leafy green vegetables, whole grains, legumes, and feed powders.

  • Species Requirements:

  • Dog Requirement: $7.1\,\mu\text{g per kg BW}^{0.75}$

  • Cat Requirement: $19\,\mu\text{g per kg BW}^{0.67}$

  • Active Form & Functions: Tetrahydrofolate (THF). One-carbon unit carrier required for purine and pyrimidine synthesis, DNA methylation, and embryonic neural tube development.

  • Deficiency Manifestations: Secondary to malabsorption from upper small intestinal disorders. Results in embryonic development failure, neural tube defects in puppies/kittens, and Macrocytic/Megaloblastic Non-Regenerative Anemia.

  • Toxicity: Rare.

• Vitamin B12 (Cobalamin):

  • Dietary Sources: Organ meats, dairy, eggs, fish, meat products, feed-grade supplements.

  • Species Requirements:

  • Dog Requirement: $0.92\,\mu\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.44\,\mu\text{g per kg BW}^{0.67}$

  • Enzymatic Reactions:

  • Methionine Synthase: Converts Homocysteine to Methionine while regenerating Tetrahydrofolate.

  • Methylmalonyl-CoA Mutase: Converts Methylmalonyl-CoA to Succinyl-CoA.

  • Deficiency Manifestations: Secondary to distal small intestinal diseases or Exocrine Pancreatic Insufficiency (EPI) due to lack of intrinsic factor binding. Results in Macrocytic/Megaloblastic Non-Regenerative Anemia and metabolic acidemia (methylmalonic aciduria).

  • Toxicity: Rare.

• Vitamin C (Ascorbate):

  • Sources & Biosynthesis: Vegetables, fruits, synthetic feed powders; endogenously converted from glucose in hepatic tissue of dogs and cats.

  • Physiological Functions: Water-soluble antioxidant, essential cofactor for proline and lysine hydroxylases in collagen synthesis, conversion of dopamine to norepinephrine, carnitine biosynthesis, peptide amidation, and epigenetic modulation.

  • Deficiency Manifestations: Extremely rare due to endogenous hepatic synthesis.

  • Toxicity: Excessive intake yields breakdown into oxalic acid, increasing risk for Calcium Oxalate Urolithiasis.

Vitamin B-Complex Metabolic Interaction Pathway

• Interdependent Methylation & Transsulfuration Cycle:

  • Step 1 (Folate Activation): Vitamin B9 (Folate) is converted to Tetrahydrofolate (THF), which is methylated to $5,10\text{-methylene tetrahydrofolate}$.

  • Step 2 (Conversion via B6 & B2):

  • Conversion of THF to $5,10\text{-methylene THF}$ utilizes the enzyme Serine Hydroxymethyltransferase, which requires Vitamin B6.

  • $5,10\text{-methylene THF}$ is converted to $5\text{-methyl THF}$ via Methylenetetrahydrofolate Reductase (MTHFR), which relies on Vitamin B2 (Flavin Adenine Dinucleotide, FAD) as a cofactor.

  • Step 3 (Methionine Regeneration via B12): $5\text{-methyl THF}$ donates its methyl group to Homocysteine to form Methionine, catalyzed by Methionine Synthase, which requires Vitamin B12. This step simultaneously regenerates free Tetrahydrofolate.

  • Step 4 (Epigenetics): Methionine is converted to S-Adenosylmethionine (SAMe), the primary methyl donor for downstream cellular methylation reactions and epigenetic modulation.

  • Step 5 (Transsulfuration via B6): Excess Homocysteine is permanently cleared via conversion to Cystathionine and subsequently Cysteine by Cystathionine $\beta\text{-synthase}$ and Cystathionine $\gamma\text{-lyase}$, both of which strictly require Vitamin B6.

Major Minerals

• Sodium ($\text{Na}^+$):

  • Localization & Sources: Dominant extracellular fluid (ECF) cation. Dietary salt, fish, meat, dairy products, and mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.03\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.02\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Maintenance of ECF volume and plasma osmolarity, systemic blood pressure regulation, generation of cell membrane resting and action potentials, nutrient co-transport, and acid-base equilibrium.

  • Hormonal Regulation: Controlled via Aldosterone (promotes renal retention) and Atrial Natriuretic Peptide (ANP, promotes renal excretion).

  • Imbalances & Clinical Applications:

  • Primary dietary imbalances are rare.

  • Concurrent Hyponatremia and Hyperkalemia in dogs is pathognomonic for Hypoadrenocorticism (Addison's Disease).

  • Rapid correction of serum sodium concentrations must be avoided to prevent Central Pontine Myelinolysis (Osmotic Demyelination Syndrome) or Cerebral Edema.

• Potassium ($\text{K}^+$):

  • Localization & Sources: Primary intracellular fluid (ICF) cation. Molasses, organ meals, and mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.14\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.15\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Maintenance of intracellular volume, resting membrane potential, neuromuscular transmission, cardiac electrical conduction, vascular tone, and renal acid-base balance.

  • Hormonal Regulation: Regulated primarily by Aldosterone.

  • Imbalances:

  • Hypokalemia: In cats, presents with severe generalized muscle weakness and pathognomonic Cervical Ventroflexion (inability to lift the head).

  • Hyperkalemia: Induces severe cardiac conduction defects, bradycardia, tall tented T-waves, ventricular arrhythmias, and cardiac arrest.

• Calcium ($\text{Ca}^{2+}$):

  • Distribution & Sources: $99\%$ sequestered in skeletal bone and teeth; $1\%$ in circulation and soft tissues. Limestone, marine shells, meat and bone meal, mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.14\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.10\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Skeletal mineralization, cellular signal transduction, excitation-contraction coupling in muscle, exocytosis of neurotransmitters, enzymatic co-factor, and blood coagulation cascade activity.

  • Hormonal Regulation: Regulated by Parathyroid Hormone (PTH, elevates serum Ca), Calcitriol (elevates serum Ca), and Calcitonin (lowers serum Ca).

  • Developmental Toxicity: Excess dietary calcium in growing large and giant breed dogs causes osteochondrosis and severe developmental orthopedic diseases due to impaired endochondral ossification.

• Phosphorus ($\text{P}$):

  • Distribution & Sources: $85\%$ stored in bones and teeth. Grains, oilseed meals, meat, bone meal, and mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.11\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.06\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Bone/teeth structure, cellular energy buffering via ATP, phospholipid membrane bilayers, nucleic acid backbone synthesis, and acid-base buffering.

  • Hormonal Regulation: Parathyroid Hormone (PTH), Calcitriol, and Fibroblast Growth Factor 23 (FGF23).

  • Renal Pathology Connection: In Chronic Kidney Disease (CKD), diminished glomerular filtration reduces phosphorus excretion, leading to Hyperphosphatemia. High serum phosphorus drives secondary hyperparathyroidism.

• Magnesium ($\text{Mg}^{2+}$):

  • Dietary Sources: Limestone, legumes, and mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.02\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.01\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Natural calcium channel blocker preventing neural overexcitation; structural cofactor for ATP-dependent enzymes and phospholipid synthesis.

  • Pathology & Imbalances:

  • Hypermagnesemia: Occurs secondary to severe renal failure, causing cardiorespiratory depression.

  • Hypomagnesemia: Inhibits PTH secretion and end-organ responsiveness, while activating renal potassium secretion pathways; makes refractory hypokalemia and hypocalcemia impossible to resolve without magnesium repletion.

  • Urolithiasis: Magnesium is a core constituent of Struvite (Magnesium Ammonium Phosphate) urinary crystals and uroliths.

• Chloride ($\text{Cl}^-$):

  • Dietary Sources: Salt, meat, fish, milk, seaweed, and mineral mixes.

  • Species Requirements:

  • Dog Requirement: $0.04\,\text{g per kg BW}^{0.75}$

  • Cat Requirement: $0.03\,\text{g per kg BW}^{0.67}$

  • Physiological Functions: Gastric hydrochloric acid ($\text{HCl}$) production, osmolarity maintenance, acid-base balance, and the chloride shift (Hamburger Phenomenon) during $CO_2$ transport in red blood cells.

  • Clinical Pathophysiology: Upper GI vomiting results in massive loss of $H^+$ and $Cl^-$ ions, producing Hypochloremic Metabolic Alkalosis.

• Sulfur ($\text{S}$):

  • Dietary Sources: Meat, eggs, dairy, and sulfur-containing amino acids (Methionine, Cysteine).

  • Species Requirements: Not specifically established for dogs or cats.

  • Physiological Functions: Component of structural proteins (disulfide bonds), vitamins (Thiamine, Biotin), and endogenous antioxidants (Glutathione).

  • Species Peculiarity: Cats possess limited enzymatic pathway capacity to convert Cysteine to Taurine, necessitating direct dietary intake of sulfur-derived Taurine.

Pathophysiology of Nutritional Secondary Hyperparathyroidism

• Etiological Triggers:

  1. Inadequate Dietary Intake: Low calcium diets, high phosphorus diets, or an unbalanced (narrow or inverted) Dietary $Ca:P$ ratio.

  2. Intestinal Malabsorption: Generalized gastrointestinal disease, Exocrine Pancreatic Insufficiency (EPI), or severe malabsorptive states.

  3. Vitamin D Insufficiency: Inadequate dietary Vitamin D, chronic hepatic dysfunction (impaired 25-hydroxylase), or lack of UV exposure.

• Cascade Mechanism:

  • Low serum calcium levels (Hypocalcemia) or hypophosphatemia trigger chronic hypersecretion of Parathyroid Hormone (PTH) from the parathyroid glands.

• Downstream Physiological Responses to Chronic Elevated PTH:

  • Bone Demineralization: Sustained PTH activity hyper-activates osteoclasts, stripping calcium and phosphorus from the skeletal matrix, leading to Fibrous Osteodystrophy ("rubber jaw"), severe bone pain, pathological fractures, and skeletal deformities.

  • Renal Compensations: Increases renal tubular reabsorption of calcium, decreases phosphate reabsorption, and upregulates renal $1\alpha\text{-hydroxylase}$ to generate active Calcitriol.

  • Intestinal Compensations: Calcitriol increases active intestinal transport and absorption of calcium and phosphorus, attempting to restore serum calcium concentrations.

Trace Minerals

• Iron ($\text{Fe}$):

  • Dietary Sources: Forage, fish/meat/blood meal, whole grains, and inorganic iron salts.

  • Species Requirements:

  • Dog Requirement: $1\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $2\,\text{mg per kg BW}^{0.67}$

  • Physiological Functions: Oxygen binding in hemoglobin and myoglobin, core constituent of mitochondrial electron transport enzymes (Aconitase, Succinate Dehydrogenase).

  • Systemic Regulation: Stored bound to Ferritin; transported via Transferrin. Systemically controlled by Hepcidin (hepatic hormone that blocks enterocyte iron export) and Erythroferrone (erythroblast hormone that inhibits hepcidin to increase iron release).

  • Deficiency: Chronic blood loss or inadequate intake causes Microcytic, Hypochromic Anemia.

  • Toxicity: Ingestion of excess iron overwhelms binding proteins, causing direct mucosal corrosion, severe lipid peroxidation, systemic shock, and acute hepatic failure.

• Zinc ($\text{Zn}$):

  • Dietary Sources: Meat, fish, whole grains, legumes, and oilseed meals.

  • Species Requirements:

  • Dog Requirement: $2\,\text{mg per kg BW}^{0.75}$

  • Cat Requirement: $1.88\,\text{mg per kg BW}^{0.67}$

  • Physiological Functions: Immune cell function, tissue repair, zinc metalloproteases, gustatory sensation via Gustin (Carbonic Anhydrase VI), and carbohydrate metabolism.

  • Deficiency Manifestations: Zinc-Responsive Dermatosis. Seen as an inherited mucosal absorption defect in Northern breeds (Siberian Husky, Alaskan Malamute), presenting with hyperkeratosis and parakeratotic scaling around facial mucocutaneous junctions.

  • Toxicity: Acute ingestion (e.g., zinc metallic objects/pennies) causes severe gastroenteritis, intravascular hemolysis, and hemolytic anemia.

• Selenium ($\text{Se}$):

  • Dietary Requirements:

  • Wet Food Formulations: Dog: $6.4\,\mu\text{g per kg BW}^{0.75}$; Cat: $6.6\,\mu\text{g per kg BW}^{0.67}$

  • Dry Food Formulations: Dog: $5.2\,\mu\text{g per kg BW}^{0.75}$; Cat: $5.3\,\mu\text{g per kg BW}^{0.67}$

  • Physiological Functions: Catalytic subunit of Glutathione Peroxidase and Thioredoxin Reductase enzymes; cofactor for iodothyronine deiodinases in thyroid hormone conversion.

  • Deficiency: Dual Vitamin E and Selenium deficiency causes severe Nutritional Myopathy (white muscle disease/myodegeneration).

  • Toxicity: Cellular damage presenting as gastrointestinal distress, alopecia, nail lesions, and neurological abnormalities.

• Iodine ($\text{I}$):

  • Dietary Sources: Mineral mixes, seaweeds, kelp.

  • Species Requirements:

  • Dog Requirement: $30\,\mu\text{g per kg BW}^{0.75}$

  • Cat Requirement: $30\,\mu\text{g per kg BW}^{0.67}$

  • Physiological Functions: Essential substrate for the thyroid gland synthesis of thyroxine ($T_4$) and triiodothyronine ($T_3$).

  • Deficiency & Toxicity Manifestations: Inadequate intake suppresses thyroid hormone output, elevating TSH and causing thyroid gland hyperplasia (Goiter) and hypothyroidism. Excessive intake similarly disrupts thyroid feedback regulation, precipitating hypothyroid-like syndromes.

• Manganese ($\text{Mn}$):

  • Dietary Sources: Grains, legumes, mineral mixes.

  • Species Requirements:

  • Dog Requirement: $160\,\mu\text{g per kg BW}^{0.75}$

  • Cat Requirement: $130\,\mu\text{g per kg BW}^{0.67}$

  • Physiological Functions: Cofactor for Manganese Superoxide Dismutase (MnSOD), Pyruvate Carboxylase, Phosphoenolpyruvate Carboxykinase (PEPCK), Glutamine Synthetase, and glycosyltransferases for glycoprotein and cartilage matrix synthesis.

  • Deficiency & Toxicity: Clinical presentations are exceptionally rare.

• Chromium ($\text{Cr}$):

  • Dietary Sources: Brewer's yeast, meat, whole grains, soybean meal, mineral mixes.

  • Active Form & Functions: Chromodulin; enhances cellular sensitivity to insulin, potentiating glucose uptake.

  • Toxicity: Trivalent Chromium [$Cr(III)$] in dietary supplements displays very low toxicity; Hexavalent Chromium [$Cr(VI)$] is a potent environmental toxicant causing severe renal, hepatic, and gastrointestinal necrosis upon accidental ingestion.

• Cobalt ($\text{Co}$):

  • Dietary Sources: Fish, meat, organ meats, shells, mineral mixes.

  • Physiological Functions: Structural atom at the center of the corrin ring of Vitamin B12 (Cobalamin). Essential for all B12-dependent metabolic pathways.

  • Toxicity: Non-dietary industrial exposures yield acute gastrointestinal illness.

• Copper ($\text{Cu}$):

  • Dietary Sources: Grains, mineral mixes.

  • Species Requirements:

  • Dog Requirement: $200\,\mu\text{g per kg BW}^{0.75}$

  • Cat Requirement: $130\,\mu\text{g per kg BW}^{0.67}$

  • Physiological Functions: Essential cofactor for Ceruloplasmin (iron transport), Cytochrome c Oxidase (ATP generation), Tyrosinase (melanin synthesis), Lysyl Oxidase, and Lysine Hydroxylase.

  • Deficiency: Impairs iron mobilization, yielding Microcytic, Hypochromic Anemia.

  • Toxicity (Copper-Associated Hepatopathy): Genetic impairment of biliary copper excretion causes toxic accumulation in hepatocytes, progressing from chronic hepatitis to liver cirrhosis and failure. Highly prevalent in Bedlington Terriers, Labrador Retrievers, Doberman Pinschers, and West Highland White Terriers.

Differential Diagnosis of Micronutrient-Induced Anemias and Clinical Syndromes

• Microcytic, Hypochromic Anemia:

  • Etiologies: Caused by Iron Deficiency (impaired hemoglobin synthesis) or Copper Deficiency (impaired ceruloplasmin-mediated iron transport).

• Macrocytic, Megaloblastic Non-Regenerative Anemia:

  • Etiologies: Caused by Folate (Vitamin B9) Deficiency or Cobalamin (Vitamin B12) Deficiency (impaired purine/pyrimidine synthesis and delayed nuclear maturation).

• Sideroblastic Anemia:

  • Etiology: Specific to Pyridoxine (Vitamin B6) Deficiency due to failure of $\delta\text{-aminolevulinic acid}$ synthesis in early heme assembly.

• Hemolytic Anemia:

  • Etiologies: Vitamin E Deficiency in dogs (membrane oxidative lysis) or acute Zinc Toxicity (oxidative damage to erythrocytes).